Spectral Signatures of Hydrated Proton Vibrations in Water Clusters

Emerging Two-Dimensional Materials for Proton-Based Energy Storage

The rapid diffusion kinetics and smallest ion radius make protons the ideal cations toward the ultimate energy storage technology combining the ultrafast charging capabilities of supercapacitors and the high energy densities of batteries. Despite the concept existing for centuries, the lack of satisfactory electrode materials hinders its practical development. Recently, the rapid advancement of the emerging two-dimensional (2D) materials, characterized by their ultrathin morphology, interlayer van der Waals gaps, and distinctive electrochemical properties, injects promises into future proton-based energy storage systems. In this perspective, we comprehensively summarize the current advances in proton-based energy storage based on 2D materials. We begin by providing an overview of proton-based energy storage systems, including proton batteries, pseudocapacitors and electrical double layer capacitors. We then elucidate the fundamental knowledge about proton transport characteristics, including in electrolytes, at electrolyte/electrode interfaces, and within electrode materials, particularly in 2D material systems. We comprehensively summarize specific cases of 2D materials as proton electrodes, detailing their design concepts, proton transport mechanism and electrochemical performance. Finally, we provide insights into the prospects of proton-based energy storage systems, emphasizing the importance of rational design of 2D electrode materials and matching electrolyte systems.

Resource Optimization for Quantum Dynamics with Tensor Networks: Quantum and Classical Algorithms

The exponential scaling of the quantum degrees of freedom with the size of the system is one of the biggest challenges in computational chemistry and particularly in quantum dynamics. We present a tensor network approach for the time-evolution of the nuclear degrees of freedom of multiconfigurational chemical systems at a reduced storage and computational complexity. We also present quantum algorithms for the resultant dynamics. To preserve the compression advantage achieved via tensor network decompositions, we present an adaptive algorithm for the regularization of nonphysical bond dimensions, preventing the potentially exponential growth of these with time. While applicable to any quantum dynamical problem, our method is particularly valuable for dynamical simulations of nuclear chemical systems. Our algorithm is demonstrated using ab initio potentials obtained for a symmetric hydrogen-bonded system, namely, the protonated 2,2'-bipyridine, and compared to exact diagonalization numerical results.

Impact of Nuclear Quantum Effects on the Structural Properties of Protonated Water Clusters

Nuclear quantum effects (NQEs) play a crucial role in hydrogen-bonded systems due to quantum tunneling and proton fluctuation. Our understanding of how NQEs affect microstructures mainly focuses on bulk phases of liquids and solids but remains deficient for water clusters, including their hydrogen nuclei, hydrogen-bonded configurations, and temperature dependence. Here, we conducted ab initio molecular dynamics (MD) and path integral MD simulations to investigate the influence of NQEs on the structural properties of protonated water clusters H+(H2O)n (n = 3, 6, 9, 12). The results reveal that the NQEs become less evident as the cluster size increases due to the competition between NQEs and electrostatic interactions. Simulations of several H+(H2O)6 isomers at different temperatures indicate that the effect of elevated temperature on proton transfer is related to the initial structure. Interestingly, the process of proton transfer also involves the interconversion between Zundel-type and Eigen-type isomers. These findings significantly deepen our understanding of ion-water and water-water interactions, opening new avenues for the study of hydrated ion clusters and related systems.

Hydrogen bond network structures of protonated dimethylamine clusters H+(DMA)n (n = 3-7)

Infrared spectroscopy of protonated dimethylamine clusters, H+(DMA)n, (n = 3-7), and their Ar-tagged clusters was performed in the NH and CH stretching vibrational region to explore their hydrogen bond network structures. A stable isomer search and vibrational spectral simulations of the clusters were also carried out to support the interpretations of the observed spectra. Weakly hydrogen-bonded NH stretching vibrational bands, which are characteristic of cyclic structures of small-sized protonated clusters, are observed in the spectra of the Ar-tagged clusters of n ≥ 5, while only linear chain type structures are suggested for the Ar-tagged clusters of n = 3-4 and the bare clusters of all the sizes. These results demonstrate that the size and temperature dependence of the hydrogen bond network structures of the protonated dimethylamine clusters is analogous to that of protonated monohydric alcohol clusters.

Size-Dependent Fullerenes for Enhanced Interaction of l-Leucine: A Combined DFT and MD Simulations Approach

Fullerene-based biosensors have received great attention due to their unique electronic properties that allow them to transduce electrical signals by accepting electrons from amino acids. Babies with MSUD (maple syrup urine disease) are unable to break down amino acids such as l-leucine, and excess levels of the l-leucine are harmful. Therefore, sensing of l-leucine is foremost required. We aim to investigate the interaction tendencies of size-variable fullerenes (CX; X = 24, 36, 50, and 70) toward l-leucine (LEU) using density functional theory (DFT-D3) and classical molecular dynamics (MD) simulation. The C24 fullerene shows the highest affinity of the LEU biomolecule in the gas phase. Smaller fullerenes (C24 and C36) show stronger interactions with leucine due to their higher curvature in water environments. Moreover, recovery times in the ranges of 1010 and 104 s make it a viable candidate for the isolation application of LEU from the biological system. Further, the interaction between LEU and fullerenes is in line with the natural bond order (NBO) analysis, Mulliken charge analysis, quantum theory atom in molecule (QTAIM) analysis, and reduced density gradient (RDG) analysis. At 310 K, employing the explicit water model in classical MD simulations, fullerenes C24 and C36 demonstrate notably elevated binding free energies (-24.946 kJ/mol) in relation to LEU, showcasing their potential as sensors for l-leucine. Here, we demonstrate that the smaller fullerene exhibits a higher potential for l-leucine sensors than the larger fullerene.

A win-win platform: Stabilized black phosphorous nanosheets loading gallium ions for enhancing the healing of bacterial-infected wounds through synergistic antibacterial approaches

Bacterial infection is the most common complication in wound healing, highlighting an urgent need for the development of innovative antibacterial technologies and treatments to address the growing threats posed by bacterial infections. Black phosphorus nanosheets (BPNSs), as a promising two-dimensional nanomaterial, have been utilized in treating infected wounds. However, BP's limited stability restricts its application. In this study, we enhance BP's stability and its antibacterial properties by anchoring gallium ions (Ga3+) onto BP's surface, creating a novel antibacterial platform. This modification reduces BP's electron density and enhances its antibacterial capabilities through a synergistic effect. Under near-infrared (NIR) irradiation, the BP/Ga3+ combination exerts antibacterial effects via photothermal therapy (PTT) and photodynamic therapy (PDT), while also releasing Ga3+. The Ga3+ employ a 'Trojan horse strategy' to disrupt iron metabolism, significantly boosting the antibacterial efficacy of the complex. This innovative material offers a viable alternative to antibiotics and holds significant promise for treating infected wounds and aiding skin reconstruction.

Stability and Structure of Potentially Atmospherically Relevant Glycine Ammonium Bisulfate Clusters

New particle formation (NPF) is the process by which trace atmospheric acids and bases cluster and grow into particles that ultimately impact climate. Sulfuric acid concentration drives NPF, but nitrogen-containing bases promote the formation of more stable clusters via salt bridge formation. Recent computational efforts have suggested that amino acids can enhance NPF, predicting that they can stabilize new particles via multiple protonation sites, but there has yet to be experimental validation of these predictions. We used mass spectrometry and infrared spectroscopy to study the structure and stability of cationic clusters composed of glycine, sulfuric acid, and ammonia. When collisionally activated, clusters were significantly more likely to eliminate ammonia or sulfuric acid than glycine, while quantum chemical calculations predicted lower binding free energies for ammonia but similar binding free energies for glycine and sulfuric acid. These calculations predicted several low-energy structures, so we compared experimental and computed vibrational spectra to attempt to validate the computationally predicted minimum energy structure. Unambiguous identification of the experimental structure by comparison to these calculations was made difficult by the complexity of the experimental spectra and the fact that the identity of the computed lowest-energy structure depended strongly on temperature. If their vapors are present, amino acids are likely to be enriched in new particles by displacing more weakly bound ammonia, similar to the behavior of other atmospheric amines. The carboxylic acid groups were found to preferentially interact with other carboxylic acids, suggesting incipient organic/inorganic phase separation even at these small sizes.

Encoding a Many-Body Potential Energy Surface into a Grid-Based Matrix Product Operator

An efficient algorithm for compressing a given many-body potential energy surface (PES) of molecular systems into a grid-based matrix product operator (MPO) is proposed. The PES is once represented by a full-dimensional or truncated many-body expansion form, which is obtained by ab initio calculations at each grid mesh point, and then all terms in the expansion are compressed and merged into a single MPO while maintaining the bond dimension of the MPO as small as possible. It was shown that the ab initio PES of the H2CO was compressed by more than 2 orders of magnitude in the size of the site operators without loss of accuracy. By the use of grid basis, the tensor rank of the site operators of the MPO is reduced from four to three due to the diagonal nature of the position-dependent operators on grid basis, which significantly reduces the computational cost of the tensor contractions required in the real and imaginary time evolution of the matrix product state (MPS) wave functions with the grid-based MPO (Grid-MPO) Hamiltonian. Similar to other grid-based methods, Grid-MPO is easily applicable to any kinds of potentials of molecular systems, such as analytical empirical model potentials expressed by position operators and ab initio potentials, if the values at the grid points are available. Using the Grid-MPO combined with the MPS, we calculated the time correlation function of the Eigen cation H 3 O + ( H 2 O ) 3 to predict the infrared spectrum and compared with the experimental and the previous theoretical studies. The actual scaling with the size of systems was examined for the multidimensional Henon-Heiles Hamiltonian. It was shown that the method is considerably accelerated by the graphic processing unit (GPU) because the sizes of site operators were kept small and all tensors were able to be stored on the VRAM of a GPU.

Competition between Water-Water Hydrogen Bonds and Water-π Bonds in Pyrene-Water Cluster Anions

We present infrared spectra and density functional theory calculations of hydrated pyrene anion clusters with up to four water molecules. The experimental spectra were acquired by using infrared Ar messenger photodissociation spectroscopy. Water molecules form clusters on the surface of the pyrene, forming hydrogen bonds with the π-system. The structures of the water clusters and their interaction with the π-system are encoded in OH stretching vibrational modes. We find that the interactions between water molecules are stronger than the interactions between water molecules and the π-system. While all clusters show multiple conformers, three- and four-membered rings are the lowest energy structures in the larger hydrates.

Correlations between the Structures and Spectra of Protonated Water Clusters

Badger's rule-like correlations between OH stretching frequencies and intensities and the OH bond length are used to develop a spectral mapping procedure for studies of pure and protonated water clusters. This approach utilizes the vibrationally averaged OH bond lengths, which were obtained from diffusion Monte Carlo simulations that were performed using the general potential developed by Yu and Bowman. Good agreement is achieved between the spectra obtained using this approach and previously reported spectra for H+(H2O)n clusters, with n = 3, 4, and 5, as well as their perdeuterated analogues. The analysis of the spectra obtained by this spectral mapping approach supports previous work that assigned the spectrum of H+(H2O)6 to a mixture of Eigen and Zundel-like structures. Analysis of the calculated spectra also suggests a reassignment of the frequency of one of the transitions that involves the OH stretching vibration of the OH bonds in the hydronium core in the Eigen-like structure of H+(H2O)6 from 1917 cm-1 to roughly 2100 cm-1. For D+(D2O)6, comparison of the measured spectrum to those obtained by using the spectral mapping approach suggests that the carrier of the measured spectrum is one or more of the isomers of D+(D2O)6 that contain a four-membered ring and two flanking water molecules. While there are several candidate structures, the two flanking water molecules most likely form a chain that is bound to the hydronium core.

ZundEig: The Structure of the Proton in Liquid Water from Unsupervised Learning

The structure of the excess proton in liquid water has been the subject of lively debate on both experimental and theoretical fronts for the last century. Fluctuations of the proton are typically interpreted in terms of limiting states referred to as the Eigen and Zundel species. Here, we put these ideas under the microscope, taking advantage of recent advances in unsupervised learning that use local atomic descriptors to characterize environments of acidic water combined with advanced clustering techniques. Our agnostic approach leads to the observation of only one charged cluster and two neutral ones. We demonstrate that the charged cluster involving the excess proton is best seen as an ionic topological defect in water's hydrogen bond network, forming a single local minimum on the global free-energy landscape. This charged defect is a highly fluxional moiety, where the idealized Eigen and Zundel species are neither limiting configurations nor distinct thermodynamic states. Instead, the ionic defect enhances the presence of neutral water defects through strong interactions with the network. We dub the combination of the charged and neutral defect clusters as ZundEig, demonstrating that the fluctuations between these local environments provide a general framework for rationalizing more descriptive notions of the proton in the existing literature.

Graph-|Q⟩⟨C|: A Quantum Algorithm with Reduced Quantum Circuit Depth for Electronic Structure

The accurate determination of chemical properties is known to have a critical impact on multiple fundamental chemical problems but is deeply hindered by the steep algebraic scaling of electron correlation calculations and the exponential scaling of quantum nuclear dynamics. With the advent of new quantum computing hardware and associated developments in creating new paradigms for quantum software, this avenue has been recognized as perhaps one way to address exponentially complex challenges in quantum chemistry and molecular dynamics. In this paper, we discuss a new approach to drastically reduce the quantum circuit depth (by several orders of magnitude) and help improve the accuracy in the quantum computation of electron correlation energies for large molecular systems. The method is derived from a graph-theoretic approach to molecular fragmentation and enables us to create a family of projection operators that decompose quantum circuits into separate unitary processes. Some of these processes can be treated on quantum hardware and others on classical hardware in a completely asynchronous and parallel fashion. Numerical benchmarks are provided through the computation of unitary coupled-cluster singles and doubles (UCCSD) energies for medium-sized protonated and neutral water clusters using the new quantum algorithms presented here.

Thermal dependence of the hydrated proton and optimal proton transfer in the protonated water hexamer

Water is a key ingredient for life and plays a central role as solvent in many biochemical reactions. However, the intrinsically quantum nature of the hydrogen nucleus, revealing itself in a large variety of physical manifestations, including proton transfer, gives rise to unexpected phenomena whose description is still elusive. Here we study, by a combination of state-of-the-art quantum Monte Carlo methods and path-integral molecular dynamics, the structure and hydrogen-bond dynamics of the protonated water hexamer, the fundamental unit for the hydrated proton. We report a remarkably low thermal expansion of the hydrogen bond from zero temperature up to 300 K, owing to the presence of short-Zundel configurations, characterised by proton delocalisation and favoured by the synergy of nuclear quantum effects and thermal activation. The hydrogen bond strength progressively weakens above 300 K, when localised Eigen-like configurations become relevant. Our analysis, supported by the instanton statistics of shuttling protons, reveals that the near-room-temperature range from 250 K to 300 K is optimal for proton transfer in the protonated water hexamer.

Cryo IR spectroscopy and cryo kinetics of dinitrogen activation and cleavage by small tantalum cluster cations

We investigate small tantalum clusters Tan+, n = 2-4, for their capability to cleave N2 adsorption spontaneously. We utilize infrared photon dissociation (IR-PD) spectroscopy of isolated and size selected clusters under cryogenic conditions within a buffer gas filled ion trap, and we augment our experiments by quantum chemical simulations (at DFT level). All Tan+ clusters, n = 2-4, seem to cleave N2 efficiently. We confirm and extend a previous study under ambient conditions on Ta2+ cluster [Geng et al., Proc. Natl. Acad. Sci. U. S. A. 115, 11680-11687 (2018)]. Our cryo studies and the concomitant DFT simulations of the tantalum trimer Ta3+ suggest cleavage of the first and activation of the second and third N2 molecule across surmountable barriers and along much-involved multidimensional reaction paths. We unravel the underlying reaction processes and the intermediates involved. The study of the N2 adsorbate complexes of Ta4+ presented here extends our earlier study and previously published spectra from (4,m), m = 1-5 [Fries et al., Phys. Chem. Chem. Phys. 23(19), 11345-11354 (2021)], up to m = 12. We confirm the priory published double activation and nitride formation, succeeded by single side-on N2 coordination. Significant red shifts of IR-PD bands from these side-on coordinated μ2-κN:κN,N N2 ligands correlate with the degree of tilting towards the second coordinating Ta center. All subsequently attaching N2 adsorbates onto Ta4+ coordinate in an end-on fashion, and we find clear evidence for co-existence of end-on coordination isomers. The study of stepwise N2 adsorption revealed adsorption limits m(max) of [Tan(N2)m]+ which increase with n, and kinetic fits revealed significant N2 desorption rates upon higher N2 loads. The enhanced absolute rate constants of the very first adsorbate steps kabs(n,0) of the small Ta3+ and Ta4+ clusters independently suggest dissociative N2 adsorption and likely N2 cleavage into Ta nitrides.

Synthesis of Hidden Subgroup Quantum Algorithms and Quantum Chemical Dynamics

We describe a general formalism for quantum dynamics and show how this formalism subsumes several quantum algorithms, including the Deutsch, Deutsch-Jozsa, Bernstein-Vazirani, Simon, and Shor algorithms as well as the conventional approach to quantum dynamics based on tensor networks. The common framework exposes similarities among quantum algorithms and natural quantum phenomena: we illustrate this connection by showing how the correlated behavior of protons in water wire systems that are common in many biological and materials systems parallels the structure of Shor's algorithm.

Infrared spectroscopy of cations in helium nanodroplets

Here, we describe our pulsed helium droplet apparatus for spectroscopy of molecular ions. Our approach involves the doping of the droplets of about 10 nm in diameter with precursor molecules, such as ethylene, followed by electron impact ionization. Droplets containing ions are irradiated by the pulsed infrared laser beam. Vibrational excitation of the embedded cations leads to the evaporation of the helium atoms in the droplets and the release of the free ions, which are detected by the quadrupole mass spectrometer. In this work, we upgraded the experimental setup by introducing an octupole RF collision cell downstream from the electron impact ionizer. The implementation of the RF ion guide increases the transmission efficiency of the ions. Filling the collision cell with additional He gas leads to a decrease in the droplet size, enhancing sensitivity to the laser excitation. We show that the spectroscopic signal depends linearly on the laser pulse energy, and the number of ions generated per laser pulse is about 100 times greater than in our previous experiments. These improvements facilitate faster and more reproducible measurements of the spectra, yielding a handy laboratory technique for the spectroscopic study of diverse molecular ions and ionic clusters at low temperature (0.4 K) in He droplets.

Developing machine-learned potentials to simultaneously capture the dynamics of excess protons and hydroxide ions in classical and path integral simulations

The transport of excess protons and hydroxide ions in water underlies numerous important chemical and biological processes. Accurately simulating the associated transport mechanisms ideally requires utilizing ab initio molecular dynamics simulations to model the bond breaking and formation involved in proton transfer and path-integral simulations to model the nuclear quantum effects relevant to light hydrogen atoms. These requirements result in a prohibitive computational cost, especially at the time and length scales needed to converge proton transport properties. Here, we present machine-learned potentials (MLPs) that can model both excess protons and hydroxide ions at the generalized gradient approximation and hybrid density functional theory levels of accuracy and use them to perform multiple nanoseconds of both classical and path-integral proton defect simulations at a fraction of the cost of the corresponding ab initio simulations. We show that the MLPs are able to reproduce ab initio trends and converge properties such as the diffusion coefficients of both excess protons and hydroxide ions. We use our multi-nanosecond simulations, which allow us to monitor large numbers of proton transfer events, to analyze the role of hypercoordination in the transport mechanism of the hydroxide ion and provide further evidence for the asymmetry in diffusion between excess protons and hydroxide ions.

Quantum Computation of Hydrogen Bond Dynamics and Vibrational Spectra

Calculating observable properties of chemical systems is often classically intractable and widely viewed as a promising application of quantum information processing. Here, we introduce a new framework for solving generic quantum chemical dynamics problems using quantum logic. We experimentally demonstrate a proof-of-principle instance of our method using the QSCOUT ion-trap quantum computer, where we experimentally drive the ion-trap system to emulate the quantum wavepacket dynamics corresponding to the shared-proton within an anharmonic hydrogen bonded system. Following the experimental creation and propagation of the shared-proton wavepacket on the ion-trap, we extract measurement observables such as its time-dependent spatial projection and its characteristic vibrational frequencies to spectroscopic accuracy (3.3 cm-1 wavenumbers, corresponding to >99.9% fidelity). Our approach introduces a new paradigm for studying the chemical dynamics and vibrational spectra of molecules and opens the possibility to describe the behavior of complex molecular processes with unprecedented accuracy.

Magic Numbers and Stabilities of Photoionized Water Clusters: Computational and Experimental Characterization of the Nanosolvated Hydronium Ion

The stability and distributions of small water clusters generated in a supersonic beam expansion are interrogated by tunable vacuum ultraviolet (VUV) radiation generated at a synchrotron. Time-of-flight mass spectrometry reveals enhanced population of various protonated water clusters (H⁺(H₂O)_n) based upon ionization energy and photoionization distance from source, suggesting there are "magic" numbers below the traditional n = 21 that predominates in the literature. These intensity distributions suggest that VUV threshold photoionization (11.0-11.5 eV) of neutral water clusters close to the nozzle exit leads to a different nonequilibrium state compared to a skimmed molecular beam. This results in the appearance of a new magic number at 14. Metadynamics conformer searches coupled with modern density functional calculations are used to identify the global minimum energy structures of protonated water clusters between n = 2 and 21, as well as the manifold of low-lying metastable minima. New lowest energy structures are reported for the cases of n = 5, 6, 11, 12, 16, and 18, and special stability is identified by several measures. These theoretical results are in agreement with the experiments performed in this work in that n = 14 is shown to exhibit additional stability, based on the computed second-order stabilization energy relative to most cluster sizes, though not to the extent of the well-known n = 21 cluster. Other cluster sizes that show some additional energetic stability are n = 7, 9, 12, 17, and 19. To gain insight into the balance between ion-water and water-water interactions as a function of the cluster size, an analysis of the effective two-body interactions (which sum exactly to the total interaction energy) was performed. This analysis reveals a crossover as a function of cluster size between a water-hydronium-dominated regime for small clusters and a water-water-dominated regime for larger clusters around n = 17.

Approaching the "Zundel" Limit: Tuning the Vibrational Coupling in N2H+Ng, Ng = {He, Ne, Ar, Kr, Xe, and Rn}

The diazenylium ion (N₂H⁺) is a ubiquitous ion in dense molecular clouds. This ion is often used as a dense gas tracer in outer space. Most of the previous works on diazenylium ion have focused on the shared-proton stretch band, ν_H⁺. In this work, we have performed reduced-dimensional calculations to investigate the vibrational structure of N₂H⁺Ng, Ng = {He, Ne, Ar, Kr, Xe, and Rn}. We demonstrate a few interesting things about this system. First, the vibrational coupling in N₂H⁺ can be tuned to switch on interesting anharmonic effects such as Fermi resonance or combination bands by tagging it with different noble gases. Second, a comparison of the vibrational spectrum from N₂H⁺He to N₂H⁺Rn shows that the ν_H⁺ can be swept from an "Eigen-like" to a "Zundel-like" limiting case. Anharmonic calculations were performed using a multilevel approach, which utilized the MP2 and CCSD(T) levels of theories. Binding energies for the elimination of Ng in N₂H⁺Ng are also reported.

Evidence for the co-existence of isomers of water dimer radical cations and their inter-conversion in a linear ion trap

Water dimer radical cations are regarded as key intermediates in many aqueous reactions and biochemical processes. However, the structure of the water dimer radical cations, and particularly the inter-conversion between their isomers, remain difficult to investigate experimentally due to their short lifetime and low abundance under ambient conditions. Furthermore, the isomers cannot be distinguished in a full mass spectra. In this study, we report the experimental evidence for the hemi-bonded and proton-transferred isomers of gas-phase water dimer radical cations, and the inter-conversion process between them in a linear ion trap at low pressure and near room temperature. Multiple collisions of isolated water dimer radical cations with He inside the ion trap were systematically investigated; first, under different trapping times (i.e., reaction times) ranging from 0.03 to 800 ms, and then at a very low collision energies ranging from 0.1% to 10% normalized collision energy. The proton-transferred isomers were dominant at shorter trapping times (≤250 ms), while the hemi-bonded isomers were dominant at longer trapping times (250-800 ms). Moreover, the difference in symmetry of the shapes of the H₂O^•+ signal profiles and the H₃O⁺ signal profiles implied the existence of two kinds of isomers and there were small potential differences between them. Our results also suggested that by tuning the experimental parameters the hemi-bonded isomers would become dominant, which could allow the study of novel chemical reactions involving the hemi-bonded two-center-three-electron (2c-3e) structure in a linear ion trap.

Microscopic Mechanism of Proton Transfer in Pure Water under Ambient Conditions

Water molecules and the associated proton transfer (PT) are prevalent in chemical and biological systems and have been a hot research topic. Spectroscopic characterization and ab initio molecular dynamics (AIMD) simulations have previously revealed insights into acidic and basic liquids. Presumably, the situation in the acidic/basic solution is not necessarily the same as in pure water; in addition, the autoionization constant for water is only 10^-14 under ambient conditions, making the study of PT in pure water challenging. To overcome this issue, we modeled periodic water box systems containing 1000 molecules for tens of nanoseconds based on a neural network potential (NNP) with quantum mechanical accuracy. The NNP was generated by training a dataset containing the energies and atomic forces of 17 075 configurations of periodic water box systems, and these data points were calculated at the MP2 level that considers electron correlation effects. We found that the size of the system and the duration of the simulation have a significant impact on the convergence of the results. With these factors considered, our simulations showed that hydronium (H₃O⁺) and hydroxide (OH^-) ions in water have distinct hydration structures, thermodynamic and kinetic properties, e.g., the longer-lasting and more stable hydrated structure of OH^- ions than that of H₃O⁺, as well as a significantly higher free energy barrier for the OH^--associated PT than that of H₃O⁺, leading the two to exhibit completely different PT behaviors. Given these characteristics, we further found that PT via OH^- ions tends not to occur multiple times or between many molecules. In contrast, PT via H₃O⁺ can synergistically occur among multiple molecules and prefers to adopt a cyclic pattern among three water molecules, while it occurs mostly in a chain pattern when more water molecules are involved. Therefore, our studies provide a detailed and solid microscopic explanation for the PT process in pure water.

Validation of Field-Dependent Ion-Solvent Cluster Modeling via Direct Measurement of Cluster Size Distributions

Ion mobility spectrometry is widely used in analytical chemistry, either as a stand-alone technique or coupled to mass spectrometry. Ions in the gas phase tend to form loosely bound clusters with surrounding solvent vapors, artificially increasing the collisional cross section and the mass of the ion. This, in turn, affects ion mobility and influences separation. Further, ion-solvent clusters play an important role in most ionization mechanisms occurring in the gas phase. Consequently, a deeper understanding of ion-solvent cluster association and dissociation processes is desirable to guide experimental design and interpretation. A few computational models exist, which aim to describe the amount of clustering as a function of the reduced electric field strength, bath gas pressure and temperature, and the chemical species probed. It is especially challenging to model ion mobility under high reduced electrical field strengths due to the nonthermal conditions created by the field. In this work, we aim to validate a recently proposed first-principles model by comparing its predictions with direct measurements of cluster size distributions over a range of 20-120 Td as observed using a High Kinetic Energy Ion Mobility Spectrometer coupled to a mass spectrometer (HiKE-IMS-MS). By studying H⁺(H₂O)_n, [MeOH + H + n(H₂O)]⁺, [ACE + H + n(H₂O)]⁺, and [PhNH₂ + H + n(H₂O)]⁺ as test systems, we find very good agreement between model and experiment, supporting the validity of the computational workflow. Further, the detailed information gained from the modeling yields important insights into the cluster dynamics within the HiKE-IMS, allowing for better interpretation of the measured ion mobility spectra.

Isotope Effects in the Zundel-Eigen Isomerization of H+(H2O)6

The isomerization pathway between the energetically low-lying Zundel and Eigen isomers of the protonated water hexamer was investigated using high-level ab initio calculations including a treatment of zero-point corrections. On the basis of these calculations, the Zundel-Eigen isomerization was found to proceed through a stable intermediate isomer, which consists of a four-membered ring with two single acceptor water molecules. The inclusion of vibrational zero-point energy is shown to be important for accurately establishing the relative energies of the three relevant isomers involved in the Zundel-Eigen isomerization. Diffusion Monte Carlo calculations including anharmonic vibrational effects show that all three isomers of H⁺(H₂O)₆ and D⁺(D₂O)₆ have well-defined structures. The energetic ordering of the three isomers changes upon deuteration. The implications of these results for the vibrational spectra of H⁺(H₂O)₆ and D⁺(D₂O)₆ are also discussed.

Many-Body Effects in Aqueous Systems: Synergies Between Interaction Analysis Techniques and Force Field Development

Interaction analysis techniques, including the many-body expansion (MBE), symmetry-adapted perturbation theory, and energy decomposition analysis, allow for an intuitive understanding of complex molecular interactions. We review these methods by first providing a historical context for the study of many-body interactions and discussing how nonadditivities emerge from Hamiltonians containing strictly pairwise-additive interactions. We then elaborate on the synergy between these interaction analysis techniques and the development of advanced force fields aimed at accurately reproducing the Born-Oppenheimer potential energy surface. In particular, we focus on ab initio-based force fields that aim to explicitly reproduce many-body terms and are fitted to high-level electronic structure results. These force fields generally incorporate many-body effects through (a) parameterization of distributed multipoles, (b) explicit fitting of the MBE, (c) inclusion of many-atom features in a neural network, and (d) coarse-graining of many-body terms into an effective two-body term. We also discuss the emerging use of the MBE to improve the accuracy and speed of ab initio molecular dynamics.

Subpicosecond Molecular Rearrangements Affect Local Electric Fields and Auto-Dissociation in Water

Molecular simulations of auto-dissociation of water molecules in an 81,000 atom bulk water system show that the electric field variations caused by local bond length and angle variations enhance proton transfer within ∼600 fs prior to auto-dissociation. In this paper, auto-dissociation relates to the initial separation of a proton from a water molecule to another, forming the H33O+ and OH- ions. Only transfers for which a proton's initial nearest covalently bonded oxygen remained the same for at least 1 ps prior to the transfer and for which that proton's new nearest acceptor oxygen remained the same for at least 1 ps after the transfer were evaluated. Electric fields from solvent atoms within 6 Å of a transferring proton (H*) are dominant, with little contribution from farther molecules. However, exclusion of the accepting oxygen in such electric field calculations shows that the field on H* from the other solvent atoms weakens as the time to transfer becomes less than 600 fs, indicating the primary importance of the accepting oxygen on enabling auto-dissociation. All resultant OH- and H3O+ ion pairs recombined at times greater than 1 ps after auto-dissociation. A concentration of 8.01 × 1017 cm-3 for these ion pairs was observed. The simulations indicate that transient auto-dissociation in water is more common than that inferred from dc-conductivity experiments (10-5 vs 10-7) and is consistent with the results of calculations that include nuclear quantum effects. The conductivity experiments require the rearrangement of farther water molecules to form hydrogen-bonded "water wires" that afford long-range and measurable proton transport away from the reaction site. Nonetheless, the relatively large number of picosecond-lived auto-dissociation products might be engineered within 2D layers and oriented external fields to offer new energy-related systems.

The primary gas phase hydration shell of hydroxide

The number of water molecules in hydroxide's primary hydration shell has been long debated to be three from the interpretation of experimental data and four from theoretical studies. Here, we provide direct evidence for the presence of a fourth water molecule in hydroxide's primary hydration shell from a combined study based on high-resolution cryogenic experimental photoelectron spectroscopy and high-level quantum chemical computations. Well-defined spectra of OH^-(H₂O)_n clusters (n = 2 to 5) yield accurate electron binding energies, which are, in turn, used as key signatures of the underlying molecular conformations. Although the smaller OH^-(H₂O)₃ and OH^-(H₂O)₄ clusters adopt close-lying conformations with similar electron binding energies that are hard to distinguish, the OH^-(H₂O)₅ cluster clearly has a predominant conformation with a four-coordinated hydroxide binding motif, a finding that unambiguously determines the gas phase coordination number of hydroxide to be four.

Protonation of Strained Epoxy Resin under Wet Conditions via First-Principles Calculations Using the H+-Shift Method

A significant challenge in adhesive bonding is the accelerated breaking of stretched adhesives under wet conditions, which is known as cohesive failure. One group of commonly used adhesives consists of the amine-cured epoxy resins. Based on deprotonation free-energy calculations of the unstrained resin in water, it has recently been proposed that these adhesives can undergo failure through breakage originating at the protonated amine group under neutral or acidic conditions [J. Phys. Chem. B 2021, 125, 8989-8996]. In this study, we comprehensively investigated the degree of protonation of the amine group under both stretched and compressed conditions by devising a robust first-principles protonation calculation method applicable to strained materials. It was found that the amine group was partially protonated in neutral water at 298 K and that the amine group was protonated when the epoxy resin was stretched to a greater extent in water, and vice versa. These findings support the physicochemical cause of cohesive failure due to protonation of the amine group in the stretched amine-cured epoxy resins.

Fragment imaging in the infrared photodissociation of the Ar-tagged protonated water clusters H3O+-Ar and H+(H2O)2-Ar

Infrared photodissociation of protonated water clusters with an Ar atom, namely H₃O⁺-Ar and H⁺(H₂O)₂-Ar, was investigated by an imaging technique for mass-selected ions, to reveal the intra- and intermolecular vibrational dynamics. The presented system has the advantage of achieving fragment ion images with the cluster size- and mode-selective photoexcitation of each OH stretching vibration. Translational energy distributions of photofragments were obtained from the images upon the excitation of the bound (ν_b) and free (ν_f) OH stretching vibrations. The energy fractions in the translational motion were compared between ν_b^I and ν_f^I in H₃O⁺-Ar or between ν_b^II and ν_f^II in H⁺(H₂O)₂-Ar, where the labels "I" and "II" represent H₃O⁺-Ar and H⁺(H₂O)₂-Ar, respectively. In H₃O⁺-Ar, the ν_f^I excitation exhibited a smaller translational energy than ν_b^I. This result can be explained by the higher vibrational energy of ν_f^I, which enabled it to produce bending (ν₄) excited H₃O⁺ fragments that should be favored according to the energy-gap model. In contrast to H₃O⁺-Ar, the ν_b^II excitation of an Ar-tagged H₂O subunit and the ν_f^II excitation of an untagged H₂O subunit resulted in very similar translational energy distributions in H⁺(H₂O)₂-Ar. The similar energy fractions independent of the excited H₂O subunits suggested that the ν_b^II and ν_f^II excited states relaxed into a common intermediate state, in which the vibrational energy was delocalized within the H₂O-H⁺-H₂O moiety. However, the translational energy distributions for H⁺(H₂O)₂-Ar did not agree with a statistical dissociation model, which implied another aspect of the process, that is, Ar dissociation via incomplete energy randomization in the whole H⁺(H₂O)₂-Ar cluster.

Vibrational signature of hydrated protons confined in MXene interlayers

The hydration structure of protons has been studied for decades in bulk water and protonated clusters due to its importance but has remained elusive in planar confined environments. Two-dimensional (2D) transition metal carbides known as MXenes show extreme capacitance in protic electrolytes, which has attracted attention in the energy storage field. We report here that discrete vibrational modes related to protons intercalated in the 2D slits between Ti3C2Tx MXene layers can be detected using operando infrared spectroscopy. The origin of these modes, not observed for protons in bulk water, is attributed to protons with reduced coordination number in confinement based on Density Functional Theory calculations. This study therefore demonstrates a useful tool for the characterization of chemical species under 2D confinement.

Evolution of Solute-Water Interactions in the Benzaldehyde-(H2O)1-6 Clusters by Rotational Spectroscopy

The investigation on the preferred arrangement and intermolecular interactions of gas phase solute-water clusters gives insights into the intermolecular potentials that govern the structure and dynamics of the aqueous solutions. Here, we report the investigation of hydrated coordination networks of benzaldehyde-(water)n (n = 1-6) clusters in a pulsed supersonic expansion using broadband rotational spectroscopy. Benzaldehyde (PhCHO) is the simplest aromatic aldehyde that involves both hydrophilic (CHO) and hydrophobic (phenyl ring) functional groups, which can mimic molecules of biological significance. For the n = 1-3 clusters, the water molecules are connected around the hydrophilic CHO moiety of benzaldehyde through a strong CO···HO hydrogen bond and weak CH···OH hydrogen bond(s). For the larger clusters, the spectra are consistent with the structures in which the water clusters are coordinated on the surface of PhCHO with both the hydrophilic CHO and hydrophobic phenyl ring groups being involved in the bonding interactions. The presence of benzaldehyde does not strongly interfere with the cyclic water tetramer and pentamer, which retain the same structure as in the pure water cluster. The book isomer instead of cage or prism isomers of the water hexamer is incorporated into the microsolvated cluster. The PhCHO molecule deviates from the planar structure upon sequential addition of water molecules. The PhCHO-(H2O)1-6 clusters may serve as a simple model system in understanding the solute-water interactions of biologically relevant molecules in an aqueous environment.

Investigating the hydrolysis of cryogenically layered molybdenum hexafluoride through a disordered hydrogen-bonded network

Molybdenum hexafluoride (MoF₆) is used as a non-radioactive substitute for uranium to study the hydrolysis of metal hexafluorides. Molybdenum hexafluoride gas and water vapor, from the air, were sequentially layered onto a diamond substrate kept at liquid nitrogen temperature using a custom designed cryogenic cell with a copper cold finger. Reaction progress was monitored by transmission Fourier Transform Infrared Spectroscopy (FTIR) through the layers and diamond substrate over several hours while allowing the substrate to warm. Changes in the modes in the 500-1000 cm^-1 region are tracked as the reaction progresses in order to identify intermediate species. Strong absorption features are also observed in the 1000-3000 cm^-1 range, suggesting the presence of ionic dissociation intermediates trapped in a disordered H-bonded network of cryogenic hydrofluoric acid. A possible reaction pathway is proposed and the final hydrolysis product is characterized by FTIR, UV-vis, and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS).

The Competition between Hydrogen, Halogen, and Covalent Bonding in Atmospherically Relevant Ammonium Iodate Clusters

Iodine-containing clusters are expected to be central to new particle formation (NPF) events in polar and midlatitude coastal regions. Iodine oxoacids and iodine oxides are observed in newly formed clusters, and in more polluted midlatitude settings, theoretical studies suggest ammonia may increase growth rates. Structural information was obtained via infrared (IR) spectroscopy and quantum chemical calculations for a series of clusters containing ammonia, iodic acid, and iodine pentoxide. Structures for five of the smallest cationic clusters present in the mass spectrum were identified, and four of the structures were found to preferentially form halogen and/or covalent bonds over hydrogen bonds. Ammonia is important in proton transfer from iodic acid components and also provides a scaffold to template the formation of a halogen and covalent bonded backbone. The calculations executed for the two largest clusters studied suggested the formation of a covalent I₃O8- anion within the clusters.

Aquaphotomics Monitoring of Lettuce Freshness during Cold Storage

Fresh-cut leafy vegetables are one of the most perishable products because they readily deteriorate in quality even during cold storage and have a relatively short shelf life. Since these products are in high demand, methods for rigorous quality control and estimation of freshness that are rapid and non-destructive would be highly desirable. The objective of the present research was to develop a rapid, non-destructive near-infrared spectroscopy (NIRS)-based method for the evaluation of changes during cold storage of lettuce using an aquaphotomics approach to monitor the water molecular structure in lettuce leaves. The reference measurements showed that after 6 days of dark, cold storage, the weight and water activity of lettuce leaves decreased and β-carotene decreased, while chlorophylls slightly increased. Aquaphotomics characterization showed large differences in the lettuce leaves' spectra depending on their growth zone. Difference spectra, principal component analysis (PCA) and linear discriminant analysis (LDA) confirmed the differences in the inner and outer leaves and revealed that spectra change as a function of storage time. Partial least squares regression (PLSR) allowed the prediction of the time spent in storage with a coefficient of determination of R² = 0.80 and standard error of RMSE = 0.77 days for inner, and R² = 0.86 and RMSE = 0.66 days for outer leaves, respectively. The following water absorbance bands were found to provide the most information in the spectra: 1348, 1360, 1373, 1385, 1391, 1410, 1416, 1422, 1441, 1447, 1453, 1466, 1472, 1490, 1503, 1515, 1521, 1534 and 1571 nm. They were further used as water matrix coordinates (WAMACs) to define the water spectral patterns (WASPs) of lettuce leaves. The WASPs of leaves served to succinctly describe the state of lettuces during storage. The changes in WASPs during storage reveled moisture loss, damage to cell walls and expulsion of intracellular water, as well as loss of free and weakly hydrogen-bonded water, all leading to a loss of juiciness. The WASPs also showed that damage stimulated the defense mechanisms and production of vitamin C. The leaves at the end of the storage period were characterized by water strongly bound to collapsed structural elements of leaf tissues, mainly cellulose, leading to a loss of firmness that was more pronounced in the outer leaves. All of this information was reflected in the changes of absorbance in the identified WAMACs, showing that the water molecular structure of lettuce leaves accurately reflects the state of the lettuce during storage and that WASPs can be used as a multidimensional biomarker to monitor changes during storage.

Graph-Theoretic Molecular Fragmentation for Potential Surfaces Leads Naturally to a Tensor Network Form and Allows Accurate and Efficient Quantum Nuclear Dynamics

Molecular fragmentation methods have revolutionized quantum chemistry. Here, we use a graph-theoretically generated molecular fragmentation method, to obtain accurate and efficient representations for multidimensional potential energy surfaces and the quantum time-evolution operator, which plays a critical role in quantum chemical dynamics. In doing so, we find that the graph-theoretic fragmentation approach naturally reduces the potential portion of the time-evolution operator into a tensor network that contains a stream of coupled lower-dimensional propagation steps to potentially achieve quantum dynamics with reduced complexity. Furthermore, the fragmentation approach used here has previously been shown to allow accurate and efficient computation of post-Hartree-Fock electronic potential energy surfaces, which in many cases has been shown to be at density functional theory cost. Thus, by combining the advantages of molecular fragmentation with the tensor network formalism, the approach yields an on-the-fly quantum dynamics scheme where both the electronic potential calculation and nuclear propagation portion are enormously simplified through a single stroke. The method is demonstrated by computing approximations to the propagator and to potential surfaces for a set of coupled nuclear dimensions within a protonated water wire problem exhibiting the Grotthuss mechanism of proton transport. In all cases, our approach has been shown to reduce the complexity of representing the quantum propagator, and by extension action of the propagator on an initial wavepacket, by several orders, with minimal loss in accuracy.

Near-Infrared Metabolomic Fingerprinting Study of Lichen Thalli and Phycobionts in Culture: Aquaphotomics of Trebouxia lynnae Dehydration

Near-infrared spectroscopy (NIRS) is an accurate, fast and safe technique whose full potential remains to be exploited. Lichens are a paradigm of symbiotic association, with extraordinary properties, such as abiotic stress tolerance and adaptation to anhydrobiosis, but subjacent mechanisms await elucidation. Our aim is characterizing the metabolomic NIRS fingerprints of Ramalina farinacea and Lobarina scrobiculata thalli, and of the cultured phycobionts Trebouxia lynnae and Trebouxia jamesii. Thalli collected in an air-dry state and fresh cultivated phycobionts were directly used for spectra acquisition in reflectance mode. Thalli water peaks were associated to the solvation shell (1354 nm) and sugar-water interactions (1438 nm). While northern-southern orientation related with two hydrogen bonded (S₂) water, the site was related to one hydrogen bonded (S₁). Water, lipids (saturated and unsaturated), and polyols/glucides contributed to the profiles of lichen thalli and microalgae. R. farinacea, with higher desiccation tolerance, shows higher S₂ water than L. scrobiculata. In contrast, fresh phycobionts are dominated by free water. Whereas T. jamesii shows higher solvation water content, T. lynnae possesses more unsaturated lipids. Aquaphotomics demonstrates the involvement of strongly hydrogen bonded water conformations, polyols/glucides, and unsaturated/saturated fatty acids in the dehydration process, and supports a "rubbery" state allowing enzymatic activity during anhydrobiosis.

The Consequences of Water Interactions with Nitrogen-Containing Carbonaceous Quantum Dots-The Mechanistic Studies

Despite the importance of quantum dots in a wide range of biological, chemical, and physical processes, the structure of the molecular layers surrounding their surface in solution remains unknown. Thus, knowledge about the interaction mechanism of Nitrogen enriched Carbonaceous Quantum Dots' (N-CQDs) surface with water-their natural environment-is highly desirable. A diffusive and Stern layer over the N-CQDs, characterized in situ, reveals the presence of anionic water clusters [OH(H2O)n]-. Their existence explains new observations: (i) the unexpectedly low adsorption enthalpy (ΔHads) in a pressure range below 0.1 p/ps, and ΔHads being as high as 190 kJ/mol at 0.11 p/ps; (ii) the presence of a "conductive window" isolating nature-at p/ps below 0.45-connected to the formation of smaller clusters and increasing conductivity above 0.45 p/ps, (iii) Stern layer stability; and (iv) superhydrophilic properties of the tested material. These observables are the consequences of H2O dissociative adsorption on N-containing basic centers. The additional direct application of surfaces formed by N-CQDs spraying is the possibility of creating antistatic, antifogging, bio-friendly coatings.

Aquaphotomic Study of Effects of Different Mixing Waters on the Properties of Cement Mortar

The mixing water used for cement concrete has a significant effect on the physical properties of the material after hardening; however, other than the upper limit for the mixed impurities, not enough consideration has been given to the functions and characteristics of water at the molecular level. In this study, we investigated the effect of four different types of water (two spring-, mineral waters, tap water and distilled water) on the drying shrinkage of the hardened cement by comparing the material properties of the concrete specimens and analyzing the molecular structure of the water and cement mortar using aquaphotomics. The near infrared (NIR) spectra of waters used for mixing were acquired in the transmittance mode using a high-precision, high-accuracy benchtop spectrometer in the range of 400-2500 nm, with the 0.5 nm step. The NIR spectra of cement paste and mortar were measured in 6.2 nm increments in the wavelength range of 950 nm to 1650 nm using a portable spectrometer. The measurements of cement paste and mortar were performed on Day 0 (immediately after mixing, cement paste), 1 day, 3 days, 7 days, and 28 days after mixing (cement mortar). The spectral data were analyzed according to the aquaphotomics' multivariate analysis protocol, which involved exploration of raw and preprocessed spectra, exploratory analysis, discriminating analysis and aquagrams. The results of the aquaphotomics' analysis were interpreted together with the results of thermal and drying shrinkage measurements. Together, the findings clearly demonstrated that the thermal and drying shrinkage properties of the hardened cement material differed depending on the water used. Better mechanical properties were found to be a result of using mineral waters for cement mixing despite minute differences in the chemical content. In addition, the aquaphotomic characterization of the molecular structure of waters and cement mortar during the initial hydration reaction demonstrated the possibility to predict the characteristics of hardened cement at a very early stage. This provided the rationale to propose a novel evaluation method based on aquaphotomics for non-invasive evaluation and monitoring of cement mortar.

From Local Covalent Bonding to Extended Electric Field Interactions in Proton Hydration

Seemingly simple yet surprisingly difficult to probe, excess protons in water constitute complex quantum objects with strong interactions with the extended and dynamically changing hydrogen-bonding network of the liquid. Proton hydration plays pivotal roles in energy transport in hydrogen fuel cells and signal transduction in transmembrane proteins. While geometries and stoichiometry have been widely addressed in both experiment and theory, the electronic structure of these specific hydrated proton complexes has remained elusive. Here we show, layer by layer, how utilizing novel flatjet technology for accurate x-ray spectroscopic measurements and combining infrared spectral analysis and calculations, we find orbital-specific markers that distinguish two main electronic-structure effects: Local orbital interactions determine covalent bonding between the proton and neigbouring water molecules, while orbital-energy shifts measure the strength of the extended electric field of the proton.

Proton-transfer spectroscopy beyond the normal-mode scenario

A stochastic theory is developed to predict the spectral signature of proton-transfer processes and is applied to infrared spectra computed from ab initio molecular-dynamics simulations of a single [Formula: see text] cation. By constraining the oxygen atoms to a fixed distance, this system serves as a tunable model for general proton-transfer processes with variable barrier height. Three spectral contributions at distinct frequencies are identified and analytically predicted: the quasi-harmonic motion around the most probable configuration, amenable to normal-mode analysis, the contribution due to transfer paths when the proton moves over the barrier, and a shoulder for low frequencies stemming from the stochastic transfer-waiting-time distribution; the latter two contributions are not captured by normal-mode analysis but exclusively reported on the proton-transfer kinetics. In accordance with reaction rate theory, the transfer-waiting-contribution frequency depends inversely exponentially on the barrier height, whereas the transfer-path-contribution frequency is rather insensitive to the barrier height.

Aquaphotomics for monitoring of groundwater using short-wavelength near-infrared spectroscopy

Water spectrum of any aqueous system contains information about OH covalent and hydrogen bonds that are highly influenced by the environment and the rest of the molecules in the system. When aquaphotomics is used to analyze the water near infrared (NIR) spectra, the information about the water molecular structure can be obtained as a function of internal and external factors. The objective of this research is to apply aquaphotomics analysis to evaluate different groundwaters by using their NIR unique spectral pattern, robust to external influences of temperature and humidity, that can potentially be used for water type identification and screening practice. Two groundwaters obtained at different depths and their mixture, differing in mineral content and molecular structure were monitored on a daily basis using portable visible/NIR (vis/NIR) spectrometer during three consecutive years. The spectra were pre-processed by smoothing and multiplicative scatter correction (MSC) to remove noise and baseline effects. Results showed that NIR spectral patterns of groundwater samples were affected by changes in environmental factors - temperature, humidity, time and others. The water absorbance bands which are highly influenced by humidity and temperature in short wavelength NIR region were identified. Their avoidance resulted in obtaining consistent spectral patterns during the entire monitoring period, unique for each groundwater, that can be used as its fingerprint and monitored over time. Consistency and uniqueness of the spectral pattern for each groundwater provide a potential to use the deviation of spectral pattern as an indicator of changes in the water. These results confirm that vis/NIR spectral pattern can be used as an integrative marker of water status, stable over time, providing the basis for an efficient cost-effective method for monitoring of water functionality.

State-resolved infrared spectrum of the protonated water dimer: revisiting the characteristic proton transfer doublet peak

The infrared (IR) spectra of protonated water clusters encode precise information on the dynamics and structure of the hydrated proton. However, the strong anharmonic coupling and quantum effects of these elusive species remain puzzling up to the present day. Here, we report unequivocal evidence that the interplay between the proton transfer and the water wagging motions in the protonated water dimer (Zundel ion) giving rise to the characteristic doublet peak is both more complex and more sensitive to subtle energetic changes than previously thought. In particular, hitherto overlooked low-intensity satellite peaks in the experimental spectrum are now unveiled and mechanistically assigned. Our findings rely on the comparison of IR spectra obtained using two highly accurate potential energy surfaces in conjunction with highly accurate state-resolved quantum simulations. We demonstrate that these high-accuracy simulations are important for providing definite assignments of the complex IR signals of fluxional molecules.

Pilot Aquaphotomic Study of the Effects of Audible Sound on Water Molecular Structure

Sound affects the medium it propagates through and studies on biological systems have shown various properties arising from this phenomenon. As a compressible media and a “collective mirror”, water is influenced by all internal and external influences, changing its molecular structure accordingly. The water molecular structure and its changes can be observed as a whole by measuring its electromagnetic (EMG) spectrum. Using near-infrared spectroscopy and aquaphotomics, this pilot study aimed to better describe and understand the sound-water interaction. Results on purified and mineral waters reported similar effects from the applied 432 Hz and 440 Hz frequency sound, where significant reduction in spectral variations and increased stability in water were shown after the sound perturbation. In general, the sound rearranged the initial water molecular conformations, changing the samples’ properties by increasing strongly bound, ice-like water and decreasing small water clusters and solvation shells. Even though there was only 8 Hz difference in applied sound frequencies, the change of absorbance at water absorbance bands was specific for each frequency and also water-type-dependent. This also means that sound could be effectively used as a perturbation tool together with spectroscopy to identify the type of bio, or aqueous, samples being tested, as well as to identify and even change water functionality.

Infrared Spectra at Coupled Cluster Accuracy from Neural Network Representations

Infrared spectroscopy is key to elucidating molecular structures, monitoring reactions, and observing conformational changes, while providing information on both structural and dynamical properties. This makes the accurate prediction of infrared spectra based on first-principle theories a highly desirable pursuit. Molecular dynamics simulations have proven to be a particularly powerful approach for this task, albeit requiring the computation of energies, forces and dipole moments for a large number of molecular configurations as a function of time. This explains why highly accurate first-principles methods, such as coupled cluster theory, have so far been inapplicable for the prediction of fully anharmonic vibrational spectra of large systems at finite temperatures. Here, we push cutting-edge machine learning techniques forward by using neural network representations of energies, forces, and in particular dipoles to predict such infrared spectra fully at "gold standard" coupled cluster accuracy as demonstrated for protonated water clusters as large as the protonated water hexamer, in its extended Zundel configuration. Furthermore, we show that this methodology can be used beyond the scope of the data considered during the development of the neural network models, allowing for the computation of finite-temperature infrared spectra of large systems inaccessible to explicit coupled cluster calculations. This substantially expands the hitherto existing limits of accuracy, speed, and system size for theoretical spectroscopy and opens up a multitude of avenues for the prediction of vibrational spectra and the understanding of complex intra- and intermolecular couplings.

Graph Theoretic Molecular Fragmentation for Multidimensional Potential Energy Surfaces Yield an Adaptive and General Transfer Machine Learning Protocol

Over a series of publications we have introduced a graph-theoretic description for molecular fragmentation. Here, a system is divided into a set of nodes, or vertices, that are then connected through edges, faces, and higher-order simplexes to represent a collection of spatially overlapping and locally interacting subsystems. Each such subsystem is treated at two levels of electronic structure theory, and the result is used to construct many-body expansions that are then embedded within an ONIOM-scheme. These expansions converge rapidly with many-body order (or graphical rank) of subsystems and have been previously used for ab initio molecular dynamics (AIMD) calculations and for computing multidimensional potential energy surfaces. Specifically, in all these cases we have shown that CCSD and MP2 level AIMD trajectories and potential surfaces may be obtained at density functional theory cost. The approach has been demonstrated for gas-phase studies, for condensed phase electronic structure, and also for basis set extrapolation-based AIMD. Recently, this approach has also been used to derive new quantum-computing algorithms that enormously reduce the quantum circuit depth in a circuit-based computation of correlated electronic structure. In this publication, we introduce (a) a family of neural networks that act in parallel to represent, efficiently, the post-Hartree-Fock electronic structure energy contributions for all simplexes (fragments), and (b) a new k-means-based tessellation strategy to glean training data for high-dimensional molecular spaces and minimize the extent of training needed to construct this family of neural networks. The approach is particularly useful when coupled cluster accuracy is desired and when fragment sizes grow in order to capture nonlocal interactions accurately. The unique multidimensional k-means tessellation/clustering algorithm used to determine our training data for all fragments is shown to be extremely efficient and reduces the needed training to only 10% of data for all fragments to obtain accurate neural networks for each fragment. These fully connected dense neural networks are then used to extrapolate the potential energy surface for all molecular fragments, and these are then combined as per our graph-theoretic procedure to transfer the learning process to a full system energy for the entire AIMD trajectory at less than one-tenth the cost as compared to a regular fragmentation-based AIMD calculation.

Visualizing Eigen/Zundel cations and their interconversion in monolayer water on metal surfaces

The nature of hydrated proton on solid surfaces is of vital importance in electrochemistry, proton channels, and hydrogen fuel cells but remains unclear because of the lack of atomic-scale characterization. We directly visualized Eigen- and Zundel-type hydrated protons within the hydrogen bonding water network on Au(111) and Pt(111) surfaces, using cryogenic qPlus-based atomic force microscopy under ultrahigh vacuum. We found that the Eigen cations self-assembled into monolayer structures with local order, and the Zundel cations formed long-range ordered structures stabilized by nuclear quantum effects. Two Eigen cations could combine into one Zundel cation accompanied with a simultaneous proton transfer to the surface. Moreover, we revealed that the Zundel configuration was preferred over the Eigen on Pt(111), and such a preference was absent on Au(111).

Solvent-Mediated Charge Transfer Dynamics of a Model Brown Carbon Aerosol Chromophore: Photophysics of 1-Phenylpyrrole Induced by Water Solvation

Nitrogen heterocycles are known to be important light-absorbing chromophores in a newly discovered class of aerosols, commonly referred to as "brown carbon" (BrC) aerosols. Due to their significant absorption and spectral overlap with the solar actinic flux, these BrC chromophores steer the physical and optical properties of aerosols. To model the local aqueous solvation environment surrounding BrC chromophores, we generated cold molecular complexes with water and a prototypical BrC chromophore, 1-phenylpyrrole (1PhPy), using supersonic jet-cooling and explored their intermolecular interactions using single-conformation spectroscopy. Herein, we utilized resonant two-photon ionization (R2PI) and UV holeburning (UV HB) double-resonance spectroscopies to obtain a molecular-level understanding of the role of water microsolvation in charge transfer upon photoexcitation of 1PhPy. Quantum chemical calculations and one-dimensional discrete variable representation simulations revealed insights into the charge transfer efficacy of 1PhPy with and without addition of a single water molecule. Taken together, our results indicate that the intermolecular interactions with water guide the geometry of 1PhPy to adopt a more twisted intramolecular charge transfer (TICT) configuration, thus facilitating charge transfer from the pyrrole donor to the phenyl ring acceptor. Furthermore, the water network surrounding 1PhPy reports on the charge transfer such that the H₂O solvent primarily interacts with the pyrrole ring donor in the ground state, whereas it preferentially interacts with the phenyl ring acceptor in the excited state. Large Franck-Condon activity is evident in the 1PhPy + 1H₂O excitation spectrum for the water-migration vibronic bands, supporting H₂O solvent reorganization upon excitation of the 1PhPy chromophore. Fluorescence measurements with increasing H₂O % volume corroborated our gas-phase studies by indicating that a polar water solvation environment stabilizes the TICT configuration of 1PhPy in the excited electronic state, from which emission is observed at a lower energy compared to the locally excited configuration.

Molecular vibrational spectral simulation connects theoretical cluster structure identification and vibrational spectral evidence

Structure identification of molecular clusters has long been a fundamental and challenging issue for cluster science. The traditional theoretical optimization on the potential energy surface heavily depends on the levels of theory and sometimes diverse identifications were reported. A solution to these disputations is to reinspect the theoretical results with the experimental data such as vibrational predissociation spectra with high sensitivity to the molecular cluster structures. Herein, the combination of global low-lying structure search and vibrational predissociation spectral simulation is proposed as an accurate and reliable approach for cluster structure identification, by which the assignments can be validated using experimental measurements. The qualitative agreement between simulated and measured vibrational spectra lends solid experimental evidence to the assignment of the cluster structures. Taking NH₄⁺(H₂O)_n (n = 2-4) as an example, we have unambiguously identified their structures and directly demonstrated the coexistence of two NH₄⁺(H₂O)₄ isomers (with 3 and 4 water molecules directly linked to NH₄⁺, respectively), which were debatable in previous studies. The developed methods would pave the way to the structure determination of the molecular clusters.