Introduction to Relativistic Electronic Structure Calculations

It is important to include relativistic effects in electronic structure calculations for many important chemical problems, including heavy-element chemistry, intersystem crossing, and zero-field splitting. The subject is old, but recent developments have been rapid. The specialized literature can be daunting for nonspecialists, and this article is intended to provide an entry to that literature, especially for the modern treatment of molecules. There are only five equations. We include discussion of the relations between four-component, two-component, and one-component treatments, the distinction between scalar relativistic effects and angular-momentum-dependent effects, approximate treatments of spin-orbit coupling, including the molecular mean-field approximations, the inclusion of electron correlation in relativistic wave functions, and zero-field splitting.

Mechanistic Foundations of the Sequential Activation of Methane by Ta+: Oxidative Addition, Ring-Opening σ-Bond Metathesis, and C-C Bond Formation

The kinetics of Ta+ + CH4 and related reactions TaCnHm+ + CH4 (n = 2-4, m = n, 2n, 3n) are measured from 300-600 K using a selected-ion flow tube apparatus. Complicated kinetics are analyzed through a novel bootstrapping methodology, and rate constants for 38 unimolecular, bimolecular, and ternary processes are reported at each of the four temperatures. As has been well-established, Ta+ efficiently dehydrogenates methane through a non-spin-conserved process. Sequential chemistry leads to the dehydrogenation of up to four methane molecules per tantalum center through the competing processes of TaCnHm+ + CH4 → TaCn+1Hm+2+ + H2 (dehydrogenation) and TaCn+1Hm+4+ (association). Supported by density functional theory calculations, the distinct mechanisms and product structures of the sequential reactions are derived. The activation energy for oxidative insertion of Ta into a C-H bond is well-predicted by a simple heuristic: whether or not the reactant tantalum atom possesses unbound valence electrons of opposite spin. TaCH2+ is predicted to have a small activation energy for oxidative insertion but can only proceed to dehydrogenation of methane via carbon-carbon bond formation, enabled by three separate intersystem crossing events. The product is determined to be the tantalapropene dihydride cation, not the more intuitive tantalapropane cation, via comparison of measured and calculated thermal dissociation rates. The TaC2H4+ tantalapropene dihydride has a prohibitive barrier to oxidative insertion. It proceeds instead through a ring-opening insertion of the entire tantalapropene moiety into a C-H bond via σ-bond metathesis; the unbroken metallacycle bond acts as a tether, preventing the activated products from separating and allowing for further isomerization, leading to dehydrogenation. This and subsequent dehydrogenation processes occur without carbon-carbon bond formation; no evidence of a tantalabutane or larger metallacycle is found.

Heterogeneous Adsorption of Volatile Organic Compounds to Aerosol Particle Surfaces Probed with In Situ Surface Vibrational Sum Frequency Scattering

Vibrational sum frequency scattering (VSFS) has shown great utility in monitoring organics at sub-micrometer droplet surfaces, but the adsorbates were present upon generation. Herein, we present the first direct observation of heterogeneous gas-phase adsorption to droplet surfaces using in situ VSFS spectroscopy. This marks a significant development in the VSFS technique by allowing direct observation of heterogeneous adsorption to droplet surfaces in situ under ambient conditions, not relying on bulk removal or uptake. Using a flow tube system, we investigated formic acid vapor adsorption under different interaction times, concentrations, and physical environments. VSFS was used to compare adsorption to droplet surfaces from the gas phase and the underlying bulk at different interaction times. We then quantified the adsorption free energies for the different adsorption processes and found no significant difference between adsorption from the aqueous particle phase and that from the gas phase to the particle surface, with similar results at depressed relative humidity.

Photophysics of a Methylated GFP Chromophore Anion in Vacuo

The photophysical properties of the isolated chromophore anion from the green fluorescent protein have been extensively studied over the years to understand the factors influencing transition energies, excited-state lifetimes, and fluorescence. A commonly used model for the protein chromophore is 4'-hydroxybenzylidene-2,3-dimethyl-imidazolinone (p-HBDI). In this work, we have spectroscopically characterized a derivative, brMe-p-HBDI, which features methylation on the carbon bridging the phenol and imidazolinone rings. Experiments were conducted on the anionic form in the gas phase and at cryogenic temperatures using the SAPHIRA ion-storage ring and the LUNA2 fluorescence mass spectrometer, both located in Aarhus. Photoinduced action spectra reveal that brMe-p-HBDI- cooled to about 20-30 K exhibits maximum absorption at 496.0 ± 0.5 nm. Vibrationally resolved bands appear at shorter wavelengths, while a featureless absorption tail extends toward longer wavelengths, up to approximately 520 nm. The methyl substituent induces a clear redshift (75 meV) in absorption as p-HBDI- absorbs maximally at 481.51 ± 0.15 nm. The excited-state lifetime of brMe-p-HBDI- is determined to be 51 ± 3 ps following 495 nm photoexcitation and probing at 800 nm, which is significantly shorter than the nanosecond lifetime previously reported for p-HBDI-. Consistent with this, no fluorescence was detected from brMe-p-HBDI- at 100 K, in contrast to p-HBDI- that is strongly fluorescent according to recent work. These findings are corroborated by (time-dependent) density-functional theory calculations: A methyl substituent at the bridge carbon is predicted to cause a redshift of 77 meV, in excellent agreement with the experimental shift. We find that brMe-p-HBDI- is planar in the ground state (S0) but undergoes a twist motion in the S1 state, leading to a lower-energy nonplanar form where the angle between the two rings is 90°. Our work reveals that even a minor alteration in molecular structure can have a significant impact on the photophysics.

Theoretical investigation of vibrational energy transfer of water at the gas/liquid interface around 3400 cm-1

Using molecular dynamics simulations based on neural network potentials and a mixed quantum/classical approach, we theoretically explore vibrational energy transfer pathways for OH groups around 3400 cm-1 at interfaces. Our calculations show that intramolecular vibrational energy transfer has a time constant of 369.2 fs, aligning with experimental findings. The reorientation time is 1624 fs. The intermolecular vibrational energy transfer rate is slower due to weak couplings between water molecules. Our results suggest that intramolecular energy transfer is the main driver of vibrational energy transfer for OH groups around 3400 cm-1 at the gas/water interface.

Experimental Quantum Simulation of Chemical Dynamics

Accurate simulation of dynamic processes in molecules and reactions is among the most challenging problems in quantum chemistry. Quantum computers promise efficient chemical simulation, but the existing quantum algorithms require many logical qubits and gates, placing practical applications beyond existing technology. Here, we carry out the first quantum simulations of chemical dynamics by employing a more hardware-efficient encoding scheme that uses both qubits and bosonic degrees of freedom. Our trapped-ion device accurately simulates the dynamics of nonadiabatic chemical processes, which are among the most difficult problems in computational chemistry because they involve strong coupling between electronic and nuclear motions. We demonstrate the programmability and versatility of our approach by simulating the dynamics of three different molecules, as well as open-system dynamics in the condensed phase, all with the same quantum resources. Our approach requires orders of magnitude fewer resources than equivalent qubit-only quantum simulations, demonstrating the potential of using hybrid encoding schemes to accelerate quantum simulations of complex chemical processes, which could have applications in fields ranging from energy conversion and storage to biology and drug design.

Lower limits on hydrogen bond strength. Charge of bridging H atom

While it is usually agreed that a major component in the composition of a H-bond is the electrostatic attraction associated with a positive charge on the bridging H, there has recently arisen some question as to whether a true H-bond can exist when this atom bears a negative charge. Quantum chemical calculations address this question for a variety of potential proton donor molecules where the H atom is bonded to atoms covering a wide range of electronegativity, including the halogen, chalcogen, pnicogen and tetrel families, as well as metal atoms. These molecules are bound to Lewis bases by a variety of noncovalent bonds, including tetrel and halogen bonds, but H-bonds are rare, and exceedingly weak when the H atom does not carry a substantial positive charge.

External electric fields drive the formation of P → C dative bonds

Chemical interactions driven by external electric fields (EFs) can serve as a catalytic force for molecular machines and linkers for smart materials. In this context, the EF-driven dative bond is demonstrated through the study of interactions between PH3 and curved carbon-based nanostructures. The P → C dative bonds emerge only in the presence of EFs, whereas the interactions in the absence of EFs lead to van der Waals (vdW) complexes. The formation of EF-driven dative bonds can be verified with distinctive signals in vibrational, carbon-13 NMR, and UV/vis spectra. The nature of EF-driven dative bonds was theoretically analyzed with the block-localized wavefunction (BLW) method and the associated energy decomposition (BLW-ED) approach. It was found that the charge transfer interaction plays a dominating role and that even in the presence of EFs, complexes dissociate to monomers once the charge transfer interaction is "turned off". Notably, the inter-fragment orbital mixing stabilizes the complexes and alters their multipoles, leading to additional stability through field-multipole interactions. This conclusion was supported by further decomposition of the charge transfer energy component, clarifying the precise role of orbital mixing. The inter-fragment orbital mixing, which occurs exclusively in the presence of EFs, was elucidated using "in situ" orbital correlation diagrams. Specifically, both external EFs and intermolecular perturbations remarkably reduce the energy gap between the frontier orbitals of the monomers, thereby facilitating inter-fragment orbital interactions. Significant covalency was confirmed through ab initio valence bond (VB) theory calculations of the EF-driven dative bonds, aligning with the crucial role of the charge transfer interaction. This pronounced covalency emerges as a key feature of EF-driven interactions, setting them apart from traditional dative bonds studied in parallel throughout this work.

Ab initio simulation of spin-vibronic spectra of methoxy radical

Despite the fact that experimental and theoretical work on the spectrum of methoxy has stretched from the microwave to the ultraviolet and proceeded for nearly 50 years, parts of the spectrum have remained a challenge to simulate theoretically and make reliable line-by-line assignments. The spectral complexity arises because the radical has a non-zero electron spin and significant vibronic coupling between the two electronic components of the ground state due to the presence of a conical intersection. This work describes a completely ab initio effort to understand and assign the spin-vibronic levels of the X̃2E state from 0 to above 3000 cm-1, a region that includes the fundamental transitions of the C-H symmetric and asymmetric stretches that have not previously been identified uniquely. A potential energy surface for methoxy was calculated at the equation-of-motion (EOM)-coupled cluster singles, doubles, and triples (CCSDT)/atomic natural orbital (ANO1) level of theory. Subsequently, this potential energy surface was fit to a quartic power series expansion of all nine vibrational normal coordinates (as determined at the minimum of the conical intersection) by the use of a machine-learning-based algorithm. After the addition of spin-orbit coupling, the spin-vibronic problem was solved using both the Krylov-Schur and Lanczos algorithms with the SOCJT3 software to converge eigenvalues up to 3500 cm-1 and their eigenvectors. The latter were used, in conjunction with the calculated dipole moment and its derivatives (calculated using finite differences at the EOM-CCSDT/ANO1 level), to determine spectral intensities for the spin-vibronic spectra. The calculated transition frequencies and intensities were used to simulate and assign the observed transitions of the spin-vibronic spectra of the radical. The credibility of the assignments and their significance is discussed in detail.

A perspective marking 20 years of using permutationally invariant polynomials for molecular potentials

This Perspective is focused on permutationally invariant polynomials (PIPs). Since their introduction in 2004 and first use in developing a fully permutationally invariant potential for the highly fluxional cation CH5+, PIPs have found widespread use in developing machine learned potentials (MLPs) for isolated molecules, chemical reactions, clusters, condensed phase, and materials. More than 100 potentials have been reported using PIPs. The popularity of PIPs for MLPs stems from their fundamental property of being invariant with respect to permutations of like atoms; this is a fundamental property of potential energy surfaces. This is achieved using global descriptors and, thus, without using an atom-centered approach (which is manifestly fully permutationally invariant). PIPs have been used directly for linear regression fitting of electronic energies and gradients for complex energy landscapes to chemical reactions with numerous product channels. PIPs have also been used as inputs to neural network and Gaussian process regression methods and in many-body (atom-centered, water monomer, etc.) applications, notably for gold standard potentials for water. Here, we focus on the progress and usage of PIPs since 2018, when the last review of PIPs was done by our group.

Controlling the spatial distribution of electronic excitation in asymmetric D-A-D' and symmetric D'-A-D-A-D' electron donor-acceptor molecules

Understanding how electronic energy is funnelled towards a specific location in a large conjugated molecule is of primary importance for the development of a site-specific photochemistry. To this end, we investigate here how electronic excitation redistributes spatially in a series of electron donor-acceptor (D-A) molecules containing two different donors, D and D', and organised in both linear D-A-D' and symmetric double-branch D'-A-D-A-D' geometries. Using transient IR absorption spectroscopy to probe the alkyne spacers, we show that for both types of systems in non-polar solvents, excitation remains delocalised over the whole molecule. In polar media, charge-transfer (CT) exciton in the linear D-A-D' systems localises rapidly at the end with the strongest donor. For the double-branch systems, excited-state symmetry breaking occurs and the CT exciton localises at the end of one of the two branches, even if the D' terminal donor is not the strongest one. This unexpected behaviour is explained by considering that the energy of a CT state depends not only on the electron donating and withdrawing properties of the donor and acceptor constituents, but also on the solvation energy. This study demonstrates the possibility to control the location of CT excitons in large conjugated systems by varying the nature of the donors and acceptors, the distance between them as well as the environment.

Effect of microhydration on the aromatic charge resonance interaction: the case of the pyrrole dimer cation

Charge resonance (CR) interactions between aromatic molecules are amongst the strongest intermolecular forces and responsible for many phenomena in chemistry and biology. Microhydration of an aromatic radical dimer cation allows investigation of the strong effects of stepwise solvation on the charge distribution and strength of the CR. We characterise herein the microhydration process of the pyrrole dimer cation (Py2+), a prototypical aromatic homodimer with a strong CR. The NH and OH stretch vibrations (νNH/OH) of mass-selected bare and colder Ar-tagged hydrated clusters of Py2+, Py2+(H2O)nArm (n ≤ 3, m ≤ 1), recorded by infrared photodissociation (IRPD) spectroscopy provide detailed insight into the preferred cluster growth and strengths of the various intermolecular interactions by comparison to dispersion-corrected density functional theory calculations. The analysis of systematic frequency shifts, structural parameters, binding energies, and charge distributions allows for a quantitative evaluation of the drastic effects of stepwise hydration on the strength and symmetry of the aromatic CR, the strengths of the various hydrogen bonds (H-bonds), and the competition between slightly noncooperative interior ion hydration and strongly cooperative formation of a H-bonded solvent network. The most stable Py2+H2O structure exhibits a strong NH⋯O ionic H-bond of H2O to the antiparallel stacked Py2+(a) core, thereby breaking the symmetry of the CR. Py2+(H2O)2 prefers a highly symmetric C2h structure with two equivalent NH⋯O H-bonds of Py2+(a) and an optimised CR. Starting from n = 3, clusters with a parallel configuration, Py2+(p), are more stable than those with Py2+(a), further highlighting the strong impact of (micro-)solvation on the structural motif of the aromatic CR. The spectral and computational data demonstrate a linear correlation of νNH of the free Py unit with its partial charge, illustrating that IR spectroscopy is a powerful tool for probing the charge distribution in aromatic CR cluster cations. Comparison of Py2+(H2O)n with neutral Py2(H2O)n and Py+(H2O)n reveals the impact of the magnitude of positive charge and the number of acidic proton donors on the structure of the microhydration shell and strength of the various competing intermolecular bonds.

Effect of Hydrogen Bonding on Ultrafast Intersystem Crossing in 7-Diethylaminothiocoumarin

Thiocarbonyls exhibit unique photophysical properties, characterized by rapid intersystem crossing (ISC) due to favorable singlet-triplet energetics and enhanced spin-orbit coupling. However, the role of hydrogen bonding in modulating the ISC remains underexplored. This study investigates the effect of solvent-solute hydrogen bonding on the ISC dynamics of 7-(diethylamino)-4-methyl-2-sulfanylidene-2H-chromen-2-one (thiocoumarin 1, TC1) using steady-state and time-resolved spectroscopy, complemented by theoretical calculations. Experimental data reveal that in methanol, hydrogen bonding leads to increased fluorescence quantum yield, prolonged singlet-state lifetime, and reduced triplet yield compared to aprotic acetonitrile. Time-resolved spectroscopy identifies an additional long-lived emissive singlet state in methanol, attributed to a hydrogen-bonded state, which slows ISC. Theoretical calculations demonstrate that hydrogen bonding alters the electronic structure and constrains ISC along key nuclear coordinates, including the C═S bond vibration and dihedral angles, leading to decreased triplet formation. These findings provide mechanistic insights into hydrogen-bonding-mediated control of ISC in thiocoumarins, with implications for designing functional materials with tunable photophysical properties.

Nonlinear photonics in glass systems doped with quantum dots and plasmonic nanoparticles

Glass, one of the most important optical materials, is highly transparent, structurally amorphous, and optically isotropic. Unlike many crystals without centrosymmetry, most glass systems exhibit centrosymmetry without second-order optical nonlinearity and weak intrinsic optical nonlinearity, including nonlinear absorption and refraction. However, as glass systems often have a wide composition range, their nonlinear absorption and refraction can be judiciously engineered by doping their active centers and nanocrystals with different optical functionalities. In this review, we discuss the recent advances in the engineering of glass systems for nonlinear photonics, with a focus on oxide glass systems doped with quantum dots (QDs) and plasmonic nanoparticles (NPs). After briefly introducing the relevant nonlinear optics theory, we present a short overview of glass systems doped with QDs and plasmonic NPs oriented for nonlinear optical (NLO) applications. Subsequently, we discuss the investigations conducted on the NLO properties of these glass systems and their application in optical switching for pulse laser generation in detail, mostly in the visible and near-infrared (NIR) regions. Finally, we present a short summary of the development of NLO properties and applications based on the discussed glass systems and a brief perspective for future research directions.

Path Integral Monte Carlo Simulation on Molecular Systems with Multiple Electronic Degrees of Freedom

We present an imaginary time path-integral formalism for molecular systems including nuclear and electronic degrees of freedom based on the previous work of [Schmidt, J. R.; Tully, J. C. J. Chem. Phys. 2007, 127, 094103]. To sample the resulting path integral expression efficiently, a path integral Monte Carlo scheme is proposed, allowing the computation of finite temperature equilibrium properties of molecular systems including multiple low-lying electronic states directly from ab initio potential energy surfaces. Finally, we show how this generalized approach in combination with the Monte Carlo scheme can reproduce exact results for a simple model system including nonadiabatic couplings as well as thermodynamic equilibrium properties of H2 and C2. Our implementation of the algorithm is available as an open-source code.

Density Matrix Renormalization Group Approach Based on the Coupled-Cluster Downfolded Hamiltonians

This study integrates a Hermitian coupled-cluster downfolding technique with the Density Matrix Renormalization Group (DMRG) method to accurately treat both static and dynamic correlations in complex electronic systems. By calculating ground-state energies of active-space Hamiltonians via DMRG, we achieve efficient and accurate simulations of strongly correlated systems, demonstrated on molecular benchmarks including N2, benzene, porphyrin, and tetramethyleneethane. This combined approach offers a promising advancement in computational chemistry for complex chemical processes.

Molecular Response Properties, Electron Correlation, and Quantum Entanglement

There is an ever-increasing interest in studying the properties and main characteristics of entangled atomic and molecular quantum states. As a matter of fact, merging two different areas of research like information theory and quantum physics/chemistry gives new insights to understand from a different framework some of the most basic quantum phenomena. In line with this, the calculation and analysis of the electronic origin of some molecular response properties, like the NMR J-coupling, require the consideration of electron correlation (quantum and classical) and the fact that some response properties could arise from nonlocal interactions. In the case of J-couplings, the change of energy due to the flip-flop of one nuclear magnetic dipole moment that is influenced (directly or indirectly) by the flip-flop of another one has its correlate in the NMR spectra. Besides, from a theoretical perspective, this J-coupling interaction is described and calculated using the electronic framework. In the past few years, we started the development of a theory that introduces a new kind of entanglement that occurs among pairs of excitations of molecular orbitals (MOs). In this work, we give the most general expression of such a theory showing that the entanglement is not dependent on the spin-dependence of the external perturbations. We applied this theory to the analysis of vicinal J-couplings between fluorine nuclei in 1,2-difluoroethane, and we show that there is an entanglement between electron-spin-dependent mechanisms (known as FC and SD) and electron-spin-independent mechanisms (PSO). This entanglement remains lightly dependent on the degree of electron correlation considered (up to the higher RPA level), which confirms previous explanations regarding the physical origin of the empirical Karplus rule. Besides, we show new results for the vicinal J(H, H) coupling in ethane that confirm a direct relationship between the Karplus rule and the entanglement among some coupling pathways that contain a couple of excitations of MOs that are close to the coupled nuclei.

The Influence of Water Molecules on the π* Shape Resonances of the Thymine Anion

Low-energy electrons have been shown to resonantly attach to DNA, inducing strand breakages and other damaging lesions. While computational studies have suggested that the nucleobase moieties can serve as the initial attachment site, there remains ambiguity over the exact character of the temporary anion resonances that form due to the unestablished role of the surrounding environment. Here, we investigate the influence of an aqueous environment on the low-lying anion shape resonances of the π* character of the thymine anion by applying frequency-resolved photoelectron spectroscopy to thymine-water cluster anions, T-(H2O)n, with an increasing degree of hydration, n. Our results indicate that spontaneous solvent rearrangement will stabilize the π2* and π3* states into bound electronic states, and we observe evidence for internal conversion to the anion ground state, further aiding long-term electron capture via these resonances.

Molecular dynamics simulations of atmospherically relevant molecular clusters: a case study of nitrate ion complexes

The formation and decomposition of complexes of ions with atmospheric analyte molecules are key processes in chemical ionization mass spectrometry (CIMS) instruments, as well as in atmospheric new particle formation (NPF). In this study, we conduct extensive molecular dynamics (MD) simulations of the decomposition of already-formed molecular complexes with nitrate ions (NO3-). We study purely thermal decomposition in vacuo and in the presence of nitrogen gas, as well as the decomposition driven by electric-field induced collisions with nitrogen gas. Our findings are relevant to improving the understanding of basic processes taking place in CIMS as well as in other MS instruments more generally.

Deconstructing 1H NMR Chemical Shifts in Strong Hydrogen Bonds: A Computational Investigation of Solvation, Dynamics, and Nuclear Delocalization Effects

This study provides the first quantitative dissection of the factors influencing 1H NMR chemical shifts δH in strong hydrogen-bonded systems, focusing on solvation, nuclear dynamics, and nuclear delocalization. A novel computational framework was developed, combining static quantum chemical calculations (nonrelativistic and relativistic), ab initio molecular dynamics (AIMD), and three-dimensional numerical solutions of the Schrödinger equation. This multiscale approach was applied to three model systems: the bifluoride anion (FHF)-, the Zundel cation (H5O2)+, and the pyridine-pyridinium cation (PyHPy)+. Our results reveal that nuclear dynamics and delocalization are the dominant factors determining δH in complexes with short, strong hydrogen bonds. Solvation effects, while critical for defining the hydrogen-bonding environment, play a secondary role. By isolating the contributions of each factor, we demonstrate that traditional methods often underestimate the quantum mechanical nature of the proton. The application of three-dimensional Schrödinger equation solutions represents a significant methodological advancement, enabling deeper insights into proton behavior in hydrogen bonds. This work not only enhances our understanding of NMR parameters in challenging systems but also establishes a robust framework for modeling complex interactions in chemical and biological environments.

Probing Host-Guest Interactions of the Dual Anion Receptor Harmane with Halide and HSO4- Anions

Harmane is a polycyclic amine that can recognize F- and HSO4- via the ═N-H or ≡N binding site. The active binding site depends on whether the solvent is protic or aprotic, but the underlying molecular mechanism remains unclear. As a first step toward obtaining such mechanisms in solutions, we investigated the interactions of harmane with halide anions (F-, Cl-, Br-, and I-) and HSO4- in the gas phase using negative ion photoelectron spectroscopy combined with theoretical calculations. The adiabatic/vertical detachment energies for deprotonated harmane and harmane·X- (X = F, Cl, Br, I, and HSO4) were determined to be 2.72/2.79, 3.25/3.38, 4.19/4.43, 4.35/4.40, 3.93/3.99, and 4.49/4.75 eV, respectively, with an uncertainty of ±0.05 eV. All the X- anions were found to form hydrogen bonds with harmane through the ═N-H site. A nearly complete proton transfer was observed within the harmane·F- complex anion. Larger halide anions in other harmane-halide complexes remain relatively intact. Four closely lying isomers of harmane·HSO4- were identified. The photodetachment locations of the harmane complex anions were also revealed by electronic state calculations and molecular orbital analyses. The current work lays out a foundation for future work on microsolvated clusters to probe how solvent molecules influence the harmane-anion binding motif.

Development of a Transferable Density-Functional Tight-Binding Model for Organic Molecules at the Water/Platinum Interface

A computationally efficient and transferable approach for modeling reactions at metal/water interfaces could significantly accelerate our understanding and ultimately the development of new catalytic transformations, particularly in the context of the emerging field of biomass conversion. Here, we present a parametrization of Pt-X (X = H, O, C) density-functional tight-binding (DFTB) for addressing this need. We first constructed Pt-H, Pt-O, and Pt-C repulsive potential splines. These pairwise parameters were then augmented to include many-body interactions using the Chebyshev Interaction Model for Efficient Simulation (ChIMES). We compare the geometrical and energetic performances of both DFTB and DFTB/ChIMES methods with DFT reference data across a variety of organic molecules at the platinum surface from nanoparticles to single-crystal surfaces. DFTB shows limited transferability between extended crystal surfaces and small nanoparticles. This transferability is significantly improved through the introduction of three-body interactions with Pt in DFTB/ChIMES, which provides consistent results across various systems, with reductions in the RMSD from around 30 kcal/mol in DFTB to around 10 kcal/mol. We demonstrate the stability and reliability of the obtained parameters by performing metadynamic simulations for the adsorption of phenol on Pt(111). We observe that DFTB itself is undersolvating the surface, leading to only one or two chemisorbed water molecules in a c(4 × 6) unit cell. In contrast, DFTB/ChIMES leads to a coverage of about 0.5 ML and successfully captures the chemisorbed mode of phenol at both the solid/liquid and the solid/gas interfaces. Furthermore, in agreement with experimental measurements, the adsorption at the solid/liquid interface is significantly weaker than that at the solid/gas interface. Furthermore, we highlight that even with DFTB, where we can accumulate dynamics for more than 1 ns for a given system, the simulations are not fully converged.

Large-Scale Non-Adiabatic Dynamics Simulation Based on Machine Learning Hamiltonian and Force Field: The Case of Charge Transport in Monolayer MoS2

We present an efficient and reliable large-scale non-adiabatic dynamics simulation method based on machine learning Hamiltonian and force field. The quasi-diabatic Hamiltonian network (DHNet) is trained in the Wannier basis based on well-designed translation and rotation invariant structural descriptors, which can effectively capture both local and nonlocal environmental information. Using the representative two-dimensional transition metal dichalcogenide MoS2 as an illustration, we show that density functional theory (DFT) calculations of only ten structures are sufficient to generate the training set for DHNet due to the high efficiency of Wannier analysis and orbital classification in sampling the interorbital couplings. DHNet demonstrates good transferability, thus enabling direct construction of the electronic Hamiltonian matrices for large systems. Compared with direct DFT calculations, DHNet significantly reduces the computational cost by about 5 orders of magnitude. By combining DHNet with the DeePMD machine learning force field, we successfully simulate electron transport in monolayer MoS2 with up to 3675 atoms and 13475 electronic levels by using a state-of-the-art surface hopping method. The electron mobility is calculated to be 110 cm2/(V s), which is in good agreement with the extensive experimental results in the range of 3-200 cm2/(V s) during 2013-2023. Due to the high performance, the proposed DHNet and large-scale non-adiabatic dynamics methods have great potential to be applied to study charge carrier dynamics in a wide range of material systems.

OH Roaming as a Key Pathway in the Anti-CH3CHOO + H2O Reaction Yielding CH3COOH and H2O

The reaction of anti-CH3CHOO with H2O is a crucial atmospheric process, resulting in the end products CH3COOH + H2O through the dissociation of the intermediate hydroxyethyl hydroperoxide (CH3CH(HO)OOH, HEHP). Based on an accurate full-dimensional PES, we have obtained detailed dynamics information for this reaction through quasi-classical trajectory simulations. We report two reaction mechanisms for the CH3COOH + H2O product channel: one involving a direct mechanism through the transition state and the other an intriguing OH roaming mechanism. The roaming pathway proceeds via the dissociation of HEHP into OH and the hydroxyethoxy radical (CH3CH(HO)O, HEO), where the OH radical roams near HEO and abstracts a hydrogen atom, subsequently forming H2O and CH3COOH. The presence of this roaming pathway significantly increases the yield of CH3COOH + H2O. This work provides new dynamical support for the study of the anti-CH3CHOO + H2O reaction and enriches our understanding of atmospheric chemistry.

Double Aromaticity and Inverse-Sandwich Structures in Double-Lanthanide-Doped Boron Clusters: Excitation from f-Electrons to d-Electrons

To elucidate the size-dependent formation mechanism of inverse-sandwich structures in double-lanthanide-doped boron clusters, we systematically investigated the geometric and electronic properties of M2B10- (M = La, Ce, Pr, and Nd) clusters through density functional theory calculations. Theoretical optimizations reveal that all studied systems adopt inverse-sandwich configurations with two lanthanide atoms sandwiching a B9 ring at n = 9. Theoretical binding energies (BEs) show excellent agreement with experimental values (BEs < 5 eV), providing the robust validation of the predicted inverse-sandwich configurations. Adaptive natural density partitioning (AdNDP) analysis uncovers a unique bonding mechanism involving f → d electronic excitation in lanthanides, which facilitates the formation of directional d-p δ bonds between metal centers and the boron framework. This electron redistribution simultaneously satisfies the Hückel 4n + 2 rule for both σ- and π-aromaticity, establishing the first example of double aromaticity in lanthanide-doped clusters. The excitation-induced electronic configuration (f + d - 1) dictates the ground-state spin multiplicity while reducing metal-boron binding energy. This energy penalty explains the structural transition from inverse-sandwich to conventional geometries at n = 10, as evidenced by comparative studies on La2B10-. Our findings establish n = 9 as the critical size limit for inverse-sandwich stability in double-lanthanide systems and propose a general electronic design rule for tailoring aromaticity in heavy metal-doped clusters.

Ultrafast photophysics of the cyan fluorescent protein chromophore in solution

Incorporation of fluorescent proteins (FPs) into biological systems has revolutionised bioimaging and the understanding of cellular processes. Ongoing developments of FPs are driving efforts to characterise the fundamental photoactive unit (chromophore) embedded within the protein. Cyan FP has a blue emitting chromophore and is widely used in Förster resonance energy transfer studies. Here, we probe the ultrafast photophysics of the cyan FP chromophore in solution using time-resolved fluorescence up-conversion and transient absorption spectroscopies. The ultrafast dynamics are characterised by two lifetimes, sub-picosecond τ1 (or τF) associated with loss of the fluorescent Franck-Condon state, and lifetime τ2 on the order of several picoseconds that is linked with cooling of a hot ground state. MRSF-TDDFT calculations show that the relaxed S1 state equilibrium geometry is classified as a partial twisted intramolecular charge-transfer state, and lies close in energy to a conical intersection seam associated with torsion about the central double bond leading to facile internal conversion. The excited state dynamics exhibit only a weak viscosity dependence, consistent with a barrierless and near-volume-conserving non-radiative decay mechanism. Fluorescence lifetimes for the deprotonated anion are twice those for the neutral.

Magnetic Deflection of High-Spin Sodium Dimers Formed on Helium Nanodroplets

Spectroscopic data on alkali-atom dimers residing on the surface of liquid helium nanodroplets have revealed that they are detected primarily in the weakly bound, metastable, spin-triplet state. Here, by measuring the magnetic Stern-Gerlach deflection of a sodium-doped nanodroplet beam, we transparently demonstrate the abundance of high-magnetic-moment dimers. Their electron spins thermalize with the cryogenic superfluid droplets and become fully oriented by the external magnetic field.

Femtosecond-and-atom-resolved solvation dynamics of a Na+ ion in a helium nanodroplet

Recently, it was shown how the primary steps of solvation of a single Na+ ion, instantly created at the surface of a nanometer-sized droplet of liquid helium, can be followed at the atomic level [Albrechtsen et al., Nature 623, 319 (2023)]. This involved measuring, with femtosecond time resolution, the gradual attachment of individual He atoms to the Na+ ion as well as the energy dissipated from the local region of the ion. In this current work, we provide a more comprehensive and detailed description of the experimental findings of the solvation dynamics and present an improved Poisson-statistical analysis of the time-resolved yields of the Na+Hen ions recorded. For droplets containing an average of 5200 He atoms, this analysis gives a binding rate of 1.84 ± 0.09 atoms/ps for the binding of the first five He atoms to the Na+ ion. In addition, thanks to accurate theoretical values for the evaporation energies of the Na+Hen ions, obtained by path integral Monte Carlo methods using a new potential energy surface presented here for the first time, we improve the determination of the time-dependent removal of the solvation energy from the region around the sodium ion. We find that it follows Newton's law of cooling for the first 5 ps. Measurements were carried out for three different average droplet sizes, ⟨ND⟩ = 9000, 5200, and 3600 helium atoms, and differences between these results are discussed.

Water-soluble BODIPY dyes: a novel approach for their sustainable chemistry and applied photonics

The BODIPY family of organic dyes has emerged as a cornerstone in photonics research development, driving innovation and advancement in various fields of high socio-economic interest. However, the majority of BODIPY dyes exhibit hydrophobic characteristics, resulting in poor solubility in water and other hydrophilic solvents. This solubility is paramount for their optimal utilization in a myriad of photonic applications, particularly in the realms of biology and medicine. Furthermore, it facilitates safer and more sustainable manipulation and chemical modification of these expansive dyes. Nevertheless, bestowing BODIPYs with water solubility while preserving their other essential properties, notably their photophysical signatures, poses a significant challenge. In this context, we present a straightforward general chemical modification aimed at converting conventional hydrophobic BODIPYs into highly hydrophilic variants, thus enabling their efficient solubilization in water and other hydrophilic solvents with minimal disruption to the dye's inherent photophysics. The efficacy of this methodology is demonstrated through the synthesis of a number of water-soluble BODIPY dyes featuring diverse substitution patterns. Furthermore, we showcase their utility in a spectrum of photonics-related applications, including in-water BODIPY chemistry and dye-laser technology, and fluorescence microscopy.

Analysis of variants of non-adiabatic ring polymer molecular dynamics for calculating excited state dynamics

The non-adiabatic ring polymer molecular dynamics (NRPMD) method, which combines the path-integral ring polymer molecular dynamics framework for the nuclei with the Meyer-Miller-Stock-Thoss mapping of the electronic states, is a powerful tool for simulating non-adiabatic dynamics including nuclear quantum effects. However, challenges arise in utilizing NRPMD associated with zero-point energy leakage between the electronic and nuclear degrees of freedom and ambiguities in how to apply the method under non-equilibrium conditions. Here, we explore several variants of NRPMD and compare their performance using a set of benchmark systems for excited-state electronic population dynamics. Within this context, we adopt an idea from recent work on the linearized semi-classical initial value representation and derive a new NRPMD correlation function for the population of the electronic states in terms of a trace-less operator and the identity operator. The in-depth analysis of the different choices when utilizing NRPMD provides new insight into the practical implementation of the method and related techniques.

Quantum Nature of Ubiquitous Vibrational Features Revealed for Ethylene Glycol

Vibrational properties of molecules are of widespread interest and importance in chemistry and biochemistry. The reliability of widely employed approximate computational methods is questioned here against the complex experimental spectrum of ethylene glycol. Comparisons between quantum vibrational self-consistent field and virtual-state configuration interaction (VSCF/VCI), adiabatically switched semiclassical initial value representation (AS-SCIVR), and thermostatted ring polymer molecular dynamics (TRPMD) calculations are made using a full-dimensional, machine-learned potential energy surface. Calculations are done for five low-lying conformers and compared with the experiment, with a focus on the high-frequency, OH-stretches, and CH-stretches, part of the spectrum. Fermi resonances are found in the analysis of VSCF/VCI eigenstates belonging to the CH-stretching band. Results of comparable accuracy, quality, and level of detail are obtained by means of AS SCIVR. The current VSCF/VCI and AS-SCIVR power spectra largely close the gaps between the experiment and TRPMD and classical MD calculations. Analysis of these results provides guidance on what level of accuracy to expect from TRPMD and classical MD calculations of the vibrational spectra for ubiquitous CH- and OH-stretching bands. This work shows that even general vibrational features require a proper quantum treatment, usually not achievable by the most popular theoretical approaches.

Enhanced Bioactivity of Natural Products by Halogenation: A Database Survey and Quantum Chemistry Calculation Study

Natural products (NPs) have long been the cornerstone of drug discovery. Halogenated organic NPs are limited, while around one-fourth of approved chemical drugs are organohalogens. This suggests that the introduction of halogens into NPs may enhance their potential for transformation into drugs. In this study, we utilized a matched molecular pair (MMP) approach alongside a database survey to investigate the impact of halogenation on this transformation. The study revealed that halogenation increased the bioactivity of 70.3% of NPs, with 50.3% exhibiting at least a 2-fold enhancement. Halogen bonds (XBs) are prevalent between organohalogens and their targets. To explore whether halogenated NPs could form XBs with their targets, computational studies were performed and demonstrated that halogenated NPs or NP-derived drugs formed strong XBs with their targets, resulting in improved binding affinities. This study highlights the considerable potential of introducing halogens into NPs as a strategic approach for enhancing bioactivity and facilitating the development of drugs.

A Cost-Effective Treatment of Spin-Orbit Couplings in the State-Averaged Driven Similarity Renormalization Group Second-Order Perturbation Theory

We present an economical approach to treat spin-orbit coupling (SOC) in the state-averaged driven similarity renormalization group second-order perturbation theory (SA-DSRG-PT2). The electron correlation is first introduced by forming the SA-DSRG-PT2 dressed spin-free Hamiltonian. This Hamiltonian is then augmented with the Breit-Pauli Hamiltonian and diagonalized using spin-pure reference states to obtain the SOC-corrected energy spectrum. The spin-orbit mean-field approximation is also assumed to reduce the cost associated with the two-electron spin-orbit integrals. The resulting method is termed BP1-SA-DSRG-PT2c, and it possesses the same computational scaling as the non-relativistic counterpart, where only the one- and two-body density cumulants are required to obtain the vertical transition energy. The accuracy of BP1-SA-DSRG-PT2c is assessed on a few atoms and small molecules, including main-group diatomic molecules, transition-metal atoms, and actinide dioxide cations. Numerical results suggest that BP1-SA-DSRG-PT2c performs comparably to other internally contracted multireference perturbation theories with SOC treated using the state interaction scheme.

Insight into the atomic-level structure of γ-alumina using a multinuclear NMR crystallographic approach

The combination of multinuclear NMR spectroscopy with 17O isotopic enrichment and DFT calculations provided detailed insight into both the bulk and surface structure of γ-Al2O3. Comparison of experimental 17O NMR spectra to computational predictions confirmed that bulk γ-Al2O3 contains Al cations primarily in "spinel-like" sites, with roughly equal numbers of alternating AlVI and AlIV vacancies in disordered "chains". The work showed that overlap of signals from OIV and OIII species complicates detailed spectral analysis and highlighted potential problems with previous work where structural conclusions are based on an unambiguous assignment (and quantification) of these signals. There was no evidence for the presence of H, or for any significant levels of O vacancies, in the bulk structure of γ-Al2O3. Computational predictions from structural models for different surfaces showed a wide variety of protonated and non-protonated O species occur. Assignment of signals for two types of protonated O species was achieved using variable temperature CP and TRAPDOR experiments, with the sharper and broader resonances attributed to more accessible surface sites that interact more strongly with water and less accessible aluminols, respectively. DFT-predicted 1H NMR parameters confirmed the 1H shift increases with denticity but is also dependent on the coordination number of the next nearest neighbour Al species. Spectral assignments were also supported by 1H-27Al RESPDOR experiments, which identified spectral components resulting from μ1, μ2 and μ3 aluminols. Combining these with 1H-27Al D-HMQC experiments showed that (i) μ1 aluminols are more likely to be bound to AlIV, (ii) μ2 aluminols are coordinated to all three types of Al, but with a higher proportion bound to similar types of Al and (iii) μ3 aluminols are most likely bound to higher coordinated Al species. 1H DQ MAS spectroscopy confirmed no aluminols exist exclusively in isolation but showed that the closest proximities are between bridging aluminols coordinated to AlIV and/or AlV species.

Superfluidity of quasi-2D 4He droplets on graphite

The superfluid response of nanoscale size quasi-2D 4He droplets adsorbed on a graphite substrate is investigated by computer simulations. It is found that clusters comprising as few as 7 atoms are stable at temperatures of ≲0.15 K. Clusters of ∼20 atoms or less are liquid-like and ∼100% superfluid. As the size is increased, the central region crystallizes, forming the commensurate C1/3 phase, with a quasi-1D surface layer of superfluidity possibly evolving into a Luttinger liquid or a transverse quantum superfluid with increasing cluster size. The relevance to quasi-2D molecular spectroscopy is discussed.

Three-Dimensional Atomic-Level Structure of an Amorphous Glucagon-Like Peptide-1 Receptor Agonist

Amorphous formulations are increasingly used in the pharmaceutical industry due to their increased solubility, but their structural characterization at atomic-level resolution remains extremely challenging. Here, we characterize the complete atomic-level structure of an amorphous glucagon-like peptide-1 receptor (GLP-1R) agonist using chemical shift driven NMR crystallography. The structure is determined from measured chemical shift distributions for 17 of the 32 carbon atoms and 16 of the 31 hydrogen atoms in the molecule. The chemical shifts are able to provide a detailed picture of the atomic-level conformations and interactions, and we identify the structural motifs that play a major role in stabilization of the amorphous form. In particular, hydrogen bonding of the carboxylic acid proton is strongly promoted, although no carboxylic acid dimer is formed. Two orientations of the benzodioxole ring are promoted in the NMR structure, corresponding to a significant stabilization mechanism. Our observation that inclusion of water leads to stabilization of the carboxylic acid group might be used as a strategy in future formulations where hydrogen bonding between neighboring molecules may otherwise be hindered by sterics.

Observation of the interaction between Au- and CO2 in Au(CO2)n- anions: Physisorption as the dominant mechanism

In this study, we investigated the structure and bonding of Au(CO2)n- (n = 2, 3) using photoelectron spectroscopy analysis, quantum chemical calculations, and weak interaction analysis. Quantum chemical calculations revealed that the geometries of the physisorbed structures closely aligned with experimental data, suggesting that these configurations were the most stable under the experimental conditions. Conversely, while chemisorbed structures exhibit stronger interactions and considerable CO2 activation, they show less agreement with the observed spectroscopic data. Using the interaction region indicator method, our weak interaction analysis confirmed that van der Waals forces were the dominant interaction in the physisorbed structures. Our experimental results indicate that these physically adsorbed structures are more stable under the conditions of this study. These findings shed light on the interaction mechanisms of Au(CO2)n- (n = 2, 3) at the molecular level and provide new insights into the potential for transition metals to catalytically activate CO2.

Room Temperature Gas-Phase Detection and Formation Gibbs Energy of the Water Dimethyl Ether Bimolecular Complex

Hydrated complexes are of general interest for understanding nucleation processes in the atmosphere, where water is abundant, especially as the role of whether water enhances or inhibits nucleation is still uncertain. We have recorded the Fourier transform infrared absorption spectrum of the water dimethyl ether bimolecular complex in the gas phase at room temperature. Four distinct bands are observed and assigned. The equilibrium constant of complex formation is determined from the experimental integrated absorbance of the bands and the corresponding calculated intensities. The calculated band intensities are obtained with a 9D reduced-dimensional variational local mode model with the CCSD(T)-F12a/cc-pVDZ-F12 potential energy and dipole moment surfaces. A similar equilibrium constant for a majority of the observed bands is obtained, with an average value of 0.042 ± 0.003 at T = 298 K. The water dimethyl ether complex studied here is similar to the water dimer, and our determined equilibrium constant may serve as a reasonable estimate for that of water dimer, which is especially relevant in atmospheric chemistry.

Unraveling the Reactivity of SiO2-Supported Nickel Catalyst in Ethylene Copolymerization with Polar Monomers: A Theoretical Study

Understanding the catalytic behavior of heterogeneous systems for the copolymerization of ethylene with polar monomers is essential for developing advanced functional polyolefins. In this study, we conducted a quantum chemical investigation of the SiO2-supported Ni-allyl-α-imine ketone catalyst (Ni-OH@SiO2) to uncover the factors governing monomer insertion, selectivity, and reactivity. Using DFT calculations and energy decomposition analysis (ALMO-EDA), we evaluated the coordination and insertion of six industrially relevant polar monomers, comparing their behavior to ethylene homopolymerization. Our results show that special polar monomers (SPMs) with aliphatic spacers, such as vinyltrimethoxysilane (vTMS) and 5-hexenyl acetate (AMA), exhibit favorable insertion profiles due to enhanced electrostatic and orbital interactions with minimal steric hindrance. In contrast, fundamental polar monomers (FPMs), including methyl acrylate (MA) and vinyl chloride (vCl), show higher activation barriers and increased Pauli repulsion due to strong electron-withdrawing effects and conjugation with the vinyl group. AMA displayed the lowest activation barrier (7.4 kcal/mol) and highest insertion thermodynamic stability (-17.6 kcal/mol). These findings provide molecular-level insight into insertion mechanisms and comonomer selectivity in Ni-allyl catalysts supported on silica, extending experimental understanding. This work establishes key structure-reactivity relationships and offers design principles for developing efficient Ni-based heterogeneous catalysts for polar monomer copolymerization.

Vibrational Spectroscopy Through Time Averaged Fourier Transform of Autocorrelated Molecular Dynamics Data: Introducing the Free SEMISOFT Web-Platform

Vibrational spectroscopy calculations based on classical molecular dynamics simulations are widely employed in a variety of popular fields, for instance, computational biochemistry and materials science. These calculations commonly rely on the Fourier transform of the velocity autocorrelation function. One major drawback of the method is that calculated spectra are difficult to interpret due to the large number of closely spaced signals. In this paper, we show how theory can help to overcome this issue by means of a time-average technique, and we introduce free software to perform such calculations for anyone who may take advantage of it. The studies presented here involve the classical vibrational spectra of aniline microsolvated by a water molecule and gas-phase deoxyguanosine. The software is made available in the form of a free web-platform, named SEMISOFT (http://semisoft.unimi.it/), whereby, upon upload of the classical trajectory, the user gets the corresponding time-averaged spectrum. Furthermore, since the evaluation of response functions through autocorrelated data is quite a general approach, the web-platform can be directly employed in many other research fields besides chemistry.

Comprehensive analysis of Syzygium cumini L. pomace extract as an α-amylase inhibitor: In vitro inhibition, kinetics, and computational studies

Type 2 diabetes mellitus (T2DM) is a widespread metabolic disorder characterized by impaired regulation of blood glucose levels. Jamun (Syzygium cumini L.) fruits and seeds have been traditionally used in Ayurveda to manage diabetes. While fruit and seed extracts have been extensively studied for their anti-α-amylase properties, pomace, a byproduct of juice extraction, remains under explored. This study investigated the α-amylase inhibitory potential of jamun pomace (JP) extract by using in vitro and in silico methods. Enzyme inhibition assays revealed an half-maximal inhibitory concentration (IC₅₀) value of 85.68 ± 5.22 μg/mL for the JP extract, comparable to acarbose (64.28 ± 7.15 μg/mL). The extract exhibited mixed-mode inhibition, whereas acarbose showed competitive mode inhibition. At 10 μg/mL, the Vmax of JP extract was half that of acarbose, demonstrating significant inhibition. GC-MS analysis identified 11 volatile compounds (R1-R11) in the JP extract. Density Functional Theory (DFT) and ADMET analyses confirmed the chemical reactivity of the volatiles, drug-like properties, and low toxicity. Molecular docking revealed a high binding score for R11 (-8.0 kcal/mol), similar to acarbose (-8.2 kcal/mol). Molecular dynamics simulations further demonstrated the stability of α-amylase complexes with R11, R3, and R8, with R11 showing the lowest binding energy (-28.75 ± 6.25 kcal/mol). These findings suggest that R11 and JP extracts hold promise as anti-diabetic agents. Utilizing JP extract as a nutraceutical offers the dual benefit of diabetes management and sustainable waste valorization in jamun juice production.

Designing a 3-Hydroxypropane-1-Sulfonate Deep-Ultraviolet Nonlinear Optical Crystal

Although 3-hydroxypropane-1-sulfonates are discovered several decades ago, their nonlinear optical (NLO) properties have remained unexplored, mainly because most of them crystallize in centrosymmetric space groups. Herein, by incorporating polarizable rare-earth metal La3+ cations, a novel non-centrosymmetric 3-hydroxypropane-1-sulfonate, namely La[SO3(CH2)3OH]3 is designed and synthesized, whose crystal structure features 3D framework constructed by highly distorted [LaO9] polyhedra and [SO3(CH2)3OH]- anions. Experimental results reveal that La[SO3(CH2)3OH]3 is NLO-active with a moderate second-harmonic generation response of 0.6 times that of the benchmark KH2PO4 and a wide absorption edge below 190 nm. Structural analyses and first-principles calculations reveal that the non-centrosymmetric nature and NLO properties are closely related to highly polarizable cationic [LaO9] polyhedra. This work highlights 3-hydroxypropane-1-sulfonates as promising candidates for deep-ultraviolet NLO materials and provides some inspiration for designing non-centrosymmetric structures.

Using phase-resolved vibrational sum-frequency imaging to probe the impact of head-group functionality on hierarchical domain structure in lipid membranes

The substantial diversity in phospholipids within a plasma membrane, varying in tail length, degree of saturation, and head-group functionality, generates widespread structural heterogeneity. This exists both laterally across the membrane through the spontaneous formation of condensed domains that differ from their surrounding expanded phase in density, composition, and molecular packing order, as well as between its two leaflets, which normally maintain significant compositional asymmetry. Of particular importance is the exposure of phosphatidylserine (PS) lipids which is a marker for important physiological processes e.g. apoptosis. Despite this, the molecular-level alterations to the phase-structure of the membrane that result from PS exposure remain generally unknown. In this work, we utilise recently developed phase-resolved azimuthal-scanned sum-frequency generation (SFG) microscopy to investigate structural changes that occur heterogeneously across model membranes as a result of PS-lipid exposure. Specifically, by probing mixed monolayers of 1,2-dipalmitoylphosphatidylcholine (DPPC) and deuterated 1-palmitoyl-2-oleoylphosphatidylcholine (dPOPC) in both the C-H and C-D stretching regions as well as equivalent films with DPPC exchanged with DPPS, we analyse the variations in the apparent phase distributions and domain morphologies, and quantitatively extract the density, composition, and relative out-of-plane packing order for both mixtures. We find that, in these mixtures, DPPS shows vast differences in the domain growth and coalescence behaviour compared to DPPC, as well as in the relative compositions and molecular ordering within each phase. This demonstrates the critical role the head-group plays in the heterogeneous phase structure of the membrane and may give insights into their impact on important physiological processes.

Two-dimensional isotopic shifts for steroid isomer delineation with high-resolution cyclic ion mobility separations

Recently, the use of mass distribution-based isotopic shifts in high-resolution ion mobility spectrometry-mass spectrometry-based separations have enabled isomer delineation by measuring the relative arrival times of their heavy and light isotopologues. However, all previous efforts to induce such shifts have focused solely on the introduction of one type of isotopic substitution for a given molecule or isomer set. Herein, for the first time, we present a two-dimensional isotopic labeling strategy where two unique derivatizations are performed on various steroid isomer molecules to induce two distinct isotopic shifts and thus simultaneously measure them in a single ion mobility separations experiment. Derivatization strategies were chosen to target two specific functional groups in these steroids (i.e., hydroxyl and carbonyl), and heavy-labeled versions of the derivatizing reagents were used to induce isotopic shifts at each of these positions. We found that isotopic shifts were orthogonal to one another, diagnostic for certain steroid isomers, and that the simultaneous analysis of two different isotopic shifts was necessary for complete characterization of each steroid isomer set. We envision this multidimensional isotopic shift strategy as a new method for delineating amongst isomeric molecules, especially those with several different functional groups causing their isomerism.

A Combined Crossed Molecular Beam and Theoretical Investigation of the Elementary Reaction of Tricarbon (C3(X1Σg+)) with Diacetylene (C4H2(X1Σg+)): Gas Phase Formation of the Heptatriynylidyne Radical (l-C7H(X2Π))

An elucidation of the underlying formation pathways to acyclic hydrocarbons such as polyynes (CnH2), cumulenes (CnH2), and linear resonantly stabilized linear radicals (l-CnH) is indispensable to understand the hydrocarbon chemistry in extreme low- and high-temperature environments. In this study, we exploited the crossed molecular beam technique to investigate the reaction of tricarbon C3(X1Σg+) with diacetylene (butadiyne; HCCCCH; X1Σg+) at a collision energy of 47 ± 1 kJ mol-1. The experimental data were merged with ab initio calculations of the singlet C7H2 potential energy surface (PES) revealing that the reaction is initiated via the formation of an initial van der Waals reactant complex in the entrance channel. Subsequent rearrangements lead to various carbene-type and cyclic intermediates via ring-opening, ring-closure, and hydrogen migration processes, eventually forming acyclic C7H2 isomers prior to their barrierless unimolecular decomposition to the most stable linear isomer, heptatriynylidyne (C7H, X2Π) in an overall endoergic reaction (+57 kJ mol-1). The reaction exhibits strong similarities to the tricarbon-acetylene (C3-C2H2). The significant energy threshold suggests that the tricarbon reaction with (poly)acetylenes forming resonantly stabilized linear radicals is open in high-temperature environments such as combustion flames and circumstellar envelopes of carbon stars and planetary nebulae as their descendants; however, these reactions are closed in low-temperature environments as in cold molecular clouds and hydrocarbon-rich atmospheres of planets and their moons such as in Titan.

Revisiting the Half-and-Half Functional

Hybrid density functionals typically provide significantly better accuracy than semilocal functionals. Conventional wisdom holds that incorporating more than 20-25% exact exchange is deleterious to thermochemical properties and should only be used as a last resort, for problems that are dominated by self-interaction error. In such cases, the Becke-Lee-Yang-Parr "half-and-half" functional (BH&H-LYP) has emerged as a go-to choice, especially in time-dependent density functional theory calculations for excitation energies. Here, we examine the assumption that 50% Hartree-Fock exchange sacrifices thermochemical accuracy. Using a sequence of functionals B(α)LYP, with different percentages of exact exchange (0 ≤ α ≤ 100), we find that BH&H-LYP (with α = 50) is nearly optimal and affords accuracy similar to B3LYP for thermochemistry, barrier heights, and excitation energies. Although BH&H-LYP is significantly less accurate than B3LYP for atomization energies, this emerges as the sole rationale for the taboo against values α > 25. Overall, BH&H-LYP is a reasonable choice for problems that are dominated by self-interaction error, including charge-transfer complexes and core-level excitation energies. While B3LYP remains more accurate for valence excitation energies, the use of 50% exact exchange appears to be an acceptable compromise, and BH&H-LYP can be used without undue concern over its diminished accuracy for ground-state properties.

Infrared Photodissociation Spectroscopy of Cationic Nitric Oxide Clusters, [(NO)n]+, and [NO2(NO)n]. J Phys Chem A 2025;129:3867-3875. [PMID: 40258304 PMCID: PMC12051196 DOI: 10.1021/acs.jpca.5c01377] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Photofragmentation spectroscopy provides a powerful method for the determination of structures and bonding in isolated gas-phase clusters. Here we report infrared action spectra of mass-selected cationic nitric oxide clusters, (NO)n+ (n = 3-8), and mixed NO2(NO)n+ clusters which are interpreted with the help of quantum chemical calculations. Despite the rich potential energy landscape which exhibits very many calculated low-energy isomers, clear structural motifs are observed. Important differences between our (NO)n+ spectra and others published previously are interpreted in terms of the qualitatively different experimental techniques employed in the initial formation of the clusters in each study. Finally, spectra recorded in different fragmentation channels provide clear evidence for intracluster chemistry leading to the formation of mixed nitrous oxide/nitrogen dioxide/nitric oxide complexes, (N2O)(NO2)(NO)n+.

Temperature-Dependent Mechanistic Control of Nonadiabatic Tunnelling in Triplet Carbenes

Experiments on three chemically similar triplet carbenes observed the reaction of one at 10 K, another only when heated to 65 K, whereas the third remained stable despite heating. As the products are singlets, it is clear that the reactions involve intersystem crossing in addition to intramolecular hydrogen transfer. Here, instanton theory is used to study various possible reaction mechanisms, including sequential and concerted pathways. The latter describes a new reaction mechanism which involves changing spin state (a nonadiabatic process) while heavy atoms tunnel underneath a barrier (an adiabatic process). In each case, we find that the concerted pathway dominates the rate at low temperatures, but at higher temperatures it switches to a sequential mechanism. The existence of a crossover temperature is the key to explaining the experimental observations and demonstrates that temperature can control the reactivity of triplet carbenes via nonadiabatic tunnelling.

Superior Differential Ion Mobility Spectrometry of Pendular Macromolecules Using Low-Frequency Rectangular Waveforms

Ion mobility spectrometry (IMS) can delineate gas-phase ions and probe their geometries. Coupling with electrospray ionization and MS has brought IMS to structural biology, revealing the macromolecular folding and subunit connectivity. However, the orientational averaging of ion-molecule collision cross sections (Ω) in the linear and field asymmetric waveform IMS (FAIMS) diminishes the resolution and structural specificity. In the novel low-field differential (LOD) IMS, a field too weak for ion heating (and thus FAIMS) aligns strong macrodipoles, capturing their magnitudes and directional Ω across the dipole (Ω⊥). However, the bisinusoidal waveforms (from FAIMS) have compromised the resolution, measurement accuracy, and correlation to the ion properties. Large ions amenable to LODIMS have low mobility and diffuse slowly, allowing the waveform frequencies down to ∼10 kHz. The low field and frequency permit generating the ideal rectangular waveforms with a flexible frequency and duty cycle by direct switching (impractical for FAIMS) in a miniature low-power format. This new IMS stage is evaluated for the exemplary large protein albumin (66 kDa) previously studied using the bisinusoidal waveform. The flat voltages and greater form factor initiate the differential IMS effect at lower fields, expand the separation space, and enable the quantification of Ω⊥ values by varying the duty cycle.

Viscosity and Phase State of Wildfire Smoke Particles in the Stratosphere from Pyrocumulonimbus Events: An Initial Assessment

Understanding the viscosity and phase state of biomass-burning organic aerosol (BBOA) from wildfires and pyrocumulonimbus (pyroCb) events in the stratosphere is critical for predicting their role in stratospheric multiphase chemistry and ozone depletion. However, the viscosity and phase state of BBOA under stratospheric conditions, including interactions with sulfuric acid (H2SO4), remain largely unquantified. In this study, we combine laboratory data with a thermodynamic model to predict the viscosity and phase state of BBOA under stratospheric conditions. Our results suggest that BBOA with a H2SO4-to-BBOA mass ratio of 0.37─an estimated upper limit for pyroCb smoke in the lower stratosphere after two months of aging─is highly viscous and frequently exists in a glassy state. Even at a higher H2SO4-to-BBOA mass ratio of 0.79─an estimated upper limit after nine months of aging─BBOA can still transition to a glassy state under certain stratospheric conditions. In the glassy state, bulk reactions are suppressed, and multiphase chemistry may be limited to the particle surfaces. We also highlight key areas for future research needed to better constrain the viscosity and phase state of BBOA in the stratosphere and its subsequent impact on ozone.