Hydrated Electrons in Phase-Matching Generation of Second-Order Stokes X-Waves in Water

Two components of X-waves, near-axis and off-axis, were observed in the generation of second-order Stokes around 550 nm, excited by intense 400 nm, 100 fs pump pulses in a 50 cm water cuvette. The emission angles of these two X-waves exhibited different evolutions; when the pump energy increased, the emission angle of the near-axis X-wave increased, while that of the off-axis X-wave decreased. These abnormal features of second-order X-waves came from the four-wave mixing process, accompanied by induced intense hydrated electrons via cascade ionization. The induced wave vector from high-density hydrated electrons led to angle-dependent phase-matching for the generation of the off-axis X-wave. However, for the generation of the near-axis X-wave, the induced wave vector from hydrated electrons initially compensated for the phase mismatch at a low pump energy, but as the energy increased, the phase mismatch also increased. Moreover, anomalous Raman shifts at second-order Stokes wavelengths (3262 cm-1 and 3350 cm-1) exhibited a similar evolutionary process to the anomalous Raman peaks at the Stokes wavelengths. The shifts arose from excess electrons being injected into the hydrogen bond network of water clusters.

Stabilities Referring to the f Electron Effect in Erbium-Based Endohedral Metallofullerenes: Revisiting Er2@C80 and the Missing Er2C2@C78

Endohedral metallofullerenes (EMFs) involving lanthanide elements exhibit unique physicochemical properties, with one of the key factors being the presence of 4f electrons. These 4f electrons contribute to complex metal-metal and metal-cage interactions. Despite extensive research on their properties, a thorough theoretical interpretation of the effect of 4f electrons on the EMF stability and structure remains lacking. In this study, building on previous findings that highlighted Er2@C80 as a potential magnetic molecule, the stabilities and metal-metal/metal-cage interactions of the Er2C80 system have been explored using density functional theory calculations combined with statistical thermodynamic analyses. Three thermodynamically stable isomers are identified: Er2@Ih(31924)-C80, Er2C2@C1(23349)-C78, and Er2C2@C1(22595)-C78. The Er2@Ih-C80 isomer features a two-center one-electron Er-Er bond stabilized by significant charge transfer to the C80 cage. Extended Transition State-Natural Orbitals for Chemical Valence (ETS-NOCV) analysis reveals electron redistribution within the cage and stabilization of the Er-Er bond, which corroborates earlier model conclusion of the magnetic properties of Er2@C80. Analyses of different electronic configurations of the main Er2C80 isomers and Er2 cluster reveal a significant impact on the exchange interaction of 4f electrons to the stability of the whole Er2C80 molecule.

Exploring quantum control landscape and solution space complexity through optimization algorithms and dimensionality reduction

Understanding the quantum control landscape (QCL) is important for designing effective quantum control strategies. In this study, we analyze the QCL for a single two-level quantum system (qubit) using various control strategies. We employ Principal Component Analysis (PCA), to visualize and analyze the QCL for higher dimensional control parameters. Our results indicate that dimensionality reduction techniques such as PCA, can play an important role in understanding the complex nature of quantum control in higher dimensions. Evaluations of traditional control techniques and machine learning algorithms reveal that Genetic Algorithms (GA) outperform Stochastic Gradient Descent (SGD), while Q-learning (QL) shows great promise compared to Deep Q-Networks (DQN) and Proximal Policy Optimization (PPO). Additionally, our experiments highlight the importance of reward function design in DQN and PPO demonstrating that using immediate reward results in improved performance rather than delayed rewards for systems with short time steps. A study of solution space complexity was conducted by using Cluster Density Index (CDI) as a key metric for analyzing the density of optimal solutions in the landscape. The CDI reflects cluster quality and helps determine whether a given algorithm generates regions of high fidelity or not. Our results provide insights into effective quantum control strategies, emphasizing the significance of parameter selection and algorithm optimization.

Intermolecular Vibrational and Orientational Dynamics of Deep Eutectic Solvents Composed of Lithium Bis(trifluoromethylsulfonyl)amide and Organic Amides Revealed by Dynamic Kerr Effect Spectroscopy

In this study, we investigated the intermolecular dynamics, including intermolecular vibration and orientational dynamics, of five deep eutectic solvents (DESs) consisting of lithium bis(trifluoromethylsulfonyl)amide and organic amides, such as acetamide, propanamide, N-methylacetamide, butyramide, and urea, at a mole ratio of 1:3 using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and subpicosecond optical Kerr effect spectroscopy (ps-OKES). The fs-RIKES results showed that the line shape of the low-frequency band of the N-methylacetamide was trapezoidal, while that of the other organic amide-based DESs was bimodal. The peak and first moment of the intermolecular vibrational band appearing in the frequency range less than 250 cm-1 for the acetamide- and urea-based DESs were in a higher-frequency region compared to the other three DESs, indicating stronger intermolecular interactions. Furthermore, analysis of the intramolecular vibrational bands of the bis(trifluoromethylsulfonyl)amide anion showed that the population of the transoid conformer of the anion was slightly higher in the urea-based DES than in the other organic amide-based DESs, suggesting that urea solvate lithium cations more than the other organic amides. The slow relaxation dynamics of all five DESs were captured for up to 1 ns using ps-OKES. The slow relaxation dynamics also depended on the organic amide species. However, the slow relaxation time constant did not show a clear correlation with the viscosity; therefore, the relaxation dynamics of the DESs did not follow the Stokes-Einstein-Debye hydrodynamic model. The densities, viscosities, surface tensions, and electrical conductivities of the DESs were also measured for comparison with spectroscopic results.

Multiset Variational Quantum Dynamics Algorithm for Simulating Nonadiabatic Dynamics on Quantum Computers

Accelerating quantum dynamical simulations with quantum computing has received considerable attention but remains a significant challenge. In variational quantum algorithms for quantum dynamics, designing an expressive and shallow-depth parametrized quantum circuit (PQC) is a key difficulty. Here, we propose a multiset variational quantum dynamics algorithm (MS-VQD) tailored for nonadiabatic dynamics involving multiple electronic states. The MS-VQD employs multiple PQCs to represent the electronic-nuclear coupled wave function, with each circuit adapting to the motion of the nuclear wavepacket on a specific potential energy surface. By simulating excitation energy transfer dynamics in molecular aggregates described by the Frenkel-Holstein model, we demonstrate that the MS-VQD achieves the same accuracy as the traditional VQD while requiring significantly shallower PQCs. Notably, its advantage increases with the number of electronic states, making it suitable for simulating nonadiabatic quantum dynamics in complex molecular systems.

Unraveling Electron Transfer in the Oxidation of Yttrium Metal Atoms: Dual Pathways from Reactive and Nonreactive Imaging

The intricate mechanisms underlying electron transfer and structural evolution are essential to understanding the oxidation dynamics of transition metal atoms; however, accurately measuring the mechanisms remains challenging. In this study, utilizing laser ablation-crossed beam and time-sliced ion velocity imaging techniques, we identified two distinct electron transfer mechanisms in the reactions of Y and O2 based on reactive and nonreactive scattering measurements across varying collision energies. (1) Low-barrier end-on pathway: Electron transfer occurs through a collinear Y-O-O geometry with a low activation barrier, evidenced by rebound scattering of YO products at low collision energy and backward scattering of Y reactants at higher energies. (2) High-barrier side-on pathway: Electron transfer proceeds through a side-on geometry, presenting a higher activation barrier that facilitates the formation of long-lived O-Y-O intermediates, which is characterized by the backward-forward peaking angular distribution of YO products and broad energy distributions of O2 reactants at high collision energy.

Exploring Vinylidene/Acetylene Isomerization by Photoelectron Spectroscopy of Vibrationally Excited Vinylidene Anions

In order to explore the vibrational levels of vinylidene (H2CC) and their possible couplings with acetylene (HCCH), we investigate the effect of infrared (IR) vibrational pre-excitation on the high-resolution photoelectron spectra of the vinylidene anion (H2CC-) and its deuterated isotopologue (D2CC-). The photoelectron spectra are obtained using slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI); here, cold anions are vibrationally excited by an IR laser pulse prior to photodetachment (IR cryo-SEVI). Infrared action spectra of the anion CH2 stretching fundamentals are measured by monitoring growth and depletion of features in photoelectron spectra as the IR laser is tuned, yielding excitation frequencies of the symmetric (ν1) and antisymmetric (ν5) CH2 stretching modes of 2590 ± 2 and 2658 ± 2 cm-1, respectively. We then use IR cryo-SEVI to explore the effect of vibrational excitation of the two modes on the anion photoelectron spectrum. Interpretation of these spectra is facilitated by quantum calculations performed for each isotopologue on accurate six-dimensional potential energy surfaces of both neutral and anionic vinylidene. IR cryo-SEVI spectra resulting from excitation of these two close-lying anion vibrations are noticeably different. Excitation of the ν1 mode leads to several new features that appear in the photoelectron spectra which closely match the Franck-Condon allowed transitions predicted by theory. Excitation of the ν5 mode in H2CC- reveals complicated spectral features in the vicinity of the 511 sequence band that are not seen for D2CC-. These are explained by a combination of anharmonic coupling between ν5 and ν6 (CH2 rock) states in neutral H2CC and possible coupling to the HCCH isomer.

Precise Determination of Excited State Rotational Constants and Black-Body Thermometry in Coulomb Crystals of Ca+ and CaH. J Phys Chem A 2025;129:3624-3629. [PMID: 40237448 DOI: 10.1021/acs.jpca.5c00229] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We present high-resolution rovibronic spectroscopy of calcium monohydride molecular ions (CaH+) cotrapped in a Coulomb crystal with calcium ions (40Ca+), focusing on rotational transitions in the | X 1 Σ + , ν″ = 0⟩ → |A1Σ+, ν' = 2⟩ manifold. By resolving individual P and R branch transitions with record precision and using Fortrat analysis, we extract key spectroscopic constants for the excited state |A1Σ+, ν' = 2⟩, specifically, the band origin, the rotational constant, and the centrifugal correction. Additionally, we demonstrate the application of high-resolution rotational spectroscopy of CaH+ presented here as an in situ probe of the local environmental temperature. We correlate the relative amplitudes of the observed transitions to the underlying thermalized ground-state rotational population distribution and extract the blackbody radiation (BBR) temperature.

Modeling the Superlattice Phase Diagram of Transition Metal Intercalation in Bilayer 2H-TaS2

Van-der-Waals hosts intercalated with transition-metal (TM) ions exhibit a range of magnetic properties strongly influenced by the structural order of the intercalants. However, predictive computational models for the intercalant ordering phase diagram are lacking, complicating experimental pursuits to target key structural phases. Here we use density functional theory (DFT) to construct a pairwise lattice model and Monte Carlo to determine its associated thermodynamic phase diagram. To circumvent the complexities of modeling magnetic effects, we use the diamagnetic ions Zn2+ and Sc3+ as computationally accessible proxies for divalent and trivalent species of interest (Fe2+, Co2+, V3+ and Cr3+), which provide insights into the thermodynamic phase diagram well above the paramagnetic transition temperature. We find that electrostatic coupling between intercalants is almost entirely screened, so the pairwise lattice model represents a coarse-grained charge density reorganization. The resulting phase diagram reveals that entropically favored √3 × √3 ordering and coexisting locally ordered √3 × √3 and 2 × 2 domains persist across a range of temperatures and stoichiometries. This occurs even at quarter-filling of interstitial sites (corresponding to bulk stoichiometries of M0.25TaS2; M = intercalant) where there are experimental reports of 2 × 2 and √3 × √3 coexistence but a thermodynamic preference for long-range 2 × 2 order is often assumed. Accompanying Raman spectra revealing quenching-induced √3 × √3 order in 2H-V0.25NbS2 supports that deficient √3 × √3 domains may dominate 2 × 2 at high temperatures in a range of TM-intercalated transition metal dichalcogenides.

Tip-Enhanced Sum Frequency Generation for Molecular Vibrational Nanospectroscopy

Vibrational sum frequency generation (SFG) is a nonlinear spectroscopic technique widely used to study the molecular structure and dynamics of surface systems. However, the spatial resolution achieved by far-field observations is constrained by the diffraction limit, obscuring molecular details in inhomogeneous structures smaller than the wavelength of light. To overcome this limitation, we developed a system for tip-enhanced SFG (TE-SFG) spectroscopy based on a scanning tunneling microscope. We successfully detected vibrational TE-SFG signals from adsorbed molecules on a gold substrate under ambient conditions. The phase analysis of interferometric SFG spectra provided information on molecular orientation. Furthermore, the observed TE-SFG signal was confirmed to originate from a highly localized region within a gap between the tip apex and the sample substrate. This method offers a novel platform for nonlinear optical nanospectroscopy, paving the way for the investigation of surface molecular systems beyond the diffraction limit.

Nonadiabatic Field: A Conceptually Novel Approach for Nonadiabatic Quantum Molecular Dynamics

Reliable trajectory-based nonadiabatic quantum dynamics methods at the atomic/molecular level are critical for the practical understanding and rational design of many important processes in real large/complex systems, where the quantum dynamical behavior of electrons and that of nuclei are coupled. The paper reports latest progress of nonadiabatic field (NaF), a conceptually novel approach for nonadiabatic quantum dynamics with independent trajectories. Substantially different from the mainstreams of Ehrenfest-like dynamics and surface hopping methods, the nuclear force in NaF involves the nonadiabatic force arising from the nonadiabatic coupling between different electronic states, in addition to the adiabatic force contributed by a single adiabatic electronic state. NaF is capable of faithfully describing the interplay between electronic and nuclear motion in a broad regime, which covers where the relevant electronic states keep coupled in a wide range or all the time and where the bifurcation characteristic of nuclear motion is essential. NaF is derived from the exact generalized phase space formulation with coordinate-momentum variables, where constraint phase space (CPS) is employed for discrete electronic-state degrees of freedom (DOFs) and infinite Wigner phase space is used for continuous nuclear DOFs. We propose efficient integrators for the equations of motion of NaF in both adiabatic and diabatic representations. Since the formalism in the CPS formulation is not unique, NaF can in principle be implemented with various phase space representations of the time correlation function (TCF) for the time-dependent property. They are applied to a suite of representative gas-phase and condensed-phase benchmark models where numerically exact results are available for comparison. It is shown that NaF is relatively insensitive to the phase space representation of the electronic TCF and will be a potential tool for practical and reliable simulations of the quantum mechanical behavior of both electronic and nuclear dynamics of nonadiabatic transition processes in real systems.

Orbital Surface Hopping from the Orbital Quantum-Classical Liouville Equation for Nonadiabatic Dynamics of Many-Electron Systems

Accurate simulation of the many-electron nonadiabatic dynamics process at metal surfaces remains a significant challenge. In this work, we present an orbital surface hopping (OSH) algorithm rigorously derived from the orbital quantum-classical Liouville equation (o-QCLE) to address nonadiabatic dynamics in many-electron systems. This OSH algorithm is closely connected to the popular independent electron surface hopping (IESH) method, which has demonstrated remarkable success in addressing these nonadiabatic phenomena, except that electrons hop between orbitals. We compare the OSH approach with the IESH method and benchmark these two algorithms against the surface hopping method using a full configuration interaction (FCI) wave function. Our approach shows strong agreement with IESH and FCI-SH results for molecular orbital populations and kinetic energy relaxation, while also exhibiting high efficiency, thereby demonstrating the capability of the new OSH method to capture key aspects of many-electron nonadiabatic dynamics.

Control of optically dark nσ* state mediated photodissociation of thioanisole

The methyl photodissociation of thioanisole molecule has been investigated within the mathematical framework of optimal control theory (OCT) considering a reduced dimensional model of three electronic states (ππ, ππ*, and nσ*) and two vibrational modes (S-CH3 stretching and S-CH3 torsional angle). The model includes two bound states (ground state and the ππ* state) and one repulsive state (nσ* state). The effect of the initial vibrational excitation on the branching ratio of the two dissociation channels is examined. In the presence of optimal pulse, methyl photodissociation predominantly occurs via two successive conical intersections. For further justification of the findings of OCT calculations, the field-free quantum dynamics calculations, where the initial wave packet is vertically excited to the ππ* state, are also examined under different initial conditions. A comparative study of these two types of calculations reveals that in the field-free case, photodissociation occurs via the adiabatic path, whereas it follows the nonadiabatic path in the presence of optimal pulse. Therefore, the branching ratio also changes accordingly with the initial conditions for these two types of dynamical events.

Equation-of-motion internally contracted multireference unitary coupled-cluster theory

The accurate computation of excited states remains a challenge in electronic structure theory, especially for systems with a ground state that requires a multireference treatment. In this work, we introduce a novel equation-of-motion (EOM) extension of the internally contracted multireference unitary coupled-cluster framework (ic-MRUCC), termed EOM-ic-MRUCC. EOM-ic-MRUCC follows the transform-then-diagonalize approach, in analogy to its non-unitary counterpart [Datta and Nooijen, J. Chem. Phys. 137, 204107 (2012)]. By employing a projective approach to optimize the ground state, the method retains additive separability and proper scaling with system size. We show that excitation energies are size-intensive if the EOM operator satisfies the "killer" and the projective conditions. Furthermore, we propose to represent changes in the reference state upon electron excitation via projected many-body operators that span the active orbitals and show that the EOM equations formulated in this way are invariant with respect to active orbital rotations. We test the EOM-ic-MRUCC method truncated to single and double excitations by computing the potential energy curves for several excited states of a BeH2 model system, the HF molecule, and water undergoing symmetric dissociation. Across these systems, our method delivers accurate excitation energies and potential energy curves within 5 mEh (∼0.14 eV) from full configuration interaction. We find that truncating the Baker-Campbell-Hausdorff series to fourfold commutators contributes negligible errors (on the order of 10-5Eh or less), offering a practical route to highly accurate excited-state calculations with reduced computational overhead.

Electronic quenching of N(2D) in collision with CO(1Σ+) via spin-forbidden transitions

In nitrogen-rich atmospheres under extreme conditions, the N2 molecule dissociates into atomic nitrogen in different electronic states. In particular, N(2D) is known to be reactive and to drive a complex chemistry in such regimes. If the atmosphere also contains carbon monoxide (such as Earth and Mars), several collisional processes with nitrogen-, carbon-, and oxygen-bearing species are relevant. Here, we employ a set of three accurate and global potential energy surfaces for the CNO system to study the N(2D) + CO → N(4S) + CO electronic quenching process, using the quasiclassical trajectories approach coupled with two different surface hopping schemes. Experimental measurements of the quenching rate coefficient are available only at room temperature, and our computational predictions show good agreement. We further provide the temperature dependence of the rate coefficients for the first time, extending to the hyperthermal regime. The effect of the initial rovibrational state of CO on the reactivity, as well as the distribution of energy in the products, is also unveiled.

IR Spectroscopy of 4-Aminobenzonitrile+-Arn (n = 0-2): Determination of the Activation Barrier for the π → NH Site-Switching Reaction

Information about the intermolecular potential energy surface for the interaction between solute and solvent molecules is required to understand the impact of solvation on reaction mechanisms and dynamics. In this study, we measured vibrational-specific infrared (IR) spectra of 4-aminobenzonitrile-(argon)n cation clusters, 4ABN+-Arn (n = 1, 2), in the NH stretching range to elucidate the energetics of the photoionization-induced π → NH migration of Ar. The IR spectra of 4ABN+-Arn generated by resonant photoionization of neutral π-bonded clusters display the hydrogen-bonded NH2 stretching vibration (νNH2) only when intermolecular vibrational levels are excited. This is the first observation of Ar migration from the aromatic ring toward the NH2 group upon photoionization in the n = 1 cluster. From the vibrational-level dependence of the IR spectra, the activation barrier heights are determined to be 21-47 (34 ± 13) and <27 cm-1 for 4ABN+-Ar1 and 4ABN+-Ar2, respectively. The potential energy surfaces and mechanism of the Ar migration are discussed with the help of complementary density functional theory calculations.

High-Frequency Tails in Spectral Densities

Recent advances in numerically exact quantum dynamics methods have brought the dream of accurately modeling the dynamics of chemically complex open systems within reach. Path-integral-based methods, hierarchical equations of motion, and quantum analog simulators all require the spectral density (SD) of the environment to describe its effect on the system. Here, we focus on the decoherence dynamics of electronically excited species in solution in the common case where nonradiative electronic relaxation dominates and is much slower than electronic dephasing. We show that the computed relaxation rate is highly sensitive to the choice of SD representation─such as the Drude-Lorentz or Brownian modes─or strategy used to capture the main SD features, even when early-time dephasing dynamics remains robust. The key reason is that electronic relaxation is dominated by the resonant contribution from the high-frequency tails of the SD, which are orders of magnitude weaker than the main features of the SD and can vary significantly between strategies. This finding highlights an important, yet overlooked, numerical challenge: obtaining an accurate SD requires capturing its structure over several orders of magnitude to ensure correct decoherence dynamics at both early and late times. To address this, we provide a simple transformation that recovers the correct relaxation rates in quantum simulations constrained by algorithmic or physical limitations on the shape of the SD. Our findings enable a comparison of different numerically exact simulation methods and expand the capabilities of analog simulations of open quantum dynamics.

On the Role of a Polymer Matrix in Enhancing Energy Transfer Efficiency Using Coumarin 6 and Rhodamine B as Donor and Acceptor Pairs

This study investigates the critical role of polymer matrices in optimizing luminescence and energy transfer, utilizing the commercial dyes Coumarin 6 (C6) and Rhodamine B (RhB) as a donor-acceptor pair. Solution-phase experiments revealed a dependence of energy transfer efficiency on solvent dielectric constant. Furthermore, embedding the dyes within Poly(methyl methacrylate) (PMMA) or Poly(vinyl butyral) (PVB) matrices significantly enhance energy transfer due to increased molecular proximity. A modified approach for calculating the overlap integral J(λ) was employed to analyze energy transfer in the solid state, providing insights beyond traditional Förster-type calculations. Photostability studies demonstrated superior performance of PVB films compared to PMMA films under exposure to monochromatic (405 nm, 515 nm) and white light irradiation. Finally, a proof-of-concept device was developed to harness this enhanced energy transfer for light-to-electricity conversion, achieving a maximum efficiency of 7.28 %.

Reactivity of syn-CH3CHOO with H2O enhanced through a roaming mechanism in the entrance channel

Criegee intermediates are highly reactive species that play a pivotal role in the chemistry of the atmosphere, substantially impacting global climate and air quality. They are formed through the reaction of ozone with alkenes and considerably influence the formation of hydroxyl radicals and aerosols through their unimolecular decomposition and their reaction with key atmospheric components, respectively. However, their interaction with water vapour, a major atmospheric component, remains inadequately characterized. Here, using both time-dependent laser-induced fluorescence experiments and full-dimensional dynamics calculations, we investigate the reaction of syn-CH3CHOO, a prevalent Criegee intermediate, with water vapour. Our results reveal a much higher reaction rate than previously estimated, challenging the conventional notion that unimolecular decomposition dominates syn-CH3CHOO removal. Notably, we uncover a complex mechanism involving a roaming process that enhances reactivity. Our findings necessitate a revised assessment of reactions involving syn-mono- and di-substituted Criegee intermediates with water, which are crucial for accurately estimating the OH budget derived from these intermediates.

P[triple bond, length as m-dash]B and As[triple bond, length as m-dash]B triple bonds in the linear PB2O- and AsB2O- species

Due to its electron deficiency, boron triple bonds are relatively scarce. We use high-resolution photoelectron imaging to investigate the structures and bonding of the EB2O- (E = P, As) type of clusters, which are found to have [E[triple bond, length as m-dash]B-B[triple bond, length as m-dash]O]- closed-shell linear structures with E[triple bond, length as m-dash]B triple bonds. The B atoms in the linear EB2O- species undergo sp hybridization, while the E atoms also undergo sp hybridization to form a σ bond with the sp orbital of B along with two π bonds formed by the p x and p y orbitals. The high-resolution photoelectron imaging data reveal detachment transitions from the EB2O- (1Σ+) anions to the EB2O (2Π) neutrals. The electron affinities of PB2O and AsB2O are measured to be 3.592(1) eV and 3.432(1) eV, respectively; the vibrational frequencies for the E-B, B-B, and B-O stretching modes are measured for both species. The spin-orbit splitting of the 2Π state to 2Π3/2 and 2Π1/2 is measured to be 153 cm-1 and 758 cm-1 for PB2O and AsB2O, respectively.

Microsolvation of cationic alkali dimers in helium: quantum delocalization and solid-like/liquid-like behaviors of He shells

We performed path-integral molecular dynamics (PIMD) simulations in the NVT ensemble to investigate the quantum solvation of Li2+ in He nanoclusters at a low temperature of 2 K. The interaction potentials were modeled using a sum-of-potentials approach, incorporating automated learning ab initio-based models up to three-body terms. Additionally, the semiclassical quadratic Feynman-Hibbs approach was applied to incorporate quantum effects into classical computations effectively, enabling the study of HeNLi2+ complexes with up to 50 He atoms. The quantum simulations revealed strong evidence of local solid-like behavior in the He atoms within the first solvation shell surrounding the Li2+ dimer cation. In contrast, the second and third solvation shells displayed delocalized He densities, allowing for the interchange of He atoms between these layers, indicative of a liquid-like structure. Our findings align with earlier studies of He-doped clusters, particularly in systems where the charged impurity interacts strongly with the solvent medium, significantly impacting the helium environment at the microscopic level.

Proton Pachinko: Probing Excited-State Intramolecular Proton Transfer of St. John's Wort-Derived Fluorescent Photosensitizer Hypericin with Ultrafast Spectroscopy

Hypericin from St. John's wort has been used as a potent photosensitizer, but its working mechanism remains elusive which hinders its rational design for improved functionality. We implement ultrafast spectroscopy and quantum calculations to track the excited-state dynamics in an intricate hydrogen-bonding network of hypericin in solution. Using femtosecond transient absorption (fs-TA), we track excited state intramolecular proton transfer (ESIPT) via a previously unreported blueshift of a long-wavelength stimulated emission (SE) band with excitation-dependent dynamics in various solvents, owing to the dominant Q7,14 tautomer that undergoes bidirectional ESIPT. This finding is corroborated by ground-state femtosecond stimulated Raman spectroscopy (GS-FSRS) and density functional theory (DFT) calculations. Moreover, contrasting the neutral and anionic forms of hypericin enables us to reveal an intramolecular charge transfer step underlying ESIPT. We demonstrate UV and visible excitations as an integral platform to provide direct insights into the photophysics and origin for phototoxicity of hypericin. Such mechanistic insights into the excited state of hypericin will power its future development and use.

Stomach cancer identification based on exhaled breath analysis: a review

Early prediction of cancer is crucial for effective treatment decisions. Stomach cancer is one of the worst malignancies in the world because it does not reveal the growth in symptoms. In recent years, non-invasive diagnostic methods, particularly exhaled breath analysis, have attracted interest in detecting stomach cancer. This review discusses invasive and non-invasive diagnostic methods for stomach cancer, with a special emphasis on breath analysis and electronic nose (e-nose) technology. Various analytical methods have been used to analyze volatile organic compounds (VOCs) associated with stomach cancer. Gas chromatography-mass Spectrometry is one of the most widely used techniques. These techniques enable the detection and analysis of VOCs, offering a promising route for early stomach cancer diagnosis. The e-nose system has been introduced as a cost-effective and portable alternative for VOC detection in stomach cancer to overcome the challenges associated with conventional methods. This review discusses the advantages and disadvantages of the e-nose system. This review recommends that e-nose sensors, combined with advanced pattern recognition techniques, be utilized to enable rapid and reliable diagnosis of stomach cancer.

The evolution of machine learning potentials for molecules, reactions and materials

Recent years have witnessed the fast development of machine learning potentials (MLPs) and their widespread applications in chemistry, physics, and material science. By fitting discrete ab initio data faithfully to continuous and symmetry-preserving mathematical forms, MLPs have enabled accurate and efficient atomistic simulations in a large scale from first principles. In this review, we provide an overview of the evolution of MLPs in the past two decades and focus on the state-of-the-art MLPs proposed in the last a few years for molecules, reactions, and materials. We discuss some representative applications of MLPs and the trend of developing universal potentials across a variety of systems. Finally, we outline a list of open challenges and opportunities in the development and applications of MLPs.

Energy conservation in real-time nuclear-electronic orbital Ehrenfest dynamics

Real-time nuclear-electronic orbital Ehrenfest (RT-NEO-Ehrenfest) dynamics methods provide a first-principles approach for describing nonadiabatic molecular processes with nuclear quantum effects. For an efficient description of proton transfer within RT-NEO-Ehrenfest dynamics, the basis function center associated with the quantum proton can be allowed to move classically. This traveling proton basis (TPB) approach effectively captures proton quantum dynamics, although its energy conservation behavior is not yet fully satisfactory. Two recently proposed TPB approaches, in principle, conserve the extended energy, which includes both the system energy and the kinetic energy associated with the proton basis function center. Herein, a thermostatted TPB approach is proposed to improve the conservation of the system energy, excluding the kinetic energy associated with the proton basis function center. In this approach, the quantum proton dynamics are modulated by dynamically rescaling the proton momentum operator to maintain the system energy conservation. With the excited-state intramolecular proton transfer of o-hydroxybenzaldehyde as an example, this approach is shown to significantly improve the system energy conservation while preserving the accuracy of the quantum proton dynamics as achieved in the original TPB approach.

High-resolution photoelectron spectroscopy of cryogenically cooled SiC

We report high-resolution photoelectron spectroscopy of SiC- anions using a cryogenic ion trap combined with the slow-electron velocity-map imaging method. Photodetachment transitions to the neutral SiC ground state X 3Π and excited states (a 1Σ+, b 1Π) were observed, revealing the fine structure of the X 3Π band. A long-lived excited state (A 3Π) of SiC- was observed at 3380(101) cm-1 above the anionic ground state. The electron affinity (EA) was determined as EA(SiC) = 19 327(15) cm-1 or 2.396(2) eV. The spectroscopic constants of the ground state (X' 2Σ+) of SiC- were derived as ωe = 1016(21) cm-1 and ωexe = 5.20(22) cm-1.

Low-Energy Elastic Scattering of Electrons from 2H-Pyran and 4H-Pyran with Time Delay Analysis of Resonances

Elucidating the significance of low-energy electrons in the rupture of electron-accepting biomolecules and the process involved in it is crucial in understanding different biochemical processes. Capturing of the incident electron in one of the empty molecular orbitals and the formation of a temporary negative ion (TNI) is considered to be a stepping stone towards the lesion of the molecule. This TNI formation manifests itself as a resonance peak in the cross-sections determined for the electron-molecule interaction. In the present work, we have reported the integral (ICS), differential (DCS), and momentum transfer (MTCS) cross-sections for the elastic scattering of low-energy electrons from the isomers, 2H-pyran and 4H-pyran( C 5 H 6 O ) ${({\rm{C}}_5 {\rm{H}}_6 {\rm{O}})}$ , which are an important structural subunits of polyphenolic compounds. The single-center expansion method has been employed for the scattering calculations. Further, we have used the time delay approach to identify and analyze the resonance peaks. Our results for the ICS and DCS compare well with the data available in the literature. MTCS data for 2H-pyran and 4H-pyran have been reported for the first time. Moreover, we have also identified an extra peak for each molecule, from time delay analysis, which might be a potential resonance.

Extending atomic decomposition and many-body representation with a chemistry-motivated approach to machine learning potentials

Most widely used machine learning potentials for condensed-phase applications rely on many-body permutationally invariant polynomial or atom-centered neural networks. However, these approaches face challenges in achieving chemical interpretability in atomistic energy decomposition and fully matching the computational efficiency of traditional force fields. Here we present a method that combines aspects of both approaches and balances accuracy and force-field-level speed. This method utilizes a monomer-centered representation, where the potential energy is decomposed into the sum of chemically meaningful monomeric energies. The structural descriptors of monomers are described by one-body and two-body effective interactions, enforced by appropriate sets of permutationally invariant polynomials as inputs to the feed-forward neural networks. Systematic assessments of models for gas-phase water trimer, liquid water, methane-water cluster and liquid carbon dioxide are performed. The improved accuracy, efficiency and flexibility of this method have promise for constructing accurate machine learning potentials and enabling large-scale quantum and classical simulations for complex molecular systems.

Ion-dip and laser photoexcitation spectroscopy of high Rydberg states in N2

N2 molecules in pulsed supersonic beams have been laser photoexcited from their X Σg+1 ground electronic state to selected singlet np and nf Rydberg states using a (2 + 1') two-color three-photon excitation scheme. This required the competition between (2 + 1) resonance-enhanced multiphoton ionization and (2 + 1') Rydberg state photoexcitation to be carefully balanced. This was achieved by performing ion-dip spectroscopy in which the signal from the N2+ cations generated by direct photoionization was selectively detected and seen to reduce under conditions in which the predissociative np Rydberg states, or long-lived nf Rydberg states, were populated. The predissocation rates of the np Rydberg states were determined from the n-dependence of the spectral widths of the transitions to them. The long-lived Rydberg states populated by excitation on nf resonances are suitable for deceleration and electrostatically trapping cold samples of N2 using inhomogeneous electric fields.

First-Generation Products of Trans-2-Hexenal Ozonolyis: A New Look at the Mechanism

The ozonolysis reaction of trans-2-hexenal was studied theoretically on the basis of highly accurate CCSD(T)-F12b/AVTZ energy values obtained in M06-2X/AVTZ preoptimized nuclear configurations. The kinetics was modeled with the help of the master equation solver MESMER. Apart from the expected stable oxidation products 1-butanal (17%) and glyoxal (35%), a secondary ozonide is formed on the glyoxal channel, which is the principal first-generation product (49%). It is further shown that glyoxal is created on two competing pathways, one of which leads to simultaneous production of the ester propylformate (18%). The inclusion of all of these mechanisms explains the experimental findings and identifies for the first time the origin of the experimental carbon deficit.

Directional Light Scattering of a Single Si Nanoparticle Revealed by Three-Dimensional Near-Field Optical Microscopy

Manipulation of light propagation is indispensable for the development of photonic circuits and nano-optical devices. In this study, we investigated the optical properties of Mie resonances in a single Si nanoparticle via dark field and near-field optical microscopy. The dark field spectrum of the nanoparticle exhibits electric and magnetic dipolar modes in the visible to near-infrared spectral region. The near-field spectrum shows red-shifted and enhanced peaks due to these resonances. From the electromagnetic simulation, we revealed that these unique near-field characteristics originate from the constructive interaction of the incident and scattered fields. We also performed three-dimensional near-field microscopy and demonstrated that the magnetic dipolar mode results in wider spatial extension and directional forward scattering than does the electric dipolar mode does. These findings indicate that the interaction of the magnetic dipolar mode with near-field light is desirable for controlling light propagation in various applications.

Identification of the Elusive Methyl-Loss Channel in the Crossed Molecular Beam Study of Gas-Phase Reaction of Dicarbon Molecules (C2; X1Σg+/a3Πu) with 2-Methyl-1,3-butadiene (C5H8; X1A')

The crossed molecular beam technique was utilized to explore the reaction of dicarbon C2 (X1Σg+/a3Πu) with 2-methyl-1,3-butadiene (isoprene, CH2C(CH3)CHCH2; X1A') at a collision energy of 28 ± 1 kJ mol-1 using a supersonic dicarbon beam generated via photolysis (248 nm) of helium-seeded tetrachloroethylene (C2Cl4). Experimental data combined with previous ab initio calculations provide evidence of the detection of the hitherto elusive methyl elimination channels leading to acyclic resonantly stabilized hexatetraenyl radicals: 1,2,4,5-hexatetraen-3-yl (CH2CC•CHCCH2) and/or 1,3,4,5-hexatetraen-3-yl (CH2CHC•CCCH2). These pathways are exclusive to the singlet potential energy surface, with the reaction initiated by the barrierless addition of a dicarbon to one of the carbon-carbon double bonds in the diene. In combustion systems, both hexatetraenyl radicals can isomerize to the phenyl radical (C6H5) through a hydrogen atom-assisted isomerization─the crucial reaction intermediate and molecular mass growth species step toward the formation of polycyclic aromatic hydrocarbons and soot.

Electronic Instability and Solvation Stabilization of Oxocarbon Dianions (CnOn)2- (n = 4-6)

Oxocarbon dianions (CnOn)2- have been recently found to be promising candidates in the design of high-capacity and fast rechargeable batteries but are intrinsically unstable in the isolated form. Fundamental understandings of their electronic structures, solvent stabilization, and interactions with solvents and counterions are crucial in comprehending their electron transfer reactions occurring in batteries. In this article, we employed microsolvated dianionic clusters as models and combined negative ion photoelectron spectroscopy (NIPES) and theoretical computations to probe the electronic instability and solvation stabilization of (CnOn)2- (n = 4-6) dianions. Through the smallest observable members in each series of microhydrated dianions and their recorded adiabatic and vertical detachment energies (ADEs and VDEs), the minimum numbers of H2O molecules required to stabilize (CnOn)2- dianions are determined to be 4, 3, and 2 for n = 4, 5, and 6, respectively, while 3 and 2 water molecules can make (C4O4)2- and (C5O5)2- metastable and detectable. Using theoretical calculations, we determined the lowest energy structures of each complex. The first few H2O molecules prefer to be directly hydrogen bonded to two adjacent O atoms around the oxocarbon ring. The water binding strengths are generally comparable when each H2O molecule is bound at a separate binding pocket, but the binding strengths decrease when all binding pockets are occupied, in parallel with the observed ADE and VDE shift trends. Moreover, hydrated (C4O4)2- dianions are found to possess distinct electronic structures compared to its (C5O5)2- and (C6O6)2- analogues due to its near-degenerate HOMO and HOMO-1, while there exists a larger gap for the latter two dianions. Upon hydration, the overall electronic structure patterns are maintained without much distortion, but fine changes are noticeable, which warrant future studies.

Gas-phase, conformer-specific infrared spectra of 3-chlorophenol and 3-fluorophenol

Conformational isomerism of phenol derivatives has been a subject of extensive spectroscopic study. Combining the capabilities of the widely tuneable infrared free-electron laser FELIX with molecular beam technologies allows for revisiting existing data and gaining additional insights into far-IR spectroscopy of halogenated phenols. Here we present conformer-resolved infrared spectra of the syn and anti conformers of 3-chlorophenol and 3-fluorophenol recorded via IR-UV ion-dip spectroscopy. The experimental work is complemented by density functional theory calculations to aid in assignment of the observed bands. The experimental spectra for the two conformers of each molecule show overall a great similarity, but also include some distinct conformer-specific bands in the spectral range investigated. Our spectra confirm previously reported OH torsional mode frequencies for the syn and anti conformers of 3-chlorophenol (3CP) at 315 cm-1, (Manocha et al., J. Phys. Chem., 1973, 77, 2094) but reverse their assignment of the 311 and 319 cm-1 bands for 3-fluorophenol. 1D torsional mode calculations were performed for 3CP to help assign possible OH torsion overtones.

High solar-to-hydrogen efficiency in Z-scheme AlN/GaO heterojunctions for visible light water splitting

Hydrogen production from solar energy is an important means to solve the problems of fossil fuel consumption and environmental pollution, and the efficiency of hydrogen production from solar energy is an important indicator. Photocatalytic water decomposition technology driven by solar energy is an ideal way to create clean energy. In this paper, a new Z-scheme AlN/GaO van der Waals heterojunction is proposed. Through first-principles calculations, we have systematically studied the electronic properties and photocatalytic hydrogen production performance of the AlN/GaO heterostructure. The calculation results show that the lattice mismatch rate of the AlN/GaO heterojunction is only 0.48%. At the same time, it not only performs well in terms of thermodynamics, kinetics and mechanical stability, but also has an appropriate band gap of 1.45 eV with an electron mobility of up to 2753.48 cm2 V-1 s-1. Under light irradiation, the transfer of internal photogenerated carriers forms a built-in electric field from AlN to GaO, which forms a typical Z-scheme, and leads to the hydrogen evolution reaction on AlN with strong reduction ability. It is worth noting that the AlN/GaO heterojunction shows a high absorption coefficient in the visible light absorption range and has an excellent solar-to-hydrogen efficiency of 60.1%. These advantages demonstrate that the AlN/GaO heterojunction, as a promising photocatalyst, has significant application potential and offers a novel approach to address the energy crisis and environmental pollution challenges.

Ultrafast excited-state dynamics of 4-hydroxychalcone: role of intramolecular charge transfer and photoacidity

Diarylketones such as benzophenones, oxybenzones, chalcones and their derivatives exhibit promising applications as UV filters/sunscreen agents due to their effective absorption in the UV region and dissipation through non-radiative pathways. However, elucidation of the underlying photoreactive mechanism is non-trivial due to the ultrafast lifetimes of transient species, involvement of non-adiabatic curve crossings among the potential surfaces, etc. In this context, we investigate the excited-state photoreaction dynamics of 4-hydroxychalcone (4-HC) under various environments through femtosecond-transient absorption (fs-TA), nanosecond-transient absorption (ns-TA), and femtosecond-fluorescence upconversion (fs-FL) measurements. Steady-state fluorescence measurements of 4-HC in the presence of 1-methylimidazole (MI)/tert-butylamine (TBA) exhibit dual band emission. The fs-TA measurements of 4-HC in the presence of MI/TBA exhibit distinct spectral and associated lifetimes as compared to 4-HC alone indicating a significant interaction of the hydroxyl proton with bases and influencing the reaction dynamics. The 4-HC:MI/TBA adduct undergoes excited-state intermolecular proton transfer within a time scale of ∼500 fs and subsequently relaxes back to the ground state through a long-lived triplet state. The experimental observations of excited-state reaction dynamics of 4-HC in the presence of MI/TBA bases have been well corroborated with the computational analysis.

Parity violation effects on the electric field gradient

The parity violation (PV) effects on the electric field gradient (EFG) and the nuclear quadrupole coupling constant (NQCC) of a wide variety of chiral systems are studied in a four-component (4c) framework. Formal expressions and calculations of the PV effects on the EFG are presented for the first time at 4c Dirac Hartree-Fock level. The chiral systems studied are XHFClY (X = C, Sn; Y = Br, I, At) molecules together with NUHXY (X, Y = F, Cl, Br, I) and NUF XY (X, Y = Cl, Br, I) uranium containing systems. We found that for the latter, calculations of PV effects on NQCC are two orders of magnitude lower than the current experimental precision and they are suitable candidates for future PV measurements in NQCC, in particular the NUHFCl chiral molecule. The dependence on the basis set, the nuclear charge distribution model and the kinetic balance prescription related to the negative-energy states is also analysed.

A new natural Cyperol A together with five known compounds from Cyperus rotundus L.: isolation, structure elucidation, DFT analysis, insecticidal and enzyme-inhibition activities and in silico study

One new natural benzaldehyde derivative (1), together with five known compounds, was isolated from the methanolic extract of the whole plant of Cyperus rotundus L., which is a globally distributed noxious weed. The structure of compound (1) (named Cyperol A) was determined using various NMR methods, including 1H, 13C, COSY, HMBC, HSQC and NOESY, and mass spectrometric techniques, including EIMS. The newly isolated compound (1) was subjected to optimization using computer-assisted calculation via DFT methods for natural bond orbital (NBO) and frontier molecular orbital (FMO) analyses and compared with carbofuran, which is used to control the pest brown planthopper. The in vitro insecticidal efficacy of compounds 1-6 was evaluated against Nilaparvata lugens. Compound 1 demonstrated exceptional lethal and notable enzyme inhibitory effects. Furthermore, compound 1 was investigated in silico for its anti-pesticidal activities targeting the BPH (Nilaparvata lugens (Stål)) key enzymes, such as glutathione S-transferase (GST) and acetylcholinesterase (AChE). Compound 1 showed good docking scores of -9.75 kcal mol-1 against GST, forming hydrogen bonds with its active site, and -10.56 kcal mol-1 with AChE owing to its high potential for hydrogen bonding.

Comparative Analysis of Reinforcement Learning Algorithms for Finding Reaction Pathways: Insights from a Large Benchmark Data Set

The identification of kinetically feasible reaction pathways that connect a reactant to its product, including numerous intermediates and transition states, is crucial for predicting chemical reactions and elucidating reaction mechanisms. However, as molecular systems become increasingly complex or larger, the number of local minimum structures and transition states grows, which makes this task challenging, even with advanced computational approaches. We introduced a reinforcement learning algorithm to efficiently identify a kinetically feasible reaction pathway between a given local minimum structure for the reactant and a given one for the product, starting from the reactant. The performance of the algorithm was validated using a benchmark data set of large-scale chemical reaction path networks. Several search policies were proposed, using metrics based on energetic or structural similarity to the product's goal structure, for each local minimum structure candidate found during the search. The performances of baseline greedy, random, and uniform search policies varied substantially depending on the system. In contrast, exploration-exploitation balanced policies such as Thompson sampling, probability of improvement, and expected improvement consistently demonstrated stable and high performance. Furthermore, we characterized the search mechanisms that depend on different policies in detail. This study also addressed potential avenues for further research, such as hierarchical reinforcement learning and multiobjective optimization, which could deepen the problem setting explored in this study.

Photocatalytic Nitrogen Reduction for Ammonia Synthesis Accelerated by Overcoming Photo-Dember Effect

During photocatalytic nitrogen fixation for ammonia synthesis, the photo-Dember effect causes direct transmission of photogenerated electrons from the illuminated surface to the bottom of photocatalyst, thus significantly reducing the number of charge carriers migrating on the surface and nitrogen fixation efficiency. Herein, a bismuth oxychloride material with largely exposed (101) crystal plane and rich oxygen vacancies (BOC(101)-OVs) is synthesized, exhibiting a high NH3 yield of 591.94 µmol g-1 h-1) after photocatalytic N2 reduction under simulated solar light irradiation. The designed (101)/(001) interface in BOC(101)-OVs generates a self-built electric field (Eself) on the material surface due to different atomic arrangements. Therefore, the newly developed material achieved >95% of photogenerated electrons changing the transfer path, i.e., from bulk phase transfer to surface lateral transfer path, thus escaping confinement by the photo-Dember effect. Meanwhile, after OVs construction, each adsorbed N2 molecule simultaneously bonds with three Bi atoms of material through N 2p-Bi 6p bonding, accelerating the filling of high-energy electrons into the π* orbital of N2, leading to a new nitrogen reduction path with combined alternating hydrogenation and terminal hydrogenation. This study greatly advances the beneficial effect of charge carrier migration through overcoming the photo-Dember effect for ammonia synthesis.

Study of clopidogrel and clonidine interactions for cardiovascular formulations: progress from DFT modeling

The drugs clopidogrel and clonidine are frequently used to treat cardiovascular diseases, which are the leading cause of mortality worldwide. Since these medications are frequently taken in combination, it is crucial to examine their molecular interactions. Therefore, herein, the bandgap energy, chemical potential, chemical hardness and softness parameters were calculated using a density functional theory (DFT)-based method. In addition, infrared (IR) spectrum, natural bond orbital (NBO), molecular electrostatic potential (MEP), electron localization function (ELF) and total density of states (TDOS) plots complemented the analysis. Clonidine exhibited greater sensitivity to electrophilic attack, while the electronic affinity of clopidogrel was slightly higher. According to the MEP map, negative charge density was located on the oxygen atoms of clopidogrel, and the positive charge was located on the nitrogen atoms of clonidine. Notably, both the drugs exhibited similar reactivity in water. Clopidogrel was less reactive than clonidine, and the interaction between the molecules occurred via physisorption, which was in agreement with the TDOS plot. NBO analysis revealed a low charge variation, in accordance with the physical adsorption-like bonding between the drugs. The lowest energy for the clopidogrel-clonidine interaction was attained via the formation of four H bonds, as indicated by a significant intensive peak at 3360 cm-1 in the IR spectrum. Hydrogen bonds played a crucial role in the controlled drug delivery application as it allowed moderate and reversible drug adsorption, facilitating drug release in the biological environment. IR spectra also supported the absence of degradation or chemical reaction between the drugs, confirming the preservation of the individual active pharmaceutical ingredient.

Targeted Transferable Machine-Learned Potential for Linear Alkanes Trained on C14H30 and Tested for C4H10 to C30H62

Given the great importance of linear alkanes in fundamental and applied research, an accurate machine-learned potential (MLP) would be a major advance in computational modeling of these hydrocarbons. Recently, we reported a novel, many-body permutationally invariant model that was trained specifically for the 44-atom hydrocarbon C14H30 on roughly 250,000 B3LYP energies (Qu, C.; Houston, P. L.; Allison, T.; Schneider, B. I.; Bowman, J. M. J. Chem. Theory Comput. 2024, 20, 9339-9353). Here, we demonstrate the accuracy of the transferability of this potential for linear alkanes ranging from butane C4H10 up to C30H62. Unlike other approaches for transferability that aim for universal applicability, the present approach is targeted for linear alkanes. The mean absolute error (MAE) for energy ranges from 0.26 kcal/mol for butane and rises to 0.73 kcal/mol for C30H62 over the energy range up to 80 kcal/mol for butane and 600 kcal/mol for C30H62. These values are unprecedented for transferable potentials and indicate the high performance of a targeted transferable potential. The conformational barriers are shown to be in excellent agreement with high-level ab initio calculations for pentane, the largest alkane for which such calculations have been reported. Vibrational power spectra of C30H62 from molecular dynamics calculations are presented and briefly discussed. Finally, the evaluation time for the potential is shown to vary linearly with the number of atoms.

Learning Multiple Potential Energy Surfaces by Automated Discovery of a Compatible Representation

Creating analytic representations of multiple potential energy surfaces for modeling electronically nonadiabatic processes is a major challenge being addressed in various ways by the chemical dynamics community. In this work, we introduce a new method that can achieve convenient learning of multiple potential energy surfaces (PESs) and their gradients (negatives of the forces) for a polyatomic system. This new method, called compatibilization by deep neural network (CDNN), is demonstrated to be accurate and, even more importantly, to be automatic. The only required input is a database with geometries and potential energies. The method produces a matrix, called the compatible potential energy matrix (CPEM), that may be interpreted as the electronic Hamiltonian in an implicit nonadiabatic basis, and the analytic adiabatic potential energy surfaces and their gradients are obtained by diagonalization and automatic differentiation. We show that the CPEM, which is neither adiabatic nor necessarily diabatic, can be discovered automatically during the learning procedure by the special design of a CDNN architecture. We believe that the CDNN method will be very useful in practice for learning coupled PESs for polyatomic systems because it is accurate and fully automatic.

Nitrogen Dioxide Monitoring by Means of a Low-Cost Autonomous Platform and Sensor Calibration via Machine Learning with Global Data Correlation Enhancement

Air quality significantly impacts the environment and human living conditions, with direct and indirect effects on the economy. Precise and prompt detection of air pollutants is crucial for mitigating risks and implementing strategies to control pollution within acceptable thresholds. One of the common pollutants is nitrogen dioxide (NO2), high concentrations of which are detrimental to the human respiratory system and may lead to serious lung diseases. Unfortunately, reliable NO2 detection requires sophisticated and expensive apparatus. Although cheap sensors are now widespread, they lack accuracy and stability and are highly sensitive to environmental conditions. The purpose of this study is to propose a novel approach to precise calibration of the low-cost NO2 sensors. It is illustrated using a custom-developed autonomous platform for cost-efficient NO2 monitoring. The platform utilizes various sensors alongside electronic circuitry, control and communication units, and drivers. The calibration strategy leverages comprehensive data from multiple reference stations, employing neural network (NN) and kriging interpolation metamodels. These models are built using diverse environmental parameters (temperature, pressure, humidity) and cross-referenced data gathered by surplus NO2 sensors. Instead of providing direct outputs of the calibrated sensor, our approach relies on predicting affine correction coefficients, which increase the flexibility of the correction process. Additionally, a calibration stage incorporating global correlation enhancement is developed and applied. Demonstrative experiments extensively validate this approach, affirming the platform and calibration methodology's practicality for reliable and cost-effective NO2 monitoring, especially keeping in mind that the predictive power of the enhanced sensor (correlation coefficient nearing 0.9 against reference data, RMSE < 3.5 µg/m3) is close to that of expensive reference equipment.

Buffer gas enhancing of power conversion efficiency of a continuous-wave acetylene-filled fiber gas laser at the 3 μm wavelength

In this paper, we report the use of ammonia as the buffer gas in the acetylene-filled hollow-core fiber gas laser (A-HCFGL) in which the power conversion efficiency of continuous-wave (CW) operation reaches a record of 35.74%, the highest reported so far to the best of our knowledge. The intermolecular collision with ammonia assists the depopulation of lower laser levels of acetylene, which otherwise relies on the non-radiation relaxation by collision with the fiber core only. About 3.9 W CW laser output power is achieved at the 3.1 μm wavelength. A numerical model illustrates the buffer gas enhancement effect and explore the optimization of A-HCFGL for high output power and slope efficiency.

Reaction of NHOs with Bisphosphanes - Designing Diradicaloids, Zwitterions and Radicals

The linkage of an imidazole-based N-heterocyclic olefin (NHO), containing a terminal CH2 donor group, with a phosphorus-centered diradical molecular fragment leads to an open-shell singlet diphospha-indenylide system, a new class of P-heterocycles, which can be interpreted both as a phosphorus-centered diradicaloid and as a zwitterion with a permanent, overall charge separation between the N- and P-heterocyclic ring systems. The rotation of the imidazole ring, which is thermally possible due to a central C-C bond with a weakened π-component, changes both the charge separation and diradical character depending on the dihedral angle, as quantum mechanical calculations indicate. By varying the bulkiness of substituents at the imidazole-based NHO, it was possible to obtain different diphospha-indenylide species with different rotation angles in the solid state and hence varying diradical character. Imidazolium-diphospha-indenylides represent a new class of NHO-based zwitterions with diradical character. Their synthesis, structure, and activation chemistry are described, as well as the quantum mechanical description of the electronic structure in these unusual heterocycles. In addition, along the synthesis route to diphospha-indenylide, we also succeeded in isolating a highly reactive monoradical anion, which was also fully characterized.

Unveiling next-generation organic photovoltaics: Quantum mechanical insights into non-fullerene donor-acceptor compounds

Organic photovoltaics (OPVs) have improved greatly in recent years in pursuit for efficient and sustainable energy conversion methods. Specifically, utilizing quantum chemistry approaches such as density functional theory (DFT), the electronic structures, energy levels, and charge transport characteristics of donor-π-acceptor (D-π-A) systems based on non-fullerene donor and acceptor molecules have been examined and synthesized. Non-fullerene acceptors offer several advantages over traditional fullerene-based materials, such as enhanced light absorption, modifiable energy levels, and reduced recombination losses. Quantum mechanical simulations are helpful in the design and development of these materials because they can accurately predict the energy level alignment, molecule interactions, and charge transport properties needed for the high-efficiency of OPVs. The research begins through the selection of electron-donating and electron-accepting non-fullerene polymeric molecules using the unique properties of non-fullerene derivatives and non-fullerene acceptors. The theory uses the B3LYP-D3 method with a 6-31+G (d,p) basis set. PY-IT is used as the reference molecule, and eight molecules PY-IT01-PY-IT08, has been created by changing the end caps of the acceptor units. The created compound has superior photovoltaic characteristics. Focus has been specifically given to the frontier molecular orbitals (FMOs), natural bond order (NBO) analysis, reorganization energies (RE), and absorption spectra in order to assess the viability of charge separation and efficient light absorption. Finally, the molecular electrostatic potential (MEP) analysis, transition density matrix (TDM) analysis, and improved open circuit voltage (Voc) all have been computed. The results of the findings provide new insight to design organic solar cells (OSCs) with improved photovoltaic and solar energy conversion capabilities, which has great potential for the future development of more dependable and efficient OSCs.

Improvement of Fourteen Coupled Global Potential Energy Surfaces of 3A' States of O + O2

We improved the potential energy surfaces for 14 coupled 3A' states of O3 by using parametrically managed diabatization by deep neural network (PM-DDNN) with three improvements: (1) We used a new functional form for the parametrically managed activation function, which ensures the continuity of the coordinates used in the parametric management. (2) We used higher weighting for low-lying states to achieve smoother potential energy surfaces. (3) The asymptotic behavior of the coupled potential energy surfaces was further refined by utilizing a better low-dimensional potential. As a result of these improvements, we obtained significantly smoother potentials that are better suited for dynamics calculations. For the new version of 14 coupled 3A' surfaces, the entire set of 532,560 adiabatic energies are fit with a mean unsigned error (MUE) of 45 meV, which is only 0.7% of the mean energy in the data set, which is 6.24 eV.

Structural evolution and electronic properties of boron sulfides (B2S3)n (n = 1-6): insights from DFT calculations

The low-energy isomers of boron sulfides (B2S3)n (n = 1-6) were constructed by using B2S3 as the building block, and their structure, stability, and reactivity toward small molecules have been explored by density functional theory (DFT) calculations. It is found that the low-energy isomers of (B2S3)n clusters are of rich bonding characteristics for their structural constituents, such as [S-B-S], [B2S3], [B3S3], [S], etc. The planar or non-planar B2S2 rings, as the basic structural units, are bridged together by the S atoms to form the most stable structures of (B2S3)n clusters with n ≥ 2. These low-energy (B2S3)n clusters are predicted to be stable both structurally and electronically, and their boron centers show relatively high activity to the binding of small molecules NH3 and CO, until all boron atoms are ligated by NH3 or CO. The present results provide new insights into the structure and properties of (B2S3)n clusters, which are beneficial to the development of boron chemistry and the application of boron-based materials.

Interaction of Small Nitriles Occurring in the Atmosphere of Titan with Metal Ions of Meteoric Origin

Meteoric material injected into the atmosphere of Titan, Saturn's moon, can react with nitriles and other organic compounds that constitute Titan's atmosphere. However, specific chemical outcomes have not been fully explored. To understand the fates of meteoric metal ions in the Titan environment, reactions of Mg+ and Al+ with CH3CN (acetonitrile) and C2H5CN (propionitrile) were carried out using a drift cell ion reactor at room temperatures (300 K) and reduced temperatures (∼193 K) and modeled using density functional theory and coupled-cluster theory. Analysis of reactant ion electronic state distributions via electronic state chromatography revealed that Mg+ was produced in our instrument exclusively in its ground (2S) state, whereas Al+ was produced in both its 1S ground state and 3P first excited state. Mg+(2S) and Al+(1S) produce association products exclusively with both CH3CN and C2H5CN. Primary association reactions with C2H5CN occurred with higher reaction efficiencies than those with CH3CN. Mg+(2S) sequentially associates up to four nitrile ligands, and Al+(1S) associates up to three, each via the nitrile nitrogen. Computed binding energies are strongest for the first ligand and diminish with subsequent nitriles. Mg+(2S) exhibits a stronger preference for binding nitriles than Al+(1S) because its unpaired electron delocalizes to the nitrile ligands through back-bonding, whereas the lone pair on Al+(1S) remains localized on the metal center. Al+(3P) exhibited evidence of bimolecular product formation with both nitriles. Computational modeling of Al+(3P) with CH3CN suggests that the major product, AlCH3+, is kinetically favored over the more energetically stable product, Al+(HCN).