Quasiclassical Molecular Dynamics Research Articles

Surprising Dynamics Phenomena in the Diels-Alder Reaction of C60 Uncovered with AI.

We performed an extensive artificial intelligence-accelerated quasi-classical molecular dynamics investigation of the time-resolved mechanism of the Diels-Alder reaction of fullerene C60 with 2,3-dimethyl-1,3-butadiene. In a substantial fraction (10%) of reactive trajectories, the larger C60 noncovalently attracts the 2,3-dimethyl-1,3-butadiene long before the barrier so that the diene undergoes the series of complex motions including roaming, somersaults, twisting, and twisting somersaults around the fullerene until it aligns itself to pass over the barrier. These complicated processes could be easily missed in typically performed quantum chemical simulations with shorter and fewer trajectories. After the barrier is passed, the bonds take longer to form compared to the simplest prototypical Diels-Alder reaction of ethene with 1,3-butadiene despite high similarities in transition states and barrier widths evaluated with intrinsic reaction coordinate (IRC) calculations. C60 is mainly responsible for these differences as its reaction with 1,3-butadiene is similar to the reaction with 2,3-dimethyl-1,3-butadiene: the only substantial difference being that the extra methyl groups double the probability of the prolonged alignment phase in dynamics. These additional calculations of C60 with 1,3-butadiene could be performed via active learning more easily by reusing the data generated for the other two reactions, showing the potential for larger-scale exploration of the effects of different substrates in the same types of reactions.

Physics-Informed Active Learning for Accelerating Quantum Chemical Simulations.

Quantum chemical simulations can be greatly accelerated by constructing machine learning potentials, which is often done using active learning (AL). The usefulness of the constructed potentials is often limited by the high effort required and their insufficient robustness in the simulations. Here, we introduce the end-to-end AL for constructing robust data-efficient potentials with affordable investment of time and resources and minimum human interference. Our AL protocol is based on the physics-informed sampling of training points, automatic selection of initial data, uncertainty quantification, and convergence monitoring. The versatility of this protocol is shown in our implementation of quasi-classical molecular dynamics for simulating vibrational spectra, conformer search of a key biochemical molecule, and time-resolved mechanism of the Diels-Alder reaction. These investigations took us days instead of weeks of pure quantum chemical calculations on a high-performance computing cluster.

Evidence for Dearomatizing Spirocyclization and Dynamic Effects in the Quasi-stereospecific Nitrogen Deletion of Tetrahydroisoquinolines.

Selectivity in organic chemistry is generally presumed to arise from energy differences between competing selectivity-determining transition states. However, in cases where static density functional theory (DFT) fails to reproduce experimental product distributions, dynamic effects can be examined to understand the behavior of more complex reaction systems. Previously, we reported a method for nitrogen deletion of secondary amines which relies on the formation of isodiazene intermediates that subsequently extrude dinitrogen with concomitant C-C bond formation via a caged diradical. Herein, a detailed mechanistic analysis of the nitrogen deletion of 1-aryl-tetrahydroisoquinolines is presented, suggesting that in this system the previously determined diradical mechanism undergoes dynamically controlled partitioning to both the normal 1,5-coupling product and an unexpected spirocyclic dearomatized intermediate, which converges to the expected indane by an unusually facile 1,3-sigmatropic rearrangement. This mechanism is not reproduced by static DFT but is supported by quasi-classical molecular dynamics calculations and unifies several unusual observations in this system, including partial chirality transfer, nonstatistical isotopic scrambling at the ethylene bridge, the isolation of spirocyclic dearomatized species in a related heterocyclic series, and the observation that introduction of an 8-substituent dramatically improves enantiospecificity.

Computational Research on Ag(I)-Catalyzed Cubane Rearrangement: Mechanism, Metal and Counteranion Effect, Ligand Engineering, and Post-Transition-State Desymmetrization.

Ag(I) salts have demonstrated superior catalytic activity in the cubane-cuneane rearrangement. This research presents a comprehensive mechanistic investigation using high-level computations. The reaction proceeds via oxidative addition (OA) of Ag(I) to the C-C bond, followed by C-Ag bond cleavage and subsequent dynamically concerted carbocation rearrangement. The OA of Ag(I) exhibits significant more electrophilic nature than classical transition metal-induced OA, and the superior catalytic activity of Ag(I) is attributed to the accessibility of a highly electrophilic "bare" Ag+ center and a relatively weak Ag-C bond. However, the highly Lewis acidic nature of the Ag(I) center limits the substrate scope. To address this problem, ligand and counteranion screening was conducted, revealing that chiral biarylether ligands in combination with BF4- as the counteranion offer both enhanced reactivity and improved chemoselectivity while suppressing the Lewis acidity. Additionally, quasi-classical molecular dynamics simulations indicate the possibility of a novel desymmetrization pathway through post-transition-state dynamics in the biarylether-Ag(I)-BF4- system, thereby providing a potential avenue for enantioselective cuneane synthesis.

ANT 2023: A program for adiabatic and nonadiabatic trajectories

ANT 2023 is a program for quasiclassical and semiclassical trajectories, both single-surface trajectories for which the Born-Oppenheimer approximation is valid and multi-surface calculations with electronic state changes, i.e., for electronically adiabatic and electronically nonadiabatic trajectories. There are several methods available for multisurface problems: surface hopping with or without time uncertainty and with or without decoherence, semiclassical Ehrenfest, self-consistent decay of mixing, and coherent switching with decay of mixing (CSDM). The potential surface for single-surface problems may be an analytic potential function supplied by the user, or one may use direct dynamics. To use the adiabatic representation (i.e., electronically adiabatic basis functions) for electronically nonadiabatic dynamics, the user may either provide the adiabatic surfaces and nonadiabatic couplings by direct dynamics, or the program may calculate them from diabatic surfaces and diabatic couplings, which are usually analytic. One can also use analytic fits to the surfaces and couplings to carry out calculations entirely in the diabatic representation. The curvature-driven approximation is available as an option for use with CSDM and trajectory surface hopping. The ANT 2023 program is especially recommended for calculations with analytic potential energy surfaces and couplings because of its high efficiency for such calculations. Program SummaryProgram Title: ANT 2023, revision BCPC Library link to program files:https://doi.org/10.17632/gr74wwckcp.1Developer's repository link: https://doi.org/10.5281/zenodo.10011563Licensing Provisions: Apache-2.0Programming language: Fortran 90Interfaces to other programs: For direct dynamics, the code has well tested interfaces to the Gaussian 09 [1], Gaussian 16 [2], MOPACmn [3], and Molpro [4] electronic structure codes. The interfaces are included, but these electronic structure programs must be obtained separately. Users may also provide their own interfaces to other electronic structure programs.[1] M. J. Frisch et al. Gaussian 09, Gaussian, Inc., Wallingford CT, 2016.[2] M. J. Frisch et al., Gaussian 16, Gaussian, Inc., Wallingford CT, 2016.[3] J. J. P. Stewart et al., MOPAC 5.022mn, University of Minnesota, Minneapolis, MN, 2015; Zenodo. https://doi.org/10.5281/zenodo.8153079[4] H.-J. Werner et al., J. Chem. Phys. 152 (2020) 144107.Nature of problem: classical, quasiclassical, or semiclassical molecular dynamics for electronically adiabatic or electronically nonadiabatic collisions and unimolecular processes of atoms, molecules, and clustersSolution method: The program integrates the classical or semiclassical equations of motion for the motion of the nuclei. ANT 2023 can use anharmonic initial-state selection for initial states of diatomics or harmonic initial-state selection for general polyatomics. The equations of motion for electronically adiabatic processes are Hamilton's equations with potential energy functions corresponding to the Born-Oppenheimer approximation. The equations of motion for electronically nonadiabatic processes (non-Born-Oppenheimer processes) include trajectory surface hopping (fewest switches with or without time uncertainty and with or without decoherence) or several possible mean-field methods, including the semiclassical Ehrenfest method (also called the time-dependent Hartree method), self-consistent decay of mixing, and coherent switching with decay of mixing. Electronically nonadiabatic processes may be treated using either the adiabatic representation or a diabatic representation, and the potential surfaces and couplings may be obtained from analytic fits in a user-supplied subroutine or by direct dynamics, which involves requesting energies, gradients, and couplings as needed from a quantum chemistry electronic structure program. In the adiabatic representation, the coupling may be nonadiabatic coupling vectors (based on gradients of electronic wave functions), time-derivative couplings (based on overlaps of electronic wave functions), or curvature-driven couplings (based on the curvatures of the adiabatic potential energy surfaces).Additional comments: The ANT 2023 program authors are Y. Shu, L. Zhang, J. Zheng, Z. H. Li, A. W. Jasper, D. A. Bonhommeau, R. Valero, R. Meana-Pañeda, S. L. Mielke, Z. Varga, and D. G. Truhlar [5]; the authors of the article [6] describing ANT 2023 are Y. Shu, L. Zhang, and D. G. Truhlar.

Molecular Dynamics of Iron Porphyrin-Catalyzed C-H Hydroxylation of Ethylbenzene.

Quasi-classical molecular dynamics (MD) simulations were carried out to study the mechanism of iron porphyrin-catalyzed hydroxylation of ethylbenzene. The hydrogen atom abstraction from ethylbenzene by iron-oxo species is the rate-determining step, which generates the radical pair of iron-hydroxo species and the benzylic radical. In the subsequent radical rebound step, the iron-hydroxo species and benzylic radical recombine to form the hydroxylated product, which is barrierless on the doublet energy surface. In the gas-phase quasi-classical MD study on the doublet energy surface, 45% of the reactive trajectories lead directly to the hydroxylated product, and this increases to 56% in implicit solvent model simulations. The percentage of reactive trajectories leading to the separated radical pair is 98-100% on high-spin (quartet/sextet) energy surfaces. The low-spin state reactivity dominates in the hydroxylation of ethylbenzene, which is dynamically both concerted and stepwise, since the time gap between C-H bond cleavage and C-O bond formation ranges from 41 to 619 fs. By contrast, the high-spin state catalysis is an energetically stepwise process, which has a negligible contribution to the formation of hydroxylation products.

Computational Design of a Tetrapericyclic Cycloaddition and the Nature of Potential Energy Surfaces with Multiple Bifurcations.

An ambimodal transition state (TS) that leads to formation of four different pericyclic reaction products ([4 + 6]-, [2 + 8]-, [8 + 2]-, and [6 + 4]-cycloadducts) without any intervening minima has been designed and explored with DFT computations and quasiclassical molecular dynamics. Direct dynamics simulations propagated from the ambimodal TS show the evolution of trajectories to give the four cycloadducts. The topography of the PES is a key factor in product selectivity. A good correlation is observed between geometrical resemblance of the products to the ambimodal TS (measured by the RMSD) and the ratio of products formed in the dynamics simulations.

Exploring the adsorption behavior of pyrazinamide on the surface of X12Y12(X = B, Al; Y = N, P) nanocages: A in-silico study

Comprehending the adsorption behavior of pharmaceutical molecules on the surface of fullerene-like nanostructures is fundamental to their applications in medical engineering. Herein, density functional theory (DFT) and quasiclassical molecular dynamics (MD) calculations have been performed to explore the adsorption behavior of pyrazinamide (PZA) on the surface of X12Y12 (X = B, Al; Y = N, P) nanocages so as to screen the promising nanocages. The interaction energy and atoms in molecules (AIM) analysis for the most stable adsorption configuration of PZA showed that there were partially covalent interactions, covalent interactions and electrostatic interactions between PZA and B12Y12, Al12N12, Al12P12 nanocage, respectively. Upon adsorption, the HOMO-LUMO energy gap (Ebg) for all X12Y12-PZA complexes decreased, with the largest reduction (50.30 %) found in B12N12-PZA complex. The MD simulations of adsorption process indicated the structure of Al12Y12 was destroyed due to the reaction with PZA, while B12Y12 still maintained its cage-like structure. Further analysis of four typical adsorption configurations of PZA on B12N12 confirmed different B12N12-PZA complexes have relatively close Eint, large fluctuations in their HOMO-LUMO energy gap and partially covalent interactions between B12N12 and PZA. All the results suggest that B12N12 nanocage may act as superior sensor and drug delivery candidate for PZA.

Ambimodal Bispericyclic [6 + 4]/[4 + 6] Transition State Competes with Diradical Pathways in the Cycloheptatriene Dimerization: Dynamics and Experimental Characterization of Thermal Dimers.

The thermal dimerization of cycloheptatriene is predicted to occur by a concerted [6 + 4] cycloaddition via an ambimodal [6 + 4]/[4 + 6] transition state (TS) and a competing stepwise diradical (6 + 2) cycloaddition; both dimers subsequently undergo intramolecular [4 + 2] cycloadditions to afford thermally stable tetracyclic products. The ambimodal TS is the 10π-electron version of the prototype bispericyclic dimerization of cyclopentadiene discovered by Caramella et al. in 2002. Quantum mechanical studies using several common DFT functionals and post-HF methods, ωB97X-D, M06-2X, DLPNO-CCSD(T), NEVPT2, and PWPB95-D3(BJ), and quasiclassical molecular dynamics simulations provide details of bond timing and bifurcation pathways. By comparing the ambimodal [6 + 4]/[4 + 6] TS for cycloheptatriene dimerization with the ambimodal [4 + 2]/[2 + 4] TS of cyclopentadiene dimerization, we found that the high distortion energy in cycloheptatriene dimerization is the key to its relatively high energy barrier. The computational investigations were coupled with experimental studies of the cycloheptatriene dimerization, which resulted in the isolation of the two tetracyclic dimers. At lower temperature, the product from the predicted exo-[6 + 4]/[4 + 6] cycloaddition, followed by a subsequent intramolecular [4 + 2] cycloaddition, predominantly forms, while at higher temperature, the diradical (6 + 2) cycloadduct is the major product.

New Insights and Predictions into Complex Homogeneous Reactions Enabled by Computational Chemistry in Synergy with Experiments: Isotopes and Mechanisms

ConspectusHomogeneous catalysis and biocatalysis have been widely applied in synthetic, medicinal, and energy chemistry as well as synthetic biology. Driven by developments of new computational chemistry methods and better computer hardware, computational chemistry has become an essentially indispensable mechanistic "instrument" to help understand structures and decipher reaction mechanisms in catalysis. In addition, synergy between computational and experimental chemistry deepens our mechanistic understanding, which further promotes the rational design of new catalysts. In this Account, we summarize new or deeper mechanistic insights (including isotope, dispersion, and dynamical effects) into several complex homogeneous reactions from our systematic computational studies along with subsequent experimental studies by different groups. Apart from uncovering new mechanisms in some reactions, a few computational predictions (such as excited-state heavy-atom tunneling, steric-controlled enantioswitching, and a new geminal addition mechanism) based on our mechanistic insights were further verified by ensuing experiments.The Zimmerman group developed a photoinduced triplet di-π-methane rearrangement to form cyclopropane derivatives. Recently, our computational study predicted the first excited-state heavy-atom (carbon) quantum tunneling in one triplet di-π-methane rearrangement, in which the reaction rates and 12C/13C kinetic isotope effects (KIEs) can be enhanced by quantum tunneling at low temperatures. This unprecedented excited-state heavy-atom tunneling in a photoinduced reaction has recently been verified by an experimental 12C/13C KIE study by the Singleton group. Such combined computational and experimental studies should open up opportunities to discover more rare excited-state heavy-atom tunneling in other photoinduced reactions. In addition, we found unexpectedly large secondary KIE values in the five-coordinate Fe(III)-catalyzed hetero-Diels-Alder pathway, even with substantial C-C bond formation, due to the non-negligible equilibrium isotope effect (EIE) derived from altered metal coordination. Therefore, these KIE values cannot reliably reflect transition-state structures for the five-coordinate metal pathway. Furthermore, our density functional theory (DFT) quasi-classical molecular dynamics (MD) simulations demonstrated that the coordination mode and/or spin state of the iron metal as well as an electric field can affect the dynamics of this reaction (e.g., the dynamically stepwise process, the entrance/exit reaction channels).Moreover, we unveiled a new reaction mechanism to account for the uncommon Ru(II)-catalyzed geminal-addition semihydrogenation and hydroboration of silyl alkynes. Our proposed key gem-Ru(II)-carbene intermediates derived from double migrations on the same alkyne carbon were verified by crossover experiments. Additionally, our DFT MD simulations suggested that the first hydrogen migration transition-state structures may directly and quickly form the key gem-Ru-carbene structures, thereby "bypassing" the second migration step. Furthermore, our extensive study revealed the origin of the enantioselectivity of the Cu(I)-catalyzed 1,3-dipolar cycloaddition of azomethine ylides with β-substituted alkenyl bicyclic heteroarenes enabled by dual coordination of both substrates. Such mechanistic insights promoted our computational predictions of the enantioselectivity reversal for the corresponding monocyclic heteroarene substrates and the regiospecific addition to the less reactive internal C═C bond of one diene substrate. These predictions were proven by our experimental collaborators. Finally, our mechanistic insights into a few other reactions are also presented. Overall, we hope that these interactive computational and experimental studies enrich our mechanistic understanding and aid in reaction development.

Theoretical studies on triplet-state driven dissociation of formaldehyde by quasi-classical molecular dynamics simulation on machine-learning potential energy surface.

The H-atom dissociation of formaldehyde on the lowest triplet state (T1) is studied by quasi-classical molecular dynamic simulations on the high-dimensional machine-learning potential energy surface (PES) model. An atomic-energy based deep-learning neural network (NN) is used to represent the PES function, and the weighted atom-centered symmetry functions are employed as inputs of the NN model to satisfy the translational, rotational, and permutational symmetries, and to capture the geometry features of each atom and its individual chemical environment. Several standard technical tricks are used in the construction of NN-PES, which includes the application of clustering algorithm in the formation of the training dataset, the examination of the reliability of the NN-PES model by different fitted NN models, and the detection of the out-of-confidence region by the confidence interval of the training dataset. The accuracy of the full-dimensional NN-PES model is examined by two benchmark calculations with respect to ab initio data. Both the NN and electronic-structure calculations give a similar H-atom dissociation reaction pathway on the T1 state in the intrinsic reaction coordinate analysis. The small-scaled trial dynamics simulations based on NN-PES and ab initio PES give highly consistent results. After confirming the accuracy of the NN-PES, a large number of trajectories are calculated in the quasi-classical dynamics, which allows us to get a better understanding of the T1-driven H-atom dissociation dynamics efficiently. Particularly, the dynamics simulations from different initial conditions can be easily simulated with a rather low computational cost. The influence of the mode-specific vibrational excitations on the H-atom dissociation dynamics driven by the T1 state is explored. The results show that the vibrational excitations on symmetric C-H stretching, asymmetric C-H stretching, and C=O stretching motions always enhance the H-atom dissociation probability obviously.

Mechanisms and Dynamics of Synthetic and Biosynthetic Formation of Delitschiapyrones: Solvent Control of Ambimodal Periselectivity.

The mechanism and dynamics for the formation of the delitschiapyrone family of natural products are studied by density functional theory (DFT) calculations and quasiclassical molecular dynamics simulations with DFT and xTB. In the uncatalyzed reaction, delitschiapyrones A and B are formed by Diels-Alder reactions through a single transition state and a post-transition state bifurcation that favors formation of delitschiapyrone B. In water and most likely in the enzyme, the acidic hydroxyquinone ionizes, and the resulting conjugate base undergoes cycloaddition preferentially to delitschiapyrone A. We demonstrate a new type of biosynthetic transformation and variable selectivity from a (4 + 2)/(4 + 3) ambimodal transition state.

Post-Transition State Bifurcation in Iron-Catalyzed Arene Aminations

Recently, an iron-catalyzed meta-selective arene amination strategy was reported to show excellent regiocontrol, operational simplicity, and high atom and cost efficiencies. In this work, density functional theory (DFT) calculations and quasiclassical molecular dynamics (MD) simulations were carried out to study the mechanism of this reaction. Our computations revealed an unreported ambimodal attack of arene by hydroxylamine-O-sulfate, which forms not only σ-complex via electrophilic addition but also an aziridinium intermediate through energetically concerted electrophilic–nucleophilic additions. Quasiclassical trajectories suggest the orientation of the attacking NH2+ group with respect to arene control: (a) site selectivity of the post-transition state nucleophilic attack and (b) σ-complex/aziridinium selectivity on the bifurcating surface. The final amination products can be favorably generated via the ring opening of aziridinium intermediates, rearrangement of σ-complexes, and deprotonation of the resultant carbocations. Overall, our study illustrates a novel pathway in which an ambimodal reaction is incorporated into the standard SEAr mechanism.

Reaction mechanisms of H(D) → D(H) + Pt(111) interaction system: Quasiclassical molecular dynamics simulations

Reaction mechanisms of the H(or D) → D(or H) + Pt(111) interaction system have been proposed by using quasiclassical molecular dynamics simulations. First, the adsorbate atoms are dispersed randomly over the surface’s adsorption sites to form 0.18 ML, 0.25 ML, and 0.50 ML of coverages. Since the surface is considered to be resilient, thanks to imitating the multi-layer slab by using a function of many-body embedded-atom potential energy, the slab atoms can move because of the implemented external forces. Thus, energy transfer from the incident atom to surface atoms and adsorbates has been considered a real collision system. Moreover, the London–Eyring–Polanyi–Sato function is modified to model interaction between the adsorbates and slab atoms. In addition to desorption of HD and H2(or D2) after the collision of the incoming H(or D) atom with the surface, subsurface penetration, sticking on the surface, and inelastic reflection of the incident atom have been investigated in detail as the reaction mechanisms on the surface. In addition, isotopic effects on reaction mechanisms have been analyzed in depth and shown. Also, hot-atom and Eley–Rideal mechanisms have been examined and explained. The hot-atom mechanism is responsible for the formation of H2/D2 products. Furthermore, the sticking rate on the surface is lower than the rate of subsurface penetration.

High-Dimensional Atomistic Neural Network Potential to Study the Alignment-Resolved O2 Scattering from Highly Oriented Pyrolytic Graphite.

A high dimensional and accurate atomistic neural network potential energy surface (ANN-PES) that describes the interaction between one O2 molecule and a highly oriented pyrolytic graphite (HOPG) surface has been constructed using the open-source package (aenet). The validation of the PES is performed by paying attention to static characteristics as well as by testing its performance in reproducing previous ab initio molecular dynamics simulation results. Subsequently, the ANN-PES is used to perform quasi-classical molecular dynamics calculations of the alignment-dependent scattering of O2 from HOPG. The results are obtained for 200 meV O2 molecules with different initial alignments impinging with a polar incidence angle with respect to the surface normal of 22.5° on a thermalized (110 and 300 K) graphite surface. The choice of these initial conditions in our simulations is made to perform comparisons to recent experimental results on this system. Our results show that the scattering of O2 from the HOPG surface is a rather direct process, that the angular distributions are alignment dependent, and that the final translational energy of end-on molecules is around 20% lower than that of side-on molecules. Upon collision with the surface, the molecules that are initially aligned perpendicular to the surface become highly rotationally excited, whereas a very small change in the rotational state of the scattered molecules is observed for the initial parallel alignments. The latter confirms the energy transfer dependence on the stereodynamics for the present system. The results of our simulations are in overall agreement with the experimental observations regarding the shape of the angular distributions and the alignment dependence of the in-plane reflected molecules.

Cycloadditions of Cyclopentadiene and Cycloheptatriene with Tropones: All Endo-[6+4] Cycloadditions Are Ambimodal.

The cycloadditions of cyclopentadiene and cycloheptatriene with tropone are some of the earliest published examples of [6+4] cycloaddition reactions. We report quantum mechanical studies (ωB97X-D and DLPNO-CCSD(T)) of transition structures and products of these reactions, as well as quasi-classical molecular dynamics simulations of reaction trajectories. The study reveals that these cycloadditions involve ambimodal transition states resulting in a web of products by pericyclic interconversion pathways. Combined with these studies, calculations of simple parent systems and a thorough meta-analysis of literature examples reveal the general concept that all endo-[6+4] cycloadditions are ambimodal.

Advances on Quasi-classical Molecular Dynamics of Organic Reaction Mechanisms

Advances on Quasi-classical Molecular Dynamics of Organic Reaction Mechanisms

Observation of the Low‐Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces

AbstractUsing the helium nanodroplet isolation setup at the ultrabright free‐electron laser source FELIX in Nijmegen (BoHeNDI@FELIX), the intermolecular modes of water dimer in the frequency region from 70 to 550 cm−1 were recorded. Observed bands were assigned to donor torsion, acceptor wag, acceptor twist, intermolecular stretch, donor torsion overtone, and in‐plane and out‐of‐plane librational modes. This experimental data set provides a sensitive test for state‐of‐the‐art water potentials and dipole moment surfaces. Theoretical calculations of the IR spectrum are presented using high‐level quantum and approximate quasiclassical molecular dynamics approaches. These calculations use the full‐dimensional ab initio WHHB potential and dipole moment surfaces. Based on the experimental data, a considerable increase of the acceptor switch and a bifurcation tunneling splitting in the librational mode is deduced, which is a consequence of the effective decrease in the tunneling barrier.

Observation of the Low-Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces.

Using the helium nanodroplet isolation setup at the ultrabright free‐electron laser source FELIX in Nijmegen (BoHeNDI@FELIX), the intermolecular modes of water dimer in the frequency region from 70 to 550 cm−1 were recorded. Observed bands were assigned to donor torsion, acceptor wag, acceptor twist, intermolecular stretch, donor torsion overtone, and in‐plane and out‐of‐plane librational modes. This experimental data set provides a sensitive test for state‐of‐the‐art water potentials and dipole moment surfaces. Theoretical calculations of the IR spectrum are presented using high‐level quantum and approximate quasiclassical molecular dynamics approaches. These calculations use the full‐dimensional ab initio WHHB potential and dipole moment surfaces. Based on the experimental data, a considerable increase of the acceptor switch and a bifurcation tunneling splitting in the librational mode is deduced, which is a consequence of the effective decrease in the tunneling barrier.

Pressure effects on the vibrational and rotational relaxation of vibrationally excited OH (ν, J) in an argon bath.

Quasi-classical molecular dynamics simulations were used to study the energy relaxation of an initially non-rotating, vibrationally excited (ν = 4) hydroxyl radical (OH) in an Ar bath at 300 K and at high pressures from 50 atm to 400 atm. A Morse oscillator potential represented the OH, and two sets of interaction potentials were used based on whether the Ar-H potential was a Buckingham (Exp6) or a Lennard-Jones (LJ) potential. The vibrational and rotational energies were monitored for 25 000-90 000 ps for Exp6 trajectories and 5000 ps for LJ trajectories. Comparisons to measured vibrational relaxation rates show that Exp6 rates are superior. Simulated initial vibrational relaxation rates are linearly proportional to pressure, implying no effect of high-pressure breakdown in the isolated binary collision approximation. The vibrational decay curves upward from single-exponential decay. A model based on transition rates that exponentially depend on the anharmonic energy gap between vibrational levels fits the vibrational decay well at all pressures, suggesting that anharmonicity is a major cause of the curvature. Due to the competition of vibration-to-rotation energy transfer and bath gas relaxation, the rotational energy overshoots and then relaxes to its thermal value. Approximate models with adjustable rates for this competition successfully reproduced the rotational results. These models show that a large fraction of the vibrational energy loss is initially converted to rotational energy but that fraction decreases rapidly as the vibrational energy content of OH decreases. While simulated rates change dramatically between Exp6 and LJ potentials, the mechanisms remain the same.