Showing papers on "Potential energy surface published in 2012"

Explicitly correlated electrons in molecules.

Abstract: Explicitly Correlated Electrons in Molecules Christof H€attig, Wim Klopper,* Andreas K€ohn, and David P. Tew Lehrstuhl f€ur Theoretische Chemie, Ruhr-Universit€at Bochum, D-44780 Bochum, Germany Abteilung f€ur Theoretische Chemie, Institut f€ur Physikalische Chemie, Karlsruher Institut f€ur Technologie, KIT-Campus S€ud, Postfach 6980, D-76049 Karlsruhe, Germany Institut f€ur Physikalische Chemie, Johannes Gutenberg-Universit€at Mainz, D-55099 Mainz, Germany School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom

Decoherence-induced surface hopping.

Abstract: A simple surface hopping method for nonadiabatic molecular dynamics is developed. The method derives from a stochastic modeling of the time-dependent Schrodinger and master equations for open systems and accounts simultaneously for quantum mechanical branching in the otherwise classical (nuclear) degrees of freedom and loss of coherence within the quantum (electronic) subsystem due to coupling to nuclei. Electronic dynamics in the Hilbert space takes the form of a unitary evolution, intermittent with stochastic decoherence events that are manifested as a localization toward (adiabatic) basis states. Classical particles evolve along a single potential energy surface and can switch surfaces only at the decoherence events. Thus, decoherence provides physical justification of surface hopping, obviating the need for ad hoc surface hopping rules. The method is tested with model problems, showing good agreement with the exact quantum mechanical results and providing an improvement over the most popular surface hopping technique. The method is implemented within real-time time-dependent density functional theory formulated in the Kohn-Sham representation and is applied to carbon nanotubes and graphene nanoribbons. The calculated time scales of non-radiative quenching of luminescence in these systems agree with the experimental data and earlier calculations.

The fourth age of quantum chemistry: molecules in motion

Abstract: Developments during the last two decades in nuclear motion theory made it possible to obtain variational solutions to the time-independent, nuclear-motion Schrodinger equation of polyatomic systems as “exact” as the potential energy surface (PES) is. Nuclear motion theory thus reached a level whereby this branch of quantum chemistry started to catch up with the well developed and widely applied other branch, electronic structure theory. It seems to be fair to declare that we are now in the fourth age of quantum chemistry, where the first three ages are principally defined by developments in electronic structure techniques (G. Richards, Nature, 1979, 278, 507). In the fourth age we are able to incorporate into our quantum chemical treatment the motion of nuclei in an exact fashion and, for example, go beyond equilibrium molecular properties and compute accurate, temperature-dependent, effective properties, thus closing the gap between measurements and electronic structure computations. In this Perspective three fundamental algorithms for the variational solution of the time-independent nuclear-motion Schrodinger equation employing exact kinetic energy operators are presented: one based on tailor-made Hamiltonians, one on the Eckart–Watson Hamiltonian, and one on a general internal-coordinate Hamiltonian. It is argued that the most useful and most widely applicable procedure is the third one, based on a Hamiltonian containing a kinetic energy operator written in terms of internal coordinates and an arbitrary embedding of the body-fixed frame of the molecule. This Hamiltonian makes it feasible to treat the nuclear motions of arbitrary quantum systems, irrespective of whether they exhibit a single well-defined minimum or not, and of arbitrary reduced-dimensional models. As a result, molecular spectroscopy, an important field for the application of nuclear motion theory, has almost black-box-type tools at its disposal. Variational nuclear motion computations, based on an exact kinetic energy operator and an arbitrary PES, can now be performed for about 9 active vibrational degrees of freedom relatively straightforwardly. Simulations of high-resolution spectra allow the understanding of complete rotational–vibrational spectra up to and beyond the first dissociation limits. Variational results obtained for H2O, H+3, NH3, CH4, and H2CCO are used to demonstrate the power of the variational techniques for the description of vibrational and rotational excitations. Some qualitative features of the results are also discussed.

Comparison of vertical and adiabatic harmonic approaches for the calculation of the vibrational structure of electronic spectra.

Abstract: The calculation of the vibrational structure associated to electronic spectra in large molecules requires a Taylor expansion of the initial and final state potential energy surface (PES) around some reference nuclear structure. Vertical (V) and adiabatic (A) approaches expand the final state PES around the initial-state (V) or final-state (A) equilibrium structure. Simplest models only take into account displacements of initial- and final-state minima, intermediate ones also allow for difference in frequencies and more accurate models introduce the Dushinsky effect through the computation of the Hessians of both the initial and final state. In this contribution we summarize and compare the mathematical expressions of the complete hierarchy of V and A harmonic models and we implement them in a numerical code, presenting a detailed comparison of their performance on a number of prototypical systems. We also address non-Condon effects through linear expansions of the transition dipole as a function of nuclear coordinates (Herzberg–Teller effect) and compare the results of expansions around initial and final state equilibrium geometries. By a throughout analysis of our results we highlight a number of general trends in the relative performance of the models that can provide hints for their proper choice. Moreover we show that A and V models including final state PES Hessian outperform the simpler ones and that discrepancies in their predictions are diagnostic for failure of harmonic approximation and/or of Born–Oppenheimer approximation (existence of remarkable geometry-dependent mixing of electronic states).

Dynamic Electron Correlation Effects on the Ground State Potential Energy Surface of a Retinal Chromophore Model.

Abstract: The ground state potential energy surface of the retinal chromophore of visual pigments (e.g., bovine rhodopsin) features a low-lying conical intersection surrounded by regions with variable charge-transfer and diradical electronic structures. This implies that dynamic electron correlation may have a large effect on the shape of the force fields driving its reactivity. To investigate this effect, we focus on mapping the potential energy for three paths located along the ground state CASSCF potential energy surface of the penta-2,4-dieniminium cation taken as a minimal model of the retinal chromophore. The first path spans the bond length alternation coordinate and intercepts a conical intersection point. The other two are minimum energy paths along two distinct but kinetically competitive thermal isomerization coordinates. We show that the effect of introducing the missing dynamic electron correlation variationally (with MRCISD) and perturbatively (with the CASPT2, NEVPT2, and XMCQDPT2 methods) leads, invariably, to a stabilization of the regions with charge transfer character and to a significant reshaping of the reference CASSCF potential energy surface and suggesting a change in the dominating isomerization mechanism. The possible impact of such a correction on the photoisomerization of the retinal chromophore is discussed.

The Water Hexamer: Cage, Prism, or Both. Full Dimensional Quantum Simulations Say Both

Abstract: State-of-the-art quantum simulations on a full-dimensional ab initio potential energy surface are used to characterize the properties of the water hexamer. The relative populations of the different isomers are determined over a wide range of temperatures. While the prism isomer is identified as the global minimum-energy structure, the quantum simulations, which explicitly include zero-point energy and quantum thermal motion, predict that both the cage and prism isomers are present at low temperature down to almost 0 K. This is largely consistent with the available experimental data and, in particular, with very recent measurements of broadband rotational spectra of the water hexamer recorded in supersonic expansions.

Chemical dynamics simulations of X- + CH3Y → XCH3 + Y- gas-phase S(N)2 nucleophilic substitution reactions. Nonstatistical dynamics and nontraditional reaction mechanisms.

Abstract: Extensive classical chemical dynamics simulations of gas-phase X(-) + CH(3)Y → XCH(3) + Y(-) S(N)2 nucleophilic substitution reactions are reviewed and discussed and compared with experimental measurements and predictions of theoretical models. The primary emphasis is on reactions for which X and Y are halogen atoms. Both reactions with the traditional potential energy surface (PES), which include pre- and postreaction potential energy minima and a central barrier, and reactions with nontraditional PESs are considered. These S(N)2 reactions exhibit important nonstatistical atomic-level dynamics. The X(-) + CH(3)Y → X(-)---CH(3)Y association rate constant is less than the capture model as a result of inefficient energy transfer from X(-)+ CH(3)Y relative translation to CH(3)Y rotation and vibration. There is weak coupling between the low-frequency intermolecular modes of the X(-)---CH(3)Y complex and higher frequency CH(3)Y intramolecular modes, resulting in non-RRKM kinetics for X(-)---CH(3)Y unimolecular decomposition. Recrossings of the [X--CH(3)--Y](-) central barrier is important. As a result of the above dynamics, the relative translational energy and temperature dependencies of the S(N)2 rate constants are not accurately given by statistical theory. The nonstatistical dynamics results in nonstatistical partitioning of the available energy to XCH(3) +Y(-) reaction products. Besides the indirect, complex forming atomic-level mechanism for the S(N)2 reaction, direct mechanisms promoted by X(-) + CH(3)Y relative translational or CH(3)Y vibrational excitation are possible, e.g., the roundabout mechanism.

On the quantum theory of molecules

Abstract: Transition state theory was introduced in 1930s to account for chemical reactions. Central to this theory is the idea of a potential energy surface (PES). It was assumed that such a surface could be constructed using eigensolutions of the Schrodinger equation for the molecular (Coulomb) Hamiltonian but at that time such calculations were not possible. Nowadays quantum mechanical ab initio electronic structure calculations are routine and from their results PESs can be constructed which are believed to approximate those assumed derivable from the eigensolutions. It is argued here that this belief is unfounded. It is suggested that the potential energy surface construction is more appropriately regarded as a legitimate and effective modification of quantum mechanics for chemical purposes.

A neural network potential-energy surface for the water dimer based on environment-dependent atomic energies and charges.

Abstract: Understanding the unique properties of water still represents a significant challenge for theory and experiment. Computer simulations by molecular dynamics require a reliable description of the atomic interactions, and in recent decades countless water potentials have been reported in the literature. Still, most of these potentials contain significant approximations, for instance a frozen internal structure of the individual water monomers. Artificial neural networks (NNs) offer a promising way for the construction of very accurate potential-energy surfaces taking all degrees of freedom explicitly into account. These potentials are based on electronic structure calculations for representative configurations, which are then interpolated to a continuous energy surface that can be evaluated many orders of magnitude faster. We present a full-dimensional NN potential for the water dimer as a first step towards the construction of a NN potential for liquid water. This many-body potential is based on environment-dependent atomic energy contributions, and long-range electrostatic interactions are incorporated employing environment-dependent atomic charges. We show that the potential and derived properties like vibrational frequencies are in excellent agreement with the underlying reference density-functional theory calculations.

Ring polymer molecular dynamics with surface hopping.

Abstract: We propose a ring polymer molecular dynamics method for the calculation of chemical rate constants that incorporates nonadiabatic effects by the surface-hopping approach. Two approximate ring polymer electronic Hamiltonians are formulated and the time-dependent Schrodinger equation for the electronic amplitudes is solved self-consistently with the ring polymer equations of motion. The beads of the ring polymer move on a single adiabatic potential energy surface at all times except for instantaneous surface hops. The probability for a hop is determined by the fewest-switches surface-hopping criterion. During a surface hop all beads switch simultaneously to the new potential energy surface with positions kept unchanged and momenta adjusted properly to conserve total energy. The approach allows the evaluation of total rate coefficients as well as electronic state-selected contributions. The method is tested against exact quantum mechanical calculations for a one-dimensional, two-state model system that mimics a prototypical nonadiabatic bimolecular chemical reaction. For this model system, the method reproduces quite accurately the tunneling contribution to the rate and the distribution of reactants between the electronic states.

Potential energy surface for graphene on graphene: Ab initio derivation, analytical description, and microscopic interpretation

Abstract: We derive an analytical expression that describes the interaction energy between two graphene layers identically oriented as a function of the relative lateral and vertical positions, in excellent agreement with first principles calculations. Thanks to its formal simplicity, the proposed model allows for an immediate interpretation of the interactions, in particular of the potential corrugation. This last quantity plays a crucial role in determining the intrinsic resistance to interlayer sliding and its increase upon compression influences the frictional behavior under load. We show that, for these weakly adherent layers, the corrugation possesses the same nature and $z$ dependence of Pauli repulsion. We investigate the microscopic origin of these phenomena by analyzing the electronic charge distribution: We observe a pressure-induced charge transfer from the interlayer region toward the near-layer regions, with a much more consistent depletion of charge occurring for the $AA$ stacking than for the $AB$ stacking of the two layers.

Communication: A chemically accurate global potential energy surface for the HO + CO → H + CO2 reaction

Abstract: We report a chemically accurate global potential energy surface for the HOCO system based on high-level ab initio calculations at ∼35 000 points. The potential energy surface is shown to reproduce important stationary points and minimum energy paths. Quasi-classical trajectory calculations indicated a good agreement with experimental data.

Theoretical study of the validity of the polanyi rules for the late-barrier Cl + CHD3 reaction

Abstract: The Polanyi rules, which state that vibrational energy is more efficient in promoting a late-barrier reaction than translational energy, were questioned recently by an experimental unexpected finding that the CH stretch excitation is no more effective in promoting the late-barrier Cl + CHD3 reaction than the translational energy. However, the present quantum dynamics study on the best-available potential energy surface for the title reaction reveals that the CH stretch excitation does promote the reaction significantly, except at low collision energies. Further studies should be carried out to solve the disagreements between theory and experiment on the reaction.

Molecular Relaxation in Liquids

Abstract: Chapter 1. Basic Concepts 1.1 Introduction 1.2 Response Functions and Fluctuations 1.3 Time Correlation Functions 1.4 Linear Response Theory 1.5 Fluctuation-Dissipation Theorem 1.6 Diffusion, Friction and Viscosity Chapter 2. Phenomenological Description of Relaxation in Liquids 2.1 Introduction 2.2 Langevin Equation 2.3 Fokker-Planck Equation 2.4 Smoluchowski Equation 2.5 Master Equations 2.6 The Special Case of Harmonic Potential Chapter 3. Density and Momentum Relaxation in Liquids 3.1 Introduction 3.2 Hydrodynamics at Large Length Scales 3.2.1 Rayleigh-Brillouin Spectrum 3.3 Hydrodynamic Relation Self-diffusion Coefficient and Viscosity 3.4 Slow Dynamics at Large Wavenumbers: de Gennes Narrowing 3.5 Extended Hydrodynamics: Dynamics at Intermediate Length Scale 3.6 Mode Coupling Theory Chapter 4. Relationship between Theory and Experiment 4.1 Introduction 4.2 Dynamic Light Scattering: Probe of Density Fluctuation at Long Length Scales 4.3 Magnetic Resonance Experiments: Probe of Single Particle Dynamics 4.4 Kerr Relaxation 4.5 Dielectric Relaxation 4.6 Fluorescence Depolarization 4.7 Solvation Dynamics (Time Dependent Fluorescence Stokes Shift) 4.8 Neutron Scattering: Coherent and Incoherent 4.9 Raman Lineshape Measurements 4.10 Coherent Anti-Stokes Raman Scattering (CARS) 4.11 Echo Techniques 4.12 Ultrafast Chemical Reactions 4.13 Fluorescence Quenching 4.14 Two-dimensional Infrared (2D IR) Spectroscopy 4.15 Single Molecule Spectroscopy Chapter 5. Orientational and Dielectric Relaxation 5.1 Introduction 5.2 Equilibrium and Time-Dependent Orientational Correlation Functions 5.3 Relationship with Experimental Observables 5.4 Molecular Hydrodynamic Description of Orientational Motion 5.4.1 The Equations of Motion 5.4.2 Limiting Situations 5.5 Markovian Theory of Collective Orientational Relaxation: Berne Treatment 5.5.1 Generalized Smoluchowski Equation Description 5.5.2 Solution by Spherical Harmonic Expansion 5.5.3 Relaxation of Longitudinal and Transverse Components 5.5.4 Molecular Theory of Dielectric Relaxation 5.5.5 Hidden Role of Translational Motion in Orientational Relaxation 5.5.6 Orientational de Gennes Narrowing at Intermediate Wave Numbers 5.5.7 Reduction to the Continuum Limit 5.6 Memory Effects in Orientational Relaxation 5.7 Relationship between Macroscopic and Microscopic Orientational Relaxations 5.8 The Special Case of Orientational Relaxation of Water Chapter 6. Solvation Dynamics in Dipolar Liquids 6.1 Introduction 6.2 Physical Concepts and Measurement 6.2.1 Measuring Ultrafast, Sub-100 fs Decay 6.3 Phenomenological Theories: Continuum Model Descriptions 6.3.1 Homogeneous Dielectric Models 6.3.2 Inhomogeneous Dielectric Models 6.3.3 Dynamic Exchange Model 6.4 Experimental Results: A Chronological Overview 6.4.1 Discovery of Multi-exponential Solvation Dynamics: Phase-I (1980-1990) 6.4.2 Discovery of Sub-ps Ultrafast Solvation Dynamics: Phase-II (1990-2000) 6.4.3 Solvation Dynamics in Complex Systems: Phase III (2000 - ) 6.5 Microscopic Theories 6.5.1 Molecular Hydrodynamics Description 6.5.2 Polarization and Dielectric Relaxation of Pure Liquid 6.5.2.1 Effects of Translational Diffusion in Solvation Dynamics 6.6 Simple Idealized Models 6.6.1 Overdamped Solvation: Brownian Dipolar Lattice 6.6.2 Underdamped Solvation: Stockmayer Liquid 6.7 Solvation Dynamics in Water, Acetonitrile and Methanol Revisited 6.7.1 The Sub 100 fs Ultrafast Component: Microscopic Origin 6.8 Effects of Solvation on Chemical Processes in the Solution Phase 6.8.1 Limiting Ionic Conductivity of Electrolyte Solutions: Control of a Slow Phenomenon by Ultrafast Dynamics 6.8.2 Effects of Ultrafast Solvation in Electron Transfer Reactions 6.8.3 Non-equilibrium Solvation Effects in Chemical Reaction 6.8.3.1 Strong Solvent Forces 6.8.3.2 Weak Solvent Forces 6.9 Solvation Dynamics in Several Related Systems 6.9.1 Solvation in Aqueous Electrolyte Solutions 6.9.2 Dynamics of Electron Solvation 6.9.3 Solvation Dynamics in Super-Critical Fluids 6.9.4 Nonpolar Solvation Dynamics 6.10 Computer Simulation Studies: Simple and Complex Systems Chapter 7. Activated Barrier Crossing Dynamics in Liquids 7.1 Introduction 7.2 Microscopic Aspects 7.2.1 Stochastic Models: Understanding from Eigenvalue Analysis 7.2.2 Validity of a Rate Law Description: Role of Macroscopic Fluctuations 7.2.3 Time Correlation Function Approach: Separation of Transient Behavior from Rate Law 7.3 Transition State Theory 7.4 Frictional Effects on Barrier Crossing Rate in Solution: Kramers' Theory 7.4.1 Low Friction Limit 7.4.2 Limitations of Kramers' Theory 7.4.3 Comparison of Kramers' Theory with Experiments 7.4.4 Comparison of Kramers' Theory with Computer Simulations 7.5 Memory Effects in Chemical Reactions: Grote-Hynes Generalization of Kramers' Theory 7.5.1 Frequency Dependence of Friction: General Aspects 7.5.1.1 Frequency Dependent Friction from Hydrodynamics 7.5.1.2 Frequency Dependent Friction from Mode Coupling Theory 7.5.2 Comparison of Grote-Hynes Theory with Experiments and Computer Simulations 7.6 Variational Transition State Theory 7.7 Multidimensional Reaction Surface 7.7.1 Multidimensional Kramers' Theory 7.8 Transition Path Sampling 7.9 Quantum Transition State Theory Appendix Chapter 8. Barrierless Reactions in Solutions 8.1 Introduction 8.2 Standard Models of Barrierless Reactions 8.2.1 Exactly Solvable Models for Photochemical Reactions 8.2.1.1 Oster-Nishijima Model 8.2.1.2 Staircase Model 8.2.1.3 Pinhole Sink Model 8.2.2 Approximate Solutions for Realistic Models 8.2.2.1 Delta Function Sink 8.2.2.2 Gaussian Sink 8.3 Inertial Effects in Barrierless Reactions: Viscosity Turnover of Rate 8.4 Memory Effects in Barrierless Reactions 8.5 Main Features of Barrierless Chemical Reactions 8.5.1 Excitation Wavelength Dependence 8.5.2 Negative Activation Energy 8.6 Multidimensional Potential Energy Surface 8.7 Analysis of Experimental Results 8.7.1 Photoisomerization and Ground State Potential Energy Surface 8.7.2 Decay Dynamics of Rhodopsin and Isorhodopsin 8.7.3 Conflicting Crystal Violet Isomerization Mechanism Chapter 9. Dynamical disorder, Geometric Bottlenecks and Diffusion Controlled Bimolecular Reactions 9.1 Introduction 9.2 Passage through Geometric Bottlenecks 9.2.1 Diffusion in a Two Dimensional Periodic Channel 9.2.2 Diffusion in a Random Lorentz Gas 9.3 Dynamical Disorder 9.4 Diffusion over a Rugged Energy Landscape 9.5 Diffusion Controlled Bimolecular Reactions Chapter 10. Electron Transfer Reactions 10.1 Introduction 10.2 Classification of Electron Transfer Reactions 10.2.1 Classification of Electron Transfer Reactions Based on Ligand Participation 10.2.2 Classification Based on Interactions between Reactant and Product Potential Energy Surfaces 10.3 Marcus Theory 10.3.1 Reaction Coordinate 10.3.2 Free Energy Surfaces: Force Constant of Polarization Fluctuation 10.3.3 Derivation of The Electron Transfer Reaction Rate 10.3.4 Experimental Verification Of Marcus Theory 10.4 Dynamical Solvent Effects on Electron Transfer Reactions (One Dimensional Descriptions) 10.5 Role of Vibrational Modes in Weakening Solvent Dependence 10.5.1 Role of Classical Intramolecular Vibrational Modes: Sumi-Marcus Theory 10.5.2 Role of High-Frequency Vibration Modes 10.5.3 Hybrid Model of Electron Transfer Reactions: Crossover from Solvent to Vibrational Control 10.6 Theoretical Formulation of Multi-Dimensional Electron Transfer 10.7 Effects of Ultrafast Solvation on Electron Transfer Reactions 10.7.1 Absence of Significant Dynamic Solvent Effects on ETR in Water, Acetonitrile & Methanol Appendix Chapter 11. Forster Resonance Energy Transfer 11.1 Introduction 11.2 A Brief Historical Perspective 11.3 Derivation of Forster Expression 11.3.1 Emission (or, Fluorescence) Spectrum 11.3.2 Absorption Spectrum 11.3.3 The Final Expression of Forster 11.4 Applications of Forster Theory in Chemistry, Biology and Material Science 11.4.1 FRET Based Glucose Sensor 11.4.2 FRET and Macromolecular Dynamics 11.4.3 FRET and Single Molecule Spectroscopy 11.4.4 FRET and Conjugated Polymers 11.5 Beyond Forster Formalism 11.5.1 Orientation Factor 11.5.2 Point Dipole Approximation 11.5.3 Optically Dark States Chapter 12. Vibrational Energy Relaxation 12.1 Introduction 12.2 Isolated Binary Collision (IBC) Model 12.3 Landau-Teller Expression: The Classical Limit 12.4 Weak Coupling Model: Time Correlation Function Representation of Transition Probability 12.5 Vibrational Relaxation at High Frequency: Quantum Effects 12.6 Experimental Studies of Vibrational Energy Relaxation 12.7 Computer Simulation Studies of Vibrational Energy Relaxation 12.7.1 Vibrational Energy Relaxation of Water 12.7.2 Vibrational Energy Relaxation in Liquid Oxygen and Nitrogen 12.8 Interference Effects on Vibrational Energy Relaxation on a Three Level Systems: Breakdown of the Rate Equation Description 12.9 Vibrational Life Time Dynamics in Supercritical Fluids Chapter 13. Vibrational Phase Relaxation 13.1 Introduction 13.2 Kubo-Oxtoby Theory of Vibrational Lineshapes 13.3 Homogeneous vs. Inhomogeneous Linewidths 13.4 Relative Role of Attractive and Repulsive Forces 13.5 Vibration-Rotation Coupling 13.6 Experimental Results of Vibrational Phase Relaxation 13.6.1 Semi-Quantitative Aspects of Dephasing Rates in Solution 13.6.2 Sub-Quadratic Quantum Number Dependence 13.7 Vibrational Dephasing Near Gas-Liquid Critical Point 13.8 Multidimensional IR Spectroscopy Chapter 14. Epilogue

Enhancing dissociative chemisorption of H2O on Cu(111) via vibrational excitation

Abstract: The dissociative chemisorption of water is an important step in many heterogeneous catalytic processes. Here, the mode selectivity of this process was examined quantum mechanically on a realistic potential energy surface determined by fitting planewave density functional calculations spanning a large configuration space. The quantum dynamics of the surface reaction were characterized by a six-dimensional model including all important internal coordinates of H2O and its distance to the surface. It was found that excitations in all three vibrational modes are capable of enhancing reactivity more effectively than increasing translational energy, consistent with the “late” transition state in the reaction path.

Mode selectivity for a "central" barrier reaction: Eight-dimensional quantum studies of the O(3P) + CH4 → OH + CH3 reaction on an ab initio potential energy surface

Abstract: The dynamics of a combustion reaction, namely, O(P-3) + CH4 -> OH + CH3, is investigated with an eight-dimensional quantum model that includes representatives of all vibrational modes of CH4 and with a full-dimensional quasi-classical trajectory (QCT) method. The calculated excitation functions for the ground vibrational state CH4 agree well with experiment. Both quantum and QCT results suggest that excitation of the stretching modes of CH4 enhances the reaction, while the bending and umbrella modes have a smaller impact on reactivity, again consistent with experimental findings. However, none of the vibrational excitations has comparable efficiency in promoting the reaction as translational energy.

Experimental and theoretical investigations of energy transfer and hydrogen-bond breaking in the water dimer

Abstract: The hydrogen bonding in water is dominated by pairwise dimer interactions, and the predissociation of the water dimer following vibrational excitation is reported here. Velocity map imaging was used for an experimental determination of the dissociation energy (D0) of (D2O)2. The value obtained, 1244 ± 10 cm–1 (14.88 ± 0.12 kJ/mol), is in excellent agreement with the calculated value of 1244 ± 5 cm–1 (14.88 ± 0.06 kJ/mol). This agreement between theory and experiment is as good as the one obtained recently for (H2O)2. In addition, pair-correlated water fragment rovibrational state distributions following vibrational predissociation of (H2O)2 and (D2O)2 were obtained upon excitation of the hydrogen-bonded OH and OD stretch fundamentals, respectively. Quasi-classical trajectory calculations, using an accurate full-dimensional potential energy surface, are in accord with and help to elucidate experiment. Experiment and theory find predominant excitation of the fragment bending mode upon hydrogen bond breaking...

Calibration-quality adiabatic potential energy surfaces for H-3(+) and its isotopologues

Abstract: Calibration-quality ab initio adiabatic potential energy surfaces (PES) have been determined for all isotopologues of the molecular ion H3+. The underlying Born–Oppenheimer electronic structure computations used optimized explicitly correlated shifted Gaussian functions. The surfaces include diagonal Born–Oppenheimer corrections computed from the accurate electronic wave functions. A fit to the 41 655 ab initio points is presented which gives a standard deviation better than 0.1 cm−1 when restricted to the points up to 6000 cm−1 above the first dissociation asymptote. Nuclear motion calculations utilizing this PES, called GLH3P, and an exact kinetic energy operator given in orthogonal internal coordinates are presented. The ro-vibrational transition frequencies for H3+, H2D+, and HD 2+ are compared with high resolution measurements. The most sophisticated and complete procedure employed to compute ro-vibrational energy levels, which makes explicit allowance for the inclusion of non-adiabatic effects, reproduces all the known ro-vibrational levels of the H3+ isotopologues considered to better than 0.2 cm−1. This represents a significant (order-of-magnitude) improvement compared to previous studies of transitions in the visible. Careful treatment of linear geometries is important for high frequency transitions and leads to new assignments for some of the previously observed lines. Prospects for further investigations of non-adiabatic effects in the H3+ isotopologues are discussed. In short, the paper presents (a) an extremely accurate global potential energy surface of H3+ resulting from high accuracy ab initio computations and global fit, (b) very accurate nuclear motion calculations of all available experimental line data up to 16 000 cm−1, and (c) results suggest that we can predict accurately the lines of H3+ towards dissociation and thus facilitate their experimental observation.

Reaction of HO with CO: Tunneling Is Indeed Important.

Abstract: The potential energy surface and chemical kinetics for the reaction of HO with CO, which is an important process in both combustion and atmospheric chemistry, were computed using high-level ab initio quantum chemistry in conjunction with semiclassical transition state theory under the limiting cases of high and zero pressure. The reaction rate constants calculated from first principles agree extremely well with all available experimental data, which range in temperature over a domain that covers both combustion and terrestrial atmospheric chemistry. The role of quantum tunneling is confirmed to be extremely important, which supports recent work by Continetti and collaborators regarding the loss of hydrogen atoms from vibrationally excited states of HOCO. A sensitivity analysis has been carried out and serves as the basis for a plausible estimate of uncertainty in the calculations.

Complete nuclear motion Hamiltonian in the irreducible normal mode tensor operator formalism for the methane molecule

Abstract: A rovibrational model based on the normal-mode complete nuclear Hamiltonian is applied to methane using our recent potential energy surface [A. V. Nikitin, M. Rey, and Vl. G. Tyuterev, Chem. Phys. Lett. 501, 179 (2011)]. The kinetic energy operator and the potential energy function are expanded in power series to which a new truncation-reduction technique is applied. The vibration-rotation Hamiltonian is transformed systematically to a full symmetrized form using irreducible tensor operators. Each term of the Hamiltonian expansion can be thus cast in the tensor form whatever the order of the development. This allows to take full advantage of the symmetry properties for doubly and triply degenerate vibrations and vibration-rotation states. We apply this model to variational computations of energy levels for (12)CH(4), (13)CH(4), and (12)CD(4).

Dynamics of the O(3P) + CHD3(vCH = 0,1) reactions on an accurate ab initio potential energy surface

Abstract: Recent experimental and theoretical studies on the dynamics of the reactions of methane with F and Cl atoms have modified our understanding of mode-selective chemical reactivity. The O + methane reaction is also an important candidate to extend our knowledge on the rules of reactivity. Here, we report a unique full-dimensional ab initio potential energy surface for the O((3)P) + methane reaction, which opens the door for accurate dynamics calculations using this surface. Quasiclassical trajectory calculations of the angular and vibrational distributions for the ground state and CH stretching excited O + CHD(3)(v(1) = 0,1) → OH + CD(3) reactions are in excellent agreement with the experiment. Our theory confirms what was proposed experimentally: The mechanistic origin of the vibrational enhancement is that the CH-stretching excitation enlarges the reactive cone of acceptance.

Decoding the energy landscape: extracting structure, dynamics and thermodynamics.

Abstract: Describing a potential energy surface in terms of local minima and the transition states that connect them provides a conceptual and computational framework for understanding and predicting observable properties. Visualizing the potential energy landscape using disconnectivity graphs supplies a graphical connection between different structure-seeking systems, which can relax efficiently to a particular morphology. Landscapes involving competing morphologies support multiple potential energy funnels, which may exhibit characteristic heat capacity features and relaxation time scales. These connections between the organization of the potential energy landscape and structure, dynamics and thermodynamics are common to all the examples presented, ranging from atomic and molecular clusters to biomolecules and soft and condensed matter. Further connections between motifs in the energy landscape and the interparticle forces can be developed using symmetry considerations and results from catastrophe theory.

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Cl(2P, 2P3/2) + CH4 → HCl + CH3 and H + CH3Cl reactions

Abstract: We report a high-quality, ab initio, full-dimensional global potential energy surface (PES) for the Cl(2P, 2P3/2) + CH4 reaction, which describes both the abstraction (HCl + CH3) and substitution (H + CH3Cl) channels. The analytical PES is a least-squares fit, using a basis of permutationally invariant polynomials, to roughly 16 000 ab initio energy points, obtained by an efficient composite method, including counterpoise and spin-orbit corrections for the entrance channel. This composite method is shown to provide accuracy almost equal to all-electron CCSD(T)/aug-cc-pCVQZ results, but at much lower computational cost. Details of the PES, as well as additional high-level benchmark characterization of structures and energetics are reported. The PES has classical barrier heights of 2650 and 15 060 cm−1 (relative to Cl(2P3/2) + CH4(eq)), respectively, for the abstraction and substitution reactions, in good agreement with the corresponding new computed benchmark values, 2670 and 14 720 cm−1. The PES also accu...

Force reversed method for locating transition states

Abstract: To identify the transition state accurately and efficiently on a high-dimensional potential energy surface is one of the most important topics in kinetic studies on chemical reactions. We present here an algorithm to search the transi- tion state by so-called force reversed method, which only requires a rough reaction direction instead of knowing the initial state and final state. Compared to the nudged elastic band method and the dimer method that require multiple images, the present algorithm with only single image required saves significantly the computational cost. The algorithm was implemented in the first-principle periodic total energy cal- culation package and applied successfully to several prototype surface processes such as the adsorbate diffusion and disso- ciation on metal surfaces. The results indicate that the force reversed method is efficient, robust to identify the transition state of various surface processes.

Tunneling in Hydrogen-Transfer Isomerization of n-Alkyl Radicals

Abstract: The role of quantum tunneling in hydrogen shift in linear heptyl radicals is explored using multidimensional, small-curvature tunneling method for the transmission coefficients and a potential energy surface computed at the CBS-QB3 level of theory. Several one-dimensional approximations (Wigner, Skodje and Truhlar, and Eckart methods) were compared to the multidimensional results. The Eckart method was found to be sufficiently accurate in comparison to the small-curvature tunneling results for a wide range of temperature, but this agreement is in fact fortuitous and caused by error cancellations. High-pressure limit rate constants were calculated using the transition state theory with treatment of hindered rotations and Eckart transmission coefficients for all hydrogen-transfer isomerizations in n-pentyl to n-octyl radicals. Rate constants are found in good agreement with experimental kinetic data available for n-pentyl and n-hexyl radicals. In the case of n-heptyl and n-octyl, our calculated rates agree well with limited experimentally derived data. Several conclusions made in the experimental studies of Tsang et al. (Tsang, W.; McGivern, W. S.; Manion, J. A. Proc. Combust. Inst. 2009, 32, 131-138) are confirmed theoretically: older low-temperature experimental data, characterized by small pre-exponential factors and activation energies, can be reconciled with high-temperature data by taking into account tunneling; at low temperatures, transmission coefficients are substantially larger for H-atom transfers through a five-membered ring transition state than those with six-membered rings; channels with transition ring structures involving greater than 8 atoms can be neglected because of entropic effects that inhibit such transitions. The set of computational kinetic rates were used to derive a general rate rule that explicitly accounts for tunneling. The rate rule is shown to reproduce closely the theoretical rate constants.

Fine and hyperfine excitation of C2H by collisions with He at low temperature

Abstract: Modelling of molecular emission from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. From a new, highly correlated, two-dimensional potential energy surface, rotational excitation of the C2H(X2Σ+) molecule by collision with He is investigated. State-to-state collisional excitation cross-sections between the 25 first fine structure levels of C2H are calculated for energies up to 800 cm−1 which yields after thermal averaging rate coefficients up to T= 100 K. The exact spin splitting of the energy levels is taken into account. The recoupling technique introduced by Alexander & Dagdigian allows us to deduce the corresponding temperature-dependent hyperfine state-to-state rate coefficients. Propensity rules are discussed.

An ab initio based full-dimensional global potential energy surface for FH2O(X2A') and dynamics for the F + H2O → HF + HO reaction.

Abstract: A global potential energy surface (PES) for the ground electronic state of FH2O is constructed based on more than 30 000 ab initio points at the multi-reference configuration interaction level. The PES features a pre-reaction van der Waals well and two post-reaction hydrogen-bonded complexes, as well as a “reactant-like” transition state with a classical barrier of 3.8 kcal/mol. The adiabatic F + H2O → HF + OH reaction dynamics on this PES was investigated using a standard quasi-classical trajectory method. In agreement with experiment, the HF product contains significant vibrational excitation with limited rotational excitation, while the OH product is internally cold, reflecting its spectator role in the reaction. The products are primarily scattered in the backward direction, consistent with a direct abstraction mechanism.

Multipath variational transition state theory: rate constant of the 1,4-hydrogen shift isomerization of the 2-cyclohexylethyl radical.

Abstract: We propose a new formulation of variational transition state theory called multipath variational transition state theory (MP-VTST). We employ this new formulation to calculate the forward and reverse thermal rate constant of the 1,4-hydrogen shift isomerization of the 2-cyclohexylethyl radical in the gas phase. First, we find and optimize all the local-minimum-energy structures of the reaction, product, and transition state. Then, for the lowest-energy transition state structures, we calculate the reaction path by using multiconfiguration Shepard interpolation (MSCI) method to represent the potential energy surface, and, from this representation, we also calculate the ground-state vibrationally adiabatic potential energy curve, the reaction-path curvature vector, and the generalized free energy of activation profile. With this information, the path-averaged generalized transmission coefficients are evaluated. Then, thermal rate constant containing the multiple-structure anharmonicity and torsional anharmonicity effects is calculated using multistructural transition state theory (MS-TST). The final MP-VTST thermal rate constant is obtained by multiplying k(MS-T)(MS-TST) by . In these calculations, the M06 density functional is utilized to compute the energy, gradient, and Hessian at the Shepard points, and the M06-2X density functional is used to obtain the structures (conformers) of the reactant, product, and the saddle point for computing the multistructural anharmonicity factors.

Contrasting the excited state reaction pathways of phenol and para-methylthiophenol in the gas and liquid phases

Abstract: To explore how the solvent influences primary aspects of bond breaking, the gas and solution phase photochemistries of phenol and of para-methylthiophenol are directly compared using, respectively, H (Rydberg) atom photofragment translation spectroscopy and femtosecond transient absorption spectroscopy. Approaches are demonstrated that allow explicit comparisons of the nascent product energy disposals and dissociation mechanisms in the two phases. It is found, at least for the case of the weakly perturbing cyclohexane environment, that most aspects of the primary reaction dynamics of the isolated molecule are reproduced in solution. Specifically, in the gas phase, both molecules can undergo fast X–H (XO, S) bond dissociation upon excitation with short wavelengths (193 < λpump < 216 nm), following population of the dissociative S2 (11πσ*) state. Product electronic branching, vibrational and translational energy disposals are determined. Photolysis of phenol and para-methylthiophenol in solution at 200 nm results in formation of vibrationally excited radicals on a timescale shorter than 200 fs. Excitation of para-methylthiophenol at 267 nm reaches close to the S1 (11ππ*)/S2 (11πσ*) conical intersection (CI): ultrafast dissociation is observed in both the isolated and solution systems—again indicating direct dissociation on the S2 potential energy surface. Comparing results for this precursor at different excitation energies, the extent of geminate recombination and the derived H-atom ejection lengths in the condensed phase photolyses are in qualitative agreement with the translational energy release measured in the gas phase studies. Conversely, excitation of phenol at 267 nm prepares the system in its S1 state at an energy well below its S1/S2 CI; the slow O–H bond fission inferred in the gas phase experiments is observed directly in the time-resolved studies in cyclohexane solution via the appearance of phenoxyl radical absorption after ∼1 ns, with only S1 excited state absorption discernible at earlier delay times. The slow O–H bond fission in solution provides additional evidence for a tunnelling dissociation mechanism, where the H atom tunnels beneath the lower diabats of the S2/S1 CI. Finally, the photodissociation of phenol clusters in solution is considered, where evidence is presented that the O–H dissociation coordinate is impeded in H-bonded dimers.

Computational study on the working mechanism of a stilbene light-driven molecular rotary motor: sloped minimal energy path and unidirectional nonadiabatic photoisomerization.

Abstract: The working mechanism of a geometrically overcrowded, chiral stilbene light-driven molecular rotary motor [(2R,2R)-2,2′,7,7′-tetramethyl-1,1′-bis(indanylidene), 3] has been investigated by a potential energy surface (PES) study. The reaction paths of the two photoinitiated cis–trans (or E/Z) isomerization processes, namely, (P,P)-stable-cis→(M,M)-unstable-trans-3 and (P,P)-stable-trans→(M,M)-unstable-cis-3, have been explored at the CASPT2//CASSCF level of theory. The minimal energy reaction paths (MEPs) of these two processes are nearly parallel on the PESs, separated by a ridge of high inversion barrier. The MEPs have a remarkably steep slope, which drives C═C bond rotation unidirectionally. The asymmetric bias on the excited-state MEPs is caused by the substituents on the “fjord” region as well as by the phenyl moieties. The overall photoisomerization reaction can be described as a three-state (1B→2A→1A) multimode mechanism: The molecule excited to the 1B state first crosses one of the sloped 1B/2A sea...