Showing papers on "Potential energy surface published in 2009"

Chemically accurate simulation of a prototypical surface reaction: H2 dissociation on Cu(111).

Abstract: Methods for accurately computing the interaction of molecules with metal surfaces are critical to understanding and thereby improving heterogeneous catalysis. We introduce an implementation of the specific reaction parameter (SRP) approach to density functional theory (DFT) that carries the method forward from a semiquantitative to a quantitative description of the molecule-surface interaction. Dynamics calculations on reactive scattering of hydrogen from the copper (111) surface using an SRP-DFT potential energy surface reproduce data on the dissociative adsorption probability as a function of incidence energy and reactant state and data on rotationally inelastic scattering with chemical accuracy (within ~4.2 kilojoules per mole).

Modeling ethanol decomposition on transition metals: a combined application of scaling and Brønsted-Evans-Polanyi relations.

Abstract: Applying density functional theory (DFT) calculations to the rational design of catalysts for complex reaction networks has been an ongoing challenge, primarily because of the high computational cost of these calculations. Certain correlations can be used to reduce the number and complexity of DFT calculations necessary to describe trends in activity and selectivity across metal and alloy surfaces, thus extending the reach of DFT to more complex systems. In this work, the well-known family of Bronsted-Evans-Polanyi (BEP) correlations, connecting minima with maxima in the potential energy surface of elementary steps, in tandem with a scaling relation, connecting binding energies of complex adsorbates with those of simpler ones (e.g., C, O), is used to develop a potential-energy surface for ethanol decomposition on 10 transition metal surfaces. Using a simple kinetic model, the selectivity and activity on a subset of these surfaces are calculated. Experiments on supported catalysts verify that this simple model is reasonably accurate in describing reactivity trends across metals, suggesting that the combination of BEP and scaling relations may substantially reduce the cost of DFT calculations required for identifying reactivity descriptors of more complex reactions.

Electronic acceleration of atomic motions and disordering in bismuth

Abstract: The development of X-ray and electron diffraction methods with ultrahigh time resolution has made it possible to map directly, at the atomic level, structural changes in solids induced by laser excitation. This has resulted in unprecedented insights into the lattice dynamics of solids undergoing phase transitions. In aluminium, for example, femtosecond optical excitation hardly affects the potential energy surface of the lattice; instead, melting of the material is governed by the transfer of thermal energy between the excited electrons and the initially cold lattice. In semiconductors, in contrast, exciting approximately 10 per cent of the valence electrons results in non-thermal lattice collapse owing to the antibonding character of the conduction band. These different material responses raise the intriguing question of how Peierls-distorted systems such as bismuth will respond to strong excitations. The evolution of the atomic configuration of bismuth upon excitation of its A(1g) lattice mode, which involves damped oscillations of atoms along the direction of the Peierls distortion of the crystal, has been probed, but the actual melting of the material has not yet been investigated. Here we present a femtosecond electron diffraction study of the structural changes in crystalline bismuth as it undergoes laser-induced melting. We find that the dynamics of the phase transition depend strongly on the excitation intensity, with melting occurring within 190 fs (that is, within half a period of the unperturbed A(1g) lattice mode) at the highest excitation. We attribute the surprising speed of the melting process to laser-induced changes in the potential energy surface of the lattice, which result in strong acceleration of the atoms along the longitudinal direction of the lattice and efficient coupling of this motion to an unstable transverse vibrational mode. That is, the atomic motions in crystalline bismuth can be electronically accelerated so that the solid-to-liquid phase transition occurs on a sub-vibrational timescale.

First principles dynamics and minimum energy pathways for mechanochemical ring opening of cyclobutene.

Abstract: We use ab initio steered molecular dynamics to investigate the mechanically induced ring opening of cyclobutene. We show that the dynamical results can be considered in terms of a force-modified potential energy surface (FMPES). We show how the minimal energy paths for the two possible competing conrotatory and disrotatory ring-opening reactions are affected by external force. We also locate minimal energy pathways in the presence of applied external force and show that the reactant, product, and transition state geometries are altered by the application of external force. The largest effects are on the transition state geometries and barrier heights. Our results provide a framework for future investigations of the role of external force on chemical reactivity.

A variationally computed T = 300 K line list for NH3.

Abstract: Calculations are reported on the rotation-vibration energy levels of ammonia with associated transition intensities. A potential energy surface obtained from coupled cluster CCSD(T) calculations and subsequent fitting against experimental data is further refined by a slight adjustment of the equilibrium geometry, which leads to a significant improvement in the rotational energy level structure. A new accurate ab initio dipole moment surface is determined at the frozen core CCSD(T)/aug-cc-pVQZ level. The calculation of an extensive ammonia line list necessitates a number of algorithmic improvements in the program TROVE that is used for the variational treatment of nuclear motion. Rotation-vibration transitions for (NH3)-N-14 involving states with energies up to 12000 cm(-1) and rotational quantum number J = 20 are calculated. This gives 3.25 million transitions between 184400 energy levels. Comparisons show good agreement with data in the HITRAN database but suggest that HITRAN is missing significant ammonia absorptions, particularly in the near-infrared.

Comparison of quantum dynamics and quantum transition state theory estimates of the H + CH4 reaction rate.

Abstract: Thermal rate constants are calculated for the H + CH(4) --> CH(3) + H(2) reaction employing the potential energy surface of Espinosa-Garcia (Espinosa-Garcia, J. J. Chem. Phys. 2002, 116, 10664). Two theoretical approaches are used. First, we employ the multiconfigurational time-dependent Hartree method combined with flux correlation functions. In this way rate constants in the range 225-400 K are obtained and compared with previous results using the same theoretical method but the potential energy surface of Wu et al. (Wu, T.; Werner, H.-J.; Manthe, U. Science 2004, 306, 2227). It is found that the Espinosa-Garcia surface results in larger rate constants. Second, a harmonic quantum transition state theory (HQTST) implementation of instanton theory is used to obtain rate constants in a temperature interval from 20 K up to the crossover temperature at 296 K. The HQTST estimates are larger than MCTDH ones by a factor of about three in the common temperature range. Comparison is also made with various tunneling corrections to transition state theory and quantum instanton theory.

Nonadiabatic dynamics at metal surfaces: Independent-electron surface hopping

Abstract: Recent experiments have shown convincing evidence for nonadiabatic energy transfer from adsorbate degrees of freedom to surface electrons during the interaction of molecules with metal surfaces. In this paper, we propose an independent-electron surface hopping algorithm for the simulation of nonadiabatic gas-surface dynamics. The transfer of energy to electron-hole pair excitations of the metal is successfully captured by hops between electronic adiabats. The algorithm is able to account for the creation of multiple electron-hole pairs in the metal due to nonadiabatic transitions. Detailed simulations of the vibrational relaxation of nitric oxide on a gold surface, employing a multistate potential energy surface fit to density functional theory calculations, confirm that our algorithm can capture the underlying physics of the inelastic scattering process.

The reaction between propene and hydroxyl

Abstract: Stationary points on the C3H7O potential energy surface relevant to the title reaction are calculated employing RQCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) quantum chemical calculations. Rate coefficients at 50–3000 K temperature and from zero to infinite pressure are calculated using an RRKM-based multiwell master equation. Due to the topography of the entrance channel an effective two-transition-state model is used to calculate accurate association rate coefficients. Our calculations are in excellent agreement with the available experimental data. We predict ∼5% vinyl alcohol branching above 1000 K, the allyl radical formation being the main channel at high temperatures.

A reactant-coordinate-based time-dependent wave packet method for triatomic state-to-state reaction dynamics: application to the H + O2 reaction.

Abstract: A new time-dependent wavepacket method is developed to study the A + BC -> AB + C, AC + B reaction at the state-to-state level. The method only requires propagation of the wavepacket in reactant Jacobi coordinates by extracting S-matrix information on a dividing surface right before the absorption potential in the product region. It has particular advantages for reactions with deep wells and long-range attractive interactions in the product channels in which the wavepacket in the product channels can only be absorbed sufficiently far away from the interaction potential. Demonstration made on the benchmark H + H-2 reaction shows that the method is rather efficient in dealing with a direct reaction at high collision energy. The method is applied to study the very challenging H + O-2 (v(0) = 0, j(0) = 0, 1) reaction, with state-to-state differential cross sections obtained for the first time for collision energies up to 1.1 eV. The calculations not only show the power and accuracy of the new approach in dealing with complex-forming reactions but also shed light on the dynamics of the H + O-2 reaction.

Accurate ab initio potential energy surface, dynamics, and thermochemistry of the F+CH4→HF+CH3 reaction

Abstract: An accurate full-dimensional global potential energy surface (PES) for the F+CH4→HF+CH3 reaction has been developed based on 19 384 UCCSD(T)/aug-cc-pVTZ quality ab initio energy points obtained by an efficient composite method employing explicit UCCSD(T)/aug-cc-pVDZ and UMP2/aug-cc-pVXZ [X=D,T] computations. The PES contains a first-order saddle point, (CH4- -F)SP, separating reactants from products, and also minima describing the van der Waals complexes, (CH4- - -F)vdW and (CH3- - -HF)vdW, in the entrance and exit channels, respectively. The structures of these stationary points, as well as those of the reactants and products have been computed and the corresponding energies have been determined using basis set extrapolation techniques considering (a) electron correlation beyond the CCSD(T) level, (b) effects of the scalar relativity and the spin-orbit couplings, (c) diagonal Born–Oppenheimer corrections (DBOC), and (d) zero-point vibrational energies and thermal correction to the enthalpy at 298 K. The ...

Rotational excitation of ortho-H$_{2}$O by para-H$_{2}$ (j$_{2}$ = 0, 2, 4, 6, 8) at high temperature

Abstract: Aims. Our objective is to obtain the best possible set of rotational (de)-excitation state-to-state and effective rate coefficients for temperatures up to 1500 K. We present state-to-state rate coefficients among the 45 lowest levels of o-H2O with H2(j2 = 0) and Δj2 = 0, +2, as well as with H2(j2 = 2) and Δj2 = 0, −2. In addition and only for the 10 lowest energy levels of o-H2O, we provide state-to-state rate coefficients involving j2 = 4 with Δj2 = 0, − 2a ndj2 = 2 with Δj2 =+ 2. We give estimates of effective rate coefficients for j2 = 6, 8. Methods. Calculations are performed with the close coupling (CC) method over the whole energy range, using the same 5D potential energy surface (PES) as the one employed in our latest publication on water. We perform comparisons with coupled states (CS) calculations, with thermalized quasi-classical trajectory (QCT) calculations using the same PES and with previous quantum calculations obtained between T = 20 K and T = 140 K with a different PES. Results. We find that the CS approximation fares extremely badly even at high energy for j2 different from zero. Comparisons with thermalized QCT calculations show large factors at intermediate temperatures and factors from 1 to 3 at high temperature for the strongest rate coefficients. Finally we stress that scaled collisional rate coefficients obtained with He cannot be used in place of collisional rate coefficients with H2.

Potential Energy Surface of Methanol Decomposition on Cu(110)

Abstract: Combining the dimer saddle point searching method and periodic density functional theory calculations, the potential energy surface of methanol decomposition on Cu(110) has been mapped out. Each elementary step in the methanol decomposition reaction into CO and hydrogen occurs via one of three possible mechanisms: O−H, C−H, or C−O bond scission. Multiple reaction pathways for each bond scission have been identified in the present work. Reaction pathway calculations are started from an initial (reactant) state with methanol adsorbed in the most stable geometry on Cu(110). The saddle point and corresponding final state of each reaction or diffusion mechanism were determined without assuming the reaction mechanism. In this way, the reaction paths are determined without chemical intuition. The harmonic pre-exponential factor of each identified reaction is calculated from a normal-mode analysis of the stationary points. Then, using harmonic transition state theory, the rate constant of each identified reaction...

Structure and Stability of the Water−Graphite Complexes

Abstract: The interaction of the water molecule with benzene, polycyclic aromatic hydrocarbons, graphene, and graphite is investigated at the density-functional/coupled-cluster (DFT/CC) level of theory. The accuracy of the DFT/CC method is first demonstrated by a comparison of the various interaction energies on the potential energy surface of water−benzene, water−naphthalene, and water−anthracene complexes with the data calculated at the coupled-cluster level at the basis set limit. The potential energy surface of water−graphene and water−graphite is relatively flat with diffusion barriers of about 1 kJ/mol. The structure with both hydrogen atoms of water pointing toward the graphene plane (denoted as a circumflex structure) above the center of the six-member ring is the global minimum characterized with an electronic interaction energy of −13 and −15 kJ/mol for graphene and graphite, respectively. The OH···π complexes (with one OH pointing toward the surface and the other OH being oriented along the surface) are ...

Some improvements of the activation-relaxation technique method for finding transition pathways on potential energy surfaces

Abstract: The activation-relaxation technique nouveau is an eigenvector following method for systematic search of saddle points and transition pathways on a given potential energy surface. We propose a variation in this method aiming at improving the efficiency of the local convergence close to the saddle point. The efficiency of the method is demonstrated in the case of point defects in body centered cubic iron. We also prove the convergence and robustness of a simplified version of this new algorithm.

Combined crossed molecular beam and theoretical studies of the N(2D) + CH4 reaction and implications for atmospheric models of Titan.

Abstract: The dynamics of the H-displacement channel in the reaction N(2D) + CH4 has been investigated by the crossed molecular beam (CMB) technique with mass spectrometric detection and time-of-flight (TOF) analysis at five different collision energies (from 22.2 up to 65.1 kJ/mol). The CMB results have identified two distinct isomers as primary reaction products, methanimine and methylnitrene, the yield of which significantly varies with the total available energy. From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms, the product energy partitioning and the relative branching ratios of the competing reaction channels leading to the two isomers have been obtained. The interpretation of the scattering results is assisted by new ab initio electronic structure calculations of stationary points and product energetics for the CH4N ground state doublet potential energy surface. Differently from previous theoretical studies, both insertion and H-abstraction p...

Full dimensional (15-dimensional) quantum-dynamical simulation of the protonated water-dimer III: Mixed Jacobi-valence parametrization and benchmark results for the zero point energy, vibrationally excited states, and infrared spectrum.

Abstract: Quantum dynamical calculations are reported for the zero point energy, several low-lying vibrational states, and the infrared spectrum of the H(5)O(2)(+) cation. The calculations are performed by the multiconfiguration time-dependent Hartree (MCTDH) method. A new vector parametrization based on a mixed Jacobi-valence description of the system is presented. With this parametrization the potential energy surface coupling is reduced with respect to a full Jacobi description, providing a better convergence of the n-mode representation of the potential. However, new coupling terms appear in the kinetic energy operator. These terms are derived and discussed. A mode-combination scheme based on six combined coordinates is used, and the representation of the 15-dimensional potential in terms of a six-combined mode cluster expansion including up to some 7-dimensional grids is discussed. A statistical analysis of the accuracy of the n-mode representation of the potential at all orders is performed. Benchmark, fully converged results are reported for the zero point energy, which lie within the statistical uncertainty of the reference diffusion Monte Carlo result for this system. Some low-lying vibrationally excited eigenstates are computed by block improved relaxation, illustrating the applicability of the approach to large systems. Benchmark calculations of the linear infrared spectrum are provided, and convergence with increasing size of the time-dependent basis and as a function of the order of the n-mode representation is studied. The calculations presented here make use of recent developments in the parallel version of the MCTDH code, which are briefly discussed. We also show that the infrared spectrum can be computed, to a very good approximation, within D(2d) symmetry, instead of the G(16) symmetry used before, in which the complete rotation of one water molecule with respect to the other is allowed, thus simplifying the dynamical problem.

Quantum Tunnelling in Enzyme-Catalysed Reactions

Abstract: Introduction. Preface: Beyond the Historical Perspective on Hydrogen and Electron Transfers. Chapter 1: The Transition State Theory Description of Enzyme Catalysis for Classically Activated Reactions: Introduction Quantifying the Catalytic Activity of Enzymes Free Energy Analysis of Enzyme Catalysis Transition State Stabilisation or Ground State Destabilisation? Selective Stabilisation of Transition Structures by Enzymes Enzyme Flexibility and Dynamics. Chapter 2: Introduction to Quantum Behavior - A Primer: Introduction Classical Mechanics Quantum Mechanics Heisenberg Uncertainty Principle The Schr/dinger Equation Electronic Structure Calculations Born-Oppenheimer Approximation Hartree-Fock Theory Basis sets Zero-point Energy Density Functional Theory DFT Calculations of Free Energies of Activation of Enzyme Models DFT Calculations of Kinetic Isotope Effects Quantum Mechanics/Molecular Mechanics Methods Summary and Outlook. Chapter 3: Quantum Catalysis in Enzymes: Introduction Theory Variational Transition State Theory The Transmission Coefficient One-Dimensional Tunneling Multidimensional Tunneling Ensemble Averaging Examples Liver Alcohol Dehydrogenase Dihydrofolate Reductase Soybean-Lipoxygenase-1 and Methylmalonyl-CoA Mutase Other Systems and Perspectives Concluding Remarks. Chapter 4: Selected Theoretical Models and Computational Methods for Enzymatic Tunneling: Introduction Vibronically Nonadiabatic Reactions: Proton-coupled Electron Transfer Theory Application to Lipoxygenase Predominantly Adiabatic Reactions: Proton and Hydride Transfer Theory Application to Dihydrofolate Reductase Emerging Concepts About Enzyme Catalysis. Chapter 5: Kinetic Isotope Effects from Hybrid Classical and Quantum Path Integral Computations: Introduction Theoretical Background Path Integral Quantum Transition State Theory Centroid Path Integral Simulations Kinetic Isotope Effects Sequential Centroid Path Integral and Umbrella Sampling (PI/UM) The PI-FEP/UM Method Kleinert's Variational Perturbation (KP) Theory Potential Energy Surface Combined QM/MM Potentials The MOVB Potential Computational Details Illustrative Examples Proton Transfer between Viscosity Multiple Reactive Configurations and a Place for Single-Molecule Measurements. Chapter 10. Computational Simulations of Tunnelling Reactions in Enzymes Introduction Molecular Mechanical Methods Quantum Mechanical Methods Combined Quantum Mechanical/Molecular Mechanical Methods Improving Semiempirical QM Calculations Calculation of Potential Energy Surfaces and Free Energy Surfaces Simulation of the H-tunnelling Event Calculation of H-tunnelling Rates and Kinetic Isotope Effects Analysing Molecular Dynamics Trajectories A Case Study: Aromatic Amine Dehydrogenase (AADH) Preparation of the System Analysis of the H-tunnelling Step in AADH Analysis of the Role of Promoting Motions in Driving Tunnelling Comparison of Short-range Motions in AADH with Long Range Motions in Dihydrofolate Reductase Summary. Chapter 11. Tunneling Does Not Contribute Significantly to Enzyme Catalysis, But Studying Temperature Dependence of Isotope Effects is Useful Introduction Methods Simulating Temperature Dependence of KIEs in Enzymes Concluding Remarks. Chapter 12: The Use of X-Ray Crystallography to Study Enzymic H-Tunnelling Introduction X-Ray Crystallography: A Brief Overview Accuracy of X-Ray Diffraction Structures Dynamic Information from X-Ray Crystallography Examples of H-tunnelling Systems Studied by Crystallography Crystallographic Studies of AADH Catalytic Mechanism Crystallographic Studies of MR Conclusions. Chapter 13: The Strengths and Weaknesses of Model Reactions for the Assessment of Tunneling in Enzymic Reactions Model Reactions for Biochemical Processes Model Reactions Relevant to Enzymic Tunneling Isotope Effect Temperature Dependences and the Configurational-Search Framework (CSF) for their Interpretation The Traditionally Dependent Category The Underdependent Tunneling Category The Overdependent Tunneling Category Example 1. Hydride Transfer in a Thermophilic Alcohol Dehydrogenase The Kirby-Walwyn Intramolecular Model Reaction The Powell-Bruice Tunneling Model Reaction Enzymic Tunneling in Alcohol Dehydrogenases Model Reactions and the Catalytic Power of Alcohol Dehydrogenase Example 2. Hydrogen-atom Transfer in Methylmalonyl Coenzyme A Mutase (MCM) Non-enzymic Tunneling in the Finke Model Reactions for MCM Enzymic Tunneling in MCM Model Reactions and MCM Catalytic Power The Roles of Theory in the Comparison of Model and Enzymic Reactions Model Reactions, Enzymic Accelerations, and Quantum Tunneling. Chapter 14: Long-Distance Electron Tunneling in Proteins: Introduction Electronic Coupling and Tunneling Pathways Direct Method Avoided Crossing Application of Koopmans' Theorem Generalized Mulliken-Hush Method The Propagator Method Protein Pruning Tunneling Pathways The Method of Tunneling Currents General Relations Many-Electron Picture Calculation of Current Density. Hartree-Fock Approximation Interatomic Tunneling Currents Many-Electron Aspects One Tunneling Orbital (OTO) Approximation and Polarization Effects The Limitation of the SCF Description of Many-Electron Tunneling Correlation Effects. Polarization Cloud Dynamics. Beyond Hartree-Fock Methods Quantum Interference Effects. Quantized Vertices Electron Transfer or Hole Transfer? Exchange Effects Dynamical Aspects.Chapter 15. Proton-coupled Electron Transfer: The Engine that Drives Radical Transport and Catalysis in Biology Introduction PCET Model Systems Unidirectional PCET Networks Bidirectional PCET Networks PCET Biocatalysis PCET in Enzymes: A Study of Ribonucleotide Reductase The PCET Pathway in RNR PCET in the ?2 Subunit of RNR PCET in ?2 Subunit of RNR: PhotoRNRs A Model for PCET in RNR Concluding Remarks.

Energetics and dynamics of H(2) adsorbed in a nanoporous material at low temperature.

Abstract: Molecular hydrogen adsorption in a nanoporous metal-organic framework structure (MOF-74) is studied via van der Waals density-functional calculations. The primary and secondary binding sites for H(2) are confirmed. The low-lying rotational and translational energy levels are calculated, based on the orientation and position dependent potential energy surface at the two binding sites. A consistent picture is obtained between the calculated rotational-translational transitions for different H(2) loadings and those measured by inelastic neutron scattering exciting the singlet to triplet (para to ortho) transition in H(2). The H(2) binding energy after zero-point energy correction due to the rotational and translational motions is predicted to be approximately 100 meV in good agreement with the experimental value of approximately 90 meV.

Symmetry-Adapted Perturbation Theory Applied to Endohedral Fullerene Complexes: A Stability Study of H2@C60 and 2H2@C60.

Abstract: Because of difficulties in a description of host-guest interactions, various theoretical methods predict different numbers of hydrogen molecules which can be inserted into the C60 cavity, ranging from one to more than 20. On the other hand, only one H2 molecule inside the C60 fullerene has been detected experimentally. Moreover, a recently synthesized H2@C70 complex prevails in the mixture formed with 2H2@C70. To get a deeper insight into the stability of the complexes created from C60 and hydrogen molecules, we carried out highly accurate calculations for complexes of one or two hydrogen molecules with fullerene applying symmetry-adapted perturbation theory (SAPT) and a large TZVPP basis set for selected points on the potential energy surfaces of H2@C60 and 2H2@C60. The electron correlation in the host and guests has been treated by density functional theory. Our calculations yield the stability of the recently synthesized H2@C60 complex. In addition, for all tried positions of the H2 dimer inside the C60 cage, the 2H2@C60 complex has been characterized by a positive interaction energy corresponding to the instability of this species. Contrary to the conclusions of several theoretical studies, this finding, as well as model considerations and literature experimental data, indicates that only one hydrogen molecule can reside inside the C60 cage. The calculated energy components have been analyzed to identify the most important contributions to the interaction energy. Supermolecular interaction energies obtained with MP2, SCS-MP2, and DFT+Disp methods are also reported and compared to those of DFT-SAPT. The DFT-SAPT interaction energy has also been calculated for several points on the potential energy surface for a larger 2H2@C70 complex, confirming, in agreement with recent experimental findings, that this species is stable. The DFT-SAPT approach has been used for the first time to obtain interaction energies for van der Waals endohedral complexes, demonstrating that the method is capable of handling such difficult cases.

A simple model for the treatment of imaginary frequencies in chemical reaction rates and molecular liquids.

Abstract: A simple model is presented for treating local imaginary frequencies that are important in the study of quantum effects in chemical reactions and various dynamical processes in molecular liquids. It significantly extends the range of accuracy of conventional local harmonic approximations (LHAs) used in the linearized semiclassical initial value representation/classical Wigner approximation for real time correlation functions. The key idea is realizing that a local Gaussian approximation (LGA) for the momentum distribution (from the Wigner function involving the Boltzmann operator) can be a good approximation even when a LHA for the potential energy surface fails. The model is applied here to two examples where imaginary frequencies play a significant role: the chemical reaction rate for a linear model of the H+H2 reaction and an analogous asymmetric barrier—a case where the imaginary frequency of the barrier dominates the process—and for momentum autocorrelation functions in liquid para-hydrogen at two th...

Continuous and smooth potential energy surface for conductorlike screening solvation model using fixed points with variable areas.

Abstract: Rigorously continuous and smooth potential energy surfaces, as well as exact analytic gradients, are obtained for a conductorlike screening solvation model (CPCM, a variant of the general COSMO) with Hartree–Fock (RHF, ROHF, UHF, and MCSCF) and density functional theory (R-DFT, RO-DFT, and U-DFT) methods using a new tessellation scheme, fixed points with variable areas (FIXPVA). In FIXPVA, spheres centered at atoms are used to define the molecular cavity and surface. The surface of each sphere is divided into 60, 240, or 960 tesserae, which have positions fixed relative to the sphere center and areas scaled by switching functions of their distances to neighboring spheres. Analytic derivatives of the positions and areas of the surface tesserae with respect to atomic coordinates can be obtained and used to evaluate the solvation energy gradients. Due to the accurate analytic gradients and smooth potential energy surface, geometry optimization processes using these methods are stable and convergent.

Theoretical Study of Dynamics for the Abstraction Reaction H′ + HBr(v=0, j=0) → H′H + Br†

Abstract: Theoretical studies of the dynamics of the abstraction reaction, H′ + HBr (v=0,j=0) → H′H + Br, have been performed with quasiclassical trajectory method (QCT) on a new ab initio potential energy surface (Y. Kurosaki and T. Takayanagi, private communication). The calculated QCT cross sections are in good agreement with earlier quantum wave packet results over most of the collision energy range from 0.1 to 2.6 eV, and the state-resolved rotational distributions of the product H′H molecule are quantitatively consistent with the experimental results. Comparisons of the QCT-calculated rotational-state-resolved cross sections on different potential energy surfaces show that the characteristics of the potential energy surface in the region far away from the minimum energy path have a large influence on the title abstraction reaction dynamics, and the indirect reactions that do not follow the minimum energy path have little influence on the differential cross sections (DCS). The DCSs are mainly governed by the d...

Computed rate coefficients and product yields for c-C5H5 + CH3 --> products.

Abstract: Using quantum chemical methods, we have explored the region of the C6H8 potential energy surface that is relevant in predicting the rate coefficients of various wells and major product channels following the reaction between cyclopentadienyl radical and methyl radical, c-C5H5 + CH3. Variational transition state theory is used to calculate the high-pressure-limit rate coefficient for all of the barrierless reactions. RRKM theory and the master equation are used to calculate the pressure dependent rate coefficients for 12 reactions. The calculated results are compared with the limited experimental data available in the literature and the agreement between the two is quite good. All of the rate coefficients calculated in this work are tabulated and can be used in building detailed chemical kinetic models.

On the mechanism of decomposition of the benzyl radical

Abstract: The C7H7 potential energy surface was studied from first principles to determine the benzyl radical decomposition mechanism. The investigated high temperature reaction pathway involves 15 accessible energy wells connected by 25 transition states. The analysis of the potential energy surface, performed determining kinetic constants of each elementary reaction using conventional transition state theory, evidenced that the reaction mechanism has as rate determining step the isomerization of the 1,3-cyclopentadiene, 5-vinyl radical to the 2-cyclopentene,5-ethenylidene radical and that the fastest reaction channel is dissociation to fulvenallene and hydrogen. This is in agreement with the literature evidences reporting that benzyl decomposes to hydrogen and a C7H6 species. The benzyl high-pressure decomposition rate constant estimated assuming equilibrium between the rate determining step transition state and benzyl is k1(T) = 1.44 × 1013T0.453exp(−38400/T) s−1, in good agreement with the literature data. As fulvenallene reactivity is mostly unknown, we investigated its reaction with hydrogen, which has been proposed in the literature as a possible decomposition route. The reaction proceeds fast both backward to form again benzyl and, if hydrogen adds to allene, forward toward the decomposition into the cyclopentadienyl radical and acetylene with high-pressure kinetic constants k2(T) = 8.82 × 108T1.20exp(1016/T) and k3(T) = 1.06 × 108T1.35exp(1716/T) cm3/mol/s, respectively. The computed rate constants were then inserted in a detailed kinetic mechanism and used to simulate shock tube literature experiments.

Energetics of [Fe(NCH)6]2+ via CASPT2 calculations: a spin-crossover perspective.

Abstract: The importance of basis sets and active spaces in the determination of the potential energy curves and relevant energy differences in the O(h)-symmetry model system [Fe(NCH)(6)](2+) is analyzed using the Complete Active Space Self-Consistent Field (CASSCF) method and subsequent second-order perturbative treatment (CASPT2). By comparison of a series of atomic basis sets contraction, it is concluded that a balanced description of the Fe 7s6p5d3f2g1h and N 4s3p1d partners is needed to reach convergence upon the potential energy surface descriptions. Since the spin-crossover phenomenon involves the simultaneous change in the spin nature and expansion of the coordination sphere of the metal ion (i.e., lengthening of the Fe-N distances), the standard 10 electrons/12 orbitals complete active space is confronted to a chemically intuitive 18 electrons/15 orbitals picture. The role of a second d-shell is finally examined using the extended RAS strategy. Using a valence-bond type analysis, it is shown that the so-called d(') orbitals allow for a significant charge redistribution (approximately 0.5 electron) along the transition. Our calculations are compared to reference coupled-cluster estimations.

Coupled translation-rotation eigenstates of H2 in C60 and C70 on the spectroscopically optimized interaction potential: Effects of cage anisotropy on the energy level structure and assignments

Abstract: We have developed a quantitatively accurate pairwise additive five-dimensional (5D) potential energy surface (PES) for H2 in C60 through fitting to the recently published infrared (IR) spectroscopic measurements of this system for H2 in the vibrationally excited ν=1 state. The PES is based on the three-site H2–C pair potential introduced in this work, which in addition to the usual Lennard-Jones (LJ) interaction sites on each H atom of H2 has the third LJ interaction site located at the midpoint of the H–H bond. For the optimal values of the three adjustable parameters of the potential model, the fully coupled quantum 5D calculations on this additive PES reproduce the six translation-rotation (T-R) energy levels observed so far in the IR spectra of H2@C60 to within 0.6%. This is due in large part to the greatly improved description of the angular anisotropy of the H2-fullerene interaction afforded by the three-site H2–C pair potential. The same H2–C pair potential spectroscopically optimized for H2@C60 wa...

Mechanism and thermal rate constants for the complete series reactions of chlorophenols with H.

Abstract: Reactions of chlorophenols with atomic H are important initial steps for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in incinerators. Detailed insight into the mechanism and kinetic properties of crucial elementary steps is a prerequisite for understanding the formation of PCDD/Fs. In this paper, the complete series reactions of 19 chlorophenol congeners with atomic H have been studied theoretically using the density functional theory (DFT) method and the direct dynamics method. The profiles of the potential energy surface were constructed at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. Modeling of the PCDD/Fs formation requires kinetic information about the elemental reactions. The rate constants were deduced over a wide temperature range of 600∼1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT). The rate-temperature formulas were fitted for the first time. This study shows that the substitution pa...

Hot-electron-mediated desorption rates calculated from excited-state potential energy surfaces

Abstract: We present a model for desorption induced by (multiple) electronic transitions [DIET (DIMET)] based on potential energy surfaces calculated with the delta self-consistent field extension of density-functional theory. We calculate potential energy surfaces of CO and NO molecules adsorbed on various transition-metal surfaces and show that classical nuclear dynamics does not suffice for propagation in the excited state. We present a simple Hamiltonian describing the system with parameters obtained from the excited-state potential energy surface and show that this model can describe desorption dynamics in both the DIET and DIMET regimes and reproduce the power-law behavior observed experimentally. We observe that the internal stretch degree of freedom in the molecules is crucial for the energy transfer between the hot electrons and the molecule when the coupling to the surface is strong.

Water Dimer Radical Cation: Structures, Vibrational Frequencies, and Energetics

Abstract: Fourteen stationary points for the water dimer radical cation on its doublet electronic state potential energy surface have been characterized using coupled cluster theory with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)]. This is done in conjunction with Dunning’s correlation consistent polarized valence basis sets (cc-pVXZ and aug-cc-pVXZ, X = D, T, Q). Two stationary points are found to be local minima, isomer 1 (C1 symmetry) with H3O+···OH character (hydrogen-bonded system), and isomer 7 (C2 symmetry) with [H2O···H2O]+ character (hemibonded system). Among the other stationary points, seven are transition states, and the remaining five are higher order saddle points. The fourteen water dimer radical cation structures lie within 45 kcal mol−1 of isomer 1. Structure 1, transition states 2 (Cs symmetry) and 3 (Cs symmetry) are related through torsion of the OH group; these three stationary points fall within one kcal mol−1, demonstrating the low energy barri...