Global Analytical Potential Energy Surface Research Articles

Kinetics study of the OH + SiH4 hydrogen abstraction reaction: A theoretical analysis

AbstractThe temperature dependence of the rate constants and the kinetics isotope effects of the OH + SiH4 hydrogen abstraction reaction were theoretically studied. Three different kinetics approaches were used in this study: variational transition state theory with multidimensional tunneling (VTST/MT) corrections, quasi‐classical trajectory (QCT) calculations, and ring polymer molecular dynamics (RPMD). These studies were performed on a global analytical potential energy surface based on explicitly correlated high level ab initio calculations developed in the present work. While the experimental rate constants are practically independent with temperature, the VTST/MT results show a pronounced dependence, the QCT partially corrects this behavior, and RPMD rates are in better agreement with experiment. In this case, in the experimental common temperature range, 300–600 K, the T‐expression for the RPMD rate constants is: k(T) = 6.40 × 10−13.T0.52exp(−0.10/T). The kinetics isotope effects, H/D, are small in this temperature range, revealing the small importance of tunneling effects in this very exothermic and low barrier reaction. Different factors, such as anharmonicity, the zero‐point violation problem or limitations of the surface, without ruling out experimental uncertainties, were analyzed to explain the discrepancies.

Unconventional SN2 retention pathways induced by complex formation: High-level dynamics investigation of the NH2 - + CH3I polyatomic reaction.

Investigations on the dynamics of chemical reactions have been a hot topic for experimental and theoretical studies over the last few decades. Here, we carry out the first high-level dynamical characterization for the polyatom-polyatom reaction between NH2 - and CH3I. A global analytical potential energy surface is developed to describe the possible pathways with the quasi-classical trajectory method at several collision energies. In addition to SN2 and proton abstraction, a significant iodine abstraction is identified, leading to the CH3 + [NH2⋯I]- products. For SN2, our computations reveal an indirect character as well, promoting the formation of [CH3⋯NH2] complexes. Two novel dominant SN2 retention pathways are uncovered induced by the rotation of the CH3 fragment in these latter [CH3⋯NH2] complexes. Moreover, these uncommon routes turn out to be the most dominant retention paths for the NH2 - + CH3I SN2 reaction.

QCT study of the vibrational and translational role in the H + C2H6(ν1, ν2, ν5, ν7, ν9 and ν10) reactions

Two important issues were analysed in the title reaction: the effects of vibrational excitation, associated with mode selectivity, and the role of translational energy, associated with Polanyi’s rules. Based on a global analytical potential energy surface, PES-2018, recently developed in our group, quasi-classical trajectory (QCT) calculations were performed at total energy of 35 kcal mol−1, either as translation or as a combination of translation and vibration energy. Independent vibrational excitation by one quantum of any of the CH3 stretching modes in ethane leads to similar dynamics pictures of reaction cross sections and H2(v′, j′) rotovibrational and scattering distributions, ruling out mode selectivity. Normal mode analysis showed a cold, non-inverted, H2(v′) product vibrational distribution, while the C2H5(v′) co-product presented many vibrational states, all of them with a low population, practically simulating a classical behaviour. An equivalent amount of energy as translation raises reactivity somewhat less effective than vibrational energy, contrary to that found for the O(3P) + CH4 reaction. Both reactions present “central” barriers, so this opposite behaviour shows the difficulties for a straightforward application of the Polanyi′s rules. The role of vibrational and translational energy on dynamics has been rationalized by the coupling between vibrational modes, which makes analysis of vibrational excitation difficult in polyatomic systems. Finally, the role of the total energy on reactivity and mode selectivity was analysed, concluding that at lower energy, 15 kcal mol−1, translational energy is much more effective than vibrational energy to enhance reactivity, while at intermediate energy, 20 kcal mol−1, the situation is more confusing and strongly dependent on the counting methods used in the QCT calculations. Therefore, very small mode selectivity is found, and translation seems to be more effective in enhancing reactivity than vibration at low collision energies, while this behaviour is reversed as we increase the collision energy, being the turning point around 20 kcal mol−1.

Theoretical study of the complexes of dichlorobenzene isomers with argon. I. Global potential energy surface for all the isomers with application to intermolecular vibrations.

The complexes of para- (p-), meta- (m-), and ortho- (o-)dichlorobenzene (DCB) isomers with argon are studied using an ab initio method. The interaction energy in the ground electronic state of the complexes has been calculated using the CCSD(T) method (coupled cluster method including single and double excitations with perturbative triple excitations) and Dunning's double-ζ (aug-cc-pVDZ) basis set supplemented by midbond functions. Local interaction parameters have been defined and interesting relations fulfilled by them, independent of the DCB isomer, have been revealed. This finding has allowed us to construct the accurate global analytical intermolecular potential energy surface for all the DCB-Ar complexes with the same set of parameters, except for the monomer geometries. Each complex is characterized by two symmetrically equivalent global minima, one located above and the other located below the monomer plane at distances equal to 3.497 Å, 3.494 Å, and 3.485 Å for p-, m-, and o-isomers of DCB bound to Ar, respectively. Additionally, the Ar atom is shifted from the geometrical center of the DCB monomer towards the chlorine atoms by the value xe of 0.182 Å for m-isomer and 0.458 Å for o-isomer. The calculated binding energy De of 460 cm-1, 465 cm-1, and 478 cm-1 for p-, m-, and o-complex, respectively, are related to xe by simple relations. The intermolecular bending fundamentals calculated from PES depend strongly on the isomer structure. The calculated dissociation energies fit in the intervals estimated by the experiment of Gaber et al. for the S0 state [Phys. Chem. Chem. Phys. 11, 1628 (2009)].

Quasi-Classical Trajectory Dynamics Study of the Cl(2P) + C2H6 → HCl(v,j) + C2H5 Reaction. Comparison with Experiment.

To understand and simulate the dynamics behavior of the title reaction, QCT calculations were performed on a recently developed global analytical potential energy surface, PES-2017. These calculations combine the classical description of the dynamics with pseudoquantization in the reactants and products to perform a theoretical/experimental comparison on the same footing. Thus, in the products a series of constraints are included to analyze the HCl(v = 0,j) product, which is experimentally detected. At collision energies of 5.5 and 6.7 kcal mol-1 the largest fraction of available energy is deposited as translation, 67%, while the ethyl radical shows significant internal energy, 27%, and so it does not act as a spectator of the reaction, thus reproducing recent experimental evidence. The HCl(v=0, j) rotational distribution is cold, peaking at j = 2, only one unit hotter than experiment, which represents an error of 0.12 kcal mol-1. At a collision energy of 5.5 kcal mol-1 product translational distribution is slightly hotter than experiment, but at 6.7 kcal mol-1 agreement with recent experiments is practically quantitative, suggesting that the first experiments should be revised. In addition, we observe that the HCl(v=0, j) scattering distribution shifts from isotropic at low values of j to backward at high values of j, which is in agreement with experimental data. Finally, no evidence was found for the "chattering" mechanism suggested to explain the low translational energy of the HCl product in the backward scattering region. In sum, agreement with experiments of a series of sensible dynamic properties permits us to be optimistic on the quality and accuracy of the theoretical tools used in the present work, QCT and PES-2017.

Mode-specific multi-channel dynamics of the F- + CHD2Cl reaction on a global ab initio potential energy surface.

We report a detailed quasiclassical trajectory study for the dynamics of the ground-state and CH/CD stretching-excited F- + CHD2Cl(vCH/CD = 0, 1) → Cl- + CHD2F, HF + CD2Cl-, and DF + CHDCl- SN2, proton-, and deuteron-abstraction reactions using a full-dimensional global ab initio analytical potential energy surface. The simulations show that (a) CHD2Cl(vCH/CD = 1), especially for vCH = 1, maintains its mode-specific excited character prior to interaction, (b) the SN2 reaction is vibrationally mode-specific,

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the F(-) + CH3F SN2 and proton-abstraction reactions.

We develop a full-dimensional global analytical potential energy surface (PES) for the F(-) + CH3F reaction by fitting about 50 000 energy points obtained by an explicitly correlated composite method based on the second-order Møller-Plesset perturbation-F12 and coupled-cluster singles, doubles, and perturbative triples-F12a methods and the cc-pVnZ-F12 [n = D, T] basis sets. The PES accurately describes the (a) back-side attack Walden inversion mechanism involving the pre- and post-reaction (b) ion-dipole and (c) hydrogen-bonded complexes, the configuration-retaining (d) front-side attack and (e) double-inversion substitution pathways, as well as (f) the proton-abstraction channel. The benchmark quality relative energies of all the important stationary points are computed using the focal-point analysis (FPA) approach considering electron correlation up to coupled-cluster singles, doubles, triples, and perturbative quadruples method, extrapolation to the complete basis set limit, core-valence correlation, and scalar relativistic effects. The FPA classical(adiabatic) barrier heights of (a), (d), and (e) are -0.45(-0.61), 46.07(45.16), and 29.18(26.07) kcal mol(-1), respectively, the dissociation energies of (b) and (c) are 13.81(13.56) and 13.73(13.52) kcal mol(-1), respectively, and the endothermicity of (f) is 42.54(38.11) kcal mol(-1). Quasiclassical trajectory computations of cross sections, scattering (θ) and initial attack (α) angle distributions, as well as translational and internal energy distributions are performed for the F(-) + CH3F(v = 0) reaction using the new PES. Apart from low collision energies (Ecoll), the SN2 excitation function is nearly constant, the abstraction cross sections rapidly increase with Ecoll from a threshold of ∼40 kcal mol(-1), and retention trajectories via double inversion are found above Ecoll = ∼ 30 kcal mol(-1), and at Ecoll = ∼ 50 kcal mol(-1), the front-side attack cross sections start to increase very rapidly. At low Ecoll, the indirect mechanism dominates (mainly isotropic backward-forward symmetric θ distribution and translationally cold products) and significant long-range orientation effects (isotropic α distribution) and barrier recrossings are found. At higher Ecoll, the SN2 reaction mainly proceeds with direct rebound mechanism (backward scattering and hot product translation).

Global Analytical Potential Energy Surface for the Electronic Ground State of NH3 from High Level ab Initio Calculations

The analytical, full-dimensional, and global representation of the potential energy surface of NH(3) in the lowest adiabatic electronic state developed previously (Marquardt, R.; et al. J. Phys. Chem. B 2005, 109, 8439–8451) is improved by adjustment of parameters to an enlarged set of electronic energies from ab initio calculations using the coupled cluster method with single and double substitutions and a perturbative treatment of connected triple excitations (CCSD(T)) and the method of multireference configuration interaction (MRCI). CCSD(T) data were obtained from an extrapolation of aug-cc-pVXZ results to the basis set limit (CBS), as described in a previous work (Yurchenko, S.N.; et al. J. Chem. Phys 2005, 123, 134308); they cover the region around the NH3 equilibrium structures up to 20,000 hc cm(–1). MRCI energies were computed using the aug-cc-pVQZ basis to describe both low lying singlet dissociation channels. Adjustment was performed simultaneously to energies obtained from the different ab initio methods using a merging strategy that includes 10,000 geometries at the CCSD(T) level and 500 geometries at the MRCI level. Characteristic features of this improved representation are NH3 equilibrium geometry r(eq)(NH(3)) ≈ 101.28 pm, α(eq)(NH(3)) ≈ 107.03°, the inversion barrier at r(inv)(NH(3)) ≈ 99.88 pm and 1774 hc cm(–1) above the NH(3) minimum, and dissociation channel energies 41,051 hc cm(–1) (for NH(3) → ((2)B(2))NH(2) + ((2)S(1/2))H) and 38,450 hc cm(–1) (for NH(3) → ((3)Σ(–))NH +((1)Σ(g)(+))H(2)); the average agreement between calculated and experimental vibrational line positions is 11 cm(–1) for (14)N(1)H(3) in the spectral region up to 5000 cm(–1). A survey of our current knowledge on the vibrational spectroscopy of ammonia and its isotopomers is also given.

Dynamics of the F− + CH3Cl → Cl− + CH3F SN2 reaction on a chemically accurate potential energy surface

Though bimolecular nucleophilic substitution (SN2) reactions play a fundamental role in chemistry, chemically accurate full-dimensional global analytical potential energy surfaces (PESs) have not been developed for these systems. These PESs govern the motion of the atoms in a chemical reaction; thus, the knowledge of the PES is essential to study the dynamics and atomic-level mechanisms. Here, we report a full-dimensional ab initio PES for the F− + CH3Cl → Cl− + CH3F reaction, the PES has an estimated average accuracy of about 0.5 kcal mol−1. Quasiclassical trajectories on this PES reveal that the direct rebound mechanism dominates at high collision energies, whereas the reaction is mainly indirect at low collision energies, where the formation of long-lived hydrogen-bonded and C3v ion–dipole entrance-channel complexes play a major role in the dynamics. A direct stripping mechanism is also found at large impact parameters resulting in significant forward scattering, whereas the direct rebound mechanism scatters towards backward directions. At high collision energies the reaction can be controlled by orienting the reactants into a reactive F− + H3CCl orientation, whereas at low collision energies the initial orientation is not always maintained, because the long-range ion–dipole interactions efficiently steer the reactants into a reactive orientation even if F− initially approaches the non-reactive side of CH3Cl. Mode-specific vibrational distributions show that the reaction produces vibrationally hot CH3F molecules with excited CF stretching, especially at low collision energies.

Time-dependent quantum dynamics of the He + H+He reaction

Time-dependent quantum-mechanical wave packet calculations have been carried out to calculate the reaction probabilities and cross sections for the proton transfer reaction, He + H+He → HeH+ + He, in a collision energy range 0.0–0.5 eV. A global analytical potential energy surface developed by Panda and Sathyamurthy (2003 J. Phys. Chem. A 107 7125) has been used for the purpose. The probability curves are characterized by numerous oscillatory structures attributed to the presence of resonances. The calculated reaction probabilities are found to be weakly dependent on vibrational and rotational excitations of the diatom for total angular momentum (J) = 0. For higher J values, rotational excitation results in a decline in probability values for j = 1 followed by enhancements for j = 2 and 3. The oscillations seen in reaction probabilities are not fully washed out in the integral cross sections. The dependence of cross section on initial orientations of the reactants is also studied.

Global analytical potential energy surfaces for HO2(X̃A″2) based on high-level ab initio calculations

Two global analytical potential energy surfaces for the HO2(X2A") system have been developed by fitting approximately 15,000 ab initio points at the icMRCI+Qaug-cc-pVQZ level of theory, using the reproducing kernel Hilbert space method. One analytical potential is designed to give a very accurate representation of the low energy range that determines the vibrational spectrum, while the other attempts to provide a fast and uniformly accurate potential function for reaction dynamics. The quality of the fitted potential functions is confirmed by good agreement of the (J=0) HO2 vibrational spectrum and (J=0) quantum reaction probability for the H+O2(ji=0,nui=0) reaction with those obtained using the spline fitted potential. Quasiclassical trajectory calculations carried out on the new potential energy surface provided the reaction probability with a zero impact parameter (b=0), which is in reasonably good agreement with the J=0 quantum results.

Global Analytical Potential Energy Surface for Large Amplitude Nuclear Motions in Ammonia

An analytical, full-dimensional, and global representation of the potential energy surface of NH(3) in the lowest adiabatic electronic state is developed, and parameters are determined by adjustment to ab initio data and thermochemical data for several low-lying dissociation channels. The electronic structure is calculated at the CASPT2 level within an [8,7] active space. The representation is compared to other recently published potential energy surfaces for this molecule. The present representation is distinguished by giving a qualitatively correct description of the potential energy for very large amplitude displacements of the nuclei from equilibrium. Other characteristic features of the present surface are the equilibrium geometries r(eq)(NH(3)) approximately 101.24 pm, r(eq)(NH(2)) approximately 102.60 pm, alpha(eq)(NH(3)) approximately 106.67 degrees, and the inversion barrier at r(inv)(NH(3)) approximately 99.80 pm and 1781 cm(-1) above the NH(3) minimum. The barrier to linearity in NH(2) is 11,914 cm(-1) above the NH(2)((2)B(1)) minimum. While the quartic force field for NH(3) from the present representation is significantly different from that of the other potential energy surfaces, the vibrational structures obtained from perturbation theory are quite similar for all representations studied here.

Dynamics of (H−,H2) collisions: A time-dependent quantum mechanical investigation on a new ab initio potential energy surface

A global analytical potential energy surface for the ground state of H(3)(-) has been constructed by fitting an analytic function to the ab initio potential energy values computed using coupled cluster singles and doubles with perturbative triples [CCSD(T)] method and Dunning's augmented correlation consistent polarized valence triple zeta basis set. Using this potential energy surface, time-dependent quantum mechanical wave packet calculations were carried out to calculate the reaction probabilities (P(R)) for the exchange reaction H(-)+H(2)(v, j)-->H(2)+H(-), for different initial vibrational (v) and rotational (j) states of H(2), for total angular momentum equal to zero. With increase in v, the number of oscillations in the P(R)(E) plot increases and the oscillations become more pronounced. While P(R) increases with increase in rotational excitation from j=0 to 1, it decreases with further increase in j to 2 over a wide range of energies. In addition, rotational excitation quenches the oscillations in P(R)(E) plots.

Analytical global potential energy surface for the X2A′ state of HCS

Abstract We report a global analytical potential energy surface (PES) corresponding to the lowest adiabatic X 2 A ′ state of HCS. High quality ab initio calculations were performed at a multi-reference configuration interaction calculation (MRCI) level using a triple-zeta basis set. Energies were calculated on a grid of 1357 points for the lowest state. They were used to construct a single-valued global analytical PES with the double many-body expansion (DMBE) method for all dissociation channels. Special attention was paid to the description of an anisotropic long-range potential. Geometries and energies of the main stationary points of the X 2 A ′ surface are compared with other theoretical and experimental data. By using the potential energy surface we have carried out a detailed dynamical study of the C( 3 P) + SH(X 2 Π) → CS(X 1 Σ + ) + H( 2 S) exothermic reaction over a wide range of translational energies.

Variational transition state theory and quasiclassical trajectory studies of the H2+OH→H+H2O reaction and some isotopic variants

Variational transition state theory (VTST) methods and quasiclassical trajectories (QCT) have been used to study the dynamics of the OH+H2 reaction, along with the isotopic counterparts OD+H2, OH+HD, OD+H2, OD+D2, and the reverse H+H2O→H2+OH reaction. Two new global analytical potential energy surfaces (PES) for H3O are employed, Wu, Schatz, Lendvay, Fang, Harding (WSLFH) and Ochoa, Clary (OC), both of which are based on high quality electronic structure calculations. Extensive comparisons with earlier results based on the Walch, Dunning, Schatz, Elgersma (WDSE) PES are also presented. The WSLFH PES surface, in combination with our best VTST estimate (ICVT/μOMT), yields rate constants for OH+H2 in quantitative agreement with experiment, while the OC PES yields somewhat less accurate results. The agreement with the OH+D2 experimental rate constants is less quantitative, but the WSLFH PES rate constant agrees with experiment to within a factor of 2 at all temperatures for which there are measurements. The OH+HD, OD+H2, and OD+D2 WSLFH PES rate constants calculated at the ICVT/μOMT level are in very good agreement with the less detailed experimental information that is available for these isotopes. The two surfaces give comparable predictions for the reverse H+H2O reaction at high temperatures, with deviations of less than 30%. This good agreement is maintained by the WSLFH PES at room temperature, while the OC PES predicts rate constants one order of magnitude larger than experiment. The QCT excitation functions for OH+H2, OH+D2, and OH+HD are well below experiment for both potentials, as was the case for earlier accurate quantum mechanical calculations that employed the WDSE PES. The WSLFH PES improves the agreement with the experimental vibrational state selected rate constants for the OH+H2 reaction compared to the WDSE PES. OC is also less accurate and presents antithreshold behavior for H2(v=1)+OH. H2 and OH rotational excitation have opposing effects: while rotation in H2 promotes reactivity, OH rotation impedes it. This impeding effect applies likewise to HD for high rotational excitation, explaining the selectivity toward HOH+D products in the OH+HD reaction.

A theoretical study of the vibrational energy spectrum of the HOCl/HClO system on an accurate ab initio potential energy surface

A new, global analytical potential energy surface is constructed for the X 1A′ electronic ground state of HOCl that accurately includes the HClO isomer. The potential is obtained by using accurate ab initio data from a previously published surface [Skokov et al., J. Chem. Phys. 109, 2662 (1998)], as well as a significant number of new data for the HClO region of the surface at the same multireference configuration interaction, complete basis set limit level of theory. Vibrational energy levels and intensities are computed for both HOCl and HClO up to the OH+Cl dissociation limit and above the isomerization barrier. After making only minor adjustments to the ab initio surface, the errors with respect to experiment for HOCl are generally within a few cm−1 for 22 vibrational levels with the largest error being 26 cm−1. A total of 813 bound vibrational states are calculated for HOCl. The HClO potential well supports 57 localized states, of which only the first 3 are bound. The strongest dipole transitions for HClO were computed for the fundamentals—33, 2.9, and 25 km/mol for ν1, ν2, and ν3, respectively. From exact J=1 ro-vibrational calculations, state dependent rotational constants have been calculated for HClO. Lastly, resonance calculations with the new potential demonstrate that the presence of the HClO minimum has a negligible effect on the resonance states of HOCl near the dissociation threshold due to the relatively high and wide isomerization barrier.

Monte Carlo sampling methods for determining potential energy surfaces using Shepard interpolation. The O( [formula omitted])+H 2 system

Shepard interpolation provides an effective way to define global analytical potential energy surfaces using ab initio energies, gradients and hessians as input. We examine Monte Carlo techniques for sampling geometries for the ab initio calculations, including the iterative determination of points which are used with the Shepard approach to optimize selected dynamical properties. Applications are presented to the O( 1 D )+H 2 surface which demonstrate the effectiveness of the method under circumstances where trajectory-based sampling gives poor results.

Reaction dynamics of the four-centered elimination CH2OH+→CHO++H2: Measurement of kinetic energy release distribution and classical trajectory calculation

Mass-analyzed ion kinetic energy (MIKE) spectrum of CHO+ generated in the unimolecular dissociation of CH2OH+ was measured. Kinetic energy release distribution (KERD) was evaluated by analyzing the spectrum according to the algorithm developed previously. The average kinetic energy release evaluated from the distribution was extraordinarily large, 1.63 eV, corresponding to 75% of the reverse barrier of the reaction. A global analytical potential energy surface was constructed such that the experimental energetics was represented and that various features in the ab initio potential energy surface were closely reproduced. Classical trajectory calculation was carried out with the global analytical potential energy surface to investigate the causes for the extraordinarily large kinetic energy release. Based on the detailed dynamical calculations, it was found that the strained bending forces at the transition state and strengthening of the CO bond from double to triple bond character were mainly responsible for such a significant kinetic energy release. In addition, the dissociation products H2 and CHO+ ion were found to be rotationally excited in the trajectory calculations. This was attributed to the asymmetry of the transition state and the release of asymmetric bending forces. Also, the bending vibrational modes of CHO+ and the H2 stretching mode, which are coupled with the bending coordinates, were found to be moderately excited.