Seung C. Park

Potential energy surfaces for polyatomic reactions by interpolation with reaction path weight: CH2OH+→CHO++H2 reaction

An improved algorithm to construct molecular potential energy surfaces for polyatomic reactions is presented. The method uses the energies, gradients, and Hessians, which can be obtained from ab initio quantum chemical calculations. The surface is constructed by interpolating the local quadratic surfaces with reaction path weights. The method is tested with a five-atom reaction system for which an analytic potential energy surface has been reported together with classical trajectory results. An excellent agreement is achieved for energy partitioning in products obtained by trajectory calculation on the original analytic and interpolated surfaces. Reduction of error caused by the use of the reaction path weight is explained.

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 f...

Quantum mechanical study of the light‐atom transfer reactions, O(3P)+XCl→OX+Cl (X=H,D). I. Reactions in the ground vibrational states

We present a three‐dimensional quantum mechanical study of the light‐atom transfer reaction O(3P)+XCl(vi=0)→OX(vf=0)+Cl(X=H,D), where vα represents the vibrational state in the α channel. The adiabatic‐bend approximation reformulated in terms of the hyperspherical coordinates is employed to calculate the cross sections and rate constants. The potential energy surface used here is the Persky–Broida’s LEPS‐I. The results are compared with the available experimental data and quasiclassical trajectory calculations. A discrepancy is found between the present results and the quasiclassical trajectory results at low collision energies (low temperatures). This is a clear manifestation of the quantum mechanical tunneling effect. The present results of the rate constants and the kinetic isotope effect are generally in better agreement with experiment. The previously proposed constant centrifugal potential approximation (CCPA) is directly demonstrated to work well.

Quantum mechanical calculation of the CO vibrations in CO/Cu(100)

We report a calculation of the vibrational energies of CO/Cu(100) focusing on anharmonic coupling between the six CO–Cu modes, for Cu treated as a rigid, multilayer slab. A realistic many‐body potential [J. C. Tully, M. Gomez, and M. Head‐Gordon, J. Vac. Sci. Technol. A 11, 1914 (1993)] is used to obtain a fourth‐order force field in normal coordinates. The vibrational eigenvalue problem is solved using the vibrational self‐consistent field method, and the fundamental frequencies are obtained for a thermal distribution of hot bands. The absorption spectra for the CO stretch, the CO–Cu stretch, the CO frustrated rotation, and the CO frustrated translation are calculated at two temperatures. All spectra are significantly broadened due to thermal effects of intermode coupling. Agreement with experiment is generally quite good.

Quasiclassical trajectory calculations of the reaction C+C2H2-->l-C3H, c-C3H+H, C3+H2 using full-dimensional triplet and singlet potential energy surfaces.

Full-dimensional, density functional theory (B3LYP/6-311g(d,p))-based potential energy surfaces (PESs) are reported and used in quasi-classical calculations of the reaction of C with C(2)H(2). For the triplet case, the PES spans the region of the reactants, the complex region (with numerous minima and saddle points) and the products, linear(l)-C(3)H+H, cyclic(c)-C(3)H+H and c-(3)C(3)+H(2). For the singlet case, the PES describes the complex region and products l-C(3)H+H, c-C(3)H+H and l-(1)C(3)+H(2). The PESs are invariant under permutation of like nuclei and are fit to tens of thousands of electronic energies. Energies and harmonic frequencies of the PESs agree well the DFT ones for all stationary points and for the reactant and the products. Dynamics calculations on the triplet PES find both l-C(3)H and c-C(3)H products, with l-C(3)H being dominant at the energies considered. Limited unimolecular reaction dynamics on the singlet PES find both products in comparable amounts as well as the C(3)+H(2) product.

Partitioning of the nonfixed excess energy and the reverse critical energy in CH2OH+→CHO++H2: A classical trajectory study

Dynamics of the four‐centered elimination reaction CH2OH+→CHO++H2 has been investigated over the internal energy range 4.6–5.9 eV using the classical trajectory method. A realistic semiempirical potential reported previously [J. Chem. Phys. (in press, 1996)] has been used for the calculation. It has been found that the disposal of the nonfixed excess energy at the transition state and of the reverse critical energy can be considered independently as manifest in the sum rule analysis. The former is determined statistically while the latter dynamically. Based on the above idea, a method to determine the kinetic energy release distribution originating only from the reverse critical energy has been developed.

Quasiclassical trajectory studies of rigid rotor–rigid surface scattering. II. Corrugated surface

The quasiclassical trajectory method, previously applied to rigid rotor–rigid flat surface scattering [J. M. Bowman and S. C. Park, J. Chem. Phys. 77, 5441 (1982)] is applied to a rigid rotor–rigid corrugated surface, i.e., a N2–LiF(001), system. The mechanisms for rotational excitation at low and high collision energies are studied as well as their dependence on initial beam orientation and corrugation strength. A significant correlation between long‐lived trajectories and high rotational excitation is found for low energy collisions and rotational rainbows are clearly observed in the high energy regime, although these features are broadened relative to the flat surface reported previously.

On the evaluation of cross section and rate constant of atom-diatom reactions in the sudden and adiabatic approximations

The constant centrifugal potential approximation is explicitly tested and its general usefulness is demonstrated. This method is directly applied to the model reaction systems Cl + H2, Cl + HBr and O + HCl in both the sudden (RIOS) and the adiabatic-bend reduced dimensionality approximations. These two approximations are employed using the collinear-type hyperspherical coordinates reported before. This constant centrifugal potential approximations saves considerable cpu time, because the orbital angular momentum l necessary to obtain converged cross sections can easily be more than 100, and what we have to evaluate in this approximation is only the l = 0 reaction probability as a function of collision energy. Brief discussion is also presented concerning the choice of the location where the constant centrifugal potential is evaluated.

CONSTRUCTION OF A GLOBAL POTENTIAL ENERGY SURFACE FROM NOVEL AB INITIO MOLECULAR DYNAMICS FOR THE O(3P) + C3H3 REACTION

We present a global potential energy surface (PES) for the 2A state of the O(3P) + C3H3 radical reaction. The global PES is constructed mainly using direct ab initio molecular dynamics and further sampling is done using the Diffusion Monte Carlo method. The potential is fully invariant with respect to permutational symmetry of like atoms. Special techniques, based on invariant theory of finite groups, have been used to develop basis functions for fitting that display this symmetry. The resultant potential energy surface shows multiple reaction paths with six different product channels. The products of the reactions are CO + C2H3 radicals H + C3H2O radicals (with two isomers, propynal and propa-1,2-dien-1-one) and OH + C3H2 radicals (with three isomers, vinylidenecarbene, propargylene and cyclopropenylidene). Energies of the PES are in excellent agreement with ab initio energies for each stationary point, the reactants and the products. Most stationary points are fitted at the sub Kcal/mol level. The global potential surface represents all the stationary points and six different product channels correctly. Preliminary dynamics calculations show abstraction and insertion mechanisms for the OH + vinylidenecarbene channel and the H + propynal channel, respectively.

Construction of an accurate potential energy surface by interpolation for quantum dynamics studies of a three-body system

A method to construct an accurate potential energy surface (PES) by interpolation for a three-body reaction which is suitable for quantum dynamics studies is presented using Cl+H2→HCl+H as an example. Use of the exponential coordinates led to a significant improvement. Dynamics results, both classical and quantal, on the LEPS and LEPS-interpolated PESs were nearly indistinguishable. An accurate analytic PES can be constructed with the ab initio results also, as manifested with the PES contours.