Analytical Potential Energy Surface Describing Abstraction Reactions in Asymmetrically substituted Polyatomic Systems of Type CX3Y + A→ Products

Constructing Potential Energy Surfaces for Polyatomic Systems: Recent Progress and New Problems

Different methods of constructing potential energy surfaces in polyatomic systems are reviewed, with the emphasis put on fitting, interpolation, and analytical (defined by functional forms) approaches, based on quantum chemistry electronic structure calculations. The different approaches are reviewed first, followed by a comparison using the benchmark H + CH4 and the H + NH3 gas-phase hydrogen abstraction reactions. Different kinetics and dynamics properties are analyzed for these reactions and compared with the available experimental data, which permits one to estimate the advantages and disadvantages of each method. Finally, we analyze different problems with increasing difficulty in the potential energy construction: spin-orbit coupling, molecular size, and more complicated reactions with several maxima and minima, which test the soundness and general applicability of each method. We conclude that, although the field of small systems, typically atom-diatom, is mature, there still remains much work to be done in the field of polyatomic systems.

Potential Energy Surface for Asymmetrically Substituted Reactions of Type CWXYZ + A. Kinetics Study

The CHClF(2) + Cl --> ClH + CClF(2) gas-phase abstraction reaction was chosen as a model of asymmetrically substituted polyatomic reactions of type CWXYZ + A --> products. The analytical potential energy surface for this reaction was constructed with suitable functional forms to represent vibrational modes, and calibrated with respect to experimental thermal rate constants which are available over the temperature range 296-410 K. On this surface, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wider temperature range, 200-2500 K, therefore obtaining kinetics information at lower and higher temperatures than are experimentally available. This surface was also used to analyze dynamical features, such as tunneling and reaction-path curvature. In the former, the influence of the tunneling factor was important since a light hydrogen atom passes through the barrier. In the latter, it was found that vibrational excitation of the C-F and C-Cl stretching modes can be expected in the exit channel.