Theoretical studies of the dynamics of four-atom systems: NH+NO, H+N(2)O, H+CO(2), H+H(2)O, and He+CS(2).

摘要

We present detailed theoretical studies of the dynamics of reactive collisions for the {dollar}rm NH+NO, H+Nsb2O, H+COsb2,{dollar} and {dollar}rm H+Hsb2O{dollar} systems. Our interest in these reactions stems from their importance in combustion and atmospheric chemistry, as well as the challenges that four-atom systems pose to the field of theoretical chemistry. We study each reaction using the quasiclassical trajectory method and report dynamics information such as cross sections, rate constants, energy partitioning, and mechanistic details. For NH + NO, we present a new, global potential energy surface and use it to determine a product branching ratio of 13% for producing {dollar}rm OH+Nsb2.{dollar} Our result helps settle a controversy regarding the value of this ratio, which is vital in many kinetic models. We then use the potential energy surface to study the reverse reaction, {dollar}rm H+Nsb2O,{dollar} with N{dollar}sb2{dollar}O in its ground state and with up to two quanta of excitation in each of the normal modes. We identify the dependence of the reaction mechanism on translational energy, and we examine the effects of reagent vibrational excitation on various aspects of reaction. For the reaction {dollar}rm H+COsb2,{dollar} we present a modified potential surface that improves the comparisons between trajectory and experimental results. We then present details of the quasiclassical trajectory calculation of rotational angular momentum polarization information for four-atom systems, and we find that the product OH rotational angular momentum vector displays no alignment preference. Also using this type of angular momentum polarization information, we are able to distinguish two different potential energy surfaces for the reaction {dollar}rm H+Hsb2O{dollar} under circumstances where their scalar properties are nearly identical. Based on the resulting alignment information, we identify two reaction mechanisms and their dependence on translational energy. In addition, we present a new theory for the direct calculation of the energy transfer moments in inelastic, collinear {dollar}rm He+CSsb2{dollar} collisions. This new theory is based on time-dependent quantum-mechanical calculations, and the results are in good agreement with results from previous, indirect calculations.