Thermal Rate Constants for Polyatomic Reactions: First Principles Quantum Theory

Abstract

The truly accurate knowledge of molecular dynamics phenomena is generally achieved through a combination of detailed experiments and first principle theory. The complexity of such a level of description had until recently restricted accurate studies to rather small systems. However, the sophistication of theoretical methods and massive technological developments have provided remarkable progress in the detailed knowledge of reactive events during the past three decades. Moreover, significant progress towards the detailed understanding of polyatomic reaction has been made in recent years. Detailed experimental and accurate theoretical studies of reactions involving more than only three or four atoms are becoming increasingly available. In this work, aspects of the theoretical work aiming at the accurate description of polyatomic reactions are reviewed. The present article focuses on the development of the first principle theory of reaction rates. It reviews theoretical developments and benchmark applications to reactions as CH4 + H → CH3 + H2 and CH4 + O → CH3 + OH. The importance of quantum effects for the thermal rate constants in different temperature regimes is discussed in detail. The accuracy of the classical transition state theory and of different approximate quantum theories is investigated in detail. A quantum transition state concept which facilitates accurate reaction rate calculations for polyatomic reaction is described. Benchmark results for the CH4 + H → CH3 + H2 reaction are shown which demonstrate that the accuracy of thermal rate constants calculated by first principle theory can rival the accuracy of available experimental data. The perspectives offered by these developments are discussed.