Hindered rotor model for radical association reactions

Abstract

The complexity and computational effort of detailed applications of variational transition-state theory to association reactions without potential energy barriers can be prohibitive. It is therefore desirable to consider more approximate treatments which are easier to implement but nonetheless retain the essential features of a realistic potential-energy surface. An appealing model of the transition state of CH₃+ H is presented, in which the transitional modes are treated as hindered internal rotations. The model is extended, in approximate fashion, to CH₃+ CH₃. Canonical variational transition state theory techniques are applied to obtain the temperature-dependent rate coefficient for both reactions. The utility of this simplified model and several related models is assessed by comparing its predictions to those of previous flexible transition-state theory calculations.