Statistical and dynamical behaviour in the unimolecular reaction dynamics of polyatomic molecules

Abstract

The dominant theories of unimolecular reaction are statistical. A fundamental assumption is that the timescale on which energy moves about a reactant molecule is much shorter than the timescale for reaction. It is assumed that intramolecular vibrational energy redistribution (IVR) is globally rapid throughout the molecular phase space. It has been widely thought that the assumption of rapid IVR referred to above is valid for sufficiently large polyatomics. Much of the supporting evidence for this view comes from indirect experimental studies of IVR and comparisons of statistical and dynamical calculations. However, the presence of a fast IVR rate, as derived from some experiments, does not automatically ensure the reaction dynamics will be statistical. In fact, in recent studies, we have shown that even in the presence of fast IVR rates between some modes the reaction dynamics can be extremely non-statistical. Secondly, most comparisons of statistical and dynamical calculations have made simplifying assumptions which render the comparisons ambiguous. In the present paper, we investigate results of recent statistical and dynamical calculations performed on identical potential energy surfaces for a range of polyatomic molecules. Our ultimate goal is to determine how the extent and timescale of IVR plays a role in determining the statistical or non-statistical behaviour in the subsequent unimolecular reaction dynamics of locally and microcanonically excited polyatomic molecules.