Spin-forbidden dehydrogenation of methoxy cation: a statistical view

A non-adiabatic version of RRKM theory is applied to predicting the microcanonical rates for different mechanisms of spin-forbidden dissociation of methoxy cation and its isotopically substituted derivatives, to formyl cation and dihydrogen. The predictions are in agreement with experimental results on this system, and in particular with the occurrence of a “direct’' mechanism for dissociation, rather than of an indirect one via hydroxymethyl cation. Abinitio computations were used throughout to provide the parameters needed to apply the non-adiabatic RRKM theory, and the success of this strategy is shown to be promising for other applications in polyatomic systems. Finally, the kinetic energy release distribution for loss of hydrogen from methoxy and hydroxymethyl cations are computed using abinitio “direct dynamics’' classical trajectories at the HF/6-31G** level, their similarity is also in agreement with experiment.