A quantitative curve-crossing model for radical fragmentation.

The kinetics of bond fragmentation for a series of N-methoxypyridyl radicals are analyzed in terms of a simple curve-crossing model that includes bond stretching and bond bending coordinates. The model accurately reproduces the reaction surfaces calculated using density functional theory (DFT) and also the experimental reaction energy barriers. The reactions proceed on the ground state surface by avoidance of a conical intersection, which is clearly illustrated by the model. A value for the electronic coupling matrix element responsible for splitting the upper and lower surfaces of 0.9 eV is obtained. The model illustrates the molecular features that allow barrierless fragmentation from a formally pi radical.