Intersystem crossing in the exit channel

Intersystem crossing plays an important role in photochemistry. It is understood to be efficient when heavy atoms are present due to strong spin-orbit coupling, or when strongly bound long-lived complexes are formed that increase the chance of finding the singlet-triplet intersection seam. Here we present evidence for a different intersystem crossing mechanism in the bimolecular reaction of O(P-3) with alkylamines. In crossed-beam experiments, product velocity-flux maps are measured for aminoalkyl radicals produced from H abstraction from the methyl group, which also gives OH radicals as co-fragments. The low translational-energy release and isotropic angular distributions of the products indicate that such reactions undergo the formation of a complex before OH and aminoalkyl are produced. However, there is no well on the triplet potential energy surface that could support such a complex. Multi-reference ab initio calculations suggest, instead, that intersystem crossing occurs in the exit-channel region due to the long-range dipole-dipole interaction between the nascent radical product pair coupled with the vanishing singlet-triplet splitting at long range. Intersystem crossing then leads to a deep hydroxylamine well before OH elimination.