Methylene collision-induced intersystem crossing

The view that electronic states of different multiplicity need not be considered cannot easily be ruled out, since both deactivation of vibrationally excited carbenes and intersystem crossing between singlet and triplet states are brought about by collision with other molecules. The difficulty is not restricted to reactions in the gas phase in solution, collisional deactivation and collision-induced intersystem crossing can still be expected to compete with collisions leading to chemical reaction. However, the parallelism between the variation in stereospecificity in the gas-phase addition of methylene to the 2-butenes with the pressure of inert gas (Frey, 1959, I960 Anet et al., 1960 Bader and Generosa,... 

Reversible electronic relaxation has been found in smaller molecules. In methylene (for which collision-induced intersystem crossing has been extensively studied for many years ) the reversible character of singlet-triplet transitions has been evidenced in a study of pressure effect on the CH2 radical prepared either in the singlet state (by photochemical decomposition of diazirine) or in the triplet state (by photofragmentation of... 

It is also possible to induce intersystem crossing from an initially formed singlet state to a lower energy triplet by forcing the singlet state to suffer collisions with an inert medium (no easy task for a species as reactive as methylene ). 

The intersystem crossing from CH2( /4,) to (3B,) is induced by inert gases (143). A theory dealing with the collision-induced singlet to triplet transition of methylene is developed by Chu and Dahler (211). 

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