Crossing, collision-induced singlet

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). 

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,... 

Among electronic relaxation processes, the collision-induced intersystem crossing (singlet-triplet transitions in closed shell molecules) have been the most extensively studied. In addition, the intramolecular and intermolecu-lar coupling mechanisms may be more easily separated in this case than in the case of the collision-induced internal conversion. For this reason, our attention will be focused at the problems of collision-induced transitions between different spin manifolds. 

The collisional fluorescence quenching and phosphorescence induction processes have been later observed for a large number of small and medium-size molecules. The interpretation of these results was however rather confusing collision-induced intersystem crossing considered as a transition from the pure singlet to the pure triplet state is in apparent contradiction with the Wigner rule of spin conservation, at least in the case of light collision partners that cannot affect the intramolecular spin-orbit interaction. 

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 ). 

---