A theory of the collision-induced singlet to triplet transition of methylene

Abstract

This paper begins with a survey and critique of past experimental and theoretical studies of the inert gas catalysed singlet to triplet transition of methylene. Next we develop a transition-state theory in which the process of activation consists of a collisional distortion of methylene to the extent that degeneracy occurs between adiabatic singlet and triplet vibronic states. Establishment of this collision induced degeneracy is followed by a spin-orbit coupled singlet to triplet transition which is then subsequently stabilized by the splitting of the degeneracy which accompanies the continued relative motion of catalyst and radical. This theory appears to be able to account for what is presently known about the bimolecular singlet to triplet reaction. It is capable of generating rate constants with the correct order of magnitude and of accounting for the experimentally observed isotope and catalytic dependence of these constants.