Equivalence of Vinylidene and C_2H_2^*: Calculated Rate Constant for Vinylidene Abstraction from CH_4

A long-lived excited state of acetylene, denoted by C_2H_2^*, often has been invoked as the major intermediate species in the photolysis of C_2H_2. The structure of the intermediate remains obscure. Recently, Laufer, on the basis of spectroscopic evidence, has suggested that triplet vinylidene radicals (^3B_2, H_2C=C) are a possible intermediate in the vacuum ultraviolet flash photolysis of acetylene as well as the combination of two triplet CH_2 radicals. In the latter, the isotopic distribution of product acetylene supports the assignment to vinylidene.