Alkali Metal Atom Induced Acetylene-Vinylidene Rearrangement: Matrix Isolation ESR Study

Interactions between alkali metal atoms and acetylene molecules cocondensed in argon matrices were examined by ESR spectroscopy. The following are shown: (1) No reaction occurs between alkali metal atoms in the 2S ground state and acetylene molecules, but the side-on π complexes are formed between alkali metal atoms in the 2P excited state and the acetylene molecules. (2) Subsequent exposure of the π complex to light corresponding to the π → π* transition in the acetylene moiety results in its isomerization to the vinylidene form M=CH2. The g tensors, the proton, and the alkali metal-atom hyperfine coupling tensors of the observed π and vinylidene complexes and the 13C hfc tensor of the Li-acetylene π complex were determined. In the case of the Na/acetylene/argon system, only the vinylidene complex was observed due to coincidence of the 3s → 3p transition of Na atom and the π — π* transition of the π complex. © 1992, American Chemical Society. All rights reserved.