ESR Study of Ethynyl and Vinyl Free Radicals

ESR spectra have been observed for ethynyl (C≡CH) and vinyl (HC=CH2) radicals trapped in solid argon at liquid-helium temperatures. Ethynyl radical was obtained by the photolytic decomposition of acetylene. Its ESR spectrum consisted of two narrow lines corresponding to a proton hyperfine splitting of 16.1 Oe. The assignment of this spectrum to ethynyl was confirmed by a study of the photolysis of deuteroacetylene. Hydrogen atoms were not observed in the acetylene—argon photolytic system, suggesting that this photolysis is a second-order process involving two molecules of acetylene. Vinyl radical was obtained by the addition of an H atom, obtained from the photolysis of HI, to acetylene. The ESR spectrum of the vinyl radical consisted of eight broad overlapping lines resulting from the interaction of the unpaired electron with three nonequivalent protons. Addition of an H atom to deuteroacetylene at 4.2°K gave only one of the two possible structural isomers of the 1,2 dideuterovinyl radical. The ESR spectrum of this radical showed that the largest of the proton hyperfine splittings in vinyl is due to a β proton. With the aid of theory the observed hyperfine splittings were assigned to the vinyl protons as follows: Atrans,β=68 Oe, Acis,β=34 Oe, and Aα=16 Oe, where the designations cis and trans refer to the relative positions of the β proton and the unpaired electron orbital. A theoretical treatment of the α proton splitting in vinyl and similar radicals provided a qualitative explanation of the marked difference between the α proton hyperfine splittings in the vinyl and formyl radicals.