EFFECT OF SUBSTITUENTS ON THE STRUCTURE OF THE VINYL RADICAL - CALCULATIONS AND EXPERIMENTS

Literature reports indicate either a linear or a bent configuration fo r alpha-substituted vinyl radicals. In order to get a better insight i nto this structural question, calculations of the structure of some al pha-substituted vinyl radicals were conducted by the DFT approach [BLY P/6-31G(d,p) and B3LYP/ 6-311G(2d,2p)]. For vinyl radicals bearing sig ma-type substituents (Me, SH, Cl, OH, F), the bent form is found to be the minimum energy structure; the inversion barrier of the E and Z fo rms is found to markedly increase as the electronegativity of the grou p increases. Vinyl radicals bearing pi-type substituents (CH=CH2, CHO, CN, C6H5) are found to be linear. The effect of beta-substituents is much lower, and the beta-fluoro- and beta-ethenylvinyl radicals, repre sentative of sigma- and pi-type substituents, are calculated to be ben t. Comparison with other literature calculations, obtained by differen t methods for some similar vinylic radicals, supports our computationa l results. Experimental support was also sought, with particular regar ds to the inversion barrier of the bent radicals. Since a very high ba rrier is calculated for the alpha-fluorovinyl radical, its inversion r ate could be slower than the ''sampling'' time of a proper chemical re action. Indeed, the alpha-fluorovinyl radicals, generated from PhCH=C( F)Br by reaction with Bu3SnH/AIBN, retained the configuration of their precursors in the overall hydrodebromination process.