Effect of substituents on the structure of the vinyl radical: Calculations and experiments

Literature reports indicate either a linear or a bent configuration for alpha-substituted vinyl radicals. In order to get a better insight into this structural question, calculations of the structure of some alpha-substituted vinyl radicals were conducted by the DFT approach [BLYP/6-31G(d,p) and B3LYP/ 6-311G(2d,2p)]. For vinyl radicals bearing sigma-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 forms is found to markedly increase as the electronegativity of the group 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, representative of sigma- and pi-type substituents, are calculated to be bent. Comparison with other literature calculations, obtained by different methods for some similar vinylic radicals, supports our computational results. Experimental support was also sought, with particular regards to the inversion barrier of the bent radicals. Since a very high barrier is calculated for the alpha-fluorovinyl radical, its inversion rate could be slower than the ''sampling'' time of a proper chemical reaction. 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.