Asymmetrically Distorted Structure, Selective Bond Length Alternation, and Reactions of Radical Cations of Silacyclohexanes: An ESR and ab-Initio MO Study

Abstract

The structure, dynamics, and thermal reactions of the radical cations of silacyclohexanes (cSiC5) containing one Si atom in the six-membered ring have been studied by means of ESR spectroscopy and ab-initio MO calculations. The cSiC5 radical cations were generated in perfluoromethylcyclohexane matrix by ionization radiation at low temperature. By the help of selectively deuterated or methylated silacyclohexanes the 4.2 K ESR spectrum of cSiC5+ was successfully analyzed in terms of four different isotropic 1H hyperfine coupling (hfc) constants: 7.55 mT to the equatorial hydrogen (H3e) at the C(3) position, 3.45 mT to H6e, 2.85 mT to H5e, and 2.60 mT to H2e. The result strongly suggests that the cSiC5 radical cation takes an asymmetrically distorted C1 structure with one of the Si−C bonds elongated in which the unpaired electron mainly resides. Ab-initio MO calculations support the distorted structure. A suggested mechanism for the structural distortion is a pseudo-Jahn−Teller effect. Temperature dependent...