Asymmetrically distorted structure of radical cations of ethylsilanes, studied by electron spin resonance spectroscopy, ab initio and density functional theories

ESR spectra for the radical cations of SiEt2H2, SiEt3H, SiEt4 and SiEt2MeH, generated in halocarbon matrices by ionizing radiation at 77 K, consist of a doublet with a coupling constant of ca. 6.3 mT to a H-1 nucleus. Accumul ation of our data for the cations with C-Si-C bonds suggests that the catio ns adopt asymmetrically distorted structures with an unpaired electron larg ely in one of the elongated Si-C bonds and that the H-1 nucleus in the "tra ns'' position to the bond results in the large hyperfine (hf) splitting obs erved. Calculations at ab initio theory and density functional theory (DFT) levels strongly support this conclusion. The reversible spectral changes, observed for SiEt2H2+ and SiEt2MeH+ in CF2BrCF2Br matrices, have been analy zed with a three-site jumping model for the internal rotation of the methyl group having the H-1 nucleus assigned and a two-site jumping model for a s elective bond length alternation between two adjacent Si-C bonds.