Isotope effects in partially deuterated tetramethylsilane cations studied by EPR spectroscopy
Well-resolved isotropic EPR spectra were observed for Si(CH3)4+ (TMS+), Si(CH3)3CD3+ (TMS-d3+) and Si(CH3)2(CD3)2+ (TMS-d6+), generated by ionizing radiation in SnCl4 matrices at 77 K. The TMS+ radical has been reported to have a C2v structure possessing two apical methyl groups with long Si–C bonds and two equatorial methyl groups with short Si–C bonds. The observed couplings of 0.95 and 0.47 mT for TMS+ were assigned to 1H nuclei in the apical and equatorial methyl groups, respectively. The small separation of spectral lines, 0.47 mT, for TMS-d3+ and TMS-d6+ is the same as that observed for TMS+. Thus, the C2v structure is retained for these partially deuterated tetramethylsilane cations. The simple well-resolved isotropic spectra unambiguously revealed that TMS-d3+ had the CD3 group in an equatorial position and that TMS-d6+ possessed the CD3 group in each of the apical and equatorial positions. Furthermore, multiplet spectra for TMS-d3+ were observed below 20 K and were attributed to A- and E-lines, which are characteristic of quantum mechanical tunneling rotation of the methyl protons.