Isotope effects in partially deuterated tetramethylsilane cations studied by EPR spectroscopy

Abstract

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 C_2v 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 C_2v 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.