Vibrational Anisotropy and Gol'danskii–Karyagin Effect in Trimethyltin Fluoride

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Abstract
The vibrational anisotropy of the tin atom in trimethyltin fluoride has been studied by means of 119Sn Mössbauer spectroscopy in the temperature range 7 < T < 296°K. From the Gol'danskii–Karyagin effect asymmetry of the doublet spectrum, it is inferred that while in the low-temperature limit the metal-atom vibrations are essentially isotropic, at room temperature the difference in the mean-square amplitudes of vibration parallel and perpendicular to the (nearly) threefold symmetry axis defined by the ···F···Sn···F framework, is given by 〈x‖2〉 − 〈x⊥2〉 = − 3.41 × 10−18cm2. The infrared data at room and liquid-nitrogen temperature in the 600–450-cm−1 region are consistent with a small departure from true C3υ symmetry for this molecule, and it is shown that the assumption of an oblate charge distribution about the metal atom (Vzz positive) with an asymmetry parameter η ∼ 0 is only valid in the first approximation. Using free-tin-atom masses and the high-temperature limiting slope of the area under each component of the Mössbauer spectrum doublet, it is possible to calculate a Mössbauer temperature for the π and σ transitions, which for trimethyltin fluoride are 105 ± 2°K and 114 ± 2°K, respectively.