A Raman spectroscopic study of the rotational isomerism of linear permethylated stannylsilanes Me3Sn(SiMe2)nSnMe3 with n=1, 2, 3, 4

The IR and Raman vibrational spectra of the methyl(trimethylstannyl)silanes Me3Sn(SiMe2)nSnMe3 with n=1 (1), 2 (2), 3 (3) and 4 (4) were recorded at various temperatures. Compound 2 consists of a mixture of two conformers, gauche (G) and anti (A), in the liquid state. By variable temperature Raman spectroscopy, the enthalpy difference ΔH=Hgauche−Hanti was determined from the relative intensities of the symmetric SiSn stretching bands as 0.8±0.3 kJ mol−1. In the Raman spectrum of the crystalline solid, the lines of the G conformer disappear completely, facilitating the assignment of the fundamental vibrations for the A form. No indication of the presence of a third (ortho) conformer could be detected. HF-SCF ab initio calculations using electron core potentials for the heavy atoms predict the existence of three conformers on the potential surface. In the Raman spectra of 3, the symmetric SiSn stretching vibration splits into three components, which can be assigned to the G+G+, GA and AA conformers. The G+G− rotamer could not be observed. As its energy is significantly larger than that for the other conformations due to repulsive four bond interactions between the SnMe3 groups (pentane effect), the concentration in the equilibrium mixture is negligibly small. For 4, eight (23) spectroscopically distinct (classical) conformations are possible. In the room temperature Raman spectrum, the SiSn stretching vibration consists of a line at 330 cm−1 with a broad shoulder at 320 cm−1. In the solid (−60°C), a single rotamer (probably the all anti form) survives. Upon heating, several conformational states with higher energies are populated which could not be resolved by Raman spectroscopy.