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

Abstract

Abstract The IR and Raman vibrational spectra of the methyl(trimethylstannyl)silanes Me 3 Sn(SiMe 2 ) n SnMe 3 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 Δ H = H gauche − H anti 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 SnMe 3 groups (pentane effect), the concentration in the equilibrium mixture is negligibly small. For 4 , eight (2 3 ) 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.