Structural parameters, barriers to internal rotation, normal coordinate analysis and quantum mechanics calculations of 1,1,1-trimethyldisilane

Abstract

The molecular structure, vibrational frequencies and barriers to internal rotation of the staggered and eclipsed conformations of 1,1,1-trimethyldisilane, H 3SiSi(CH 3) 3, have been studied using ab initio molecular orbital calculations. In these calculations, up to 6-311+G(d,p) basis set at the levels of RHF and MP2 with full electron correlation were employed as well as the hybrid density functionals at the theoretical level of B3LYP. From these data, the energy difference between the staggered and eclipsed conformations has been determined with an average of 318 cm −1 (0.91 kcal/mol) in favor of the staggered rotamer, while, the eclipsed form has been barred due to the presence of an imaginary torsion frequency. The –SiH 3 and –CH 3 barriers of 1.05 and 1.98 kcal/mol, respectively, have been predicted from potential function scans at MP2/6-31(d) basis set with full electron correlations. The r o Si–H bond distance of 1.4885 Å has been obtained from the observed Q-branch in the gaseous infrared spectrum. The calculated infrared and Raman spectra were obtained using harmonic vibrational frequencies, infrared intensities and Raman activities from RHF/3-21G(d) in addition to MP2 and B3LYP levels at 6-31G(d) basis set. The current study provides a complete vibrational assignment for all the vibrational modes, which is supported by normal coordinate analysis, force constants and potential energy distributions. The results are discussed and theoretical values are compared to experimental values whenever possible.