Conformational stability, r0 structural parameters, barriers to internal rotation, vibrational spectra and ab initio calculations of c-C 3H 5SiH 2CH 3

Abstract

The infrared spectra (3100–40 cm −1) of gaseous and solid and Raman spectra (3200–20 cm −1) of liquid and solid c-C 3H 5SiH 2CH 3, methylsilylcyclopropane, have been recorded. Additionally, variable temperature (−55 to −100 °C) studies of the infrared spectra of the sample dissolved in xenon have been recorded. From these spectral data, two conformers have been identified with one the cis form where the methyl group is over the three-membered ring and the other the gauche form. By utilizing four conformer pairs of vibrational bands the enthalpy difference of the sample dissolved in xenon has been determined to be 98 ± 13 cm −1 (1.17 ± 0.16 kJ mol −1) with gauche the more stable conformer. It is estimated that there is approximately 23 ± 6% of the cis form present at ambient temperature. The enthalpy difference of the liquid was obtained from the Raman spectra to be 101 ± 37 cm −1 (1.21 ± 0.44 kJ mol −1) again with gauche the more stable conformer. The SiH distances of 1.488 and 1.489 Å have been determined from their stretching frequencies. By utilizing the microwave rotational constants for three isotopomers ( 28Si, 29Si, 30Si) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r 0 structural parameters have been obtained for the gauche conformer. The heavy atom distances in Å are: r 0(SiC 2) = 1.852(5); r 0(C 2C 4) = 1.518(3); r 0(C 2C 5) = 1.519(3); r 0(C 4C 5) = 1.500(3); r 0(SiC 6) = 1.871(5) and the angles in degrees are: ∠CSiC = 111.5(5); ∠SiC 2C 4 = 119.9(5); ∠SiC 2C 5 = 119.2(5). For the cis form very small differences of 0.003 and 0.002 Å from those of the gauche form are predicted for the SiC bonds whereas the other distances are predicted to be the same or differ by 0.001 Å. A complete vibrational assignment is given for both the conformers. To support the vibrational assignments, normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. Barriers to internal rotation are reported. The results are discussed and compared to the corresponding properties of some similar molecules.