Conformational stability, r0 structural parameters, barriers to internal rotation, vibrational assignments and ab initio calculations of c-C3H5GeH2CH3

The infrared spectra (3100–40 cm−1) of gaseous and amorphous solid and Raman spectra (3200–20 cm−1) of the liquid at various temperatures for methylgermylcyclopropane, c-C3H5GeH2CH3, have been obtained. 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 (syn) form where the methyl group is over the three-membered ring and the other the gauche form. By utilizing six conformer pairs of the vibrational bands the enthalpy difference of the sample dissolved in xenon has been determined to be 43 ± 11 cm−1 (0.51 ± 0.13 kJ mol−1) with the gauche form the more stable conformer. It is estimated that there is approximately 30 ± 3% of the cis form present at ambient temperature. It was not possible to achieve crystallization of the compound by annealing it, and both conformers were present at all temperatures. The Ge–H distances of 1.531 and 1.533 Å for the gauche conformer have been determined from their stretching frequencies. By utilizing the microwave rotational constants of the gauche conformer for five isotopomers (70Ge, 72Ge, 73Ge, 74Ge, 76Ge) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, the adjusted r0 structural parameters have been obtained. The heavy atom distances (Å) are: (GeC2) = 1.925(5); (C2C4) = 1.517(3); (C2C5) = 1.519(3); (C4C5) = 1.502(3); (GeC6) = 1.947(5) and the angles (°) are: ∠∠CGeC = 110.6(5); ∠∠GeC2C4 = 120.1(5); ∠∠GeC2C5 = 119.2(5). For the cis form very small differences of 0.003 and 0.001 Å from those of the gauche form are predicted for the GeC bonds whereas the other distances are predicted to be the same or differ at the most by 0.002 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 have been predicted. The results are discussed and compared to the corresponding properties of some similar molecules.