Vibrational spectra, conformational equilibrium, ab initio calculations and spectral assignments of ethylmethylgermane

The infrared spectra of ethylmethylgermane (CH3CH2GeH2CH3) have been studied as a vapour in the 4000–400 and 500–100 cm−1 regions and as an amorphous solid at 78 K. Raman spectra of the sample as a liquid were recorded at 293 K and polarization data were obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143 K, and small intensity changes of certain bands with temperature were detected. The sample was also investigated as an amorphous solid on a cold finger of copper or a CsI window at 78 K. No crystallization was ever obtained in the Raman or infrared cryostats in spite of extensive annealing.The compound exists a priori in two conformers, anti and gauche, and the variable temperature Raman spectra suggest an equilibrium in which the gauche conformer has 0.4 kJ mol−1 lower enthalpy than anti in the liquid.B3LYP and MP2 calculations with various basis sets and the CBS-QB3 and G2 models were employed, yielding an average conformational enthalpy difference ΔH(anti–gauche) = 0.24 kJ mol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the gauche and anti conformers were calculated. Instead of scaling the calculated wavenumbers in the harmonic approximation, performed in our previous publications, calculations from B3LYP/cc-pVTZ were derived in the anharmonic approximation. In most cases these values gave a good agreement with the experimental results for 34 observed modes of the gauche and 10 modes of the anti conformer.