Conformational stability, infrared and Raman spectra, and vibrational assignment of ethyl bromogermane

Abstract

The infrared spectra (3200–30 cm–1) of ethyl bromogermane, CH3CH2GeH2Br, in the gaseous phase and of amorphous and annealed solid samples as well as the Raman spectrum (3200–100 cm–1) of the liquid have been recorded. Variable-temperature (–105 to –150 °C) studies of the infrared spectra of a sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference between the gauche and trans conformers has been determined to be 158 ± 16 cm–1 (1.89 ± 0.20 kJ/mol), with the gauche conformer being the more stable form. This result is consistent with the prediction from ab initio calculations at both the Hartree–Fock level and with full electron correlation by the perturbation method to second order. It is estimated that at ambient temperature 81 ± 2% of the sample is in the gauche form. A relatively complete vibrational assignment is proposed for both the gauche and trans conformers based on infrared band contours, relative intensities, depolarization values, and group frequencies which is supported by normal coordinate calculations utilizing the force constants from the ab initio MP2/6-31G(d) calculations. The optimized geometries and conformational stabilities have also been obtained from ab initio calculations utilizing several different basis sets with full electron correlation by the perturbation method up to MP2/6-311+G(2d,2p). Additionally, the potential functions governing the conformation interchange have been estimated from ab initio MP2/6-31G(d) and density functional theory calculations by the B3LYP method, with barriers to conformational interchange of 560, 924, and 852 cm–1 from MP2/6-31G(d) and 418, 771, and 606 cm–1 from B3LYP/6-31G(d) calculations for the trans to gauche, gauche to gauche, and gauche to trans conformers, respectively. The results are discussed and compared to some corresponding quantities for several related molecules.Key words: conformational stability, Raman spectra, infrared spectra; ab initio calculations, ethyl bromogermane.