The r0 structural parameters, conformational stability, barriers to internal rotation, and vibrational assignments for trans and gauche ethanol

The infrared spectrum (4000–400 cm−1) of the gas and variable temperature (−55 to −100 °C) spectra of xenon solutions of ethanol (ethyl alcohol), CH3CH2OH, have been recorded. From these data complete vibrational assignments have been made for both the trans (anti) and gauche conformers. By utilizing the conformer pair of the CH3 rock fundamental modes in the xenon solution the enthalpy difference has been determined to be 62 ± 12 cm−1 (0.75 ± 0.14 kJ mol−1) with the trans form the more stable conformer from variable temperature spectra of xenon solutions. It is estimated that there is approximately 40 ± 1% of the trans form present at ambient temperature. By using the previously reported fifteen rational constants of the five heavy atom isotopomers for the trans conformer and the three rotational constants for the gauche form combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, the adjusted-r0 parameters have been obtained. The heavy atom structural parameters for the trans [gauche] are: the distance (Å), C–O = 1.433(3) [1.430(3)], C–C = 1.518(3) [1.523(3)]; and angle (°), CCO = 107.6(5) [112.5(5)], HOCC = 180.0 [56.9(5)]. The ΔJ, ΔJK, ΔK, δJ, δK centrifugal distortion constants have been predicted from ab initio and density functional theory calculations which are compared to the corresponding experimentally determined values. The harmonic force field, infrared intensities, band contours, and vibrational frequencies have been obtained for both conformers from ab initio MP2 (full)/6-31G(d) calculations and the results are compared to the experimental values when appropriate. The results are discussed and compared to the corresponding quantities of some similar molecules.