Vibrational spectra, conformational equilibrium and ab initio calculations of 1,2-diphenylethane

Abstract

The infrared spectra of 1,2-diphenylethane were recorded as a melt and crystalline solid in capillary films and as a pellet in KBr and polyethylene in the 4000–50 cm −1 range. Moreover, the sample was isolated in an argon matrix at ca. 5 K, and the spectra recorded in the range 3100–450 cm −1, before and after annealing. Raman spectra of the melt were recorded between 295 (supercooled) and 357 K, and spectra of the crystalline solid were obtained at ambient temperature. A number of infrared and Raman bands in the melt vanish in spectra of the crystal, and the compound undoubtedly exists as a mixture of two conformers in the melt, probably anti and gauche. Various non-coincidences between the infrared and Raman bands of the crystals agree with C 2h symmetry of the anti conformer. The intensity variations with temperature of two band pairs in the liquid phase Raman spectra were used for van't Hoff plots, giving a value of 2.4±0.5 kJ mol −1 for Δ H ( gauche–anti). The anti conformer had the lower energy and was also present in the crystal. Very small intensity changes were detected when the matrix spectra were annealed to 36 K (argon) revealing that the conformational barrier was too high to allow significant conversion from gauche to the anti conformer even at the highest annealing temperature of 36 K. Ab initio calculations were carried out with the gaussian 98 program at the RHF/6-311G* level and the vibrational frequencies for the anti and gauche conformers including infrared and Raman intensities were calculated. After appropriate scaling, a reasonably good agreement was obtained between the experimental and calculated wavenumbers for both conformers. Nearly all of the 78 fundamentals of the anti and 24 of the gauche conformers were tentatively assigned. However, strong overlap between the anti and gauche conformer bands was observed in most of the spectral region, and additional accidental degeneracy in the spectra prevented reliable assignments for all the vibrational bands.