Infrared and Raman spectra, ab initio force field and vibrational assignment of the rotational isomers of 1-butene

The conformational stability of gaseous 1-butene studied by femtosecond nonlinear spectroscopy and ab initio calculations

“…Since the dipole moment of the cis is slightly higher than that of the gauche conformer, it may be more stabilized in liquid phase. In the variable temperature study with gaseous 1-butene [3] two spectral lines assigned to the cis and the gauche conformers were recorded at two different temperatures: 298 and 370 K. The ratio of the gauche to cis intensity was found to be smaller at 298 than at 370 K, indicating that the cis form is lower in energy than the gauche one. However, at the temperature range employed in all these variable temperature studies quite many torsional states are populated and contribute to the signal.…”

“…All previous theoretical calculations [3,4,10,9] predict the energetic preference of the gauche form over the cis one with an energy difference of about 35-200 cm −1 .…”

“…Some comments should be made on the variable temperature infrared and Raman studies [2][3][4], where the opposite was found, namely that the cis conformer is the most stable form. All these experiments except for Ref.…”

“…All these experiments except for Ref. [3] have been performed in liquid 1-butene. Since the dipole moment of the cis is slightly higher than that of the gauche conformer, it may be more stabilized in liquid phase.…”

“…It was also reported that the vibrational band ascribed to the nearly planar conformer disappeared under crystallization indicating a non-planar form to be the most stable one. In subsequent infrared studies [2][3][4] the opposite was found: of the two conformers, cis and gauche, the former one is the most stable form in the liquid as well as in the gas phase. The gauche-cis enthalpy difference was determined to be 77 cm −1 in liquid [2], 26 cm −1 in gas phase [2], 73 cm −1 in gas phase [3], 73 cm −1 in krypton [4] and 64 cm −1 in xenon [4].…”