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

Abstract

The Raman spectra of the glassy phase at 77 K and of the liquid at 235 K, and the infrared spectra of the vapor at 298 and 370 K were recorded for 1-butene. Ab initio calculations were performed at the HF, MP2 and DFT levels, using the 6-31G ∗ (5D) split-valence basis sets, to obtain the equilibrium geometries, vibrational frequencies and infrared intensities for the skew and the syn isomers of the molecule. The enthalpy difference between the skew and syn isomers was measured following the temperature dependence of the infrared pair at 638.1 and 551.8 cm−1 in the vapor phase and it was found to be H°(skew) - H°(syn) = 0.21 ± 0.05 kcal mol−1; the evaluation of the separate energy contributions shows that Δ H° is nearly coincident with the electronic energy difference. A vibrational analysis was performed for both isomers, using the force field determined at the DFT level of theory. With the guide of the computed spectrum, almost all of the fundamentals, with very few exceptions, are detected and attributed. The vibrational assignment is accomplished in terms of group internal coordinates; the potential energy distribution is reported.