Infrared and Raman spectra, conformational stability, ab initio calculations and vibrational assignment of 1-hexen-4-yne

Abstract

The infrared spectra (3500–50 cm⁻¹) of the gas and solid and the Raman spectra (3500–50 cm⁻¹) of the liquid and solid have been recorded for 1-hexen-4-yne, CH₂CHCH₂CCCH₃. Also variable temperature studies over the temperature range −105 to −150 °C of the infrared spectra (3500–400 cm⁻¹) of the sample dissolved in liquid krypton have been carried out. By utilizing these data from seven cis/gauche conformer pairs, the cis conformer is found to be the lower energy form with an enthalpy difference of 233 ± 23 cm⁻¹ (2.79 ± 0.28 kJ mol⁻¹). At room temperature it is estimated that there is 39 ± 2% of the gauche conformer present. Equilibrium geometries and energies of the two conformers have been predicted from ab initio calculations by perturbation theory (MP2) with full electron correlation to second order and hybrid density functional theory (DFT) calculations by the B3LYP method with a number of basis sets. A complete vibrational assignment is proposed for the cis conformer based on the vibrational-rotational band contours, relative intensities, Raman depolarization ratios and predicted frequencies from the ab initio and DFT calculations and many of the fundamentals for the gauche form have been assigned. The observation of sub-band structure for two of the nearly degenerate methyl vibrations indicates that the CH₃ group has almost free internal rotation which is consistent with the ab initio predicted barrier of 100 cm⁻¹. From the spacings of the sub-bands the values of the Coriolis coupling constants, ζ, have been determined for the CH₃ stretch and deformation.