Infrared and Raman spectra, ab initio calculations, barriers to internal rotation, and vibrational assignment of 4-fluoro-2-pentyne

Abstract

The infrared spectra (3500–50 cm −1) of the gas and solid and the Raman spectra (3500–50 cm −1) of the gas, liquid and solid have been recorded for 4-fluoro-2-pentyne, CH 3C≡CCHFCH 3. Equilibrium geometries and energies have been determined by ab initio (HF and MP2) and hybrid density functional theory (DFT) by the method B3LYP calculations with a number of basis sets. A vibrational assignment is proposed based on the force constants, relative intensities, depolarization ratios from the ab initio and DFT calculations and rotational–vibrational band contours obtained by using the calculated equilibrium geometries. The methyl group attached to the C≡C triple bond exhibits almost completely free rotation and this is accompanied by the observation of Coriolis sub-band structure on two of the near degenerate methyl vibrations. The Coriolis coupling constants have values of 0.181±0.033 and −0.137±0.078 for the stretch and deformation, respectively. Torsional transitions of the other methyl group are evident in the far infrared spectrum from which a barrier to internal rotation of 1465±30 cm −1 is derived which agrees well with the theoretically predicted barrier value. The results are compared to the corresponding quantities of some similar molecules.