Infrared and Raman spectra, ab initio calculations of structure and vibrational assignment of 1-fluoro-2-butyne

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-fluoro-2-butyne, CH3-C-triple bond-C-CH2F. Equilibrium geometries and energies have been determined by ab initio and hybrid DFT methods using 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 on vibrational-rotational band contours obtained using the calculated equilibrium geometries. From calculated energies it is shown that the CH3 group exhibits almost completely free rotation which is in agreement with the observation of Coriolis sub-band structure in two of the degenerate methyl vibrations. The results are compared to the corresponding quantities for some similar molecules.