Infrared and Raman spectra, conformational stability, ab initio calculations of structure and vibrational assignment of 5-fluoropent-2-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 were recorded for 5-fluoropent-2-yne, CH₃–CC–CH₂CH₂F. Variable temperature studies of the infrared spectrum (3500–400 cm⁻¹) of 5-fluoropent-2-yne dissolved in liquid krypton and xenon were also recorded. Utilizing anti–gauche conformer pairs, the enthalpy difference was determined as 272 ± 11 cm⁻¹ (3.25 ± 0.13 kJ mol⁻¹) and 297 ± 31 cm⁻¹ (3.55 ± 0.37 kJ mol⁻¹) from the xenon and krypton solutions, respectively, with the anti rotamer the more stable form. Equilibrium geometries and energies of the two conformers have been determined by ab initio and hybrid density functional theory (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 sub-band structure in the degenerate methyl vibrations from which values of the Coriolis coupling constant, ζ, were determined. The results are discussed and compared with the corresponding quantities for some similar molecules.