Conformational Analysis, Barriers to Internal Rotation, ab Initio Calculations, and Vibrational Assignment of 4-Fluoro-1-butyne

Abstract

The infrared spectra (3400−300 cm-1) of gaseous, xenon and krypton solutions, and solid state and the Raman spectra (3400−10 cm-1) of the liquid and solid states have been recorded for 4-fluoro-1-butyne, CH2FCH2C⋮CH. These data have been interpreted to show that the molecule exists in the anti conformation (the C−F bond is trans to the C⋮C bond) and the gauche forms in the vapor and liquid, but only the gauche conformer is present in the solid. From variable-temperature infrared studies of xenon and krypton solutions, the anti conformation has been determined to be more stable than the gauche form by 215 ± 22 cm-1 (2.57 ± 0.26 kJ/mol) and 170 ± 17 cm-1 (2.04 ± 0.2 kJ/mol), respectively. The asymmetric torsional fundamentals have been observed at 109.4 and 116.6 cm-1 for the more stable anti and the high-energy gauche conformers, respectively. From these data the asymmetric torsional potential function governing the internal rotation about the C−CH2F bond has been determined and the potential parameters are V1 = 438 ± 14, V2 = −157 ± 12, V3 = 1137 ± 5, and V4 = 17 ± 6 cm-1. This potential function is consistent with the anti to gauche and gauche to gauche barriers of 1142 and 1364 cm-1, respectively, and the dihedral angle FCCC for the gauche conformer of 64°. These data are compared to the corresponding quantities obtained from ab initio calculations, which predict the anti conformer to be the more stable form. Vibrational assignments for the 24 normal modes for both the anti and gauche conformers are proposed. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.