Infrared and Raman spectra, conformational stability, barrier to internal rotation, and ab initio calculations of 1,1-difluoropropane

Abstract

The infrared (3200–40cm−1) and Raman (3200–20cm−1) spectra of gaseous and solid 1,1-difluoropropane, CH3CH2CHF2, have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained. Both the gauche and trans conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (−55 to −150°C) studies of the infrared spectra of the sample dissolved in liquid xenon and krypton have been recorded. From these data the enthalpy difference has been determined to be 127±6cm−1 (1.52±0.07kJ/mol) in xenon and 65±5cm−1 (0.78±0.06kJ/mol) in krypton with the gauche conformer the more stable rotamer. A complete vibrational assignment is proposed for the gauche conformer based on infrared band contours, relative intensities, depolarization values and group frequencies. Several of the fundamentals for the trans conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G(d) basis set at the levels of restricted Hartree–Fock (RHF) and with full electron correlation by the perturbation method to second order Moller–Plesset (MP) with four different basis sets up to 6-311+G(2d,2p). The frequencies for the normal modes and the potential energy terms for the conformer interconversion have been obtained from the MP2/6-31G(d) calculation. The results are discussed and compared to those obtained for some similar molecules.