Microwave, Raman, and far infrared spectra, barrier to internal rotation, and dipole moment of 2,2-difluoropropane

Abstract

The microwave spectrum of 2,2-difluoropropane has been investigated in the region from 26.5 to 40.0 GHz. Only b-type transitions were observed and these were assigned on the basis of the rigid rotor model. The rotational constants in the ground vibrational state were found to be A = 5149.79±0.04, B = 4840.11±0.02, and C = 4805.62±0.02 MHz. Two excited states were also identified and assigned as the excited states of the two methyl torsional modes. From relative intensity measurements the frequencies for these two methyl torsional modes were determined to be ν(A2) = 217±15 and ν(B2) = 250±30 cm−1. From the Stark effect the dipole moment was determined to be ‖μb‖ = ‖μt‖ = 2.40±0.02 D. No splitting due to the internal rotations of the methyl groups were observed in the ground or first excited states. From a diagnostic least-squares adjustment to fit the three rotational constants, the following heavy atom structural parameters were obtained: r(C–F) = 1.352±0.013 Å; r(C–C) = 1.532±0.013 Å; ∢FCF = 106.09±1.44°; ∢CCC = 112.54±0.79°. These parameters are compard to the corresponding ones in some related molecules. The Raman spectra (0–3500 cm−1) were recorded for the gaseous, liquid, and solid states of 2,2-difluoropropane. The far infrared spectra (40–520 cm−1) of the gas and the solid were also investigated. A revised assignment is presented for the skeletal bending modes. The methyl torsions were observed in the Raman spectrum of the solid at 255 and 285 cm−1 from which a periodic barrier of 1675 cm−1 (4.79 kcal/mole) for the methyl internal rotation was calculated for the solid state. From combination and overtone bands of the methyl torsions, the barrier for the gaseous state has been determined to be 1207 cm−1 (3.45 kcal/mole). These results are compared to the corresponding quantities in related molecules.