Vibrational spectra including matrix isolation and conformations of 1,1,2-trichloro-2,3,3-trifluorocyclobutane

Abstract

The IR and Raman spectra of 1,1,2-trichloro-2,3,3-trifluorocyclobutane have been reinvestigated, including matrix isolation IR spectroscopy in argon and nitrogen matrixes, using the hot nozzle technique. New Raman spectra of the vapour, liquid, plastic and crystalline phases were recorded, and IR high pressure spectra of the liquid and solid phases were obtained. The compound has two conformers of nearly equal stability, with an enthalpy difference (Δ H 0 ) equal to 0.0±0.9 in the vapour, 0.9±0.7 in the liquid and 0.4±0.3 kJ mol −1 in the plastic phase, while the values +0.4 (argon) and −0.2 (nitrogen) kJ mol −1 for the vapour were obtained from the unannealed matrix spectra. The argon and nitrogen matrixes favoured the more stable and the less stable conformer, respectively. A Δ H 0 value of −0.5±0.2 kJ mol −1 in the nitrogen matrix was obtained by measuring the conformational equilibrium between 30 and 33 K. The barrier to inversion was found to be about 9 kJ mol −1 in the nitrogen matrix. The Δ G 0 values for conformational change in the two matrixes were calculated from the spectra to differ by 0.8 kJ mol −1. A plastic phase containing both conformers in thermodynamic equilibrium was present between 194 and 240 K under atmospheric pressure, whereas an anisotropic phase existed below 194K. A considerable hysteresis was obtained both for the melting and phase transitions. The plastic and anisotropic crystals were also observed under pressure and studied in a polarization microscope. The more stable conformer was present both in the low temperature and in the high pressure crystals. Ab initio force constants were calculated for both conformers and an energy difference between the two of 0.79 kJ mol −1 was obtained using the 3–21 G * basis set. The scaled force constants gave wavenumbers in good agreement with the experimental values, and complete assignments of the fundamentals are presented for both conformers. From the force constant calculation it is clear that the more stable conformer is that in which the fluorine atom on the asymmetric carbon is in the equatorial position.