VIBRATIONAL-SPECTRA AND CONFORMATIONS OF 2,2-DI(FLUOROMETHYL)-1,3-DIFLUOROPROPANE AND ASSIGNMENTS BASED UPON ABINITIO FORCE-FIELDS

The IR spectra of 2,2-di(fluoromethyl)-1,3-difluoropropane were record ed in the vapour and liquid states and as a solute in solvents of diff erent polarity. Additional IR spectra were obtained for plastic and cr ystalline solids at low temperature and at high pressure, and also of tbe compound isolated in argon and nitrogen matrices, using the hot no zzle technique. Corresponding Raman spectra of the liquid, of solution s and of tbe plastic phase were recorded at different temperatures in addition to the crystalline solid. There are six possible staggered co nformations with symmetries D2d, S4, C1, C1, C2 and C(s) in this mole cule, and all but the last two were detected in the spectra. The two m ost stable conformers are S4 and D2d Which have no 1,3-parallel C-F in teractions. Among them S4 is ca. 0.5 and 1 kJ mol-1 lower in energy th an D2d in the plastic phase and in the matrices, respectively, and S4 is present in the low-temperature and high-pressure crystals. A plasti c phase, in which more conformers were present, was observed at temper atures below T(melt) and at high pressure. The C, and C, conformers both have enthalpies 5-10 kJ mol-1 higher than S4 and their bands are enhanced at increased temperatures both in the plastic crystal and in the matrices. They have dipole moments around 2-3 D, and the intensiti es of their bands increase in polar solvents. The enthalpy differences DELTAH-degrees were calculated from variable-temperature measurements in the plastic crystal and in the matrices, and confirmed by Gaussian 90 with basis sets 6-31G and 6-31G and by semi-empirical and molecul ar mechanics calculations: MNDO, AM1 and MM2. A nearly complete assign ment of the fundamentals belonging to S4 is presented. Additional band s are attributed to the D2d, C1 and C1 conformers from annealing expe riments at different temperatures in the N2 matrix and in the amorphou s phase. The assignments are based on the spectral observations and on the results of force constant calculations, involving scaled ab initi o force constants.