Vibrational spectra and conformations of 2,2‐di(fluoromethyl)‐1,3‐difluoropropane and assignments based upon ab initio force fields

The IR spectra of 2,2-di(fluoromethyl)-1,3-difluoropropane were recorded in the vapour and liquid states and as a solute in solvents of different polarity. Additional IR spectra were obtained for plastic and crystalline solids at low temperature and at high pressure, and also of the compound isolated in argon and nitrogen matrices, using the hot nozzle technique. Corresponding Raman spectra of the liquid, of solutions and of the plastic phase were recorded at different temperatures in addition to the crystalline solid. There are six possible staggered conformations with symmetries D2d, S4, C1*, C1, C2 and Cs in this molecule, and all but the last two were detected in the spectra. The two most stable conformers are S4 and D2d which have no 1,3-parallel CF interactions. Among them S4 is ca. 0.5 and 1 kJ mol−1 lower in energy than D2d in the plastic phase and in the matrices, respectively, and S4 is present in the low-temperature and high-pressure crystals. A plastic phase, in which more conformers were present, was observed at temperatures below Tmelt and at high pressure. The C1* and C1 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 intensities of their bands increase in polar solvents. The enthalpy differences ΔH° 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 semiempirical and molecular mechanics calculations: MNDO, AM1 and MM2. A nearly complete assignment of the fundamentals belonging to S4 is presented. Additional bands are attributed to the D2d, C1* and C1 conformers from annealing experiments at different temperatures in the N2 matrix and in the amorphous phase. The assignments are based on the spectral observations and on the results of force constant calculations, involving scaled ab initio force constants.