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Vibrational spectra and conformations of 2,2‐di(fluoromethyl)‐1,3‐difluoropropane and assignments based upon ab initio force fields
Author(s) -
Gatial A.,
Sablinskas V.,
Klaeboe P.,
Powell D. L.,
Nielsen C. J.
Publication year - 1993
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.1250240507
Subject(s) - conformational isomerism , raman spectroscopy , chemistry , crystallography , ab initio , phase (matter) , spectral line , infrared spectroscopy , dipole , molecule , organic chemistry , physics , astronomy , optics
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 D 2 d , S 4 , C 1 *, C 1 , C 2 and C s in this molecule, and all but the last two were detected in the spectra. The two most stable conformers are S 4 and D 2 d which have no 1,3‐parallel CF interactions. Among them S 4 is ca. 0.5 and 1 kJ mol −1 lower in energy than D 2 d in the plastic phase and in the matrices, respectively, and S 4 is present in the low‐temperature and high‐pressure crystals. A plastic phase, in which more conformers were present, was observed at temperatures below T melt and at high pressure. The C 1 * and C 1 conformers both have enthalpies 5–10 kJ mol −1 higher than S 4 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 Δ H ° 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 S 4 is presented. Additional bands are attributed to the D 2 d , C 1 * and C 1 conformers from annealing experiments at different temperatures in the N 2 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.