Reassignment of the vibrational spectra of CHF2CH3 (HFC-152a), CF3CH3 (HFC-143a), CF3CHF2 (HFC-125), and CHCl2CF3 (HCFC-123)

We provide new or revised vibrational assignments for three hydrofluorocarb ons (HFCs) and one hydrochlorofluorocarbon (HCFC) through the combined use of experimental absolute infrared intensity measurements, experimental Rama n measurements and ab initio computations of vibrational frequencies, absol ute infrared intensities, and Raman intensities. CHF2CH3 (HFC-152a), CF3CH3 (HFC-143a), CF3CHF2 (HFC-125), and CHCl2CF3 (HCFC-123) are the molecules i nvestigated in this study. We have measured the vapor infrared spectra from 400 to 4000 cm(-1) at a resolution (0.08 cm(-1)) sufficient to resolve som e overlapping fundamentals and to assign symmetry species for several bands on the basis of their rotational band contours and absolute infrared inten sities. Raman spectra were measured for the HFCs at pressures between 3.8 a nd 11.1 atm and for HCFC-123 in the liquid phase. Second-order Moller-Pless ett (MP2) level of theory and the 6-31G** basis set were used to optimize m olecular geometry and calculate harmonic vibrational frequencies and infrar ed intensities; Hartree Fock (HF) level of theory was used to calculate Ram an intensities. The higher-resolution infrared spectra and experimental abs olute infrared intensities. the Raman spectra and relative Raman intensitie s, together with the results of the computations, allow a firm assignment o f previously ambiguous bands. On the basis of the current assignments, we f ind that the scaled MP2/6-31G** frequencies are in good agreement with the observed frequencies. Furthermore, the calculated absolute infrared intensi ties and calculated Raman intensities are generally in good agreement with the experimental measurements. (C) 1999 Elsevier Science B.V. All rights re served.