AB-INITIO CALCULATIONS OF VIBRATIONAL FREQUENCIES AND INFRARED INTENSITIES FOR GLOBAL WARMING POTENTIAL OF CFC SUBSTITUTES - CF3CH2F (HFC-134A)

We have investigated the feasibility of using ab initio molecular orbital methods for predicting the global warming potential of the proposed chlorofluorocarbon (CFC) substitute CF3CH2F, HFC-134a. Various levels of theory and basis sets were used to optimize geometry and calculate harmonic vibrational frequencies and infrared intensities for the molecule using the GAUSSIAN 92 software package. In attempting to assess the quality of the computations, we found it necessary to reconsider the vibrational assignments available in the literature. We have remeasured the infrared spectrum of the vapor from 400 to 4000 cm(-1) at a resolution (0.08 cm(-1)) sufficient to resolve some overlapping fundamentals and to assign symmetry species unambiguously for several bands on the basis of their rotational contours. The higher resolution spectra and the results of the computations together permit a firm assignment of all 18 fundamentals to be made. Some bands previously assigned as fundamentals are found to be combination or impurity bands. On the basis of the current assignment, we find that for the highest level calculation, MP2/6-31G**, the calculated harmonic frequencies agree extremely well with the experimentally observed ones at frequencies below 800 cm(-1), with a systematic error toward higher calculated frequencies becoming apparent above 800 cm(-1). At lower levels of theory, the systematic error is apparent at all frequencies. The regularity of the deviation between calculated and observed frequencies makes ab initio calculations of vibrational frequencies much more useful than semiempirical calculations, which tend to show random deviations, as demonstrated with a PM3-UHF calculation in this work. The calculated absolute intensities are in good agreement with the limited experimental measurements previously reported, and the observed relative intensities for the fundamentals are also in approximate agreement with our calculated values.