Ab Initio Calculations of Vibrational Frequencies and Infrared Intensities for Global Warming Potential of CFC Substitutes: CF3CH2F (HFC-134a)

Abstract

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-' at a resolution (0.08 cm-I) 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 fm 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, MP2/6-3 1G**, the calculated harmonic frequencies agree extremely well with the experimentally observed ones at frequencies below 800 cm-I, with a systematic error toward higher calculated frequencies becoming apparent above 800 cm-I. At lower levels of theory, the systematic error is apparent at all frequencies. The regularity of the deviation between calculated and observed frequencies makes a b 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.

Infrared radiative forcing of CFC substitutes and their atmospheric reaction products

“…Details of these calculations are provided by Papasavva et al [1995Papasavva et al [ , 1996Papasavva et al [ , 1997 and ?apasavva [1997]. We have investigated a variety of levels of theory and found that the second-order MollerPiesset perturbation theory, using the 6-31G** basis set, MP2/6-31G**, represents a reasonable compromise between the accuracy of method and the limitations of size (of which the total number of electrons is the most useful gauge) of molecular systems that can be computed.…”

Assessment of theoretical methods for the study of hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation (IUPAC Technical Report)

“…Previously, this parameter was determined from degradation rates and infrared vibrational spectroscopy but recently, theoretical approaches have been used to estimate GWPs. Papassavva et al [76] used ab initio computations of vibrational frequencies and infrared intensities to calculate the GWP of CF 3 CH 2 F. The calculated harmonic frequencies using MP2/6-31G(d,p) method agreed with the experimentally observed frequencies below 800 cm -1 . The calculated absolute intensities were in good agreement (better than 10 %) with the limited experimental measurements previously reported.…”

Valence electronic structure and photofragmentation of 1,1,1,2-tetrafluoroethane (CF3-CH2F)

“…The computational results have been used to assign the structures observed in the experimental valence photoelectron spectrum of 1,1,1,2-tetrafluoroethane. Previous theoretical studies have included the calculation of the equilibrium geometry [30,31] and have been compared with experimental results, for example, with microwave spectroscopy by Ogata et al [32]. Due to 1,1,1,2-tetrafluoroethane's atmospherical abundancy, studies reported by Papasavva et al [30,31] concentrated on calculating vibrational frequencies and infrared intensities in order to estimate the global-warming potential of the molecule.…”

“…Previous theoretical studies have included the calculation of the equilibrium geometry [30,31] and have been compared with experimental results, for example, with microwave spectroscopy by Ogata et al [32]. Due to 1,1,1,2-tetrafluoroethane's atmospherical abundancy, studies reported by Papasavva et al [30,31] concentrated on calculating vibrational frequencies and infrared intensities in order to estimate the global-warming potential of the molecule. The calculated results presented by Zhou et al [28] included also the minimum-energy geometry of both the neutral and cationic ground state of 1,1,1,2-tetrafluoroethane, as well as enthalpies of formation of the molecule and its fragments.…”