Abstract. We have employed the radiative forcing function recently computed by Pinnock et al. [1995], in conjunction with infrared cross sections and vibrational frequencies calculated with ab initio quantum-mechanical methods, to estimate the instantaneous infrared radiative forcing terms-'•ot of a number of CFC substitutes and their atmospheric reaction products. The computed quantities-•>•,t are compared with those obtained by Pinnock et al. [ 1995] employing experimental infrared spectra. In particular, Gaussian-92 TM software has been utilized, at the MP2/6-31G** level of theory, to provide computational estimates of these quantities. Our results agree with the former, with a mean and a standard deviation of the difference of +3% and 15%, respectively. An error analysis is provided to assess the utility of the method. The capability of quantum-mechanical computational methods to explore structural trends in radiative and physicochemical properties is exploited for the molecular systems examined here.
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.
To gain insight into the interpretation of the atomic polar
tensors (APT) in the series of fluoroethanes, the
fluorine, carbon, and hydrogen APT components
were calculated, in the inertial-axes coordinate system,
and examined by determining the mean dipole derivatives
p̄
α, the King effective atomic charges
χα, and APT
anisotropies βα. From this, relationships between
chemical bonding environment and p̄
α
terms are clearly
discernible, with the carbon-atom charges exhibiting a labile response
to the degree and form of fluorine
substitution. We examine the dependence of the total absolute
infrared intensities, as predicted by the APT
sum rules, on the degree of fluorine substitution and molecular
structure for the entire set of fluoroethanes.
Finally, dipole-moment components were calculated (i) directly
from ab initio methods at the MP2/6-31G**
level, (ii) from the
terms, and (iii) as the first moments of the
p̄
α atomic charges at the
nuclear
equilibrium positions, and a detailed comparison was made.
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