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Halon Thermochemistry: Ab Initio Calculations of the Enthalpies of Formation of Fluoromethanes.


pdf icon Halon Thermochemistry: Ab Initio Calculations of the Enthalpies of Formation of Fluoromethanes. (957 K)
Berry, R. J.; Ehlers, C. J.; Burgess, D. R. F., Jr.; Zachariah, M. R.; Nyden, M. R.; Schwartz, M.

Journal of Molecular Structure (Theochem), Vol. 422, 89-98, 1998.

Keywords:

halons; thermochemistry; enthalpy; heat of formation

Abstract:

The ab initio G2, G2(MP2), CBS-4 and CBS-Q quantum mechanical protocols and the parameterized BAC-MP4 procedure were used to calculate the enthalpies of formation of ethane and the complete sereis of fluoroethanes. Results from all methods exhibited significant negative deviations from experiment. With the exception of the CBS-4 and BAC-MP4 procedures, the negative errors in the calculated enthalpies were observed to be linearly dependent upon the number of C-F bonds in the molecule. Application of a bond additivity correction (BAC) parameter derived in an earlier investigation of fluoro- and chlorofluormethanes, removed some although not all of the systematic deviations. Introduction of a heavy atom interaction parameter, representing the effect of an attached carbon on the C-F bond error, yielded corrected enthalpies which agree with experiment to within the reported uncertainties. The BAC-MP4 method, which has already been parameterized with generalized BACs, yields calculated enthalpies which average apprximately 10 kJ mol-1 below the experimental values of [see article] in the fluoroethanes.