HALON THERMOCHEMISTRY - AB-INITIO CALCULATIONS OF THE ENTHALPIES OF FORMATION OF FLUOROETHANES

The ab initio G2, G2(MP2), CBS-4 and CBS-Q quantum mechanical protocol s and the parameterized BAC-MP4 procedure were used to calculate the e nthalpies of formation (a PO) of ethane and the complete series of flu oroethanes, C2Hx,F6-x,x=0-5., Results from all methods exhibited signi ficant negative deviations from experiment. With the exception of the CBS-4 and BAC-MP4 procedures, the negative errors in the calculated en thalpies were observed to be linearly dependent upon the number of CF bonds in the molecule. Application of a bond additivity correction (BA G) parameter, Delta(C-F), derived in an earlier investigation of fluor o- and chlorofluoromethanes removed some although not all of the syste matic deviations. Introduction of a heavy atom interaction parameter, representing the effect of an attached carbon on the C-F bond error, y ielded corrected enthalpies which agree with experiment to within the reported uncertainties. The BAC-MP4 method, which has already been par ameterized with generalized BACs, yields calculated enthalpies which a verage approximately 10 kJ mol(-1) below the experimental values of De lta(f)H degrees in the fluoroethanes. (C) 1998 Elsevier Science B.V.