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

Atomic equivalent (AEQ), BAC-MP4, G2(MP2), G2, CBS-4, CBS-Q, and CBS-Q CI/APNO molecular orbital calculations were used to calculate enthalpi es of formation in the series of fluoromethanes, CHxF4-x, x = 0-4. Whi le the computed Delta(f)H degrees(BAC-MP4) and Delta(f)H degrees(CBS-4 ) were in close agreement with experiment, errors in enthalpies from t he other five methods were relatively high. In particular, enthalpies of formation calculated with the G2(MP2) and G2 procedures exhibited s ystematic deviations from experiment which were linearly dependent upo n the number of C-F bonds in the molecule. Application of isodesmic re action calculations yielded values of Delta(f)H degrees(G2(MP2)), Delt a(f)H degrees(G2), Delta(f)H degrees(CBS-Q), and Delta(f)H degrees(CBS -QCI/APNO) that were in remarkably close agreement with experiment. Th is technique had no significant effect on the quality of results from the AEQ, BAC-MP4, and CBS-4 methods.