HALON THERMOCHEMISTRY - CALCULATED ENTHALPIES OF FORMATION OF CHLOROFLUOROMETHANES

The ab initio G2, G2(MP2), CBS-4, CBS-Q, and BAC-MP4 methods have been used to calculate the enthalpies of formation of the series of four c hloromethanes and six chlorofluoromethanes [CHxFyCl4-x-y, x = 0-3, y = 0-3]. Calculated values of Delta(f)H degrees using the first four ab initio methods exhibit comparatively large systematic negative errors compared with experimental values, up to -50 kJ/mol, which are nearly linearly dependent upon the number of C-F and C-Cl bonds in the molecu le. It is found for the chlorofluoromethanes that the application of b ond additivity corrections (BAC's) to the ab initio enthalpies effecti vely removes systematic errors in the calculations and yields values t hat are in close agreement with experimentally derived heats. The rms deviations of the corrected calculated enthalpies from the experimenta l values are 2.4, 2.6, 3.4, 4.7, and 3.8 kJ/mol for the G2(MP2), G2, C BS-Q, CBS-4, and BAC MP4 methods, respectively. These deviations are l ower than the rms errors (6.9 kJ/mol) in the experimental enthalpies. Therefore, it is concluded that any of these calculational procedures, together with bond additivity corrections to remove systematic error, may profitably be used to obtain accurate enthalpies of formation in chlorofluorocarbon species.