Ab initio calculations of the heats of formation for (CH)6 isomers

The heats of formation for the five (CH)(6) isomers have been calculated using both the atomization and isodesmic bond separation schemes at the following six theoretical levels: G2(MP2,SVP), G2(MP2), G2(MP3), G2, CBS-Q, and CBS-APNO. It is found that, with the isodesmic scheme, all four G2-based methods give Delta H-f values that are within a few kJ mol(-1) of each other. In particular, the G2(MP2,SVP) and G2 yield the most accurate Delta H-f values for benzene, the only isomer whose listed experimental data are suitable for comparison with ab initio results. For the CBS-Q and CBS-APNO levels, the isodesmic scheme gives worse results than the atomization scheme, but the results from the two levels are more consistent in the isodesmic scheme. These results suggest that a combination of either the G2(MP2,SVP) or G2 model with the isodesmic scheme should afford accurate thermochemical data for relatively large hydrocarbon compounds. At the G2 level, using the isodesmic bond separation scheme, we obtain the following Delta H-f0 values (in kT mol(-1)) for the five (CH)(6) isomers: 100.5 for benzene (compared to the experimental value of 100.4 +/- 1), 415.5 for Dewar benzene, 567.2 for prismane, 397.5 for benzvalene, and 593.6 for 3,3'-bicyclopropenyl. At the same level and with the same scheme, the calculated Delta H-f298 values (in kT mol(-1)) are: 82.0 for benzene (compared to the experimental value of 82.9 +/- 0.3), 397.1 for Dewar benzene, 547.0 for prismane, 378.1 for benzvalene, and 578.8 for 3,3'-bicyclopropenyl. (C) 1998 Elsevier Science B.V. All rights reserved.