Abstract
Heats of formation for benzene, cyclohexane and other selected hydrocarbons, along with the heats of several isodesmic and isogyric reactions involving these species, are reported at the G2,G2(MP2) and G2(MP2,SVP) levels of theory. All three models predict heats of isodesmic reactions very accurately. G2 heats of isogyric non-isodesmic reactions are also reliable, but G2(MP2,SVP) heats of such reactions (e.g. hydrogenation) can be significantly in error. In the case of 'large' hydrocarbons, an accumulation of small component errors can cause G2,G2(MP2) and (surprisingly to a lesser extent) G2(MP2,SVP) heats of formation evaluated from atomization energies to be unreliable. However, the accumulated error in the heat of formation can be estimated in advance. In cases where the predicted error is large, more accurate heats of formation can generally be obtained with the help of isodesmic (and to a lesser degree isogyric) reactions rather than by following the standard procedure based on heats of atomization.
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