G2(MP2) and G2(MP2,SVP) Calculations of Enthalpies of Hydrogenation, Isomerization, and Formation of C₅ Hydrocarbons. 2. Substituted Cyclobutenes, Vinylcyclopropene, Spiropentane, and Methyltetrahedrane

We report the ab initio G2(MP2) and G2(MP2,SVP) standard enthalpies of hydrogenation and isomerization (Δ hyd H and Δ isom H ) at 298 K of 23 C 5 cyclic hydrocarbons, including methyl- and methylenecyclobutane, spiropentane, and related unsaturated hydrocarbons, as well as methyltetrahedrane. Δ hyd H 298 and Δ isom H 298 are found from the differences in total energy ( E 0 ) of the participants in the hydrogenation and isomerization reactions. Combination of calculated Δ hyd H 298 and Δ isom H 298 with one experimental enthalpy of formation (Δ f H 298 ) in the set yields Δ f H 298 of all the rest. In those few instances for which reliable experimental results can be found, agreement between experiment and calculation is within 1 kcal mol -1 .