The G3(MP2) method has been employed to study the 1,4-addition reactions between singlet oxygen and five acenes, including benzene, naphthalene, anthrecene, tetracene, and pentacene. In all, nine pathways between O(2) and the five acenes have been investigated. Our calculated results indicate that all nine pathways are concerted and exothermic and that the most reactive sites on the acenes are the center ring's meso-carbons. In addition, reactivity increases along the series benzene < naphthalene < anthrecene < tetracene < pentacene. This trend is identical to that of aromaticity for the five acenes. A correlation between reactivity and aromaticity is briefly rationalized with natural bond orbital (NBO) analysis and frontier molecular orbital (FMO) analysis. Furthermore, some experimental kinetics data from the literature supporting the calculated results are cited.

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Applying the G3 and G3(MP2) models and using both the isodesmic and atomization schemes, the heats of formation (ΔH f) at 0 and 298 K are calculated for mono-tert-butylmethane (2,2-dimethylpropane or neopentane, abbreviated as mono-TBM), di-tert-butylmethane (di-TBM), tri-tert-butylmethane (tri-TBM), and tetra-tert-butylmethane (tetra-TBM). Upon examining the results, it is found that all of the calculated ΔH f298 values are well within ±10 kJ mol-1 of the available experimental data for the first three compounds. Hence, for tetra-TBM, a compound that has not yet been synthesized, the G3(MP2) results reported in this work should be reliable estimates. Moreover, we have found that the atomization scheme is slightly more suitable for the study of the smaller molecules, while the isodesmic scheme is more suitable for the larger molecules. Structurally, it is found that the equilibrium structures of mono-TBM, di-TBM, tri-TBM, and tetra-TBM have T d , C 2, C 1, and T symmetry, respectively. The energy-minimized structure of each TBM molecule is determined and all structural parameters are generally in good agreement with the available experimental data. Furthermore, it is found that the innermost C−C bond lengths increase along the series mono-TBM < di-TBM < tri-TBM < tetra-TBM, a trend that is expected from steric consideration.

Applying the Gaussian-3 (G3) model and its variant G3(MP2), and using the atomization scheme, the heats of formation (?Hf) at 0 K and 298 K have been calculated for twelve monocyclic azines with the general formula Nn(CH)6-n n = 1, 2,?, 6. Upon comparing the calculated results with available experimental data, it is found that the calculated structural parameters agree very well with the experimental ones. Additionally, most of the calculated ?Hf values are well within ?10 kJ mol-1 of the available experimental data. Thus, it is concluded that the unfavorable accumulation of component errors found in the Gaussian-2 methods is greatly reduced in the G3 models. Also, the calculated ?Hf values for those azines for which no experimental data exists should be reliable estimates.