The reactions between fifteen carbonyl oxides and water have been investigated with the aim of contributing to a better understanding of the effects of the substituents in the reactivity of carbonyl oxides. We have employed density functional theory and large scale ab initio methods (CCSD(T), CASSCF, and CASPT2), combined with transition state theory, to investigate the addition of water to carbonyl oxide and, for those carbonyl oxides having a methyl substituent in syn, the hydrogen transfer from the methyl group to the terminal oxygen of carbonyl oxide. In this case, the water acts as a catalyst and this reaction can contribute to the atmospheric formation of a hydroxyl radical. Carbonyl oxides with electron withdrawing substituents and zwitterionic character have low energy barriers and react fast, whereas carbonyl oxides with electron releasing substituents have high energy barriers and react slowly. The position of the substituents plays also an important role and carbonyl oxides having a hydrogen atom substituent in syn react faster than carbonyl oxides having a hydrogen atom substituent in anti. The differences in the reactivity of different substituted carbonyl oxides raise up to ten orders of magnitude and the branching ratios for the two different reactions investigated are also reported.

Global hardness and softness and the associated hard/soft acid/base (HSAB) principle have been used to explain many experimental observed reactivity patterns and these concepts can be found in textbooks of general, inorganic, and organic chemistry. In addition, local versions of these reactivity indices and principles have been defined to describe the regioselectivity of systems. In a very recent article (Chem.–Eur. J. 2008, 14, 8652), the present authors have shown that the picture of these well-known descriptors is incomplete and that the understanding of these reactivity indices must be “reinterpreted”. In fact, the local softness and hardness contain the same “potential information” and they should be interpreted as the “local abundance” or “concentration” of their corresponding global properties. In this contribution, we analyze the implications of this new point of view for the applicability of these well-known descriptors when comparing two sites in three situations: two sites within one molecule, two sites in two different, but noninteracting molecules, and two sites in two different, but interacting, molecules. The implications on the HSAB principle are highlighted, leading to the discussion of the role of the electrostatic interaction.

There are several possible pathways in the macrocycle of large porphyrinoids and, among aromaticity indices, only AV_minis capable of recognizing the most aromatic one.

The influence of a single water molecule on the gas-phase reactivity of the HO₂ radical has been investigated by studying the reactions of SO₃ with the HO₂ radical and with the H₂O⋯HO₂ radical complex. The naked reaction leads to the formation of the HSO₅ radical, with a computed binding energy of 13.81 kcal mol⁻¹. The reaction with the H₂O⋯HO₂ radical complex can give two different products, namely (a) HSO₅ + H₂O, which has a binding energy that is computed to be 4.76 kcal mol⁻¹ more stable than the SO₃ + H₂O⋯HO₂ reactants (Δ(E + ZPE) at 0K) and an estimated branching ratio of about 34% at 298K and (b) sulfuric acid and the hydroperoxyl radical, which is computed to be 10.51 kcal mol⁻¹ below the energy of the reactants (Δ(E + ZPE) at 0K), with an estimated branching ratio of about 66% at 298K. The fact that one of the products is H₂SO₄ may have relevance in the chemistry of the atmosphere. Interestingly, the water molecule acts as a catalyst, [as it occurs in (a)] or as a reactant [as it occurs in (b)]. For a sake of completeness we have also calculated the anharmonic vibrational frequencies for HO₂, HSO₅, the HSO₅⋯H₂O hydrogen bonded complex, H₂SO₄, and two H₂SO₄⋯H₂O complexes, in order to help with the possible experimental identification of some of these species.

Note: In lieu of an abstract, this is the article's first page.