A high-level ab initio study on the oxidations of alkanes (methane, propane and isobutane) with dioxirane (DO), dimethyldioxirane (DMDO), difluorodioxirane (DFDO) and methyl(trifluoromethyl)dioxirane (TFDO) has provided a rationale for the formation of products derived from radical intermediates when dioxygen is rigorously excluded and lends strong support to the generally accepted, highly exothermic, concerted oxygen insertion mechanism for the oxidation of alkanes with dioxiranes under typical preparative conditions. At the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(3df,2p) level, the barriers for the oxidation of methane with DO, DMDO, DFDO, and TFDO are 36.5, 41.2, 21.2, and 35.0 kcal/mol. The activation barriers for the oxidation of methane, propane, and isobutane with DMDO are 44.2, 30.3, and 22.4 kcal/mol, respectively, at the CCSD-(T)/6-31G*//B3LYP/6-31G* level. These barriers are higher than the barriers for epoxidations of ethylene, propene, and isobutene (17.9, 15.9, and 15.4 kcal/mol, respectively, at the B3LYP/6-31G* level). Calculations at the G2 level have shown that C-H bond homolysis is not thermodynamically favorable in hydrogen abstraction reactions of methane, propane, and isobutane with either DMDO, its parent DO, or DFDO. If the sources of the initiating radical species are hydrogen atoms produced by breaking X-H bonds, the energy of the X-H bond should be less than 83.5 kcal/mol in its reaction with DMDO (89.5 and 87.8 kcal/mol for DO and DFDO) for the reaction to be exothermic.

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A 6.8 fold increase in the rate of epoxidation of (Z)-cyclooctene with m-chloroperbenzoic acid is observed upon addition of the catalyst trifluoroacetic acid. Kinetic and theoretical studies suggest that this increase in rate is due to complexation of the peroxy acid with the undissociated acid catalyst (HA) rather then protonation of the peroxy acid. The transition structure for oxidation of ethylene with protonated peroxyformic acid exhibits a spiro orientation of the electrophilic oxygen at the QCISD/6-31G(d) level and the complexed peroxy acid (HCO3H·HA) transition state is also essentially spiro at the ab initio and density functional levels. At the B3LYP/6-311G(d,p) level the protonated transition structure exhibits a more planar approach where the O3−H9 of the peroxy acid lies in the plane of the π-system of ethylene, and the barrier for formation of protonated oxirane is only 4.4 kcal mol-1. Epoxidation with neutral and complexed peroxyformic acid also involves a symmetrical spiro orientation affording an epoxide, and the barriers for formation of oxirane at the same level are 14.9 kcal mol-1 and 11.5 kcal mol-1, respectively. The free energy of activation for the epoxidation of ethylene by peroxyformic acid is lowered by about 3 kcal mol-1 upon complexation with the catalyst.

A theoretical study of the mechanism of decarboxylation of beta-keto acids is described. A cyclic transition structure was found with essentially complete proton transfer from the carboxylic acid to the beta-carbonyl group. The activation barrier for decarboxylation of formylacetic acid is predicted to be 28.6 kcal/mol (MP4SDTQ/6-31G//MP2/6-31G) while loss of CO(2) from its anion exhibits a barrier of only 20.6 kcal/mol (MP4SDTQ/6-31+G//MP2/6-31+G). Barrier heights of decarboxylation of malonic acid and alpha,alpha-dimethylacetoacetic acid are predicted to be 33.2 and 26.7 kcal/mol, respectively. Model enzyme studies using a thio methyl ester of malonate anion suggests that the role of malonyl-CoA is to afford a polarizable sulfur atom to stabilize the developing enolate anion in the transition structure for decarboxylation. Adjacent positively charged ammonium ions are also observed to stabilize the loss of CO(2) from a carboxylate anion by through-bond Coulombic stabilization of the transition structure.

CAS-MCSCF calculations describe the addition of singlet CF(2) and C(OH)(2) to the ethene double bond as a two-step reaction. The energy barriers that separate, in the first rate-determining step, loosely bound pi-complexes from stable CH(2)CH(2)CX(2) diradical intermediates show the expected ordering, smaller for CF(2) than for C(OH)(2). Back-dissociation of the diradicals into reactants requires the overcoming of non-negligible energy barriers. In both diradicals, the CAS-MCSCF activation energy for ring closure is smaller than that required for rotation of their terminal methylenic groups, which models, in these simple systems, an isomerization process. However, when the activation free energies are computed, in the case of the difluoro diradical the isomerization process appears to be less disfavored (and possibly competitive to some extent at higher temperatures); in contrast, in the case of the dioxy diradical, isomerization is never competitive with ring closure. The small energy barriers for ring closure of the diradicals disappear altogether when multireference MP2 energy calculations are carried out on the CAS-MCSCF critical points, casting doubts on the very existence of these intermediates. However, in contrast with the ethene reaction, the addition of singlet CF(2) to isobutene involves the formation of a diradical intermediate whose barrier for ring closure persists also at the MP2 level. These results suggest that cyclopropanation is likely to be a two-step process (with formation of a diradical intermediate) only with bulky substituted alkenes, while the attack to an unsubstituted double bond could be an asynchronous but concerted process. The analogous triplet reactions go through transition and stable structures of lower symmetry than the singlet and see the intervention of diradical intermediates. Their formation is easier than that in the singlet case and their stability with respect to back-dissociation higher. Also the isomerization processes (taking place again through rotation of the terminal methylenic group) are easier than those examined on the singlet surfaces.

The oxidation reactions of ethylene, propene, dimethyl sulfide, trimethylamine, and trimethylphosphine with peroxynitrous acid have been studied computationally with the B3LYP, MP2, MP4, CISD, QCISD, and QCISD(T) levels of theory. The activation barriers for the alkene (ethylene and propene) epoxidations (18.4 and 15.5 kcal/mol at the QCISD(T)/6-31G*//QCISD/6-31G* level, respectively) and for the oxidations of dimethyl sulfide, trimethylamine, and trimethylphosphine (8.3, 4.6, and 0.5 kcal/mol at the QCISD(T)/6-31G*//B3LYP/6-311G** level, respectively) with peroxynitrous acid are similar to the barriers of their oxidations with peroxyformic acid and dimethyldioxirane, although these peroxides have very diverse O−O bond dissociation energies. Therefore, the feasibility of alkene epoxidation and the oxidations of methyl-substituted sulfides, amines, and phosphines by peroxynitrous acid should not differ significantly from those for peroxyformic acid and dimethyldioxirane. The transition structures for the epoxidation of ethylene and propene with peroxynitrous acid are symmetrical with equal or almost equal bond distances between the spiro oxygen and the carbons of the double bond. This symmetrical approach of the electrophilic oxygen is similar to that found for alkene epoxidations with peroxyformic acid. The geometries of the transition structures calculated at the QCISD and CISD levels are quite comparable to each other. The B3LYP calculated barriers for oxidations of alkenes, as well as sulfides, amines, and phosphines, are underestimated when compared with those calculated at the QCISD(T)//QCISD levels. The most economical and accurate protocol utilizes the B3LYP (for such “σ-donors” as sulfides, amines, and phosphines) or CISD geometries with barriers calculated at the QCISD(T) level.

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The active site of histidine decarboxylase (HDC) has been modeled with both ab initio (MP2/6-31G(d)) and DFT (BH&HLYP/6-311G(d,p)) calculations. The results clearly point out the role of zwitterionic transition structures and the importance of hydrogen bonding interactions in enzymatic decarboxylation. A comparison between the gas-phase decarboxylation of aminoformylacetic acid (H(CO)CH(NH2)COOH) and the corresponding process in solution according to the supermolecule model approach with six water molecules is provided. This study analyzes the role of the proton distribution in lowering the reaction barrier in an intermediate Schiff base (H2CNCH2COOH) and its transition structure for decarboxylation (ΔE ⧧ = 29.8 kcal mol-1 at the MP2/6-31G(d) level of theory). Electronic features displayed by the intermediate imine are analyzed by making use of models of increased complexity. The iminium ion functionality has been established to be the dominant factor in lowering the barrier for the decarboxylation of the α-amino acids through Coulombic stabilization of the developing negative charge on the α-carbon and delocalization of the positive charge induced by proton transfer to the imine nitrogen along the reaction coordinate. Further extension of the model imine by an amide group (H2N(CO)CHNCH2COOH) lowers the barrier height by an additional 6.7 kcal mol-1. A net transfer of electron density to the amide functionality in the transition state is not in evidence. The stabilizing influence on the barrier height of a hydrogen bonding network with formic acid and a model peptide residue (H(CO)NHCH2CHO) is estimated to be 3.1 kcal mol-1 at the BH&HLYP/6-311G(d,p) level.