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The enthalpies, activation energies, and geometries of transition states of the decomposition of organic radicals, viz., alkyl, alkoxyl, acyl, peroxyl, etc., are reviewed. The results of semiempirical and quantum chemical calculations are compared with experimental data. Special attention is given to an analysis of physicochemical factors affecting the activation barriers and geometry of transition states of radical decomposition. [ABSTRACT FROM AUTHOR]

The enthalpies of formation of some biphenyl derivatives were determined. A "double difference" method for calculating the enthalpies of formation of aromatic radicals and the bond dissociation energies was proposed. The enthalpies of formation of the radicals biphenyl, diphenyl oxide, and phenyl oxide were determined. The energies of reorganization of these radicals as well as phenyl and 4-, 3-, and 2-pyridyls were calculated. The sums of the energies of the chemical bonds in the molecular moieties transformed into radicals upon the decomposition of chemical compounds were found to be constant for different compounds. The energies of the chemical bonds in arenes were determined. [ABSTRACT FROM AUTHOR]

Kinetic schemes for the intermolecular oxidation of four analogs of 10-dihydroartemisinin with peroxide substituents were developed. Each step of the kinetic scheme was characterized by the enthalpy, and its activation energy and rate constant were calculated using the model of intersecting parabolas. The antimalarial activity of these derivatives is proved to depend on the number of hydroxyl radicals generated by the compound upon oxidation rather than on the number of peroxide groups. An empirical dependence between the antimalarial efficiency relative to artemisinin (IC) and the number of generated hydroxyl radicals n was obtained for a series of peroxy, hydroxy, and alkoxy derivatives of 10-dihydroartemisinin (11 compounds): ln(IC) =-13.51 + 3.76• n . [ABSTRACT FROM AUTHOR]

The molecular characteristics were calculated and the structures of monoamino-substituted polyhydroxy-1,4-naphthoquinones, 3-amino-7-ethyl-2,5,6,8-tetrahydroxy-1,4-naphthoquinone (echinamine A, 1) and 2-amino-7-ethyl-3,5,6,8-tetrahydroxy-1,4-naphthoquinone (echinamine B, 2), were studied by the B3LYP/6-311G(d) method in the gas phase. The enol-enol tautomerism of these compounds caused by the transfer of hydrogen atoms of the α-OH groups and rotamerism due to the rotation of the β-OH groups around the C-O bonds are considered. The relative acidity of the β-OH groups in compounds 1 and 2 was estimated. The gas-phase reactions of compounds 1 and 2 with the hydroperoxyl radical are exothermic. The heterolysis of the O-H bond of one of the two β-OH groups in compounds 1 and 2 decreases the energy of the subsequent O-H bond homolytic dissociation of the remaining β-hydroxy group, due to which the energy gain of the reaction with the hydroperoxyl radical increases. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations of the dissociation energy of the C-H bond in the α-hydroperoxide fragment of MeCHOOH were carried out. It was shown that abstraction of H atom is accompanied by dissociation of the O-O bond. Density functional calculations of transition states of the reactions of CH, CHOO, and HO radicals with the C-H bond in the α-hydroperoxide fragment of MeCHOOH were carried out. It was established that H atom abstraction is accompanied by concerted dissociation of the O-O bond. For 45 peroxides RRCHOOH, RRCHOOR, and RRCHOOC(O)R (R, R = H, Me, Et, Ph, HC=CH), the enthalpies of H atom abstraction from the C-H bond in the a-hydroperoxide fragment with fragmentation of the peroxides at the O-O bond were calculated. The kinetic parameters for 12 classes of radical abstraction reactions with fragmentation of molecules were calculated from experimental data within the framework of the model of intersecting parabolas. The activation energies and reaction rate constants of H atom abstraction from C-H bonds of a-peroxide fragments involving peroxyl and alkyl radicals were determined for 45 peroxides of different structure. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations and the conformational analysis of dianions, radical dianions, disodium salts, and radicals of disodium salts of 7-ethyl-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone (echinochrome A) were carried out at the (U)B3LYP/6-31G(d) and (U)B3LYP/6-311G(d) levels of theory. The heats of reactions of the hydroperoxyl radical (HOO) with the isomers of dianions and disodium salts of echinochrome A with the lowest Gibbs free energies were estimated. All reactions of these isomers of dianions and disodium salts of echinochrome A with HOO in the gas phase are exothermic. The isomer of the dianion of echinochrome A with the lowest Gibbs free energy, which is formed by the heterolysis of the 2β- and 6β-OH groups, is the more effective antioxidant than the isomer of the 2,6-disodium salt with the lowest Gibbs free energy. [ABSTRACT FROM AUTHOR]

Based on the experimental results and the published data, the enthalpies of formation of ethane and propane nitro derivatives were obtained for both the standard state and gas phase. The bond dissociation energies of the ethane and propane nitro derivatives were calculated using the enthalpies of atomization and the energies of nonvalent interactions of nitro groups. The calculated values were compared with the kinetic data on thermal decomposition. The bond dissociation energies in radicals of the ethane and propane nitro derivatives were also calculated using the enthalpies of atomization and the energies of nonvalent interactions of nitro groups. Regularities of changes in the bond dissociation energies of the ethane and propane nitro derivatives and their radicals were established. [ABSTRACT FROM AUTHOR]

The enthalpies of formation in the standard state and in the gas phase were recommended for a series of secondary nitramines and n-butyldinitramine on the basis of the experimental enthalpies of combustion and vaporization and literature data. An analysis of the main thermochemical values (the enthalpies of formation in the gas phase and the enthalpies of atomization) showed that the energy properties of the nitramine group are independent of the structure of the molecules studied and of the number of functional groups in them. The enthalpies of formation of the alkylnitramine radicals were determined. The values obtained make it possible to calculate the bond dissociation energies in the nitramines and their radicals of different structures. [ABSTRACT FROM AUTHOR]

The enthalpies, activation energies, and rate constants for the reactions of 15 phenoxyl radicals derived from natural bioantioxidants with hydroperoxides, C-H bonds of linoleic acid, SH-groups of l-cysteine, and O-H bonds of α-tocopherol (60 reactions) were calculated. The activation energies were calculated using the model of intersecting parabolas. The interatomic distances in the reaction sites of the transition states of the studied reactions were calculated. The factors affecting the reactivity of these radicals are discussed. The activation energy of the reaction of oxygen with the O-H bond of the 1,2-dihydroxybenezene semiquinone radical was estimated. [ABSTRACT FROM AUTHOR]

The enthalpies of formation of nitromethane derivatives were obtained on the basis of experimental and literature data. The procedure for the calculation of the bond dissociation energies in nitromethanes from the atomization enthalpies and energies of nonvalent interactions of nitro groups was proposed. The calculated values were compared with the data on the thermal decomposition kinetics. The atomization enthalpies and energies of nonvalent interactions of nitro groups were also used for the calculation of the bond dissociation energies in radicals. [ABSTRACT FROM AUTHOR]

The C—H bond dissociation energies were calculated on the basis of the parabolic model from the rate constants of free radical reactions for more than 160 oxygen-containing compounds. The enthalpies of formation of free radicals formed from these compounds were calculated. The method was modified taking into account the influence of functional groups on the partial rate constant and for the case when the reference reaction in the reaction series belongs to another class of structurally similar reactions. [ABSTRACT FROM AUTHOR]

Energies of homolytic cleavage of O-H bonds in 33 compounds of the general formula RoH ( n = 2, 3, and 4) were calculated by the AMI method. For hydrotrioxides and hydrotetroxides, the bond dissociation energies are virtually independent of the nature of the substituent R: D(RO-H) = 92.3±0.8 kcal mol ( n = 3 and 4). [ABSTRACT FROM AUTHOR]