E. T. Denisov

Geometry of the Transition State of Radical Abstraction Reactions Involving Si-H, Ge-H, and Sn-H Bonds

Transition-state interatomic distances in the reactions C˙H3+SiH4, Si˙H3+SiH4, C˙H3+GeH4, and C˙H3+SnH4 are calculated by the B3LYP density functional and intersecting parabolas methods. A semiempirical algorithm is developed for the calculation of the Y...H and C...H distances in the transition state of the radical abstraction reactions R˙+YH involving silanes, germanes, and stannanes and the reverse reactions of silyl, germanyl, and stannyl radicals with hydrocarbons. This algorithm is used to calculate interatomic distances in these reactions. An analysis of the calculated data shows that the Y...H and C...H distances in these reactions depend on the following factors: the enthalpy of reaction, the radius of the Y atom (Y = C, Si, Ge, Sn), and four-electron repulsion during the attack of a radical on the C-H bond adjacent to the double bond. Empirical equations relating the interatomic distances to the enthalpy of reaction and to the Y-R bond length are set up.

Reactivity of quinones as alkyl radical acceptors

The enthalpies of the addition of 11 alkyl radicals to ortho-and para-benzoquinones and substituted para-benzoquinones and the enthalpies of formation of various alkoxyphenoxyl radicals have been calculated. Experimental data for the addition of alkyl radicals to quinones are analyzed in terms of the intersection of two parabolic potential curves, and parameters characterizing this class of reactions are calculated. The classical potential barrier of the thermally neutral reaction of alkyl radical addition to benzoquinone is Ee,0 = 82.1 kJ/mol. This class of reactions is compared to other classes of free-radical addition reactions. The interaction between the electrons of the reaction center and the π electrons of the aromatic ring is a significant factor in the activation energy. Activation energies, rate constants, and the geometric parameters of the transition state have been calculated for 40 reactions of alkyl radical addition to quinones. Strong polar interaction has been revealed in the addition of polar macroradicals to quinones, and its contribution to the activation energy has been estimated. Kinetic parameters, activation energies, and rate constants have been calculated for the reverse reactions of alkoxyphenoxyl radical decomposition to quinone and alkyl. The competition between chain termination and propagation reactions in alkoxyphenol-inhibited hydrocarbon oxidation is discussed.

Competition of monomolecular and bimolecular reactions of the alkyl radicals of artemisinin

The competition between monomolecular and bimolecular reactions of alkyl radicals of artemisinin is considered theoretically. The enthalpies of these reactions are calculated. The activation energies and rate constants of intramolecular hydrogen atom transfer, of the decyclization of the alkyl radicals of artemisinin, and of the bimolecular reactions of these radicals with C-H, S-H, and O-H bonds of biological substrates and their analogues are calculated in the framework of the parabolic model. The intramolecular hydrogen transfer reactions proceed at the highest rate. The bimolecular reactions occur somewhat less rapidly. The fastest of them are the reactions of the alkyl radicals with the thio groups of cysteine. The decyclization reactions of all artemisinin alkyl radicals are very slow.

Estimation of O-H bond dissociation energies in alcohols and acids from kinetic data

The O-H bond dissociation energies (DO-H) in five alcohols and six acids have been determined from experimental data (rate constants of radical reactions). The ratio of the rate constants of the reactions R1O˙+RH→R1OH+R˙ and RiO˙+RH→RiOH+R˙ and the intersecting parabolas method are used in the estimation procedure. The DO-H values are used to calculate the activation energies and rate constants for hydrogen abstraction from 2-methylbutane, butene-1, and cumene by alkoxyl and carboxyl radicals. The geometric parameters of the transition state are calculated for these reactions.

Estimation of Enthalpies of Alkoxy Radical Formation and Bond Strengths in Alcohols and Ether

Kinetic data on the thermal decomposition of peroxides were analyzed, and energies of the O–O bond dissociation were calculated. Enthalpies of formation of various alkoxy radicals and peroxides were determined. The dissociation energies for the O–H bonds in alcohols and C–O bonds in ethers were estimated. Comparative analysis of literature and obtained data was performed.

Free-radical decarboxylation of carboxylic acids as a concerted abstraction and fragmentation reaction

Two variants of the reaction of radicals with the carboxyl group of carboxylic acids, namely, RO2• + RiCOOH → ROOH + RiCO2• and ROi• + RjCOOH → ROOH + Ri• + CO2 are theoretically analyzed. It is demonstrated by the intersecting-parabolas method that if the reaction proceeded via the formation of an intermediate carboxyl radical, it would be much slower than is actually observed. Quantum-chemical calculations carried out by the density functional method using the nonempirical functional PBE have shown that the reactions of the methyl radical with the carboxyl group of acetic, butyric and vinylacetic acids include concerted H atom abstraction and C-C bond breaking. In the framework of the intersecting-parabolas model, an algorithm has been developed to calculate the activation energy and rate constant for X• + RiCOOH → XH + CO2 + Ri• reactions, where X = R•, RO•, HO•, ArO•, Ar2N• or H•

Reactivity of the Ge–H, Sn–H, P–H, and Se–H Bonds in Radical Abstraction Reactions

AbstractThe experimental data for the liquid- and gas-phase reactions of atoms and radicals with organoelement compounds Rn – 1E–Hn n

Kinetic Parameters for Direct Atomic Substitution Reactions

{text{X}}^ cdot + {text{H}} - {text{ER}}_{n - 1} to {text{XH}} + {text{R}}_{n - 1} {text{E}}^ cdot,