Semiempirical method of calculations of transition-state geometries for radical addition reactions

Transition-state geometries of the addition reactions of H^·, ^·CH₃, ^·NH₂, and ^·OCH₃ radicals to ethylene; H^· radical to acetylene, methyleneimine, acetonitrile, and formaldehyde; and ^·CH₃ radical to acetone and acetylene were determined by the density functional (B3LYP) method. The interatomic distances in the transition states of these reactions were also calculated from experimental data (enthalpies and activation energies) using the model of intersecting parabolas, the model of reduced intersecting parabolas (RIP), and the model of reduced intersecting parabola and Morse curve. The results obtained by different methods were compared and analyzed. An algorithm was elaborated for calculations of interatomic distances using experimental data, based on introduction of corrections to the RIP model. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 886–893, April, 2005.     

Radical addition reactions: factors determining the transition-state geometry

Interatomic distances in the reaction centers of the addition reactions of (i) H^· to the C=C, C=O, N≡C, and C≡C bonds, (ii) ^·CH₃ radical to the C=C, C=O, and C≡C bonds, and (iii) alkyl, aminyl, and alkoxyl radicals to olefin C=C bonds were determined using a new semiempirical method for calculating transition-state geometries of radical reactions. For all reactions of the type X^· + Y=Z → X— Y—Z^· the r ^# _X...Y distance in the transition state is a linear function of the enthalpy of reaction. Parameters of this dependence were determined for seventeen classes of radical addition reactions. The bond elongation, Δr ^# _X...Y, in the transition state decreases as the triplet repulsion, electronegativity difference between the atoms X and Y in the reaction center, and the force constant of the attacked multiple bond increase. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 894–902, April, 2005.     

The reactivity of organoboranes in radical substitution reactions

The parameters characterizing substitution reactions of the types R^· + BR₃, RO^· + BR₃, ROO^· + BR₃, and RS^· + BR₃ were calculated from the experimental data using the parabolic model of bimolecular radical reaction. Along with the enthalpy of the reaction, the following factors affect the activation energy: triplet repulsion in the transition state, difference in electronegativities of the atoms forming the reaction center, -bonds in the -position to the reaction center, steric hindrances, and force constants of the reacting bonds. The change in the dissociation energy of the B--C bond in organoboranes, in which alkyl substituents were replaced by alkoxyl and thiyl substituents, was estimated from the kinetic data. The parameters obtained make it possible to calculate the activation energies of individual reactions of four types under study.     

Competition between mono-and bimolecular reactions of artemisinin alkoxyl radicals

The competition between intramolecular and bimolecular reactions of alkoxyl radicals formed from artemisinin was theoretically analyzed. The enthalpies of these reactions were calculated. The activation energies and rate constants of reactions of intramolecular hydrogen atom transfer, decyclization, and decomposition of alkoxyl radicals of artemisinin and several its derivatives, as well as the activation energies and rate constants of reactions of these radicals with the C-H, S-H, and O-H bonds in biological substrates and their analogs were calculated by the intersecting parabolas method The fastest reactions of artemisinin alkoxyl radicals were identified. The full kinetic scheme of transformation of these radicals was proposed. Artemisinin radicals with the free valence on the carbon atom are predominantly formed due to the transformation of the artemisininoxyl radicals. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1502–1510, September, 2006.     

The reactivity of silanes in free-radical reactions: Analysis in terms of the parabolic model of the transition state

Rate constants and activation energies of free-radical reactions of silanes and silyl radicals were analyzed in terms of the parabolic model of the transition state. The kinetic parameters were estimated for 16 groups of reactions of silanes and silyl radicals. These parameters were used to calculate the activation energies for 112 free-radical reactions and to estimate the dissociation energies of the Si−H bond for 21 compounds and those of the C−Cl bond for 12 substituted benzyl chlorides. Triplet repulsion, electronegativity, and radii of atoms of the reaction center of the transition state were shown to play an important role in the formation of the activation barrier. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1311–1316, July, 1998.     

Physical factors determining the activation energy of alkyl radical addition to unsaturated compounds

A parabolic model of the transition state is used for the analysis of experimental data (rate constants and activation energies) for reactions of addition of alkyl and phenyl radicals to multiple bonds of unsaturated compounds. The parameters describing the activation energy as a function of the enthalpy of the reactions were calculated from the experimental data. The activation energy depends also on the strength of the forming C−C bond, the presence of π-bonds in the α-position near the attacked C=C bond and the presence of polar groups in the monomer and radical. The empirical dependence of the activation energy of a thermoneutral addition reactionE _e0 on the dissociation energyD _e of the forming C−C bond was obtained:E _e0=(5.95±0.06)·10⁻⁴ D _e ² kJ mol⁻¹, indicating the important role of triplet repulsion in the formation of the transition state of radical addition. The contribution of the polar interaction to the activation energy of addition of polar radicals to polar monomers was calculated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 445–450, March, 1999.     

Estimation of dissociation energies of C—H bonds in oxygen-containing compounds from kinetic data for radical abstraction reactions

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.     

Dissociation energies of O-H bonds in natural antioxidants

The dissociation energies of O-H bonds in natural antioxidants were estimated from the kinetic data (rate constants of reactions of the antioxidants with the peroxy radicals). The calculations were performed using the method of intersecting parabolas. α-Tocopherol was used as a reference phenol with D _O-H = 330 kJ mol⁻¹. The following groups of antioxidants were chosen for the estimation: tocopherols, their sulfur- and selenium-containing analogs, flavones, flavanones, and gallates. The discrepancy in the D _O-H values for the same phenol by measurements of k(RO₂ ^· + ArOH) of different authors does not exceed 2 kJ mol⁻¹. The D _O-H values were calculated for 64 phenols. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1824–1832, September, 2008.     

Influence of the size of the formed cycle on the reactivity of free radicals in cyclization reactions

Numerous experimental data for the cyclization of free radicals C·H₂(CH₂)_nCH=CH₂ cyclo-[(CH₂)_n+1CH(C·H₂)], and C·H₂(CH₂)_nCH=CHR cyclo-[(CH₂)_n+1C·HCHR] were analyzed in the framework of the parabolic model. The activation energy of thermoneutral (H _e = 0) cyclization E _e0 decreases linearly with an increase in the energy of cycle strain E _rsc: E _e0(n) (kJ mol–1) = 85.5 – 0.44E _rsc(n) (n is the number of atoms in the cycle). The activation entropy of cyclization S ^# also depends on the cycle size: the larger the cycle, the lower S ^#. A linear dependence of S ^# on the difference between the entropies of formation S° of cyclic hydrocarbon and the corresponding paraffin was found: S ^# = 1.00[S°(cycle) – S°(C_nH_2n+2)]. The E _e0 values coincide for cyclization reactions with the formation of the six-membered cycle and the bimolecular addition of alkyl radicals to olefins.     

Polar and solvation effects in reactions of oxygen atoms and hydroxyl and alkoxyl radicals with oxygen-containing compounds

Experimental data on the activation energies of reactions of H-abstraction from oxygencontaining compounds by oxygen atoms and hydroxyl and alkoxyl radicals in the gas and liquid phases have been analyzed by means of the parabolic model of the transition state. The contribution of polar interaction to the activation energies of the reactions has been calculated. The contribution of solvation to the activation energy has been calculated by comparison of the reaction parameters of the respective reaction in the liquid and gas phases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 38–42, January, 1994.     

Geometric parameters of transition states of radical abstraction reactions with C...H...C reaction center

An algorithm of calculations of interatomic distances in the transition states (TS) of reactions of hydrogen abstraction by alkyl, allyl, and benzyl radicals from C—H bonds of organic molecules using the enthalpies of the corresponding reactions is proposed. The geometric parameters of the TS of the reactions involving carbon-centered radicals with the C...H...C reaction center, calculated using experimental data, are compared with other characteristics of the reactions and reactants. The r(C...H...C) distance in the TS of the reactions of alkyl radicals with alkanes remains unchanged as the enthalpies of reactions vary, being a characteristic parameter of a given class of reactions. -Bonds adjacent to the reaction center are responsible for an increase in the parameter r(C...H...C) in the TS.     

Reactivity of cyano-substituted fluorenes in reaction with the tempo radical

The rate constants for the reaction of the TEMPO radical with fluorene, 2-cyanofluorene, and 9-cyanofluorene were determined by ESR. A comparison was made of the reactivity of the hydrocarbons in reaction with the TEMPO radical and the peroxyl radical. The quantum-chemical characteristics of the radicals and particles taking part in the reactions and also the characteristics of the transition states in the reactions of the TEMPO radical and cumenylperoxyl radical with fluorene were obtained by the PM3 method. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 19–22, January–February, 2006.