BEBO Calculations for hydrogen abstraction by 0(3P)

Rate constants for hydrogen transfer reactions from C—H bonds to O(³P) in the gas phase have been calculated by the bond-energy bond-order method. The use of a single or double triplet repulsion term is discussed. Computational results indicate that with carefully selected input parameters, it is unnecessary to use a double-repulsion term. Tunneling corrections were carried out by two different procedures. Results indicate that in this case there is not much difference between the simple Wigner's treatment and more complicated methods.     

Ab initio group contribution method for activation energies of hydrogen abstraction reactions.

The group contribution method for activation energies is applied to hydrogen abstraction reactions. To this end an ab initio database was constructed, which consisted of activation energies calculated with the ab initio CBS-QB3 method for a limited set of well-chosen homologous reactions. CBS-QB3 is shown to predict reaction rate coefficients within a factor of 2-4 and Arrhenius activation energies within 3-5 kJ mol(-1) of experimental data. Activation energies in the set of homologous reactions vary over 156 kJ mol(-1) with the structure of the abstracting radical and over 94 kJ mol(-1) with the structure of the abstracted hydrocarbon. The parameters required for the group contribution method, the so-called standard activation group additivity values, were determined from this database. To test the accuracy of the group contribution method, a large set of 88 additional activation energies were calculated from first principles and compared with the predictions from the group contribution method. It was found that the group contribution method yields accurate activation energies for hydrogen-transfer reactions between hydrogen molecules, alkylic hydrocarbons, and vinylic hydrocarbons, with the largest deviations being less than 6 kJ mol(-1). For reactions between allylic and propargylic hydrocarbons, the transition state is believed to be stabilized by resonance effects, thus requiring the introduction of an appropriate correction term to obtain a reliable prediction of the activation energy for this subclass of hydrogen abstraction reactions.     

Kinetic solvent effects on hydrogen abstraction reactions

Kinetic solvent effects on hydrogen abstractions, namely, acceleration in nonpolar solvents, have been presumed to be restricted to O-H hydrogen donors. We demonstrate that also abstractions from C-H and even Sn-H bonds by cumyloxyl radicals and n,pi*-excited 2,3-diazabicyclo[2.2.2]oct-2-ene are fastest in the gas phase and nonpolar solvents but slowest in acetonitrile. Accordingly, solvent effects on hydrogen abstractions are more general, presumably due to stabilization of the reactive oxygen or nitrogen species in polar solvents.     

Kinetics of hydrogen abstraction reactions of butene isomers by OH radical

The rate coefficients of H-abstraction reactions of butene isomers by the OH radical were determined by both canonical variational transition-state theory and transition-state theory, with potential energy surfaces calculated at the CCSD(T)/6-311++G(d,p)//BH&HLYP/6-311G(d,p) level and CCSD(T)/6-311++G(d,p)//BH&HLYP/cc-pVTZ level and quantum mechanical tunneling effect corrected by either the small-curvature tunneling method or the Eckart method. While 1-butene contains allylic, vinylic, and alkyl hydrogens that can be abstracted to form different butene radicals, results reveal that s-allylic H-abstraction channels have low and broad energy barriers, and they are the most dominant channels which can occur via direct and indirect H-abstraction channels. For the indirect H-abstraction s-allylic channel, the reaction can proceed via forming two van der Waals prereactive complexes with energies that are 2.7-2.8 kcal mol(-1) lower than that of the entrance channel at 0 K. Assuming that neither mixing nor crossover occurs between different reaction pathways, the overall rate coefficient was calculated by summing the rate coefficients of the s-allyic, methyl, and vinyl H-abstraction paths and found to agree well with the experimentally measured OH disappearance rate. Furthermore, the rate coefficients of p-allylic H abstraction of cis-2-butene, trans-2-butene, and isobutene by the OH radical were also determined at 300-1500 K, with results analyzed and compared with available experimental data.     

Some hydrogen abstraction reactions of the trichloromethyl radical

The gas phase photolysis of CCl₄ in the presence of several alkanes has been used to obtain Arrhenius parameters for the abstraction of hydrogen atoms by the CCl₃ radical: The following log k₄ values were obtained:

Absolute rate constants for some hydrogen atom abstraction reactions by a primary fluoroalkyl radical in water

A combination of laser flash photolysis and competitive kinetic methods have been used to measure the absolute bimolecular rate constants for hydrogen atom abstraction in water from a variety of organic substrates including alcohols, ethers, and carboxylic acids by the perfluoroalkyl radical, *CF(2)CF(2)OCF(2)CF(2)SO(3)(-) Na(+). Comparison, where possible, of these rate constants with those previously measured for analogous reactions in the non-polar organic solvent, 1,3-bis(trifluoromethyl)benzene (J. Am. Chem. Soc, 1999, 121, 7335) show that the alcohols react 2-5 times more rapidly in the water solvent and that the ethers react at the same rate in both solvents. A transition state for hydrogen abstraction that is more reminiscent of an "intimate ion pair" than a "solvent separated ion pair" is invoked to explain these modest solvent effects.     

Rate constants for some hydrogen abstraction reactions of the carbonate radical

Rate constants have been measured in aqueous solutions for the reactions of the carbonate radical, CO₃^˙?, with several saturated alcohols and one cyclic ether as a function of temperature. Arrhenius pre-exponential factors ranged from 2×10⁸ to 1×10⁹ ?? mol^?1 s^?1 and activation energies ranged from 16 to 29 kJ mol^?1. The results suggest that the reactions are not pure hydrogen abstraction, but involve an additional interaction of the radical with the ? OH or ? O? linkage. © 1993 John Wiley & Sons, Inc.     

Reactions of carbamyl radicals: intramolecular hydrogen abstraction reactions

ω-Phenylalkyl-N-methylcarbarnyl radicals undergo intermolecular addition to 3,3-dinethylbut-l-ene in preference to intramolecular hydrogen abstraction. Methyl N-(ω-phenylalkyl) carbanyl radicals and methyl N-pentylcarbamyi radicals readily abstract hydrogen through a six membered transition state or a seven membered transition state if the hydrogen is beniylic. The selectivities are interpreted in terms of the electrophilicity of the radical and the stereo-electronic requirements of hydrogen abstraction reactions.     

A valence bond modeling of trends in hydrogen abstraction barriers and transition states of hydroxylation reactions catalyzed by cytochrome P450 enzymes

The paper outlines the fundamental factors that govern the mechanisms of alkane hydroxylation by cytochrome P450 and the corresponding barrier heights during the hydrogen abstraction and radical rebound steps of the process. This is done by a combination of density functional theory calculations for 11 alkanes and valence bond (VB) modeling of the results. The energy profiles and transition states for the various steps are reconstructed using VB diagrams (Shaik, S. S. J. Am. Chem. Soc. 1981, 103, 3692-3701. Shaik, S.; Shurki, A. Angew. Chem. Int. Ed. 1999, 38, 586-625.) and the DFT barriers are reproduced by the VB model from raw data based on C-H bond energies. The model explains a variety of other features of P450 hydroxylations: (a) the nature of the polar effect during hydrogen abstraction, (b) the difference between the activation mechanisms leading to the Fe(IV) vs the Fe(III) electromers, (c) the difference between the gas phase and the enzymatic reaction, and (d) the dependence of the rebound barrier on the spin state. The VB mechanism shows that the active species of the enzyme performs a complex reaction that involves multiple bond making and breakage mechanisms by utilizing an intermediate VB structure that cuts through the high barrier of the principal transformation between reactants and products, thereby mediating the process at a low energy cost. The correlations derived in this paper create order and organize the data for a process of a complex and important enzyme. This treatment can be generalized to the reactivity patterns of nonheme systems and synthetic iron-oxo porphyrin reagents.     

Theoretical study on the hydrogen abstraction reactions from hydrazine derivatives by H atom

The hydrogen abstraction reactions from hydrazine and its methyl derivatives by the H atom have been investigated theoretically by using CBS-QB3//DSD-BLYP-D3(BJ)/Def2-TZVP quantum chemical calculations and transition state theory calculations coupled with various tunneling correction methods. Both the products and transition state energies of the hydrogen abstraction from the amino group were stabilized by the methyl group substitution. The substitution effect on the ^αN site was two times larger than that on the ^βN site. On the other hand, the substitution effect was negligible on the hydrogen abstraction from the methyl group. The overall rate coefficients of N₂H₄ + H reaction calculated by canonical variational transition state theory with the small-curvature tunneling correction agreed well with previously reported values, but those of CH₃NHNH₂/(CH₃)₂NNH₂ + H were slightly lower than a previous experimental value. The product-specific rate coefficients have been proposed for the kinetics modeling of these fuels’ combustion.     

Origin of the activation barrier in the process of hydrogen abstraction: the perturbation treatment

The perturbation treatment previously given is extended to explain the process of hydrogen abstraction from the various hydrogen donor molecules by the triplet nπ* state of ketones or the ground state of the alkyl or alkoxy radical. The results suggest that, as the ionization energy of the donor bonds is decreased, the reaction is accelerated and it is not influenced by the bond strength of the donor bonds. The activation barrier in such reactions arises from a weakening of the charge resonance term as the ionization energy of the donor bond increases.     

Efficient protocol for quantum Monte Carlo calculations of hydrogen abstraction barriers: Application to methanol

Accurate calculation of hydrogen abstraction reaction barriers is a challenging problem, often requiring high level quantum chemistry methods that scale poorly with system size. Quantum Monte Carlo (QMC) methods provide an alternative approach that exhibit much better scaling, but these methods are still computationally expensive. We describe approaches that can significantly reduce the cost of QMC calculations of barrier heights, using the hydrogen abstraction of methanol by a hydrogen atom as an illustrative example. By analysing the combined influence of trial wavefunctions and pseudopotential quadrature settings on the barrier heights, variance, and time‐step errors, we devise a simple protocol that minimizes the cost of the QMC calculations while retaining accuracy comparable to large‐basis coupled cluster theory. We demonstrate that this protocol is transferable to other hydrogen abstraction reactions.     

Tunneling parameters for the hydrogen atom abstraction reactions of diphenylcarbene in a low temperature toluene matrix

Tunneling reaction rate constants of diphenylcarbene in a toluene matrix can be fit by an asymmetric Eckart barrier. The barrier heights in good agreement with theory.     

Some hydrogen abstraction reactions of the CF3CCl2 radical

The gas phase photolysis of CF₃CCl₃ in the presence of several alkanes has been used to obtain Arrhenius parameters for the abstraction of hydrogen atoms by the CF₃CCl₂ radical: Activation energies of 9.6 and 8.0 kcal/mole are found for abstraction from secondary and tertiary C–H bonds, respectively. The Arrhenius parameters are compared to those for CCl₃ and CF₃ radicals.     

Recombination dynamics and hydrogen abstraction reactions of chlorine radicals in solution

We observe chlorine radical dynamics in solution following two-photon photolysis of the solvent, dichloromethane. In neat CH(2)Cl(2), one-third of the chlorine radicals undergo diffusive geminate recombination, and the rest abstract a hydrogen atom from the solvent with a bimolecular rate constant of (1.35 +/- 0.06) x 10(7) M(-1) s(-1). Upon addition of hydrogen-containing solutes, the chlorine atom decay becomes faster, reflecting the presence of a new reaction pathway. We study 16 different solutes that include alkanes (pentane, hexane, heptane, and their cyclic analogues), alcohols (methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol), and chlorinated alkanes (cyclohexyl chloride, 1-chlorobutane, 2-chlorobutane, 1,2-dichlorobutane, and 1,4-dichlorobutane). Chlorine reactions with alkanes have diffusion-limited rate constants that do not depend on the molecular structure, indicating the absence of a potential barrier. Hydrogen abstraction from alcohols is slower than from alkanes and depends weakly on molecular structure, consistent with a small reaction barrier. Reactions with chlorinated alkanes are the slowest, and their rate constants depend strongly on the number and position of the chlorine substituents, signaling the importance of activation barriers to these reactions. The relative rate constants for the activation-controlled reactions agree very well with the predictions of the gas-phase structure-activity relationships.     

Stereoselective intramolecular hydrogen abstraction by a chiral benzophenone derivative

An unprecedented stereoselective photoreduction of a chiral BZP is observed in steady state as well as in time-resolved studies.     

Understanding kinetic solvent effects on hydrogen abstraction reactions from carbon by the cumyloxyl radical

A kinetic study of the hydrogen abstraction reactions from tetrahydrofuran (THF) and cyclohexane (CHX) by the cumyloxyl radical was carried out in different solvents. With THF, a 4.5-fold decrease in rate constant (k(H)) was observed on going from isooctane to 2,2,2-trifluoroethanol. An opposite behavior was observed with CHX, where k(H) increased by a factor 4 on going from isooctane to 2,2,2-trifluoroethanol. The important role of substrate structure and of the solvent hydrogen bond donor ability is discussed.     

Activation-controlled hydrogen abstraction by benzophenone triplet

Through nanosecond flash photolysis measurements, activation energies for hydrogen abstraction by benzophenone triplet have been shown to correlate with solvent molecular structure. We demonstrate that two successive processes result in photoproduct generation. One, triplet quenching, is independent of macroscopic viscosity, and is not simply reflected in the observed quantum yield. The second process involves separation of two caged radicals, and does not occur in rigid media.     

On the possible catalysis by single water molecules of gas-phase hydrogen abstraction reactions by OH radicals

The gas phase hydrogen abstraction reaction between OH and CY(2)XH, where X = H, F, OH, or NH(2) and Y = H, CH(3) or F, in the absence and presence of a single water molecule is investigated using both density function theory, B3LYP, and explicitly correlated coupled cluster theory, CCSD(T)-F12. We find that a single water molecule could have a catalytic effect at low temperatures possible in laboratory experiments, but does not seem to catalyze these reactions at 298 K, and will not play a role under relevant atmospheric conditions.