Kinetic Isotope Effect Studies Research Articles

Exact quantum mechanical study of kinetic isotope effects in the collinear reaction Cl + H2 → HCl + H. The H2/D2 and the H2/T2 isotope effects

Exact quantum mechanical probabilities for the collinear reaction of chlorine atoms with H2, D2, and T2 in their ground and first vibrational excites states, were calculated for a semiempirical LEPS potential energy surface. These probabilities were used to calculate the kinetic isotope effects kCl+H2/kCl+D2 and kCl+H2/kCl+T2 which are compared with experimental data and with results of transition state theory calculations. The effect of the vibrational excitation of the hydrogen molecules on the rate of their reaction with chlorine atoms was also studied.

Liversidge Lecture. Recent advances in the study of kinetic hydrogen isotope effects

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Kinetic Isotope Effect Studies on the Reaction of Benzyl Nitrate with Base in Varions Solvents

The role of solvent and base on the nature of the transition state for the concerted one-step carbonyl elimination reaction of benzyl nitrate has been examined using rate and kinetic isotope effect techniques. The primary hydrogen–deuterium isotope effect was found to increase with increasing strength of the abstracting base, kH/kD = 5.94 and 6.41 at 30° for reaction with EtO−/EtOH and t-BuO−/t-BuOH, respectively. The nitrogen isotope effect decreased with increasing strength of the abstracting base, 2.07, 1.82, and 1.54% for reaction with OH−/40 vol % EtOH–H2O, EtO−/EtOH, and t-BuO−/t-BuOH, respectively. The conclusion is reached that an increase in the strength of the abstracting base leads to a more reactant-like transition state with decreased rupture of both the C—H and O—N bonds. The role of solvent on the nature of the transition state was examined by determining rates and nitrogen isotope effects for reaction in various ethanol–water and ethanol–dimethyl sulfoxide mixtures. The nitrogen effect was found to remain essentially constant in solvent mixtures ranging from 65 vol % EtOH–H2O on the one hand to 80 vol% EtOH–DMSO on the other hand where the rate constants differed by a factor of 100 fold. It is concluded that solvent plays only a very minor role in determining transition state structure for the concerted elimination reaction of a neutral substrate with base.

Oxidation of 4a,4b-Dihydrophenanthrenes. III. A Theoretical Study of the Large Kinetic Isotope Effect of Deuterium in the Initiation Step of the Thermal Reaction with Oxygen

The large kinetic isotope effect of deuterium that occurs in the reaction of perdeuterio-4a,4b-dihydrophenanthrene with O2 is studied by a semiempirical calculation. The large effect is a consequence of two significant effects, the existence of quantum mechanical tunneling and the loss of zero-point energy. The theoretical study is mainly based on the construction of the potential surface by the method of bond eigen-functions, and the treatment of the reaction as the solution of a 10-electron problem. The calculated tunneling effect, which is found by integration of all possible transmission probabilities through the potential barrier, results in factors of 11.8 at 242°K and 8.1 at 263°K. A conformational and normal mode analysis shows that the contribution of the zero-point energy effect amounts to a factor of 7.6 at 242°K and 6.4 at 263°K.

The stereochemistry of the conversion of propanediol to propionaldehyde

A combination of time-resolved techniques (part I) with kinetic isotope effects studies (part II) has been used to investigate the mechanism of the decatungstate (W10O324−) photocatalyzed oxidation of aromatic alcohols, in acetonitrile. It is demonstrated that aromatic alcohols exclusively react by hydrogen atom transfer (HAT) mechanism, like to that proposed for the analogous alkane oxidations, and not by an electron transfer (ET) which is predominant in alkenes. The primary and β-secondary isotope effects provide strong evidence for a stepwise mechanism, in which the hydrogen atom abstraction occurs in the rate-determining step of the reaction. The positive slope in Hammett kinetics suggests a radical intermediate for this reaction and supports an early transition state.

Hydrogen—Deuterium Kinetic Isotope Effect, an Experimental and Theoretical Study over a Wide Range of Temperature

An experimental study of the kinetic isotope effect for the reactionCF3+CHD3–|→F3C–H–CD3→F3CH+···→F3C–D–CHD2→F3CD+···has been carried out between 300° and 700°K, and also the Arrhenius parameters k=A exp (—E/RT) were obtained for the individual reactions. An especial effort was made to avoid systematic errors, and the precision obtained was good. These data were combined with those of Tschuikow-Roux who studied the essentially same reaction between 1000° and 1800°K, and a detailed test was made of activated complex theory utilizing the method of London—Polanyi—Eyring—Sato with corrections for quantum mechanical tunneling. Sato's parameter was evaluated from activation energy of an analogous reaction so that kH/kD and A provided an unambiguous, nonadjustable test of theory. Because reactants were identical for both reactions, all the kinetic isotope effect depended on differences in the two activated complexes. The vibrational frequencies of the two nine-atom models of the complex were evaluated by a computer, programmed to E. B. Wilson's FG-matrix methods, and the effect of replacing the full nine-atom model by simpler five-, four-, or three-atom models was tested. The Arrhenius A factor was enormously sensitive (factor of 103) to number of atoms assigned to the model; the three- or four-atom models gave A factors much too high, but the five- or nine-atom models with tunneling corrections were in fairly good agreement with experiment from 300° to 1800°K. It is judged that if one evaluates a potential-energy surface, carries out a full vibrational analysis, and applies a reasonable tunneling correction, then one has in activated complex theory an expression analogous in logic and in usefulness to the perfect-gas theory; that is, broad over-all predictions can be made (in spite of readily identified conceptual flaws) but fine points will always be missed. Any quantitative value in the simplified versions of the theory is open to grave doubt.

Further Evidence for Benzyne as an Intermediate in Nucleophilic Aromatic Substitution Reactions1

Some alternatives to the “benzyne” (elimination-addition) mechanism for aminations of non-activated halobenzenes have been disproved by the finding that the amination of iodobenzene-1-^(14)C-2,4,6^(-2)H_3 gives the same extent of ^(14)C rearrangement in the resulting aniline as does iodobenzene-1-^(14)C. The intermediacy of substituted benzynes in non-rearranging aminations of 3-substituted halobenzenes has been confirmed by a study of the hydrogen-deuterium kinetic isotope effect in reactions of 3-bromoanisole-2,4,6-^2H_3.