Darren L. Reid

H-atom abstraction from thiols by C-centered radicals. A theoretical and experimental study of reaction rates

The Arrhenius parameters and rates of reaction of three hydroxyradicals, methyl radical, and the hindered primary C-centred radical from t-butyl alcohol with dithiothreitol were measured by pulse radiolysis in water. The bimolecular rate constants were found to be in the order: ˙C(CH3)2OH > ˙CH(CH3)OH > ˙CH2OH > ˙CH3 > ˙CH2C(CH3)2OH. The reaction of three of these, ˙C(CH3)2OH, ˙CH2OH, and ˙CH3, with methanethiol were examined at the ab initio B3LYP/6311+G(d,p) level, coupled with transition state theory, both in the gas phase and in solution. The solvent effects are evaluated by two different continuum models (SCIPCM, CPCM), coupled with a novel approach to the calculation of the solution phase entropy. The reaction is discussed in terms of the charge and spin polarization in the transition state, as determined by AIM analysis, and in terms of orbital interaction theory. Rate constants, calculated by transition state theory are in good agreement with the experimental data.

H-atom abstraction by thiyl radicals from peptides and cyclic dipeptides. A theoretical study of reaction rates

The reactions of methanethiyl radicals (CH3S˙) with the cyclic anhydrides of glycine (1a), alanine (L,L-1b and D,L-1b), and sarcosine (1c), and the acyclic peptides, N-formylglycinamide (3a) and N-formylalaninamide (3b), were studied by means of theoretical calculations at the B3LYP/6-311+G(d,p) level of theory. Free energies in the gas phase were determined in the classical harmonic oscillator-rigid rotator model, and used to estimate rates of H-transfer reactions at the αC-site of the peptides. The effects of aqueous solution were estimated by the SCIPCM procedure in combination with modified experimental Arrhenius A-factors to calculate solution phase rate constants. The reactions are discussed in terms of the charge and spin polarisation in the transition state, as determined by AIM analysis. Rate constants calculated by this semiempirical Arrhenius approach are in very good agreement with available experimental data on the reaction of D3N+CH2CH2S˙ with 1a and 1c but not 1b in D2O. The rate constants for the reaction of CH3S˙ with 3a and 3b are also in very good agreement with experimental data on the N-acetyl analogues. Contrary to experiment, the cyclic alanine anhydrides, L,L-1b and D,L-1b, were predicted to react more than an order of magnitude more quickly than any of the other dipeptides.

H-Atom abstraction by C-centered radicals from cyclic and acyclic dipeptides. A theoretical and experimental study of reaction rates

Using the pulse radiolysis and other radiation chemical techniques, the reaction kinetics of ˙CH3 and ˙CH2OH radicals with alanine anhydride, glycine anhydride and sarcosine anhydride were examined in aqueous solution. The second order rate constant of reaction of ˙CH3 with alanine anhydride has been determined at 2 × 105 dm3 mol−1 s−1. A very similar value (8 × 104 dm3 mol−1 s−1) has been obtained by measuring the methane and ethane yields as a function of the alanine anhydride concentration at different dose rates and evaluation of these data by computer analysis. Similar experiments carried out with glycine anhydride and sarcosine anhydride yielded identical rate constants of 4 × 104 dm3 mol−1 s−1 for these compounds. It was found that ˙CH2OH radical does not react with alanine anhydride at an appreciable rate (k < 2 × 102 dm3 mol−1 s−1). The reactions of ˙CH3, ˙CH2OH, and ˙C(CH3)2OH with both cyclic anhydrides and acyclic glycine and alanine peptides were studied by means of theoretical calculations at the B3LYP/6-311+G(d,p) level of theory. Free energies in the gaseous phase were determined in the classical harmonic oscillator–rigid rotator model, and used to estimate rates of H-transfer reactions at the αC-site of the peptides. The effects of aqueous solution on the activation enthalpies were estimated by the SCIPCM procedure. Using the experimental A-factor for the methyl radical + D,L-alanine anhydride reaction, modified experimental Arrhenius A-factors were obtained for the reactions of the other species, and used to calculate solution-phase rate constants. The reactions are discussed in terms of the charge and spin polarisation in the transition state, as determined by AIM analysis, and in terms of orbital interaction theory. Rate constants, calculated by transition state theory are in good agreement with the available experimental data.

Remarkable aromatic substitution by a 1,5-diradical

Generation of 2,6-dioxa-3-phenylcyclohexylidene in benzene leads to 2,4-diphenyl-1,3-dioxane, the product of apparent insertion into a CH bond of benzene. However, that product arises from attack of a diradical intermediate on benzene.