CCSD(T)//BHandHLYP/6-311G(d,p) calculations have been performed to study the OH hydrogen abstraction reaction from three characteristic ketones. A previously proposed complex mechanism, involving the formation of a stable prereactive complex, is confirmed for some channels. The temperature dependence of the rate coefficients (k) is studied for all significant reaction channels over the temperature range 290−500 K, using conventional transition state theory. A good agreement between calculated and experimental k at 298 K has been obtained. The rate coefficient for the formation of the beta radical in 2-pentanone is found to be significantly larger than those of the competing channels. The explanation for this behavior, previously attributed only to the structure of the reactant complex, was found to be also a consequence of the lowering of the reaction barrier due to the presence of a hydrogen-bond-like interaction in the transition state.

Unrestricted density functional theory (BHandHLYP) calculations have been performed, using the 6-311G(d,p) basis sets, to study the gas-phase OH hydrogen abstraction reaction from methionine. The structures of the different stationary points are discussed. Ring-like structures are found for all the transition states. Reaction profiles are modeled including the formation of prereactive complexes, and negative net activation energy is obtained for the gamma H-abstraction channel. A complex mechanism is proposed, and the rate coefficients are calculated using transition state theory over the temperature range 250-350 K. The rate coefficients are proposed for the first time and it was found that in gas phase the hydrogen abstraction occurs almost exclusively from the gamma site. The large overall rate coefficient for the methionine + OH reaction compared to other free amino acids could explain the significant role of methionine in the oxidative processes. The following expressions in [L/(mol s)] are obtained for the alpha, beta, and gamma Habstraction channels, and for the overall temperature-dependent rate constants, respectively: k α = (3.42 ± 0.11) × 10 8 exp[(−1118 ± 9)/T], k β = (1.13 ± 0.03) × 10 8 exp[(−1070 ± 8)/T], k γ = (2.11 ± 0.26) × 10 7 exp[(2049 ± 34)/T], and k tot = (2.12 ± 0.26) × 10 7 exp [(2047 ±

Density functional theory has been used to model the reaction of OH with l-phenylalanine, as a free molecule and in the Gly-Phe-Gly peptide. The influence of the environment has been investigated using water and benzene as models for polar and non-polar surroundings, in addition to gas phase calculations. Different paths of reaction have been considered, involving H abstractions and addition reactions, with global contributions to the overall reaction around 10% and 90% respectively when Phe is in its free form. The ortho-adducts (o-tyrosine) were found to be the major products of the Phe+OH reaction, for all the modeled environments and especially in water solutions. The reactivity of phenylalanine towards OH radical attacks is predicted to be higher in its peptidic form, compared to the free molecule. The peptidic environment also changes the sites' reactivity, and for the Gly-Phe-Gly+OH reaction H abstraction becomes the major path of reaction. The good agreement found between the calculated and the available experimental data supports the methodology used in this work, as well as the data reported here for the first time.

A theoretical study on the mechanism of the OH + aliphatic thiols reactions is presented. Optimum geometries and frequencies have been computed at the BHandHLYP/6-311++G(2d,2p) level of theory for all stationary points. Energy values have been improved by single-point calculations at the above geometries using CCSD(T)/6-311++G(d,p). Twelve possible channels have been modeled, three of them including the possible influence of molecular oxygen, and three of them involving excess of OH. The only channels that have been found to significantly contribute to the overall reaction in the troposphere are the hydrogen abstractions from the -SH group and from the alkyl groups. Our analysis supports a stepwise mechanism involving the formation of a short-lived, weakly bonded adduct in the entrance channel, for the abstraction paths. The results proposed in the present work seem to provide a viable explanation for diverse findings previously reported from experimental investigations.

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