Reactions of benzene and 3-methylpyrrole with the •OH and •OOH radicals: an assessment of contemporary density functional theory methods.

A high-level quantum chemistry investigation has been carried out for the addition and abstraction reactions by the radicals (•)OH and (•)OOH to and from the model alkenes 3-methylpyrrole and benzene. These models were chosen to reflect the functionalities contained in the side chain of the amino acid tryptophan. The W1BD procedure was used to calculate benchmark barriers and reaction energies for the smaller model system of (•)OOH addition to ethylene. It was found that the CBS-QB3 methodology compares best with the W1BD benchmark, demonstrating a mean absolute deviation (MAD) from W1BD of 3.9 kJ mol(-1). For the reactions involving the (•)OH radical and benzene or 3-methylpyrrole, addition is favored over abstraction in all cases. In particular the CBS-QB3 calculations suggest a barrierless addition reaction of the (•)OH radical to position two of 3-methylpyrrole. For the analogous addition and abstraction reactions involving the (•)OOH radical, the same order of reactivity was found, albeit with higher barriers. A number of other processes involving the addition of the (•)OOH radical were also investigated. The main findings of these studies determined that the initial (•)OOH barrier of stepwise addition to 3-methylpyrrole (+18.8 kJ mol(-1)) is significantly smaller than the concerted addition barrier (+71.5 kJ mol(-1)). This conclusion contrasts starkly with the situation for ethylene in which it is well established that the concerted process has the smaller barrier. A considerable variety of contemporary density functional theory procedures have been tested to examine their accuracy in predicting the CBS-QB3 results. It was found that the best overall performing method was UBMK with an MAD of 7.3 kJ mol(-1). A number of other functionals additionally performed well. They included UM06, RM06, UXYG3 and RXYG3, all of which have MADs of less than 8 kJ mol(-1).