A Theoretical study on the degradation of 2-mercaptobenzothiazole and 2-mercaptobenzimidazole by [rad]OH in vacuo and aqueous media

Abstract

The hydrogen atom transfer mechanism for the reaction between 2-mercaptobenzothiazole (MBT) and OH and between 2-mercaptobenzimidazole (MBZ) and OH has been investigated theoretically for the first time for the purpose of understanding the degradation pathway for MBZ and MBT to thiyl radical species. The objectives of the work have been to elucidate the geometries of the species involved in the reaction and to characterise them with respect to their energetic, geometric, electronic and reaction-kinetic properties. More importantly, the study compares and contrasts the two hydrogen transfer mechanisms through which the degradation of MBZ and MBT could be achieved: the direct hydrogen atom transfer (hydrogen abstraction) and the OH addition followed by water elimination (addition–elimination). The study has been performed in vacuo and in aqueous solution using the DFT/ωB97X−D and DFT/M06−2X methods in conjunction with the 6-311++G(3df,2p) basis set. The results of the study indicate that the geometry of the considered species only minimally depends on the medium and is similar in the results of both calculation methods. The pre-reactive complexes for the direct H atom transfer mechanism and for the addition-elimination mechanism are significantly different. The activation energy depends on the medium; for instance, it is often greater in aqueous solution than in vacuo; the calculated values of the activation energy depend also on the calculation method. In water solution, the direct hydrogen atom abstraction mechanism is kinetically preferred to the addition-elimination mechanism, suggesting that the studied reactions most likely proceed through the direct hydrogen atom abstraction mechanism. The estimated rate constant for the MBZ + OH reaction is also in close agreement with the experimentally determined value. The spin density distribution on the thiyl species is significantly delocalized away from the centre of H atom abstraction. The SH bond dissociation enthalpy (BDE) is slightly higher for MBT than for MBZ, although both values are below the experimentally determined SH BDE value.