An appraisal of the hydrogen atom transfer mechanism for the reaction between thiourea derivatives and [formula omitted] radical: A case-study of dimethylthiourea and diethylthiourea

Abstract

Reactions between thioureas (TUs) and hydroxyl radical (OH) have wide applications in electrochemical, radiation, photochemical and biological processes. TUs+OH reactions have been suggested to proceed through either direct hydrogen abstraction or OH addition pathways. Yet, the two pathways have not been elucidated. In this study, quantum theoretical approaches are utilised to investigate the hydrogen atom transfer mechanisms for the dimethylthiourea (DMTU)+OH and diethylthiourea (DETU)+OH reactions. The objectives of the study are to identify the preferred pathway for the reaction and elucidate the nature of the species involved in the reaction as well as to characterise them with respect to their energetic, geometric, electronic and kinetic properties. The study is performed in vacuo and in water solution by utilising DFT/ωB97X−D, DFT/BHHLYP and DFT/M06−2X methods, in conjunction with the 6-311++G(3df,2p) basis sets. The results of the study suggest that direct hydrogen abstraction pathway is kinetically preferred to the OH addition pathway in both media. The activation energy values and the rate constant values for the OH addition pathway suggest that the aqueous media plays a crucial role in determining the rate determining step for the reaction. The NH bond dissociation enthalpy value is minimally dependent on the media. The spin density on the thiyl radical species is largely localised on the N atom with a little delocalisation onto the S and C for the results in water solution, while it is delocalised through the SCN region for the results in vacuo, a result which is in agreement with experimental findings. A comparison of the performance of the calculation methods suggests that there are minimal differences in their ability to estimate geometric, electronic and energetic properties and significant differences may exist in their ability to estimate reaction rate constant. Overall, the trend in the energetic (e.g., bond dissociation enthalpy) and electronic properties (e.g., spin density distribution) for the two reactions, indicating that the radical scavenging properties of thiourea derivatives is minimally influenced by the length of the alkyl chain.