Kinetic and thermodynamic aspects of reactions involving thiyl radicals :|a computational approach

Abstract

In this study, the kinetics of the addition of different phenylthio radicals on the thiol-ene reactions mechanism have been computationally investigated for the first time. The contributions of substituents are examined by Density Functional Theory (DFT) calculations. It is known that the reaction mechanism of thiol-ene reactions depends upon the kP/kCT ratio, where kP is the rate constant of phenylthio radical addition to unsaturated alkene and kCT is the rate constant of hydrogen abstraction of newly formed carbon centered radical from the thiol. The M06-2X/6-31++G(d,p) level of theory is used to carry out geometry optimizations and energetics of eleven reactions. The activation energy barrier of the addition reaction is strongly controlled by the electrophilic character of phenylthio radicals and the singlet-triplet gap (S-T gap) of the alkenes. For this reason, the functional groups on the alkene and phenylthio radical have an importance in order to understand the reaction kinetics of the propagation step. It is shown that the transition state structure is responsible for the lower activation energy barrier of the chain transfer step due to the stabilization effect of intramolecular interactions. Our study has shown that the substituents on the thiol as well as the substituents on the alkene have an important effect on the kp/kct ratio as well as the substituents on the alkene. The computational method described in this study can be applied in the synthesis of the desired polymers by modifying the substituents. In addition to the investigation of thiol-ene reaction energetics and kinetics, the solvent effect on the hydrogen abstraction reaction of alkyl radical from thiol is examined by implicit solvent model. Water and acetonitrile are chosen as solvents to show the effect of the polarity of solvent. For this purpose, three different reactions are modeled at the M06-2X/6-31++G(d,p) level of theory. The results showed that the role of solvent on the thiol-alkyl reactions can be predicted by methodology used in this thesis.