Mechanisms of acid decomposition of dithiocarbamates. 4. Theoretical calculations on the water-catalyzed reaction.

A theoretical study of the water-catalyzed dithiocarbamic acid cleavage has been performed using N-methyl- (MeDTC) and N-phenyldithiocarbamic acid (PhDTC) as model molecules. Calculations have been carried out within the Density Functional Theory (DFT) formalism, using the B3LYP hybrid functional together with medium-sized basis sets, both in gas phase and by considering solvent effects through dielectric continuum methods. According to the results obtained, both in gas phase and in solution, MeDTC decomposes through a proton-transfer step assisted by a water molecule (this being the rate-determining step), leading to a zwitterionic intermediate, followed by a fast N-C bond-breaking process. In the case of PhDTC, the theoretical results point to a one-step mechanism in which the N-C bond breaking takes place in a concerted manner with the proton transfer. The calculated Delta Delta G(++) of the proton-transfer step for MeDTC and PhDTC is 4.0 kcal mol(-1), which is similar to the experimental values. For both compounds the water-assisted intramolecular proton transfer occurs with a twisting of the CS(2) group that inhibits the resonance of the thiocarbamic group, making the nitrogen more basic and therefore favoring the proton transfer. The difference in the torsional barrier has been calculated to be ca. 5 kcal mol(-1), and it is therefore concluded that most of the activation barrier of the reaction is due to the torsional barrier of the CS(2) group.