Combined quantum mechanical and molecular mechanics studies of the electron-transfer reactions involving carbon tetrachloride in solution

The reductive dechlorination of carbon tetrachloride, CCl4, by a concerted electron transfer-bond breaking mechanism was studied using combined high level quantum mechanical and molecular mechanics (QM/MM) approach. The free energy activation barriers for the first electron-transfer step were determined from the dissociation profiles of CCl4 and (CCl4-)-C-center dot Complexes in aqueous phase using hybrid-free energy QM/MM methodologies. Both density functional and coupled cluster perturbative triples (CCSD(T)) versions of QM/MM methods were investigated. The impact of the implicit solvent description based on continuum (COSMO) solvent models was also analyzed. QM/MM calculations at the CCSD(T)/aug-ce-pVDZ/SPCE level of theory predict that the activation barriers vary from 0.7 to 35.2 kcal/mol for -2.32 and 0.93 V reduction potentials respectively. Good agreement with experimental data for oxide-free iron electrodes (-0.6 to -1.2 V reduction potentials) is observed indicating that the measured activation barriers are consistent with the concerted electron transfer-bond-breaking mechanism.