Combined Quantum Mechanical and Molecular Mechanics Studies of the Electron-Transfer Reactions Involving Carbon Tetrachloride in Solution

Journal Article · Thu Mar 27 04:00:00 EDT 2008 · Journal of Physical Chemistry A, 112(12):2713-2720

DOI:https://doi.org/10.1021/jp7104709· OSTI ID:927715

Valiev, Marat; Bylaska, Eric J; Dupuis, Michel; Tratnyek, Paul G

The reductive dechlorination of carbon tetrachloride, CC₄, was investigated using combined high level quantum mechanical and molecular mechanics (QM/MM) approach. The first electron transfer process was assumed to proceed by a concerted electron transfer-bond breaking mechanism, and reaction barriers for the first electron reduction were estimated by using the crossing point of the free energy profiles of CCl₃-Cl and CCl₃-Cl•- as a function of the CCl₃-Cl distance. The results of these calculations showed that the activation barriers for this reaction are reachable under a wide range of reduction potentials. In the gas-phase, the barrier to reduction varied from 0.8 kcal/mol for reducing agent with a -5 kcal/mol work function to 24.7 kcal/mol for a reducing agent with a 40 kcal/mol work function at the CCSD(T)/aug-cc-pVDZ level. In the aqueous phase, QM/MM calculations at the CCSD(T)/aug-cc-pVDZ level predicted that the barrier to reduction varied from 0.7 kcal/mol to 35.2 kcal/mol for -2.32 V and 0.93 V reduction potentials respectively. COSMO continuum solvation calculations were also performed for comparison. For strong reducing agents (EH < -1.5V) very little difference was seen between the QM/MM and COSMO activation barriers. For weak reducing agents (EH > 0V) the activation barriers differed by as much as 6 kcal/mol between the QM/MM and COSMO calculations. These results demonstrate that ab initio electronic structure methods coupled with explicit molecular mechanics representation of the aqueous environment offer an efficient and accurate way to calculate the free energy reaction barriers for dissociative electron transfer reactions of organochlorine compounds to identify the potentially important environmental degradation processes.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 927715
• Report Number(s): PNNL-SA-56528; 25709; 20900; 2573b; KP1504020
• Journal Information: Journal of Physical Chemistry A, 112(12):2713-2720, Journal Name: Journal of Physical Chemistry A, 112(12):2713-2720 Journal Issue: 12 Vol. 112
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
54 ENVIRONMENTAL SCIENCES
CARBON TETRACHLORIDE
CHEMICAL REACTION KINETICS
DECHLORINATION
ELECTRON TRANSFER
ELECTRONIC STRUCTURE
Environmental Molecular Sciences Laboratory
FREE ENERGY
REDUCING AGENTS
SOLVATION
WORK FUNCTIONS