The reactions of zerovalent iron with water and carbon tetrachloride are of interest for environmental remediation of contaminated water and soil. Atom-dropping experiments have shown that the reactions of iron atoms with water and CCl4 may produce HFeOH and FeCl2, respectively, but these compounds are themselves unreactive toward CCl4 at the low temperatures under which the atom-dropping experi ents were performed. We report a modeling study of these reactions using density functional theory, ab initio Hartree-Fock and couple-cluster theory, and principles of Marcus-Hush theory to characterize the underlying intrinsic barriers and rationalize the experimental results. Electron-correlated CCSD(T) calculations (at B3LYP/TZVP optimized structures) show that the transition state for Cl atom transfer from CCl 4 to HFeOH arises from crossing of electronic states in which the configuration of Fe changes from a quintet high spin state in the Fe II reactant to a sextet high spin state in the FeIII products. The crossing point is 23.8 kcal/mol above a long-range precursor complex that is 2.1 kcal/mol more stable than the separated reactants. The electronic structure changes in these Cl atom transfer reactions involve unpairing of d electrons in FeII and their recoupling with Cl-C σbond electrons. These processes can be conveniently described by invoking the self-exchange reactions HFeOH/HFeClOH, FeCl2/FeCl3, and CCl4/•CCl3 for which we determined the energy barriers to be 15.5, 13.1, 18.6 kcal/mol, respectively. For the cross reaction FeCl2/CCl4, we estimated a barrier of 16.6 kcal/mol relative to the separated reactants and 21.1 kcal/mol from the precursor complex. The magnitudes of the reaction barriers are consistent with reports of the absence of products in the atom-dropping experiments.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry