Thermochemistry of iron chlorides and their positive and negative ions

The iron chlorides FeCl, FeCl2, and FeCl3 and their ions FeCl+, FeCl2+, FeCl-, FeCl2-, and FeCl3- were investigated using MP2 and QCISD(T) calculations with double- and triple-zeta basis sets augmented with multiple sets of diffuse and polarization functions. The dissociation enthalpies for FeCl --> Fe + Cl, FeCl2 --> FeCl + Cl, and FeCl3 --> FeCl2 + Cl are predicted to be 82.5, 109.6, and 59.6 kcal/mol at 298.15 K, respectively. The calculated heats of formation of these species in the gas phase at 298.15 K are +45.3 kcal/mol for FeCl, -35.8 kcal/mol for FeCl2, and -66.8 kcal/mol for FeCl3. The calculated heat of formation of FeCl is 15 kcal/mol lower than the estimated value of +60.0 (+/-20.0) kcal/mol reported in the JANAF tables, but is in reasonably good agreement with a recent experimental determination (49.5 +/- 1.6 kcal/mol). The calculated ionization potential of FeCl is 7.89 eV and that of FeCl2 is 10.10 eV. The electron affinities are 1.54 eV for FeCl, 0.99 eV for FeCl2, and 3.90 eV for FeCl3. Comparison of the bond dissociation enthalpies in FeCln, FeCln+, and FeCln- reveals a preference for iron to exist in the +2 oxidation state (as FeCl2, FeCl+, or FeCl3-); this preference is also seen when comparing IPs and the EAs of the iron chlorides. We also evaluated the dissociation energies, IPs and EAs of the iron chloride species using the B3LYP version of density functional theory. Comparison to the high-level ab initio results shows that density functional theory with the targe basis set is accurate to 5-10 kcal/mol for these species.