High-level ab initio thermochemical data for halides of chromium, manganese, and iron

The thermochemistry of the transition-metal fluorides and chlorides MFn and MCln (M = Cr, Mn, Fe; n = 1, 2) has been characterized by high-level ab initio electronic structure methods. Geometries and harmonic vibrational frequencies were computed at the B3LYP level of theory using triple-zeta basis sets including diffuse and polarization functions. Heats of formation were computed from isogyric reaction energies at the CCSD(T) level using high-quality basis sets, including corrections for core-valence correlation and scalar relativistic effects. To investigate the possible linearity of the ground states of CrCl2 and CrF2, we performed geometry optimizations for these species at the CCSD(T) level using large basis sets. In both cases, a bent (B-5(2)) minimum structure was located, but the bent structure is only slightly below the linear form, which was found to be a transition state. For all of the investigated halides, polynomial fits were carried out for the heat capacity and the standard enthalpy and entropy in the 300-3000 K temperature range.