The elusive structure of CrCl₂ - A combined computational and gas-phase electron-diffraction study

Chromium dichloride poses a challenge to the structural chemist. Its different forms of aggregation and association display all well-known structural distortions induced by vibronic interactions. The monomeric molecule has a Renner-Teller distorted bent geometry, the crystal exhibits strong Jahn-Teller distortion, and the oligomers have slightly distorted four-membered-ring structures due to the pseudo-Jahn-Teller effect. In this paper we report on the low-energy structures of the monomer and its clusters, Cr₂Cl ₄, Cr₃Cl₆, and Cr₄Cl₈, from unrestricted Kohn-Sham (broken-symmetry) density functional calculations. CrCl₂ was also investigated at higher level, including coupled-cluster and state-average CASSCF computations. The global minima of the gas-phase clusters consist of two-dimensional, antiferromagnetically coupled chains of CrCl₂ units forming four-membered, doubly bridged Cr ₂Cl₂ rings, closely resembling the solid-state structure of α-CrCl₂. Each Cr atom in these chains has spin quantum number S = 2. This suggests that the CrCl₂ nucleation starts very early on the structural chain motif found in the solid. There is only a very small change in energy from the antiferromagnetically to the ferromagnetically coupled Cr atoms, which indicates little spin-coupling between the metal centers. There is an approximately constant change in energy, about 50 kcal mol^-1, with every new CrCl₂ unit during cluster formation. Information about the structure of these clusters was used in the re-analysis of high-temperature electron-diffraction data. The vapor at 1170K contained about 77% monomeric molecules, 19% dimers, and a small amount of trimers. Monomeric CrCl₂ was found to be bent with a bond angle of 149(10)°, in good agreement with our computations, which resulted in a Renner-Teller distortion of the lowest-energy ⁵∏_g electronic state into the bent ⁵B₂ ground state. The vibrational spectrum of chromium dichloride is discussed and the thermodynamics of cluster formation from 1000-2000 K is examined.