Abstract:
The nucleation of chromium dihalide clusters is investigated by studying clusters of the form CrnX2n (n <= 4, X = F, Cl, Br, and I) for different spin states and the corresponding low temperature solid-state modifications using density functional theory. Using both wave function based (coupled cluster) and density functional theory, we predict that in all cases the ground state of the CrX2 monomer is a bent B-5(2) state arising from a weakly Renner-Teller distorted 5 Pi(g) state of the linear CrX2 unit. These quintet units can form antiferromagnetically coupled, two-dimensional chains with chromium being bridged by two halides and a nucleation growth pattern that resembles the structural motif found for the solid state. Deviations from this two-dimensional chain growth are only found for the trimers and tetramers of CrBr2 and Crl(2), where a "triangular" three-dimensional geometry takes slight precedence over the planar ribbon motif. We find that each single CrX2 unit adds an almost constant amount of energy between 45 and 50 kcal/mol to the cluster growth. This is in accordance with the calculated sublimation energies for the solid state which gave 58 kcal/mol for CrF2, and between 41 and 46 kcal/mol for CrCl2, CrBr2, and Crl(2). The large deviation of the calculated from the experimental sublimation energy for CrF2 is due to the high electronegativity of fluorine ligand, which substantially increases the ionic interactions, resulting in a much more tightly packed solid-state structure, which is not so well described by spin-broken density functional theory. In accordance with this, CrF2 shows an unusually large bulk modulus (395 kbar) compared to the heavier halides CrCl2 (82 kbar), CrBr2 (40 kbar), and Crl(2) (18 kbar).
