The electrostatic contribution to the Mössbauer isomer shift of mercury for the series HgFn (n = 1, 2; 4) with respect to the neutral atom has been investigated in the framework of 4- and 2-component relativistic theory. Replacing the integration of the electron density over the nuclear volume by the contact density (that is, the electron density at the nucleus) leads to a 10% overestimation of the isomer shift. The systematic nature of this error suggests that it can be incorporated into a correction factor, thus justifying the use of the contact density for the calculation of the Mössbauer isomer shift. The performance of a large selection of density functionals for the calculation of contact densities has been assessed by comparing with finite-field 4-component relativistic Coupled-Cluster with Single and Double and Perturbative Triple excitations [CCSD(T)] calculations. For the absolute contact density of the mercury atom the Density Functional Theory (DFT) calculations are in error by about 0.5%, a result which must be judged against the observation that the change in contact density along the series HgFn (n = 1; 2; 4), relevant for the isomer shift, is on the order of 50 ppm with respect to absolute densities. Contrary to previous studies of the 57Fe isomer shift [F. Neese, Inorg. Chim. Acta 332 (2002) 181], for mercury DFT is not able to reproduce the trends in the isomer shift provided by reference data, in our case CCSD(T) calculations, notably the non-monotonous decrease in the contact density along the series HgFn (n = 1; 2; 4). Projection analysis shows the expected reduction of the 6s1/2 population at the mercury center with an increasing number of ligands, but also brings into light an opposing effect, namely the increasing polarization of the 6s1/2 orbital due to increasing eective charge of the mercury atom, which explains the non-monotonous behavior of the contact density along the series. The same analysis shows increasing covalent contributions to bonding along the series with the eective charge of the mercury atom reaching a maximum of around +2 for HgF4 at the DFT level, far from the formal charge +4 suggested by the oxidation state of this recently observed species. Whereas the geometries for the linear HgF2 and square-planar HgF4 molecules were taken from previous computational studies, we optimized the equilibrium distance of HgF at the 4-component Fock-space CCSD/aug-cc-pVQZ level, giving spectroscopic constants re = 2.007 Å and ωe = 513.5 cm-1.
Theoretical Chemistry Accounts, 2011, Vol 129, Issue 3-5, p. 631-650