Density functional analysis of 13C and 1H chemical shifts and bonding in mercurimethanes and organomercury hydrides:: The role of scalar relativistic, spin-orbit, and substituent effects

Relativistic and substituent effects on C-13 NMR chemical shifts in mercurimethanes and on H-1 shifts in organomercury hydrides have been studied by density functional calculations, comparing quasirelativistic and nonrelativistic effective-core potentials for mercury. The positive shift increments in the C-13 shifts as a function of HgCl or HgCN substituents in the mercurimethanes CHn(HgX)(4-n)(X = Cl, CN; n = 0-4) are due to scalar relativistic effects. The relativistic effects for a given structure and the influence of the relativistic Hg-C bond contraction partly oppose each other, in contrast to results obtained recently for O-17 shifts in oxo complexes. These differences are due to different types of metal orbitals involved in bonding, mainly of 6s-character for the mercury compounds but predominantly of 5d-character for the oxo complexes. Remaining discrepancies between computed and experimental C-13 shifts of CH3HgX for more electropositive substituents X = CH3, SiH3 and particularly between computed and experimental H-1 shifts in organomercury hydrides RHgH (R = CH3, C2H5, C2H3, C6H5, C6F5), appear to be largely due to spin-orbit coupling, as indicated by preliminary calculations of spin-orbit corrections to the chemical shifts. The spin-orbit contributions are almost entirely due to a rho(u)(X-Hg-Y) --> pi*(Hg 6(x,y))-type coupling and affect exclusively the shift tensor components perpendicular to the X-Hg-Y axis. The magnitude of the spin-orbit corrections correlates well with the inverse of the energy differences between the corresponding Kohn-Sham MOs. Thus spin-orbit coupling probably accounts in part for the increase of the C-13 shifts in CH3HgX with decreasing electronegativity of X, and for similar trends of the H-1 shifts in organomercury hydrides. In addition to the chemical shift results, analyses of the molecular and electronic structures of the mercurimethanes reveal interesting counterexamples to Bent's rule. (C) 1998 American Institute of Physics.