DENSITY-FUNCTIONAL ANALYSIS OF C-13 AND H-1 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 me rcurimethanes and on H-1 shifts in organomercury hydrides have been st udied by density functional calculations, comparing quasirelativistic and nonrelativistic effective-core potentials for mercury. The positiv e shift increments in the C-13 shifts as a function of HgCl or HgCN su bstituents in the mercurimethanes CHn(HgX)(4-n)(X = Cl, CN; n = 0-4) a re due to scalar relativistic effects. The relativistic effects for a given structure and the influence of the relativistic Hg-C bond contra ction partly oppose each other, in contrast to results obtained recent ly for O-17 shifts in oxo complexes. These differences are due to diff erent types of metal orbitals involved in bonding, mainly of 6s-charac ter for the mercury compounds but predominantly of 5d-character for th e oxo complexes. Remaining discrepancies between computed and experime ntal C-13 shifts of CH3HgX for more electropositive substituents X = C H3, SiH3 and particularly between computed and experimental H-1 shifts in organomercury hydrides RHgH (R = CH3, C2H5, C2H3, C6H5, C6F5), app ear to be largely due to spin-orbit coupling, as indicated by prelimin ary 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 tens or components perpendicular to the X-Hg-Y axis. The magnitude of the s pin-orbit corrections correlates well with the inverse of the energy d ifferences between the corresponding Kohn-Sham MOs. Thus spin-orbit co upling probably accounts in part for the increase of the C-13 shifts i n CH3HgX with decreasing electronegativity of X, and for similar trend s of the H-1 shifts in organomercury hydrides. In addition to the chem ical shift results, analyses of the molecular and electronic structure s of the mercurimethanes reveal interesting counterexamples to Bent's rule. (C) 1998 American Institute of Physics.