EVALUATION OF STERNHEIMER NUCLEAR-QUADRUPOLE SHIELDING FUNCTIONS AND THEIR APPLICATION TO THE DETERMINATION OF THE FE-57(M) NUCLEAR-QUADRUPOLE MOMENT FROM X-RAY-DETERMINED CHARGE-DENSITIES

The Sternheimer function gamma(r) describes the shielding/antishieldin g of the electric held gradient (EFG) at the nuclear position due to p olarization induced in the atomic density by the quadrupolar component s of the density distribution. The functions for Fe, Fe2+ and Fe3+ hav e been derived by means of Sternheimer's procedure [Sternheimer (1986) . Z. Naturforsch. Teil A, 41, 24-36], using a finite-difference method for solving the radial equations for the perturbed wavefunctions and numerical integration for the calculation of gamma(r). The shielding f actors R, due to the contributions from the electron density of the at om at the nucleus of which the EFG is being considered (the 'central c ontributions'), are derived from the gamma functions. Results are give n for near-Hartree-Fock atomic and ionic wavefunctions [Clementi & Roe tti (1974). At. Data Nucl. Data Tables, 14, 177-478]. Contributions to the shielding from the core and valence electrons are separated. Sinc e the X-ray multipole formalism describes a flexible valence shell but uses a frozen core, only gamma(infinity)(core) and R(core) are used i n the calculation of Mossbauer splittings from the experimental charge densities. The effect on the shielding of X-ray-determined radial exp ansion/contraction of the valence shells [Coppens, Guru Row, Leung, St evens, Becker & Yang (1979). Acta Cryst. A35, 63-72] is evaluated. The combination of spectroscopic nuclear quadrupole splittings and X-ray charge densities on iron pyrite (FeS2), sodium nitroprusside {[Na2Fe(N O)(CN)(5)].2H(2)O} and [Fe(TPP)(pyridyl)(2)] leads to unweighted and w eighted average values for Q(Fe-57(m)) of 0.12 (3) and 0.11 (2) x 10(- 28) m(2), respectively, when the core shielding factors are used.