Calibration of isomer shift and quadrupole coupling for ¹¹⁹Sn, ¹²⁷I and ¹²⁹I as derived from self-consistent charge Xα calculations and Mossbauer measurements

The self-consistent charge (SCC) X alpha method has been applied to investigate the electronic structure of molecules which contain heavy, Mossbauer-active elements (Sn and I). For a series of halogen-containing compounds, with various oxidation states of the heavy elements, the authors have calculated orbital energies and dipole moments, which compare well with corresponding experimental data. In the case of hypervalent ions the inclusion of d besides s and p orbitals is sensitive with respective to dipole moments only and leaves orbital energies practically unaffected. For tin- and iodine-containing molecules they have calculated electron charge densities rho (0) and electric field gradients (EFG) at the nuclear site of the Mossbauer atom. Comparing these results with corresponding isomer shifts and quadrupole splittings (coupling constants), which were partly measured by one of them, they derive as calibration factors the fractional change of the nuclear radius Delta R/R and the quadrupole moment Q for ¹¹⁹Sn, ¹²⁷I and ¹²⁹I: Delta R/R(¹¹⁹Sn)=1.61*10^-4, Delta R/R(¹²⁷I)=-0.345 Delta R/R(¹²⁹I), Delta R/R( ¹²⁹I)=4.18*10^-4; Q(¹¹⁹Sn)=-0.061 b, Q(¹²⁷I)=-0.60+or-0.03 b, Q(¹²⁹I)=0.701Q(¹²⁷I).