ASPHERICITY SHIFTS FROM AB-INITIO DENSITIES COMPARED WITH EXPERIMENTAL RESULTS. S. Harkema, B.J.M. Fransen, J.A. Romein and D. Feil, Chemical Physics Laboratory, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

It is well known that positional and thermal parameters derived from X-ray and neutron diffracton experiments differ. These differences are caused by deviations from local spherical symmetry in the electronic charge density due to chemical bond formation. These asphericity shifts are most pronounced for H-atoms, but have also been detected for other atoms (F.H. Allen, Acta Cryst. (1986), B42, 515). In order to study these effects in a quantitative way structure factors were calculated from ab-initio electron densities for urea-phosphoric acid, for which compound a very extensive X-ray data set is available. The calculated structure factors were used as input for a spherical atomic refinement and theoretical asphericity shifts were determined. The shifts observed are a function of S.

The functions can be compared with experimental ones, obtained by refinements in small ranges of S, taking the positions from very high angle refinements as a reference. Calculated and experimental values are in excellent agreement.

Shifts observed can be rationalized in terms of competition of bonding and lone pair difference densities: the bonding densities dominating at low S, the lone pair densities at high S.