TOPOLOGY OF THE INTERATOMIC INTERACTIONS. C. Lecomte1 and E. Espinosa1,2, LCM3B, URA CNRS ndeg. 809, Universite Henri Poincare-Nancy 1, BP 1 239, 54506 Vandoeuvre-les-Nancy Cedex, France1, Instituto de Ciencia de Materiales de Barcelona (CSIC), Campus de la l UAB, 08193 Bellaterra (Barcelona), Spain2.
The topological characterization of the electron density [[rho]](r) has been widely applied to isolated molecules and only a few theoretical studies has been devoted to hydrogen bonding (HB) in dimers. However, any experimental study of the HB topology involving both [[rho]](r) and the electrostatic potential V(r) functions has not been carried out. Comparison between crystal, procrystal (artificial crystal where the isolated molecules are placed at the crystallographic places) and IAM models lead to separate the effects involving chemical bonds and lone pairs (comparing procrystal and IAM models) from those of the molecular cohesion as HB. We will apply this method to the experimental electron density of L-arginine phosphate monohydrate (LAP) (monoclinic, P21, Z=2; X-ray and neutron, data: T=130K, 15513 and 4139 collected reflections, respectively, with Rint= 0.013 for X-ray measurements; X-(X+N) multipolar refinement: 6805 unique reflections, I>3[[sigma]](I), sin[[theta]]/[[lambda]]< 1.20Å-1, RW(F)= 0.014). Ab initio SCF theoretical models of [[rho]](r) and V(r) were calculated for the asymmetric unit of LAP with basis sets of triple-[[zeta]] quality for the valence shell, including both diffuse and polarization functions.
As hydrogen bonding is essentially an electrostatic interaction, it appears that V(r) is the observable really worth being considered and,, , checked in regions where HB's occur. The first quantitative agreement between electrostatic potential calculations from experimental X-(X+N) and theoretical Ab initio SCF models in the hydrogen bonding region (within 0.05 eÅ-1) of the unique HB existing in the asymmetric unit of LAP, allows us the study of HB effects from the comparison between crystal and procrystal models.