Estudio de la densidad electrónica y de las propiedades electroestáticas, a partir de experiencias de difracción de rayos-X a baja temperatura y alta resolución, en el ácido fosfórico de 1-arginina monohidratado

Abstract

The electron density model MOLLY, based in a multipolar development of spherical harmonic functions, has been used to study the electron density distribution p(r), the electrostatic potential V(,) and the topology of p(r) of two molecular materials: phosphonc acid and I-arginine g.i nine phosphate monohydrate.

The study of the phosphorus and oxygen radial functions has been carried out by means a carefully inspection of the residual maps. Using three sets of structure factors at several simulated temperatures, the deconvolution between thermal and electron density parameters is shown effective. Comparisons between X-(X+N), X-X and theoretical Ab initio SCF models point out that, in the intermolecular regions, al1 of them describe V(,) with similar precision.

The difference between the electrostatic potentials developed by a pseudoisolated molecule of the crystal and a procrystal molecule shows that crystalline field effects are very small. By inspection of the topo- Iogy of V(,), hydrogen bonding (HB) seems a limit case of a covalent bond, which is characterised by positive isopotential lines with sandglass shapes, as it is found in HB. In H...O region, higher positive values of V(,) stronger hydrogen bonding interactions happens. Clusters of pseudoisolated molecules develop the V(,)=O surface around both molecules and HB regions. This effect point out that, in molecular material~ w ith HBs, crystalline cohesion is a network of electrostatic interactions which take place inside the volume defined by the zero potential surface. On the other hand, the p and r values, found at the VZ p critical point, characterise the HB interaction. They are related to the HB strength and permit a quantitative approach to its partial covalent character.