Polarised neutron diffraction and molecular magnetic materials

Abstract

The topology of the electrostatic potential has been studied for single molecules using geometries and electron distributions determined from high-resolution singlecrystal X-ray diffraction experiments. The electrostatic potential gradient, which is the negative of the electric field, has been represented, revealing the position of zeroflux surfaces and critical points [1]. Through the representation of the electric field lines, the relationship between the topology of the electrostatic potential and the reactivity of the molecule is explored. Local maxima, corresponding to the nuclei, are associated to electrophilic sites, while local minima, which are related to local accumulations of electron density (i.e. in lone pairs) are identified as nucleophilic sites. Both kind of reactive sites present influence zones delimited by zeroflux surfaces. Space can be partitioned in disjoint volumes (primary bundles) [2], each of them corresponding to the intersection of the influence zones of at most one nucleophilic and one electrophilic sites, which are both situated on the surface of the primary bundle. As a result of the interaction with the whole molecule, a probe charge inside one primary bundle is directed to, or away from, the reactive sites on the surface of this volume. The bundles can be added to generate two separate partitions in nucleophilic and electrophilic influence zones, the completeness of these last partitions depending on either the neutral or ionic character of the molecule. Analogous to the bond and ring critical points in the electron density topology, electrostatic potential saddle critical points appear on the zero-flux surfaces. They are related to the limits of the electrophilic and nucleophilic influence zones, which can extend to infinite or present a finite volume. In this last case, a saddle point reveals the limit of the influence zone and the path for preferred attack on the reactive site.