On the full topology of the Laplacian of the electron density II: Umbrella inversion of the ammonia molecule

The full topology of L(r), which is defined as minus the Laplacian of the electron density, ∇2ρ, has recently been explored for the water molecule (Coord. Chem. Rev. 2000, 197, 169). In this work, we have investigated the changing topology during the "umbrella" inversion in ammonia. The maxima in L(r) are points of local charge concentration, which can be associated with the electron pairs of VSEPR theory. We examine changes in three valence shell charge concentration (VSCC) and three depletion (VSCD) graphs as a function of the angle between the C3 axis and a hydrogen. Through the use of planar graphs, the transition mechanisms can be easily rationalized. The previously noted double maxima in L(r), corresponding to the lone-pair of nitrogen, found at the transition state for inversion is shown to persist for geometries distorted considerably from planar. The transitions between structures in the valence shell charge concentration and charge depletion graphs do not occur simultaneously.