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

The full topology of L(r), which is defined as minus the Laplacian of the e lectron density, del (2)rho, has recently been explored for the water molec ule (Coord. Chem. Rev. 2000, 197, 169). In this work, we have investigated the changing topology during the "umbrella" inversion in ammonia. The maxim a 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 valen ce shell charge concentration (VSCC) and three depletion (VSCD) graphs as a function of the angle between the C-3 axis and a hydrogen. Through the use of planar graphs, the transition mechanisms can be easily rationalized. Th e 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 g raphs do not occur simultaneously.