Topological and orbital-based mechanisms of the electronic stabilization of bis(diisopropylamino)cyclopropenylidene.

Previous analysis of the topology of the electron density of bis(dimethylamino)cyclopropenylidene as a model of the stable bis(diisopropylamino)cyclopropenylidene revealed mechanisms of induction/back-polarization, sigma-aromaticity, and sigma-pi polarization to be responsible for the electronic stabilization of the divalent carbon C2 upon amino substitution on the 3MR. This work presents new data from molecular orbital calculations and a full analysis of the operative natural bond orbitals and their interactions. The discrepancies between these orbital-based stabilization mechanisms and the physical stabilization based upon the quantum theory of atoms in molecules are uncovered through the separation of electron localization and delocalization indices into contributions from orbitals of sigma- and pi-symmetry, as well as calculated nucleus-independent chemical shifts that determine the degree of sigma- and pi-delocalization/aromaticity. Graphical representations of functions of the electron density mapped onto various pi-orbital isosurfaces serve to better visualize the underlying differences between mathematical orbital space and the real space of the electron density. This work also provides new insight into the topological-based mechanism through investigation of the changes in the virial of the electronic forces acting on the interatomic surfaces--forces that govern the bonding and stabilization within a molecule.