A Description of the Chemical Bond in Terms of Local Properties of Electron Density and Energy

Chemical bonding is described in terms of the properties of the one-electron density f2 (r) and the local energy density H (r) = G (r) + V (r). Analysis of a variety of different bonds suggests that covalent bonding requires the existence of ao saddle point rP of f2 (r) in the internuclear region (necessary condition) and a predominance of the local potential energy V (r) at r P : J VP J > GP and, hence, HP< O. A covalent bond can be characterized by the position of r P, the value and the aonisotropy of f!p · These properties of f2 (r) can be used to define polarity, order and n-character of the bond. Information about concentration and depletion of electronic charge at rP is provided by the Laplacian of f2p• \1 2 f!p· Investigation of \1 2 (! (r) does not suffice to detect weak covalent bonds, an observation which is aolways valid if accumulation of electronic charge in the internuclear region is taken as the sole indicator for bonding. Interactions between closed shell systems as experienced in ionic, hydrogen bonded or van der Waals systems lead to a positive value of HP. In this case, shared electron density causes destabilization rather than stabilizaotion of the molecule.