Nicholas D. Epiotis

Professor of Chemistry
(Organic, Ph.D., Princeton University, 1972)

(206) 543-4643
epiotis@chem.washington.edu

Research Interests

The current conceptual models of chemical bonding are not supported by the computational and experimental results taken in their totality because they fail to capture the physics of the problem due to neglect of electron-electron repulsion. When this is explicitly considered in the frame of VB theory, the covalent/ionic gospel is rejected. Two electrons can create one bond by two different mechanisms: (a) By delocalizing in the same overlap region at the expense of strong electron-electron repulsion while conserving atom charge (exchange). This is T-bonding. (b) By delocalizing in different overlap regions with the benefit of reduced electron-electron repulsion while now creating charge separation (charge transfer). This is I-bonding.

This means that there exists a mechanistic continuum which is a function of atom electronegativity since this determines the efficacy of charge transfer (CT). As atom electronegativity decreases, the magnitude of bonding due to kinetic energy reduction decreases while, at the same time, the mechanism of bonding changes from T to I. Ionic bonding (E-bonding) is seen as a limiting case of impaired I-bonding. If electron-electron repulsion is either neglected or poorly treated, the intrinsic superiority of I-bonding (reduced interelectronic repulsion) and the consequent differentiation between T- and I-bonding vanishes. This is why the ab initio computation of metal-containing molecules is problematic and why the covalent/ionic gospel dominated the thinking of chemists for nearly one century. Since the three mechanisms of chemical bonding are sharply differentiated, each atom has differing affinity for each type of bonding mechanism. Chemical selectivity is a consequence of the preference of atoms which have the same mechanistic affinity to combine with each other.

The theory is developed, explained and applied by reference to VB formulae which project explicitly what is "good" and what is "bad" about the electronic structure of a molecule. At the same time, these formulae project clearly the difference between the conventional view and the new VB theory of electronic structure. Specifically, in 1913 G. N. Lewis suggested that a line connecting two electrons is one bond and the concept of the Lewis electronic formula, the cornerstone of chemical thinking, was born. Nearly a century later, Professor Epiotis suggests that two arrows affixed on electrons define one bond. Superior bonds are formed when arrows are oriented head-to-tail (HT). Furthermore, two HT arrows can form either a closed or an open loop. The first arrangement corresponds to a T-bond promoting segregation and the second arrangement an I-bond promoting aggregation because a third party is needed at the head (tail) of one arrow.

Indeed, much in the fashion of G.N. Lewis, this new VB model could have been discovered intuitively without any recourse to quantum mechanics. However, the quantal aspects become clearly visible when one focuses on the distinction between aromatic (constructive interference) and antiaromatic (destructive interference) in the frame of VB theory.