Abstract
We address the paradoxical fact that the concept of a covalent bond, a cornerstone of chemistry which is well resolved computationally by the methods of quantum chemistry, is still the subject of debate, disagreement, and ignorance with respect to its physical origin. Our aim here is to unify two seemingly different explanations: one in terms of energy, the other dynamics. We summarize the mechanistic bonding models and the debate over the last 100 years, with specific applications to the simplest molecules: H2+ and H2. In particular, we focus on the bonding analysis of Hellmann (1933) that was brought into modern form by Ruedenberg (from 1962 on). We and many others have helped verify the validity of the Hellmann–Ruedenberg proposal that a decrease in kinetic energy associated with interatomic delocalization of electron motion is the key to covalent bonding but contrary views, confusion or lack of understanding still abound. In order to resolve this impasse we show that quantum mechanics affords us a complementary dynamical perspective on the bonding mechanism, which agrees with that of Hellmann and Ruedenberg, while providing a direct and unifying view of atomic reactivity, molecule formation and the basic role of the kinetic energy, as well as the important but secondary role of electrostatics, in covalent bonding.
Highlights
Covalent chemical bonding is undoubtedly a central concept in Chemistry
We propose to employ both representations to show that this duality of views of bonding is advantageous since it makes clear: (i) that bonding is a quantum phenomenon relating to both energy and dynamics and (ii) how the rate of interatomic electron motion, i.e., delocalization and its timescale, is the key determinant of the bonding while related mechanisms of orbital contraction or electron correlation are important but secondary
These observations are in complete accord with the findings of Schmidt et al [58,59], whereby orbital provides a relative measure of the atomic and molecular contributions of the density and energy contraction enhances delocalization, i.e., the transfer of electron density to the interatomic region that changes that occur on covalent bonding
Summary
Covalent chemical bonding is undoubtedly a central concept in Chemistry. While bond formation is arguably the most fundamental chemical process, its physical origin is still the subject of debate, even today when accurate quantitative molecular electronic structure calculations of ever-increasing accuracy and complexity have become widely available. After expressing support for Slater’s electrostatic view of covalent bonding in 1939, 26 years later Feynman [88] himself, in his famous Lectures on Physics, explained covalent bond formation in H2 + , and by extension in other molecules, as the consequence of a flip-flop motion of electrons between bonded atoms, causing a corresponding drop in the electron’s kinetic energy, as a molecule forms. His simplest argument was based on the Heisenberg uncertainty principle, ∆x∆p. The combination of energetics and dynamics will provide an understanding of the covalent bond that’s both physically clear and fully consistent with the results of quantum chemistry
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