Abstract
Lowering of the electron kinetic energy (KE) upon initial encounter of radical fragments has long been cited as the primary origin of the covalent chemical bond based on Ruedenberg’s pioneering analysis of H{}_{2}^{+} and H2 and presumed generalization to other bonds. This work reports KE changes during the initial encounter corresponding to bond formation for a range of different bonds; the results demand a re-evaluation of the role of the KE. Bonds between heavier elements, such as H3C–CH3, F–F, H3C–OH, H3C–SiH3, and F–SiF3 behave in the opposite way to H{}_{2}^{+} and H2, with KE often increasing on bringing radical fragments together (though the total energy change is substantially stabilizing). The origin of this difference is Pauli repulsion between the electrons forming the bond and core electrons. These results highlight the fundamental role of constructive quantum interference (or resonance) as the origin of chemical bonding. Differences between the interfering states distinguish one type of bond from another.
Highlights
Lowering of the electron kinetic energy (KE) upon initial encounter of radical fragments has long been cited as the primary origin of the covalent chemical bond based on Ruedenberg’s pioneering analysis of Hþ2 and H2 and presumed generalization to other bonds
This result begs the question that we take up here: if the critical role of orbital contraction is to restore virial balance by raising KE and lowering PE and this does not apparently occur in bonds between nonhydrogen atoms, what is the nature of the kinetic and potential energy balance in these systems? We utilize the ALMOEDA method to demonstrate that the model in which covalent bond formation is driven by KE lowering is not universally true for all covalent bonds
The wavefunctions in absolutely localized molecular orbitals (ALMOs)-energy decomposition analysis (EDA) are mean-field for all electrons except for two orbitals which may engage in single bond formation
Summary
Lowering of the electron kinetic energy (KE) upon initial encounter of radical fragments has long been cited as the primary origin of the covalent chemical bond based on Ruedenberg’s pioneering analysis of Hþ2 and H2 and presumed generalization to other bonds. Bonds between heavier elements, such as H3C–CH3, F–F, H3C–OH, H3C–SiH3, and F–SiF3 behave in the opposite way to Hþ2 and H2, with KE often increasing on bringing radical fragments together (though the total energy change is substantially stabilizing) The origin of this difference is Pauli repulsion between the electrons forming the bond and core electrons. This was attributed to the presence of core electron pairs in such cases, which precludes significant orbital contraction due to repulsion between the contracting valence and core electrons This result begs the question that we take up here: if the critical role of orbital contraction is to restore virial balance by raising KE and lowering PE and this does not apparently occur in bonds between nonhydrogen atoms, what is the nature of the kinetic and potential energy balance in these systems? This result begs the question that we take up here: if the critical role of orbital contraction is to restore virial balance by raising KE and lowering PE and this does not apparently occur in bonds between nonhydrogen atoms, what is the nature of the kinetic and potential energy balance in these systems? We utilize the ALMOEDA method to demonstrate that the model in which covalent bond formation is driven by KE lowering is not universally true for all covalent bonds
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