Chemical Bonding with or without Tunneling.

A solid-state tunneling analysis is performed in order to assess whether a given chemical bond type is mediated by quantum mechanical electron tunneling. Four bond types are found to involve tunneling-covalent, ionic, polar covalent, and transition metal bonding. Two bond types do not rely on tunneling-free electron metal and van der Waals bonding. Cohesive energy is large for the four bonds involving tunneling due to tunneling-induced Coulombic energy storage, while it is small for the two bonds that do not involve tunneling. Coulombic energy storage is dynamic for covalent and strong polar covalent bonding, static for ionic bonding, and quasi-static for weak polar covalent bonding, where quasi-static pertains to tunneling times longer than ∼160 fs, the room-temperature vibrational attempt time. The cohesive energy of tungsten (W) is anomalously large, suggesting that chemical bonding in W is mediated by a two-electron d-d tunneling process in which charge polarity flips between WW and WW with every two-electron tunneling event. All six bonds just listed are directly connected bonds, in contradistinction to a hydrogen bond, which is a bridge bond linking two adjacent atoms. A hydrogen bond is mediated by quantum mechanical electron tunneling. However, its cohesive energy is variable and can be either relatively large or very small depending on interatomic spacing.