Modern ab initio valence bond theory calculations reveal charge shift bonding in protic ionic liquids

Charge-Shift Bonding: A New and Unique Form of Bonding

Charge-shift bonds (CSBs) constitute a new class of bonds different than covalent/polar-covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence-bond pioneers and then demonstrates that the unique status of CSBs is not theory-dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis-à-vis covalent and ionic bonds. Furthermore, the covalent-ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g., benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity.

The Carbon-Nitrogen Bonds in Ammonium Compounds Are Charge Shift Bonds

A comprehensive investigation, utilizing valence-bond- and molecular-orbital-based techniques, reveals that C-N bonds in protonated and methylated ammonium compounds are charge shift bonds. Moreover, the ammonium compounds are predominantly covalent at equilibrium distance, yet have two competing dissociation channels, which determine whether or not a state function crossing occurs during dissociation. The location of the crossing point can be predicted with relative ionization potentials, and may be tuned by altering the stabilization of the respective fragments. By closely examining the nature of the C-N bond as it is stretched, a correlation is observed between the extent of the charge shift nature and bond length. Identification of charge shift bonds in these ubiquitous organic species specifically affects understanding of their role in organocatalysis, in which the C-N bond is stretched.

Brønsted acidity of protic ionic liquids: a modern ab initio valence bond theory perspective

Modern ab initio valence bond theory calculations of representative primary and tertiary ammonium protic ionic liquids indicate that modern ab initio valence bond theory can be employed to assess the acidity and ionicity of protic ionic liquids a priori.