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.