What Is the Nature of Supramolecular Bonding? Comprehensive NBO/NRT Picture of Halogen and Pnicogen Bonding in RPH···IF/FI Complexes (R = CH, OH, CF, CN, NO).

We employ a variety of natural bond orbital (NBO) and natural resonance theory (NRT) tools to comprehensively investigate the nature of halogen and pnicogen bonding interactions in RPH···IF/FI binary complexes (R = CH, OH, CF, CN, and NO) and the tuning effects of R-substituents. Though such interactions are commonly attributed to "sigma-hole"-type electrostatic effects, we show that they exhibit profound similarities and analogies to the resonance-type 3-center, 4-electron (3c/4e) donor-acceptor interactions of hydrogen bonding, where classical-type "electrostatics" are known to play only a secondary modulating role. The general 3c/4e resonance perspective corresponds to a continuous range of interatomic A···B bond orders (), spanning both the stronger "covalent" interactions of the molecular domain (say, ≥ ½) and the weaker interactions ( ˂ ½, often misleadingly termed "noncovalent") that underlie supramolecular complexation phenomena. We show how a unified NBO/NRT-based description of hydrogen, halogen, pnicogen, and related bonding yields an improved predictive utility and intuitive understanding of empirical trends in binding energies, structural geometry, and other measurable properties that are expected to be manifested in all such supramolecular interaction phenomena.