Abstract
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 RPH2···IF/FI binary complexes (R = CH3, OH, CF3, CN, and NO2) 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 (bAB), spanning both the stronger “covalent” interactions of the molecular domain (say, bAB ≥ ½) and the weaker interactions (bAB ˂ 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.
Highlights
The present study extends the choice of species and the variety of natural bond orbital (NBO)/natural resonance theory (NRT) analysis tools to address the broader questions of covalency versus electrostatics in halogen and pnicogen bonding, as well as the general relationships to H-bonding and other types of “X-ogen bonding” that signal a grand unified picture of all such donor-acceptor phenomena
We have computationally investigated the nature of supramolecular bonding in a series of
R-substituted phosphine···dihalogen complexes (RPH2 ···IF/FI, R = CH3, OH, CF3, CN, NO2 ), focusing on the electronic origin of what might be identified as a “halogen bond,” “pnicogen bond,” or some combination of both
Summary
Recent computational [1,2,3,4] and experimental [5,6,7,8] studies have called attention to the multiplicity of halogen [9,10,11,12,13], chalcogen [14,15], pnicogen [16], and tetrel bonds [17,18,19] Molecules 2019, 24, 2090 provide the more useful starting point for describing intermolecular forces For present purposes, the former is exemplified by a natural bond orbital [25] (NBO)-based description, and the latter by is exemplified by symmetry adapted perturbation theory (SAPT) [26,27], electrostatic potential models (e.g., of CHELP [28,29] or “sigma hole” [30,31,32] types), or related classical force field [33,34] models. In contrast to the superficial “on-off” picture of H-bonding that is sometimes suggested by graphical software or inflexible analysis methods, the NBO/NRT descriptors all reflect the continuously variable character of resonance-type bonding, as quantified by fractional bond orders ranging over integer and sub-integer values All such characteristic NBO/NRT signatures are found in all known H-bonded species, including the paradoxical “anti-electrostatic” H-bonds between like-charged ions [50] that most clearly demonstrate the relatively secondary influence of “electrostatic” modulation on the authentic H-bonding phenomenon. The concluding summary emphasizes how a balanced NBO/NRT description of leading donor–acceptor interactions can lead to a unified conceptual picture of supramolecular bonding that encompasses the entire range of chemically significant (“non-innocent”) complexation and ligation phenomena
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