The Prominence of Facilitator π-Holes: The Classic N←N Pnictogen Bonding in Nitrobenzene-Ammonia Dimer with its Structural Elucidation and Experimental Characterization at Low Temperatures.

The nitrogen of nitro group is a paradigmatic pnictogen due the presence of a π-hole and a number of studies have been performed recently on prototypical nitromethane (NM). Homodimers and heterodimers of NM are sustained by π-hole driven pnictogen bonds hosted by nitrogen. To understand the effect of substitution on this π-hole and thus the pnictogen bond, heterodimers of nitrobenzene (NB; phenyl substitution in place of methyl) with ammonia (AM) have been probed, as a test case, using matrix isolation infrared spectroscopy and ab initio computations. Of the four structures optimized on the potential energy surface the energetically dominant global minimum, stabilized by π-hole driven O=N←N pnictogen bonding with co-operative N-H←O hydrogen bonding, was experimentally identified at low temperatures. A comparison of the pnictogen bonding of NB-AM dimers with NM counterpart (NM-AM dimers) divulged the dominance of electrostatic origin of pnictogen bonding in both the class of dimers. The reduced strength of pnictogen bonding in NB-AM dimers in comparison to NM-AM dimers was discerned, which has been established to be a consequence of the reduced electrostatic potential at the π-hole of NB relative to that in NM. The strength of π-hole driven pnictogen bond was directly correlated with the binding energy and the infrared shifts in the signature vibrational bands of the NB, NM and AM submolecules due to dimerization under matrix isolated conditions at low temperatures.