NO₃- anions can act as Lewis acid in the solid state

Identifying electron donating and accepting moieties is crucial to understanding molecular aggregation, which is of pivotal significance to biology. Anions such as NO₃^- are typical electron donors. However, computations predict that the charge distribution of NO₃^- is anisotropic and minimal on nitrogen. Here we show that when the nitrate's charge is sufficiently dampened by resonating over a larger area, a Lewis acidic site emerges on nitrogen that can interact favourably with electron rich partners. Surveys of the Cambridge Structural Database and Protein Data Bank reveal geometric preferences of some oxygen and sulfur containing entities around a nitrate anion that are consistent with this 'π-hole bonding' geometry. Computations reveal donor-acceptor orbital interactions that confirm the counterintuitive Lewis π-acidity of nitrate.