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

Exploiting non-covalent π interactions for catalyst design

Molecular recognition, binding and catalysis are often mediated by non-covalent interactions involving aromatic functional groups. Although the relative complexity of these so-called π interactions has made them challenging to study, theory and modelling have now reached the stage at which we can explain their physical origins and obtain reliable insight into their effects on molecular binding and chemical transformations. This offers opportunities for the rational manipulation of these complex non-covalent interactions and their direct incorporation into the design of small-molecule catalysts and enzymes