Large Proton‐Affinity Enhancements Triggered by Noncovalent Interactions

The proton affinity of the hydroxyl group of a series of compounds YHxOH (x=0-3, Y = Li, Na, Be, Mg, B, Al, Si, P, S, Cl, H) always increases upon interaction with Lewis bases LB = NH3, H2CNH, HCN, according to electronic structure calculations carried out at the G4 level of theory. The LB:YHxOH complexes experiment proton affinity enhancements from 24.8 to 304.8 kJ·mol-1 on its OH group with respect to the free compounds. This enhancement is related to the ability of these YHxOH compounds, acting as Lewis acids, to form non-covalent interactions of different kinds. Interestingly, weakly interacting systems can give place to proton affinity enhancements equal or even larger than strongly interacting systems, as observed in the halogen bond-containing complex H2CNH···ClOH, the free compound (PA = 624 kJ·mol-1) being transformed into a base (PA = 947.2 kJ·mol-1) stronger than pyridine.