Anion receptors containing NH binding sites: Hydrogen-bond formation or neat proton transfer?

When the amide-containing receptor I+ is in a solution of dimethyl sulfoxide (DMSO) in the presence of basic anions (CH3COO-, F-, H2PO4-), it undergoes deprotonation of the -NH fragment to give the corresponding zwitterion, which can be isolated as a crystalline solid. In the presence of less basic anions (Cl-, Br-, NO3-), 1(+) establishes true hydrogen-bond interactions of decreasing intensity. The less acidic receptor 2(+) undergoes neat proton transfer with only the more basic anions CH3COO- and F-, and establishes hydrogen-bond interactions with H2PO4-. An empirical criterion for discerning neutralisation and hydrogen bonding, based on UV/Vis and H-1 NMR spectra, is proposed.