2-Acylamino-6-pyridones: Breaking of an Intramolecular Hydrogen Bond by Self-association and Complexation with Double and Triple Hydrogen Bonding Counterparts. Uncommon Steric Effect on Intermolecular Interactions

2-Acylamino-6-pyridones (acyl = RCO, where R = Me, Et, i-Pr, t-Bu, and 1-adamantyl) were previously characterized by X-ray diffractometry and solid-state NMR techniques by us. One of these compounds was used recently in organocatalysis. The series is now studied in solution and by computational methods recommended for noncovalent interactions (DFT/M05). These compounds showed interesting behavior during dilution and titration experiments monitored by H-1 NMR. 2-Acylamino-6-pyridones change their conformation at higher concentrations, forming double hydrogen-bonded dimers and trimers in which an uncommon steric effect is observed. To the best of our knowledge, this is the first example of such behavior of hydrogen-bonded molecules. Heterocomplexation of the studied compounds happens via double or triple hydrogen bonding, depending on the properties of the counterpart. The computation data support and explain the effects observed experimentally, including the tautomeric, closed/open form equilibrium and intermolecular interaction preferences.