Synthon preference in O-protonated amide crystals - dominance of short strong hydrogen bondsElectronic supplementary information (ESI) available: Crystallographic table of amide salts synthesized in this work; pKa of acids and amides, ΔpKa, and C&z.dbd;O bond length. CCDC 947738-947749. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ce41271g

Although the amide functional group is weakly ionizable, there have been serendipitous discoveries of amide salts. To facilitate their systematic design, we sought to identify the preference of 12 possible synthons in oxygen protonated amide structures. An analysis of a total of 81 protonated amide crystal structures retrieved from the Cambridge Structural Database and 12 new protonated amide structures prepared by us, revealed that ∼86% of these structures involving charge assisted short strong hydrogen bonds, including ∼54% of the hydrogen bridged dimers formed between protonated and neutral amide. The probability of forming hydrogen bridged dimers increases in the order of primary amide < secondary amide < tertiary amide. The two previously missing synthons, R 2 2 (8) and R 1 2 (6), in primary amides are realized for the first time in this work. However, the two predicted ring ( R ) hydrogen bonding synthons, R 2 2 (8) and R 1 2 (6), for cis secondary amides remain to be discovered. Through both database analysis and crystallization experiments, we have quantified the distribution of 12 possible synthons in O-protonated amide crystals.