In the crystal structures of N3-protonated cytidinium and 2'-deoxycytidinium salts with composite XYn anions capable of accepting hydrogen bonds through their Y atoms, the dominating motif of cytosinium...anion interactions consists of a pair of hydrogen bonds donated from the N3+-H protonation site and from the exoamino N4-H41 group cis to N3, and accepted by two Y centers of one anion. This multi-point recognition pattern is stable and robust and thus can be classified as a supramolecular synthon. In a broader group of N3-protonated, N1-substituted cytosinium salts with composite anions it occurs with 70% frequency. The C5 side of the cytosine ring mimics the N3+-H type synthon and shows a propensity to form an analogous motif in which a C5-H5...Y hydrogen bond replaces the strong N3+-H...Y interaction. Since the C-H...Y bond is much weaker, the secondary motif shows higher deformability and is less frequent (44%).
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