The Crystal Structures of Pyridinium Chloride Revisited: Evidence for Extensive C -H ••• Cl Hydrogen-Bond Interactions

Although there is now an increasing recognition of the existence of C -H -X (X = F, Cl, Br or I) hydrogen bonds in neutral organic molecules [1 , 2], there have been surprisingly few studies of C -H -X” interactions in organic salts. We have recently demonstrated the existence of extensive hydrogen bonding between aromatic C ~ H groups and halide ions, C -H X , within a series of imidazolium salts [3-5]. It is our contention that such hydrogen bonds are present in a wide range of compounds containing cationic heterocyclic aromatic systems. Although these interactions are, energetically, relatively weak, it is im portant to be aware of their existence, as they may have a deter­ ministic part to play, both in solution (precipitation/nucleation processes), and in the solid state (geometry and packing of ions). In order to inves­ tigate this hypothesis, we decided to reexamine two recent crystal structures of two polymorphs of pyridinium chloride, in order to find possible crystallographic evidence for aromatic C -H --C l hy­ drogen bonds. Neither of the original investiga­ tions explored the possibilities of such interactions being present.