How H-bonding modifies molecular structure and pi-electron delocalization in the ring of pyridine/pyridinium derivatives involved in H-bond complexation.

[structure: see text] Analysis of the experimental geometry of 397 variously substituted pyridine and pyridinium derivatives has shown very little variation of all bond lengths in the ring and substantial changes in the values of alpha and beta angles, being in line with theoretically modeled data (at the B3LYP/6-311+G level of theory). No dependence of bond lengths and pi-electron delocalization in the ring measured by HOMA index on the changes in bond angle at the N-atom is observed. This is at variance with the patterns observed in substituted phenolate/phenol systems involved in H-bonding. Due to significant differences in magnitude of the ipso angle at the nitrogen atom that depend on the location of hydrogen atom in pyridine/pyridinium systems (N, N...H, and N-H), this angle may be used as a valuable indicator of N...H interactions since the X-ray structural analysis does not give a reliable position for the proton.