The annular tautomerism of 4(5)-phenylimidazole

A new polymorph of 4-phenylimidazole 1a has been characterised by X-ray crystallography. Both polymorphs present a secondary structure of chains, and the observed differences in the topology of their crystal packing are related to the conformational differences in the primary and secondary structural levels. NMR experiments (C-13 and N-15) reveal that tautomer 1a is the only one observed in the solid state, being also the most abundant in solution (80%) in agreement with high-level theoretical calculations which favour this tautomer over the 5-phenyl one. The results obtained for this compound are relevant for the structure of the related 4(5)-porphyrinylimidazole.