Crystal structures of а series of 1-substituted imidazol-4,5-dicarboxylic acids

Crystal structure of a series of 1-alkylimidazole-4,5-dicarboxylic acids has been determined. It is shown that an increase in the length of the alkyl group leads to drastic changes in the crystal and molecular structures. 1-Methyl and 1-ethylimidazole-4,5-dicarboxylic acid crystallize in zwitterionic form, but 1-propyl- and 1-butylimidazole-4,5-dicarboxylic acid crystallize as rare equimolar mixture of neutral and zwitterionic tautomeric forms. The observed changes in the crystal and molecular structures are apparently associated with the steric factor, which determines both the packing method and the tautomeric composition of the unit cell.