Theoretical and experimental investigations on the structure and vibrational spectra of 4-{[(1Z)-1-methyl-3-oxobut-1-enyl]amino}benzoic acid

Hartree-Fock and hybrid density functional methods have been used to determine the structure and energetics of 4-{[(1Z)-1-methyl-3-oxobut-1-enyl]amino}benzoic acid (1) tautomers. The N-H⋯O (1.750 Å) intramolecular hydrogen bonded interactions are present in the keto tautomer and relatively strong O-H⋯N (1.616 Å) interactions prevail in the enol form. The two functional groups, viz. benzoic acid and 4-amino-pent-3-ene-2-one orient mutually with 154° in the keto tautomer in agreement with the X-ray data. Extended conjugation, which renders stability to the keto tautomer, has been manifested in the topography of molecular electrostatic potential. The deepest minimum in the electrostatic potential of the lowest energy tautomer emerges near carbonyl oxygen and is located near the carboxylic oxygen of benzoic acid for the enol form. X-ray crystal structure reveals the extended three-dimensional network comprising of CO⋯H intermolecular hydrogen bonds (1.82 Å). Quantum mechanically derived molecular electrostatic potential provide insights for the formation of supramolecular assemblies. The 1699 and 1240 cm−1 CO and C-N stretching vibrations in the observed infrared spectrum agree well with those predicted from the density functional calculations. The downshift of OH stretching to ∼2986 cm−1 in the observed infrared spectrum results from the CO⋯H intermolecular hydrogen bonded interactions.