Vibrational, NMR and quantum chemical investigations of acetoacetanilde, 2-chloroacetoacetanilide and 2-methylacetoacetanilide.

The vibrational assignment and analysis of the fundamental modes of the compounds acetoacetanilide (AAA), 2-chloroacetoacetanilide (2CAAA) and 2-methylacetoacetanilide (2MAAA) have been performed. Density functional theory studies have been carried out with B3LYP method utilising 6-311++G(**) and cc-pVTZ basis sets to determine structural, thermodynamic and vibrational characteristics of the compounds and also to understand the influence of chloro and methyl groups on the characteristic frequencies of amide (CONH) group. Intramolecular hydrogen bond exists in acetoacetanilide and o-substituted acetoacetanilide molecules and the N⋯O distance is found to be around 2.7Å. The (1)H and (13)C nuclear magnetic resonance chemical shifts of the molecules were determined and the same have been calculated using the gauge independent atomic orbital (GIAO) method. The energies of the frontier molecular orbitals have been determined. In AAA, 2CAAA and 2MAAA molecules, the nN→πCO(∗) interaction between the nitrogen lone pair and the amide CO antibonding orbital gives strong stabilization of 64.75, 62.84 and 64.18kJmol(-1), respectively. The blue shift in amide-II band of 2MAAA is observed by 45-50cm(-1) than that of AAA. The steric effect of ortho methyl group significantly operating on the NH bond properties. The amide-III, the CN stretching mode of methyl and chloro substituted acetoacetanilide compounds are not affected by the substitution while the amide-V band, the NH out of plane bending mode of 2-chloroacetoacetanilide compound is shifted to a higher frequency than that of AAA. The substituent chlorine plays significantly and the blue shift in o-substituted compounds than the parent in the amide-V vibration is observed. The amide-VI, CO out of plane bending modes of 2MAAA and 2CAAA are significantly raised than that of AAA. A blue shift of amide-VI, CO out of plane bending modes of 2MAAA and 2CAAA than AAA is observed.