Structure and vibrational assignment of 3-nitro-2,4-pentanedione: A density functional theoretical study

The molecular structure of 3-nitro-2,4-pentanedione, known as α-nitro-acetylacetone (NO2AA), has been investigated by means of Density Functional Theory (DFT) calculations and the results were compared with those of acetylacetone (AA). The harmonic and anharmonic vibrational frequencies of the most stable cis-enol form were calculated using the B3LYP functional and 6-31G∗∗ and 6-311G∗∗ basis sets. The calculated frequencies and the Raman and IR intensities were compared with the experimental results. lculated geometrical parameters for NO2AA show a very strong hydrogen bond, compared with its unsubstituted parent (AA). The calculated O⋯O distance of 2.416–2.457 Å is about 0.09–0.1 Å shorter than in AA. According to the theoretical calculations, NO2AA has an asymmetric structure with a hydrogen bond strength of about 74.0 kJ/mol (calculated with 6-311G∗∗ basis set), which is 4.7 kJ/mol stronger than the hydrogen bond of AA. This increase in the hydrogen bond strength is also consistent with the experimental results. Natural Bond Orbital (NBO) analyses indicate that both electron delocalization and steric effects are responsible for the increased hydrogen bond strength in NO2AA compared with that in AA.