Synthesis, structure, tautomerism, intramolecular hydrogen bond, and vibrational assignment of 3-nitroso-2,4-pentanedione: A theoretical and experimental approach

Abstract

The synthesis and molecular structure of 3-nitroso-2,4-pentanedione, entitled to oxime-acetylacetone (oxime-AA), was studied utilizing Density Functional Theory (DFT) calculations and the results were compared with those of 3-nitro-pentane-2,4-dione (NO2AA) and 2-nitromalonaldehyde (NO2MA). The vibrational frequencies of the most stable cis-enol form were calculated using the B3LYP functional and 6-311++G(d,p) basis set. The calculated frequencies and chemical shifts of oxime-AA were compared with the experimental results. The calculated geometrical parameters for oxime-AA show a medium hydrogen bond compared with their α-substituted (NO2AA) and (NO2MA) which manifested the strong hydrogen bond. The calculated O⋯O distance of 2.460–2.561 Å is about 0.002-0.104 Å longer than in NO2AA and NO2MA. According to the theoretical calculations, oxime-AA has a structure with a hydrogen bond strength of about 21.0 kcal/mol (calculated with 6-311++G(d,p) basis set), which is 4.0–6.0 kcal/mol weaker than the hydrogen bond strength of NO2AA and NO2MA. This decrease in the hydrogen bond strength is also consistent with the experimental results. Natural Bond Orbital (NBO) and AIM analyses were applied for considering the hydrogen bond strength in oxime-AA that indicates the effect of NOH group decreases the hydrogen bond strength.