Intramolecular hydrogen bonding in 5-nitrosalicylaldehyde: IR spectrum and quantum chemical calculations

• FT-IR and FT-Raman spectral of 5-Nitrosalicylaldehyde were studied.
• The geometrical parameters and vibrational frequencies were calculated by DFT.
• The position of OH vibrational modes were determined with the aid of the isotopic deuterium shifts.
• The intramolecular O–H⋯O interaction was described by AIM topological analysis.

The structural parameters, and vibrational frequencies of 5-nitrosalicylaldehyde (5NSA) were studied by the FT-IR and Raman spectra and the quantum chemical calculations carried out at the B3LYP/6-311++G(d,p) level of theory in order to investigate the intramolecular hydrogen bonding (IHB) present in its structure. The strength and nature of IHB in the optimized structure of 5NSA were studied in detail by means of the atoms in molecules (AIM) and the natural bond orbital (NBO) approaches. The results obtained were then compared with the corresponding data for its parent molecule, salicylaldehyde (SA). Comparisons made between the geometrical structures for 5NSA and SA, their OH/OD stretching and out-of-plane bending modes, their enthalpies for the hydrogen bond, and their AIM parameters demonstrated a stronger H-bonding in 5NSA compared with that in SA.The calculated binding enthalpy (ΔHbind) for 5NSA was −10.92 kcal mol−1. The observed νOH and γOH appeared at about 3120 cm−1 and 786 cm−1 respectively. The stretching frequency shift of H-bond formation was 426 cm−1 which is consistent with ΔHbind and the strength of H-bond in 5NSA. The delocalization energies and electron delocalization indices derived by the NBO and AIM approaches indicate that the resonance effects were responsible for the stronger IHB in 5NSA than in SA.

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