Spectroscopic studies, HOMO–LUMO and NBO calculations on monomer and dimer conformer of 5-nitrosalicylic acid

In this work, FT-IR and FT-Raman spectra are recorded on the solid phase of 5-nitrosalicylic acid (abbreviated as 5-NSA). The energies of all possible conformers obtained from DFT theory with 6-311++G(d,p) basis set identified the most stable conformer of 5-NSA as C6 form. Optimized geometrical parameters, vibrational assignments, HOMO–LUMO energies and NBO calculations are performed on the stable monomer and dimer structure of 5-NSA using the same level of theory. Second order perturbation energies and electron density (ED) transfer from filled lone pairs of Lewis base to unfilled Lewis acid sites of 5-NSA are discussed on the basis of NBO analysis. Inter- and intramolecular hydrogen bonds exist between COOH and OH group, give the evidence for the formation of dimer entities in the title molecule. The variations in bond lengths, ED and vibrational wavenumbers of 5-NSA dimer are being discussed. The spectroscopic and theoretical results are compared to the corresponding properties for 5-NSA monomer and dimer of C6 conformer. Reliable vibrational modes associated with 5-NSA are made on the basis of total energy distribution (TED) results obtained from scaled quantum mechanical (SQM) method.

► Monomer and the dimer conformer of 5-NSA were determined.
► Vibrational studies and MO calculations were carried out on both monomer and dimer.
► Intermolecular hydrogen bond exist via COOH group was analyzed by NBO analysis.
► Intramolecular hydrogen bonds between OH and COOH group were also discussed.
► The changes in bond lengths, electron density of atoms of 5-NSA were analyzed.