Spectroscopic (FT-IR, FT-Raman, NMR and UV–Visible) and quantum chemical studies of molecular geometry, Frontier molecular orbital, NLO, NBO and thermodynamic properties of salicylic acid

• Molecular structure of salicylic acid molecule was studied using HF and DFT/B3LYP.
• The vibrational assignment and spectroscopic analysis have been carried out.
• The MEP, NBO and HOMO, LUMO energy gap were theoretically predicted.
• The 13C NMR and 1H NMR chemical shift analysis were made.

The solid phase FT-IR and FT-Raman spectra of 2-hydroxybenzoic acid (salicylic acid) have been recorded in the region 4000–400 and 4000–100 cm−1 respectively. The optimized molecular geometry and fundamental vibrational frequencies are interpreted with the aid of structure optimizations and normal coordinate force field calculations based on density functional theory (DFT) method and a comparative study between Hartree Fork (HF) method at 6-311++G(d,p) level basis set. The calculated harmonic vibrational frequencies are scaled and they are compared with experimentally obtained FT-IR and FT-Raman spectra. A detailed interpretation of the vibrational spectra of this compound has been made on the basis of the calculated potential energy distribution (PED). The time dependent DFT method is employed to predict its absorption energy and oscillator strength. The linear polarizability (α) and the first order hyper polarizability (β) values of the investigated molecule have been computed. The electronic properties, such as HOMO and LUMO energies, molecular electrostatic potential (MEP) are also performed. Stability of the molecule arising from hyper conjugative interaction, charge delocalization has been analyzed using natural bond orbital (NBO) analysis.

Salicylic acid (2-hydroxybenzoic acid) is used as an anti-inflammatory drug. The equilibrium geometries, harmonic vibrational frequencies, infrared intensities and Raman intensities were calculated by DFT and HF method with 6-311++G(d,p) basis set using Gaussian 03W program. A study on the electronic properties, such as HOMO and LUMO energies, molecular electrostatic potential (MEP) were also performed.Figure optionsDownload as PowerPoint slide