Spectroscopic, electronic structure and natural bond orbital analysis of o-fluoronitrobenzene and p-fluoronitrobenzene: A comparative study

Experimental FTIR, FT-Raman and FT-NMR spectroscopic studies of o-fluoronitrobenzene and p-fluoronitrobenzene have been carried out. A detailed quantum chemical calculations have been performed using DFT/B3LYP method with 6-311++G** and 6-31G** basis sets. Complete vibrational analyses of the compounds were performed. The temperature dependence of thermodynamic properties has been analysed. The atomic charges, electronic exchange interaction and charge delocalisation of the molecule have been performed by natural bond orbital (NBO) analysis. Molecular electrostatic surface potential (MESP), total electron density distribution and frontier molecular orbitals (FMOs) are constructed at B3LYP/6-311++G** level to understand the electronic properties. The charge density distribution and site of chemical reactivity of the molecules have been obtained by mapping electron density isosurface with electrostatic potential surfaces (ESP). The electronic properties, HOMO and LUMO energies were measured by time-dependent TD-DFT approach. 1H and 13C NMR spectra were recorded and 1H and 13C nuclear magnetic resonance chemical shifts of the molecule were calculated. The 1H and 13C nuclear magnetic resonance (NMR) chemical shifts of the molecules in chloroform solvent and in gas phase were calculated by using the Gauge-Independent Atomic Orbital (GIAO) method and are found to be in good agreement with experimental values. The theoretical parameters obtained at B3LYP levels have been compared with the experimental values.

Graphical abstractThe FT-IR, FT-Raman, 1H and 13C NMR spectral measurements of o-fluoronitrobenzene (o-FNB) and p-fluoronitrobenzene (p-FNB) and complete assignments of the observed spectra have been proposed. DFT calculations have been performed and the structural parameters of the compounds were determined from the optimised geometry with 6-31G(d,p) and 6-311++G(d,p) basis sets and giving energies, harmonic vibrational frequencies, depolarisation ratios, IR and Raman intensities. The geometric parameters, harmonic vibrational frequencies and chemical shifts were compared with the experimental data of the molecules. The influences of fluorine and nitro groups on the skeletal modes and proton chemical shifts have been investigated. A detailed natural bond orbital analysis was carried out.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► FTIR and FT-Raman investigations of o-fluoronitrobenzene and p-fluoronitrobenzene were carried out. ► Energy, structural, thermodynamical and vibrational characteristics were determined. ► 1H and 13C NMR spectra and chemical shifts were measured by GIAO method. ► The electronic properties by TD-DFT approach and NBO analysis were carried out. ► Effect of fluorine and nitro groups on the benzene ring parameters has been analysed.