Experimental IR and Raman spectra and quantum chemical studies of molecular structures, conformers and vibrational characteristics of nicotinic acid and its N-oxide

FTIR and Raman spectra of nicotinic acid and its N-oxide have been recorded and analyzed. The stabilities, optimized molecular geometries, APT charges and vibrational characteristics for the two possible conformers of nicotinic acid and its N-oxide have been computed using DFT method. The E (trans) conformers of both the molecules are found to be more stable and less polar than their respective Z (cis) conformers. Due to addition of an O atom at the N1 site in nicotinic acid the magnitudes of atomic charges on all the H atomic sites of the nicotinic acid N-oxide molecule are found to increase. Most of the vibrational frequencies have nearly the same magnitude for the two conformers of both the molecules. However, significant changes are noticed in their IR intensities, Raman activities and depolarization ratios of the Raman bands. The calculated frequencies have been correlated with the experimental frequencies.

Graphical abstractThe NIA (nicotnic acid) and NIANO (nicotnic acid-N-oxide) compounds are found to form white powder at room temperature. These compounds were purchased from the Sigma–Aldrich Chemical Co. (USA). The IR spectra of both the compounds were recorded in KBr pellet. Perkin Elmer RX1 spectrometer was used to record the IR spectra in the spectral range 400–4000 cm−1 with the following parameters: scans – 200, resolution – 4 cm−1, gain – 50. Raman spectra of these compounds were recorded in pure solid form, on a Jobin Yvon Horibra LABRAM Raman HG.2S spectrometer in the spectral range 50–4000 cm−1. The samples were kept on a glass plate/slide which was kept on a stand in the sample illuminator chamber. The radiation (λ – 4880 Å) from an Ar+ laser was used as the exciting source. Following were the experimental parameters during the recording of the Raman spectra: scanning speed – 100 cm−1/min, resolution – 1 cm−1, power of the laser beam – 500–600 mW, PMT voltage – 800 V.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Nicotinic acid (NIA/vitamin B3) is involved in a wide range of biological processes, including production of energy, synthesis of fatty acids and steroids, signal transduction, regulation of gene expression and maintainace of genomic integrity. ► To record the IR and Raman spectra of NIA and its N-oxide and analyze these IR and Raman spectra in light of the DFT calculations and to study the stability, optimized molecular geometries, APT charges and vibrational characteristics for the two possible conformers NIA and its N-oxide. ► Furthermore, earlier reported vibrational spectra of NIA have also been compared with that of the present investigation.