The inelastic neutron scattering spectrum of nicotinic acid and its assignment by solid-state density functional theory

The 25 K inelastic neutron scattering (INS) spectrum of nicotinic acid has been measured and assigned by solid-state density functional theory (DFT). Vibrational mode energies involving the carboxylic acid proton are found to be significantly altered due to intermolecular hydrogen-bonding. There is good overall agreement between experiment and simulation in all regions of the spectrum, with identified deviations considered in detail by spectral region: phonon (25–300 cm−1), molecular (300–1600 cm−1), and high-frequency (>2000 cm−1). The relative energies, geometries, and vibrational spectra associated with hypothesized tautomerization in the solid-state have also been investigated.