Understanding the vibrational spectra of crystalline isoniazid: Raman, IR and INS spectroscopy and solid-state DFT study.

This work presents a comprehensive spectroscopic analysis of crystalline isoniazid, one of the main drugs in tuberculosis chemotherapy, using a blend of spectroscopic and computational methods. Mid- and far-infrared, Raman, and inelastic neutron scattering spectroscopies, with contribution of isotopic substitution are combined with discrete and periodic DFT quantum chemical calculations. This combined approach successfully reproduces the whole spectral range, allowing a sound assignment of all the vibrational bands. Previous misassignments have been corrected and several spectral features of isoniazid crystal are reported for the first time. Virtues and limitations of the computational approach (periodic and discrete) are also discussed in light of the present state-of-the-art in the field.