Using solid-state density functional theory and terahertz spectroscopy to spectroscopically distinguish the various hydrohalide salts of 5-(4-pyridyl)tetrazole

The crystal structures and experimental terahertz spectroscopy of the isostructural hydrobromide and hydrochloride salts of 5-(4-pyridyl)tetrazole were reproduced using solid-state density functional theory as implemented in CRYSTAL09. The effect of the halide size was investigated to determine the influence on the low energy lattice and molecular vibrations exhibited in the region from 10 to 100 cm^-1, known as the terahertz (THz) region. Using solid-state DFT, the normal modes of these THz vibrations were determined and it was shown that the mass and size of the halide anion has a significant effect in this region. A trend was observed in the frequencies in the terahertz region, such that increasing the mass of the anion, produces an overall red shift in the frequency.