Low-Frequency Infrared and Raman Spectra of Hydrogen-Bonded Pyridinium Halides

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Abstract
The infrared spectra of crystalline C5H5NH+Cl−, C5H5NH+Br−, C5H5NH+I−, and their deuterated derivatives at room and liquid‐nitrogen temperature have been investigated in the 4000–1500‐ and 300–20‐cm−1 ranges. The Raman spectra of these compounds have been examined below 300 cm−1. All six infrared‐active and four of the nine Raman‐active lattice modes have been observed for pyridinium chloride. The lattice frequencies of pyridinium halides have been assigned in terms of translational and rotational as well as hydrogen‐bond vibrations. The hydrogen‐bond stretching frequencies have been identified at 192, 133, and 110 cm−1 for chloride, bromide, and iodide, respectively. The relationships between the NH and (NH)···X stretching frequencies and shapes of the corresponding absorption bands are discussed.