LOW-FREQUENCY INFRARED AND RAMAN SPECTRA OF HYDROGEN-BONDED PYRIDINIUM HALIDES

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.