Far-Infrared and Raman Spectra of Phosphonium Iodide and Phosphonium Iodide-d

Abstract

The infrared spectra of polycrystalline PH₄I and PD₄I have been recorded from 33 to 550 cm⁻¹ at −170°C. Only three bands were manifest in this region; two of them are translatory motions and the other is a librational mode. The Raman spectra of polycrystalline PH₄I and PD₄I have been recorded at 23°C. Two librations and four translations have been assigned both compounds. Seven internal fundamentals were observed in the Raman spectrum of PH₄I and five internal fundamentals were observed in the Raman spectrum of PD₄I. Many of the other bands found in the Raman spectra were attributed to overtones and combinations involving external and internal fundamentals. The barrier to reorientation of phosphonium ions in the lattice was calculated from observed librational frequencies, and the barrier height for PH₄ ions is 6.4 kcal/mole and for PD₄ ions is 6.2 kcal/mole. The barrier height and orientation for the NH₄ ions were compared to those for the PH₄ ions (in isomorphous phase III NH₄Br); the differences can be rationalized on the basis of a simple electrostatic interaction and steric hindrance.