Laser–Raman Spectra of PH4I, PH4Br, and PH4Cl

Publisher Website
Google Scholar
Abstract
Laser–Raman spectra have been measured at 23°C for PH4I and PH4Br and at ∼5°C for PH4Cl. Peaks observed in the spectra of all the halides are assigned to “internal” PH4+ modes, ν1(A1g), ν2(A1g, B1g), ν3(Eg), and ν4(Eg) and to torsional [L2(Eg)] and translational [T1(A1g)] lattice modes. The torsional frequencies are 326, 343 and 363 cm−1, respectively, for PH4I, PH4Br, and PH4Cl, while the translational mode is observed at 55.5, 75, and 112 cm−1, respectively. Barriers to rotation are calculated from the torsional frequencies on the simple but unrealistic assumption of a fourfold cosine-type potential for the PH4+ ions. The barriers obtained are 6.7, 7.4, and 8.2 kcal/mole, respectively, for PH4I, PH4Br, and PH4Cl. The spectral results and calculated barriers are compared in some detail with previous optical, NMR, and neutron scattering results for both the phosphonium and ammonium halides. Suggested differences in force fields between the ammonium and phosphonium salts are discussed.