Proton Magnetic Resonance and Hindered Rotation in Phosphonium Halides and Ammonium Iodide

Abstract

Proton spin–lattice relaxation times $T_1$ and second moments have been measured as a function of temperature for the phosphonium halides and ammonium iodide. Correlation times ${\tau }_c$ derived from the relaxation data were used to obtain activation energies for the reorientation of the phosphonium and ammonium ions. The activation energies for PH₄Cl, PH₄Br, and PH₄I are (35 ± 1) × 10³, (29.4 ± 0.6) × 10³, and (30.7 ± 0.8) × 10³ J/mole, (8.3 ± 0.2, 7.1 ± 0.1, 7.3 ± 0.2 kcal/mole), respectively, indicating little change in barrier with halide ion. The frequency factors, however, do appear to vary significantly from crystal to crystal. These results are in marked contrast with previous results for the ammonium halides and suggest that nonelectrostatic repulsive forces are important in the phosphonium salts. The activation energy for NH₄⁺ ion reorientation in the tetragonal phase (Phase III) of NH₄I is found to be (13.4 ± 0.4) × 10³ J/mole (3.2 ± 0.1 kcal/mole). The present results are compared with previous spectroscopic and structural results.