Neutron Diffraction Study of the Structure and Thermal Motion of Phosphonium Bromide

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Abstract
The crystal structure of PH4Br has been determined by a single‐crystal neutron diffraction study. The tetragonal structure is similar to that determined previously for PH4I with the P–H bonds of the phosphonium ions directed at the four next‐nearest‐neighbor halide ions. The corresponding P–H and H···Br distances are 1.414 and 2.65 Å, respectively, with a P–H···Br angle of 172°. The orientation of the PH4+ ions (90° away from the orientation of NH4+ in the analogous phase III (NH4Br) suggests that hydrogen bonding plays a minor role in determining the crystal structure. A “rigid‐body” analysis of the motion of the PH4+ ion was performed using the anisotropic thermal parameters of the phosphonium and hydrogen atoms derived from the PH4Br and PH4I diffraction data. The resulting mean‐square amplitudes of translational and torsional motion of the PH4+ ion lead to frequencies which are in good agreement with spectroscopic observations.

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