Hydrogen Bonding in Phosphonium Iodide: A Neutron-Diffraction Study

A three-dimensional single-crystal neutron-diffraction study of phosphonium iodide, PH₄I, has confirmed the presence of weak P-H...I hydrogen bonds in this system. Each iodine is surrounded by four hydrogen atoms at a normal van der Waals contact distance of 3.35 Å and four others at a hydrogen-bond distance of 2.87 Å. The P-H bond length is 1.414 Å (σ0.008 Å), and the P-H...I angle is 171.6°. The thermal motion parameters derived from the diffraction study lead to a predicted librational frequency of 246 cm^-1 in comparison with a value of 335 cm^-1 observed by Rush in a neutron-inelastic-scattering experiment. This agreement may be considered good in view of the fact that crystal energy calculations reported here indicate that the barrier to rotation is possibly extremely anharmonic with a very high barrier height. That the potential curve is dominated by the repulsive term rather than the electrostatic term is strong evidence that the hydrogen bond is weak.