Positive temperature coefficient of the iodine quadrupole resonance frequency in ammonium tri-iodide due to reorientation of the ammonium ion

Abstract

A large increase in the nuclear quadrupole resonance (n.q.r.) frequency of a terminal iodine atom I_a of the I₃ anion in NH₄I₃ in NH₄I₃ is explained by a simple theory which relates the reorientation of the ammonium ion to a concerted change in the relative weights of the resonance structures of the I₃ anion through formation and scission of the N—H ⋯ I_a hydrogen bond. The weight of the structure (I^–_a I—I) oscillates between 0.75 (hydrogen-bonded) and 0.48 (non-bonded) as the NH₄ cation is reoriented. The proton spin–lattice relaxation time was measured between 77 and 240 K; a minimum of 2.45 ms occurs at 80 K. The activation energy for ammonium reorientation is 7.53 kJ mol^–1. The energy of a hydrogen bond, including the effect of the change in relative weights of I₃, was derived as 5.20 kJ mol^–1.