Nuclear Magnetic Resonance of the Aquated Proton: Hydronium Perchlorate

Abstract

The deuteron and proton nuclear magnetic resonance of polycrystalline hydronium perchlorate has been examined by continuous wave and pulse techniques from 100 to 300°K particularly in the vicinity of the phase transition at 243°K. Above 243°K the deuteron resonance is a single sharp line for which relaxation times have been determined. Below 243°K the deuteron resonance is broadened by the quadrupolar interaction with a quadrupolar coupling constant (QCC) of 72 · 2 kHz. At 131°K the rigid-lattice QCC is 212 · 5 kHz. Proton spin–lattice relaxation times (T1) demonstrate that below 243°K and H3O+ ion is reorientating about the threefold axis and above 243°K is undergoing essentially isotropic reorientation. In agreement with the results of the Hennel and Pollack-Stachura the proton second moment is constant below 243° K down to 160°K and is equal to 10 · 2 G2 and below 130°K is equal to 31 · 2 G2. The hydronium ion exists as a shallow pyramid with the DOD angle equal to 118.5 · 0.7° and the OH distance 1.01 · 0.02 Å. Thermal hysteresis was encountered in the proton and dueteron resonance on traversing the phase transition at 243°K (Tc). Chlorine-35 nuclear magnetic resonance was observed above Tc as a single sharp line which disappeared below Tc. This is consistent with a large distortion of the ClO4− tetrahedron below Tc.