Proton-Magnetic-Resonance Study of the Spin-Symmetry States of Ammonium Ions in Solids

The proton-magnetic-resonance absorption spectra at 4.2 \ifmmode^\circ\else\textdegree\fi{}K have been obtained for a number of ammonium salts. Some of these solids exhibit narrow proton spectra even at 4.2 \ifmmode^\circ\else\textdegree\fi{}K. Information obtained from proton-relaxation data precludes the possibility of thermally activated molecular reorientation as the effective mechanism for line narrowing at this temperature. The results are discussed in terms of indistinguishability of the ammonium-group protons and the formation of nuclear-spin-symmetry states (spin isomerism) and are compared with the predictions of a theoretical treatment developed by Tomita. Features characteristic of spin isomerism were found in a number of spectra, but the Tomita treatment was inadequate to give quantitative agreement.