Chemical exchange in NH4HSeO4 single crystals studied by two-dimensional H-2 nuclear magnetic resonance

Processes of chemical exchange of deuterons in partially deuterated ammoniu m hydrogen selenate, NH4HSeO4 (AHSe), crystals are investigated by means of H-2 nuclear magnetic resonance (NMR) experiments over a wide temperature r ange. The temperature dependencies of the quadrupole line splittings in the one-dimensional spectra of AHSe above 350 K revealed lineshape changes whi ch are characteristic for chemical exchange processes. A detailed study of these exchange processes in AHSe is achieved by means of two-dimensional H- 2 NMR experiments. In the temperature range investigated, a chemical exchan ge occurs only between those deuteron (proton) sites which are involved in hydrogen bonds (alpha- and beta-positions). It was established that the rat es of exchange between all types of hydrogen-bound deuteron are approximate ly the same. Exchange between these positions and the deuterons in the ND4 groups could not be detected. On the basis of our findings, we finally disc uss a model for the microscopic mechanism of hydrogen transport in AHSe.