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

Processes of chemical exchange of deuterons in partially deuterated ammonium hydrogen selenate, NH4HSeO4 (AHSe), crystals are investigated by means of H-2 nuclear magnetic resonance (NMR) experiments over a wide temperature range. The temperature dependencies of the quadrupole line splittings in the one-dimensional spectra of AHSe above 350 K revealed lineshape changes which 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 exchange occurs only between those deuteron (proton) sites which are involved in hydrogen bonds (alpha- and beta-positions). It was established that the rates of exchange between all types of hydrogen-bound deuteron are approximately 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 discuss a model for the microscopic mechanism of hydrogen transport in AHSe.