Two-dimensional2H NMR exchange spectroscopy on conducting ionic crystals

Abstract

The two-dimensional (2-D) deuterium nuclear magnetic resonance (NMR) exchange spectroscopy is applied to two types of conducting ionic crystals for the study of hydrogen mobility and conductivity, viz. partially deuterated ammonium hydrogen selenate, NH4HSO4 (AHSe), and partially deuterated mixed crystals of betaine phosphate (DBP) and betaine phosphite (DBPI), DBP1−x DBPIx. In both crystals chemical exchange processes of deuterons between different hydrogen bridges occur which are studied by the 2-D-2H-NMR technique over a wide temperature range in the slow-exchange regime. For AHSe exchange only occurs between two sites which are involved in hydrogen bonds. Two Arrhenius-like exchange processes were found the activation energies of which could be determined. For the case of DBP1−x DBPIx, with several deuteron sites taking part in the exchange, the analysis of the quantitative exchange behavior required a combination of time-domain analysis of our 2-D NMR data with mixing-time- and temperature-dependent measurements. Different exchange rates for each two-site exchange, all showing Arrhenius behavior, were obtained for DBP0.3, DBPI0.7. For crystals with different phosphite concentrationx the differences towards DBP0.3, DBPI0.3 were established. With the help of conductivity data from dielectric measurements quantitative relations between the exchange and conductivity processes are obtained for AHSe as well as for DBP1−x DBPIx. Finally, an estimation of effective charge carrier densities is discussed in view of possible conductivity models for both crystals.