Proton exchange effects on pulsed nmr signals from preferentially oriented water molecules

Abstract

Preferentially oriented water molecules can give rise to doublet NMR spectra. The doublet splitting can be used as a probe to study proton exchange between H 2O molecules. Theoretical calculations of pulsed NMR signals from powder samples of similarly oriented pairs of rigid water molecules undergoing proton exchange have been made. Under suitable conditions, these calculations can be applied to preferentially oriented water. Several different methods of determining the doublet splitting constant and the proton exchange lifetime from pulsed NMR signals are given. These methods are applied to the pulsed NMR signals from water in various cation-exchanged forms of chabazite and montmorillonoid clays. The proton exchange lifetimes in several cation-exchanged chabazites decrease in the order Ca 2+ ≈ Na + > K+ > Li +. The temperature dependences are consistent with an Arrhenius equation with an apparent activation energy 11.4 kcal/mole. The proton exchange lifetime for water in the clays depends on the clay particle size as well as on the exchanged cation. The larger particle size samples exhibit the larger exchange lifetimes.