NMR in Hydrate Crystals: Structural Information from Broadened Fine-Structure Lines

Abstract

The effects of interpair broadening upon the NMRspectra of proton pairs in hydrate crystals are explored experimentally. The shape of a fine‐structure line is generally found to be asymmetric. The importance of using the position of the center of gravity of a fine‐structure line rather than the position of maximum absorption in determining structural parameters is emphasized by several experimental illustrations. In a coupled pair framework the former quantity is independent of interpair broadening to first order, the latter only to zero order (isolated pairs), but the position of maximum absorption is the quantity most commonly used in the literature. Data for a crystal not previously investigated with NMR, K2C2O4·H2O, illustrates the importance of the use of centers of gravity for a case in which the interpair interactions are relatively weak. It also demonstrates how the nature of the line asymmetry may be interpreted to give information about relative positions of neighboring water molecules. The protonspectra of CaSO4·2H2O and Li2SO4·H2O have been remeasured and are reinterpreted. For these crystals short interpair distances suggest large modifications of the zero‐order treatment given in the literature. It is demonstrated that in CASO4·2H2O a first‐order treatment gives consistent results, with minor changes in the previously derived structural parameters. Second‐order effects on the position of the center of gravity of the NMR line are largely suppressed by the relatively large number of interacting pairs. In Li2SO4·H2O it is concluded that strong interpair couplings along a chain of water molecules cast grave doubts on any interpretation of the spectra based on an analytical treatment using a coupled pair framework.