Surface-induced NMR line splittings and augmented relaxation rates in water

Nuclei in liquid water adsorbed onto substrates in layered silicates or ordered macromolecules have been shown to exhibit nuclear magnetic resonance line splittings. In aqueous protein solutions they also exhibit a dispersion in their longitudinal relaxation rate at low frequencies. Furthermore, the reduced line splittings and relaxation rates have been found to be the same for 1H, 2H, and 17O and independent of the nature of the substrate. We show that these results are consistent with a surface-induced anisotropy in the orientational probability distribution of the water molecules. Agreement between this theory and the experimental results is possible only if the values of the H–O–H angle and the coefficients for the field gradient anisotropies at the 2H and 17O nuclei are close to those found in the solid phase. The theory is consistent with a model for the water surface in which all of the molecules are statistically equivalent and rotationally mobile under a weak anisotropic constraint.