Angular dependence of 1H‐ and 2H‐NMR spectra of H2O and D2O absorbed in cellulose acetate film

Abstract 1H‐ and 2H‐NMR spectra of H2O and D2O absorbed in cellulose acetate films were observed while changing the angle between the plane of the film and the magnetic field. 1H‐NMR spectra show dipolar splittings that vary depending on the angle. The splitting has a maximum when the surface of the film is perpendicular to the magnetic field. From the angular dependence of the dipolar splitting, it is recognized that the proton‐proton dipolar axis of water molecules tends to orient perpendicularly to the surface of the film. 2H‐NMR spectra that show quadrupolar splittings also indicate that the quadrupolar axis tends to orient perpendicularly to the film. The so‐called bound water and free water in the film can exchange rapidly on the NMR time scale, so that the line width and the splitting of the NMR spectra become smaller as the amount of water in the film increases. From the temperature dependence of line widths, the apparent reorientational activation energy of water molecules in the film is estimated to be 25 kJ/mol from 1H‐NMR and 31 kJ/mol from 2H‐NMR.