Proton Chemical Shift Tensors and Hydrogen Bond Geometry:  A ¹H−²H Dipolar NMR Study of the Water Molecule in Crystalline Hydrates

We report a 1 H− 2 H dipolar NMR study of the water molecule in magnetically 1 H dilute crystalline hydrates. From the spectra obtained at two different magnetic fields, 1 H chemical shift (CS) tensors were directly determined. A clear correlation was observed between the 1 H CS tensor of hydrogen-bonded water molecules and hydrogen bond environment. Specifically, the 1 H chemical shieldings along directions parallel and perpendicular to the H···O hydrogen bond were found to change in opposite directions with respect to hydrogen bond length. The parallel component of the 1 H CS tensor becomes more shielded as the hydrogen bond length decreases, whereas the perpendicular component becomes less shielded. This fact makes the span of the 1 H CS tensor four times more sensitive to the hydrogen bond length than the isotropic 1 H chemical shift. Although it has been commonly assumed that the 1 H CS tensors in O−H···O hydrogen-bonded systems are axially symmetric, this is not the case in this study. The asymmetry parameter of the 1 H CS tensor was found to depend on the hydrogen bond angle, ∠O W −H···O, in crystalline hydrates.