17O and 1H NMR spectral parameters in isolated water molecules

Abstract

Small amounts of water enriched in oxygen-17 were studied by ¹⁷O and ¹H NMR in binary gaseous mixtures with Xe, Kr, CHF₃ and CH₃F and CO₂. The distinct linear dependences of ¹⁷O and ¹H chemical shifts and ¹J(¹⁷O,¹H) spin–spin coupling on the density of every gas solvent were measured. After the extrapolation of experimental results to zero density the relevant parameters in the isolated H₂¹⁷O molecule were determined. The same procedure was applied for H₂¹⁶O when its proton chemical shift was analyzed but the secondary isotope effect in the ¹H shielding of H₂¹⁷O and H₂¹⁶O molecules was too small for detection. As shown, all the intermolecular effects in nuclear magnetic shielding are negative and these effects are more significant for ¹⁷O nuclei than for protons. It is consistent with the appropriate gas-to-liquid shifts of water which also indicate deshielding effects for both the investigated nuclei. On the other hand, the ¹J₀(¹⁷O,¹H) coupling constant in H₂¹⁷O, which is completely free from intermolecular interactions, considerably differs from the ¹J(¹⁷O,¹H) experimental values obtained for water in liquid solutions. The present experimental data of the isolated H₂¹⁷O molecule are compared with selected results of shielding and spin–spin coupling calculations available from the literature and with the recent experimental data for a water molecule encapsulated in the C₆₀ fullerene. Additionally, on the basis of actual results the magnetic dipole moment of the ¹⁷O nucleus is revalued for greater accuracy.