17O and 183W NMR diamagnetic and paramagnetic shifts in heterodecatungstates (X = Ln, Th, U) in aqueous solutions

Heterodecatungstates of the general formula X W 10 O 36 − where X = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Th and U) have been studied by 17O and 183W NMR spectroscopy. It is shown that the structure of the heteropolyanions, where X occupies a centre of a square antiprism formed by two W5O18 ligands and which was determined in the crystalline state, is retained in aqueous solution. Completely resolved 17O and 183W NMR spectra for all atoms, even for the nearest oxygen atoms, have been recorded. From the analysis of the isotropic shifts, the contact and dipolar contributions have been estimated for all constituting atoms. The possible σ- and π-mechanisms of the spin transfer are considered. The negative spin density determines the dominant contact shift for the oxygen and tungsten nearest to X. The large pseudo-contact (ligand centred) contributions are estimated for the nearest oxygen and tungsten atoms. The positive spin density due to the σ-mechanism and large dipolar contribution gives rise to the observed chemical shift for the internal oxygen atom. The other oxygen atoms experience a mostly dipolar contribution with other terms probably cancelling each other. Temperature dependences of the chemical shifts are discussed. Copyright © 1996 Elsevier Science Ltd