O-17 AND W-183 NMR DIAMAGNETIC AND PARAMAGNETIC SHIFTS IN HETERODECATUNGSTATES XW(10)O(36)(N-) (X=LN,TH,U) IN AQUEOUS-SOLUTIONS

Heterodecatungstates of the general formula XW(10)O(36)(n-) where X = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Th and U) have been studied by O-17 an d W-183 NMR spectroscopy. It is shown that the structure of the hetero polyanions, where X occupies a centre of a square antiprism formed by two W5O18 ligands and which was determined in the crystalline state, i s retained in aqueous solution. Completely resolved O-17 and W-183 NMR spectra for all atoms, even for the nearest oxygen atoms, have been r ecorded. From the analysis of the isotropic shifts, the contact and di polar contributions have been estimated for all constituting atoms. Th e possible sigma- and pi-mechanisms of the spin transfer are considere d. The negative spin density determines the dominant contact shift for the oxygen and tungsten nearest to X. The large pseudo-contact (ligan d centred) contributions are estimated for the nearest oxygen and tung sten atoms. The positive spin density due to the sigma-mechanism and l arge dipolar contribution gives rise to the observed chemical shift fo r 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. Copyrig ht (C) 1996 Elsevier Science Ltd.