NMR and UV spectra of lanthanide decatungstates LnW 10O 36 n− and W 10O 32 4− : a study of some peculiarities in spectra by the extended Hückel MO method

Abstract

The decatungstates for Eu, Tm, Yb and Lu have been synthesized, and 183W and 17O NMR spectra have been measured for their solutions. The latter three compounds may be formed in the solid state but in aqueous solution they are practically completely decomposed. 183W NMR and UV spectra of the polyoxotungstates have been analyzed by extended Hückel molecular orbital calculations. The calculated HOMO–LUMO separation for LaW 10O 36 9− , Ce IVW 10O 36 8− , W 10O 32 4− and W 6O 19 2− corresponds to the wavelength of the lowest charge transfer band observed in UV spectra of aqueous solutions. Using the calculated energy of MOs for LaW 10O 36 9− , Ce IVW 10O 36 8− , W 10O 32 4− and W 6O 19 2− , it was found that the general trend in the change of the 183W NMR chemical shifts depends on the paramagnetic contribution which is determined by the excited states. The 183W chemical shifts is shown to linearly depend on the sum of five inverse energy separations between bonding MOs involving W5d-orbitals and five antibonding W5d*-orbitals for the nucleus under consideration. It is also shown that the charges on tungsten atoms do not play an important role in the change of the chemical shift.