Electron spin resonance studies of free and supported 12-heteropoly acids. Part 5.—Elucidation of structures of hydrated and of dehydrated heteropoly acids by quantum chemical calculations of electron spin resonance parameters

Abstract

In order to correlate the reversible changes of the e.s.r. parameters of H₄(PVMo₁₁O₄₀)·χ(H₂O) caused by thermal treatment of this heteropoly acid in its hydrated state up to 670 K with structural changes, the e.s.r. parameters of models have been calculated by means of the CNDO/2 method and perturbation theory. The experimentally observed decrease of g_⊥ and of the anisotropy, g_⊥–g_∥, and the constancy of g_∥ could be reproduced qualitatively with models which reflect the transformation of a hydrated terminal oxygen atom (vanadyl group) of this heteropoly acid anion into a proton or a hydronium attacked one, whereas with models in which a bridging oxygen atom is attacked, no meaningful e.s.r. parameters could be calculated. On the basis of these results it is concluded that owing to the loss of water of crystallization and of constitution during the thermal treatment some of the protons which had been bound before at water molecules, are located at vanadyl groups and that a flattening of the VO₆ octahedra occurs.