High-field 75As NMR study of arsenic oxysalts

Arsenic is an important environmental hazard, but there have been few NMR investigations of its molecular scale structure and dynamics, due principally to the large quadrupole moment of (75) As and consequent large quadrupole couplings. We examine here the potential of existing, single-field solid-state NMR technology to investigate solids containing arsenate and arsenite oxyanions. The results show that current techniques have significant potential for arsenates that do not contain both protonated HxAsO4-(3-x) groups and structural water molecules, but that the quadrupole couplings for the arsenites examined here are large enough that interpretation of the spectra is difficult, even at 21.1 T. Compounds that contain both structural H2O molecules and protonated arsenate groups do not yield resolvable signal, likely a result of T-2 effects related to a combination of strong quadrupolar interactions and proton exchange. Spin-echo experiments at 11.7 and 14.1 T were effective for Li3AsO4 and CsH2AsO4, as were whole-pattern spikelet experiments for arsenate oxide (As2O5) at 17.6 and 21.1 T. The central transition resonance of Ca-3(AsO4)(2)center dot 8H(2)O is similar to 6 MHz broad and required a non-conventional, histogram-style spikelet method at high field to improve acquisition efficiency. This approach reduces the acquisition time due to the sensitivity enhancement of the spikelet sequence and a reduction in the number of frequency increments required to map the resonance. Despite the large quadrupole couplings, we have identified a correlation between the (75) As isotropic chemical shift and the electronegativity of the next-nearest neighbor cation in arsenate compounds. (C) 2007 Elsevier Inc. All rights reserved.