Solid-state and solution-state self-association of dimethylarsinic acid: IR, NMR and theoretical study

The structure of dimethylarsinic acid (Me2AsOOH, DMA) in crystal and in solution in aprotic medium was studied by X-ray, spectroscopic (IR, NMR and Raman) and quantum chemical calculation (DFT) methods. Two polymorphs of DMA – hydrogen-bonded cyclic dimers or infinite chains – were obtained by growing crystals from protic and aprotic solvents. By comparison of the experimental data with the results of quantum-chemical calculations performed with periodic boundary conditions we propose ways how to distinguish two polymorphs using IR marker bands. The self-association of DMA in polar aprotic solution was studied by low-temperature NMR (T = 100 K, solvent CDF3/CDF2Cl mixture). By using H/D isotope effects on 1H NMR chemical shifts (appearing after a partial deuteration of DMA in the mobile proton sites) we were able to prove that in solution DMA forms cyclic dimers with two equivalent strong hydrogen bonds (in contrast, for example, to Me2POOH acid, which forms predominantly cyclic trimers). Based on the available IR spectra, the type of complexation does not change when the aprotic solvent is varied.