Selective production of electrostatically-bound adducts of alkyl cations/polyoxoanions by the collision-induced fragmentations of their quaternary ammonium counterparts

Solutions of the quaternary ammonium salts of a set of classic polyoxometalates (POMs) (Keggin [XM₁₂O₄₀]^n-, Dawson [P₂W₁₈O₆₂]^6-, and Lindqvist [M₆O₁₉]^2- (X = P, Si; M = W, Mo) were characterized by electrospray mass spectrometry. The gas-phase fragmentations of a series of quaternary ammonium-associated clusters were investigated by their collision-induced dissociations to elucidate their fragmentation mechanisms. It was found that the quaternary ammonium-associated clusters had distinctive dissociation characteristics. Moreover, the mono-quaternary ammonium-associated clusters, {NR₄[POMs]}^(n-1)-, shared a common fragmentation feature, that is, they decomposed exclusively into their respective alkyl cation-bound clusters irrespective of the different cation sizes and the different natures of the polyoxoanions. The optimized geometries and the binding energies of the mono cation-bound Lindqvist POM-based clusters were obtained by calculations. To the best of our knowledge, this is the first investigation of the gas-phase fragmentations of these noncovalent complexes between organic amines and inorganic POM anions by a combination of theory and mass spectrometry. [Figure not available: see fulltext.]