Grant Johnson - Influence of heteroanion and ammonium cation size on the composition and gas-phase fragmentation of polyoxovanadates

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      Publication Details (including relevant citation   information):


      Volume:   354

        Pages: 333-341

        Special Issue: SI


    Published: NOV 15 2013


        This paper describes the results of a systematic experimental   investigation of the influence of different size alkyl ammonium   cations and heteroanions on the composition, ionic charge state   and gas-phase fragmentation pathways of anionic polyoxovanadates   synthesized in solution. Four separate solutions of   polyoxometalates (POMs) were prepared using all possible   combinations of the tetraethylammonium [(C2H5)(4)N+] cation,   chloride (Cl-) heteroanion, tetrabutylammonium [(C4H9)(4)N+]   cation and acetate (CH3CO2-) heteroanion. Employing electrospray   ionization combined with high-resolution mass spectrometry   (ESI-MS) we demonstrate that POM solutions synthesized using the   small [(C2H5)(4)N+] cation and Cl- heteroanion are composed   predominately of large doubly and triply charged chlorine   containing species with a size distribution centered at 14   vanadium atoms. POM solutions prepared using the Cl- anion and a   larger [(C4H9)(4)N+] cation are shown to contain slightly larger   species with 15 and 16 vanadium atoms, thereby indicating that   the size of the ammonium cation exerts only a weak influence on   the size of polyoxovanadates formed in solution. POM solutions   prepared using (C2H5)(4)NCl and (C4H9)(4)NCl also produced peaks   consistent with the attachment of one and two ammonium cations to   the larger vanadium oxide species. Solutions prepared using the   large (CHCO2-)-C-3 heteroanion, in contrast, contain much smaller   singly and doubly charged species with a size distribution   centered at six vanadium atoms. In addition, while incorporation   of one and two ammonium cations into the smaller vanadium oxide   species was observed, no POMs containing the CH3CO2- heteroanion   were identified. The gas-phase fragmentation pathways of   representative POMs containing one and two ammonium cations were   examined using collision induced dissociation (CID) and mass   spectrometry. Similar primary fragmentation pathways involving   partial loss of the ammonium cation were observed for species   containing both one and two ammonium cations largely independent   of the size, composition and charge state of the precursor ion.   The [(C4H9)N+] was found to exhibit stronger interactions with   the core of the POMs resulting in higher abundance of fragment   ions containing (C4H9) units compared to (C2H5) units originating   from [(C2H5)(4)N+]. These results provide fundamental insight   into the interactions between anionic metal oxides, heteroanions   and ammonium cations that are responsible for the size and   composition-controlled synthesis of POMs in solution.

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