Electrospray mass spectral study of isopolyoxomolybdates

Electrospray mass spectrometry (ESMS) has been performed on aqueous solutio ns of dilute (10(-3) M) isopolyoxomolybdate systems. There is direct eviden ce that the evaporation process in the ESMS technique involves significant chemical effects, resulting in the detection of many new anions and cations . For ammonium polyoxomolybdate systems, negative-ion ESMS yields ions of t he form [HMomO3m+1](-), [MomO3m+1](2-), [MomO3m+2](4-) as well as [Mo7O24]( 6-), whereas for alkali metal polyoxomolybdate systems ions of the form [Mo (m)O(3m+1)A](-) and [Mo(m)O(4m)A(2m-2)](2-) (where A = Li+, Na+ or K+) were observed. In positive-ion mode two series of polyoxomolybdate cations, nam ely [Mo(m)O(4m)A(2m+1)](+) and [Mo(m)O(4m)A(2m+2)](2+) could be assigned. A ggregates of both the [HMomO3m+1](-) and [MomO3m+1](2-) series in the ammon ium polyoxomolybdate system can be classified in terms of open-chained stru ctures of tetrahedra that are corner shared, whereas the highly charged ani ons [MomO3m+2](4-) and [MomO3m+3](6-) are consistent with closed-packed str uctures. For the alkali metal polyoxomolybdate anion and cation systems the spectra are consistent with open-chained structures of octahedral units th at are edge shared, with a terminating tetrahedral unit. Linear correlation s suggest that the assembly of these aggregates occurs via an addition poly merization mechanism. This model, consistent with the ESMS data, may identi fy the additive moieties (MoO3, MoO22+ and Mo(2)O(8)A(4)) required for aggr egation of polyoxomolybdate species in aqueous solution.