1 Electrospray Ionization Based Methods for the Generation of Polynuclear Oxo- and Hydroxo Group 6 Anions in the Gas-Phase Rosa Llusar a, * Ivan Sorribes a and Cristian Vicent b,* a Departament de Química Física i Analítica, Universitat Jaume I, Avda. Sos Baynat s/n, E-12071, Castelló, Spain b Serveis Centrals d´Instrumentació Cientifica, Universitat Jaume I, Avda. Sos Baynat s/n, E-12071 Castelló, Spain AUTHOR EMAIL ADDRESS: * author for correspondence: ; Rosa.Llusar@qfa.uji.es ; barrera@sg.uji.es RUNNING HEAD: gas-phase ion chemistry of group 6 oxo- and hydroxo anions Abstract Electrospray ionization (ESI) of the Lindqvist (n-Bu 4 N) 2 [M 6 O 19 ] (M = Mo, W) polioxometalates provides a straightforward entry for the generation of an assortment of oxo- and hydroxo anions in the gas-phase. In particular, the series of oxo dianions of general formula [(MO 3 ) n O] 2- (n = 2-6; M = Mo, W), monoanions, namely [(MO 3 ) n O] - (n = 1, 2) and [(MO 3 ) n ] - (n = 1, 2), and the hydroxo [(MO 3 ) n (OH)] - (n = 1-6) species can be readily generated in the gas-phase upon varying the solvent composition as well as the ionisation conditions (typically the U c cone voltage). Complementary tandem mass experiments (collision induced dissociation and ion-molecule reactions) are also used aimed to investigate the consecutive dissociation of these species and their intrinsic gas-phase reactivity towards methanol. Special emphasis is paid to some of the key factors of these group 6 anions related to the gas-phase activation of methanol, such as molecular composition, open vs closed shell electronic nature and cluster size. Keywords: group 6 oxide anions; hydroxo complexes; Electrospray ionization, CID, ion-molecule reactions Journal of Cluster Science