Ozone reactions with alkaline-earth metal cations and dications in the gas phase: Room-temperature kinetics and catalysis

Room-temperature rate coefficients and product distributions are reported for the reactions of ozone with the cations and dications of the alkaline-earth metals Ca, Sr, and Ba. The measurements were performed with a selected-ion flow tube (SIFT) tandem mass spectrometer in conjunction with either ail electrospray (EST) or an inductively coupled plasma (ICP) ionization source. All the singly charged species react with ozone by O-atom transfer and form monoxide cations rapidly, k = 4.8, 6.7, and 8.7 x 10(-10) cm(3) molecule(-1) s(-1) for the reactions of Ca+, Sr+, and Ba+, respectively. Further sequential O-atom transfer occurs to form dioxide and trioxide cations. The efficiencies for all O-atom transfer reactions are greater than 10%. The data also signify the catalytic conversion of ozone to oxygen with the alkaline-earth metal and metal oxide cations serving as catalysts. Ca2+ reacts rapidly with O-3 by charge separation to form CaO+ and O2(+) with a rate coefficient of k = 1.5 x 10(-9) cm(3) molecule(-1) s(-1). In contrast, the reactions of Sr2+ and Ba2+ are found to be slow and add O-3, (k >= 1.1 x 10(-11) cm(3) molecule(-1) s(-1)). The initial additions are followed by the rapid sequential addition of up to five O-3 molecules with values of k between 1 and 5 x 10(-10) cm(3) molecule(-1) s(-1). Metal/ozone cluster ions as large as Sr2+ (O-3)(5) and Ba2+ (O-3)(4) were observed for the first time.