Cluster Phase Chemistry: Collisions of Vibrationally Excited Cationic Dicarboxylic Acid Clusters with Water Molecules Initiate Dissociation of Cluster Components
A homologous series of cationic gas-phase clusters of dicarboxylic acids (oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid) generated via electrospray ionization (ESI) are investigated using collision-induced dissociation (CID). Singly charged cationic clusters with the composition (Na^+)_(2n+1)(dicarboxylate^(2-))_n, where n = 1−5, are observed as major gas-phase species. Significant abundances of singly charged sodiated hydrogen dicarboxylate clusters with the composition (Na^+)^(2n)(dicarboxylate^(2-))_n(H^+), where n = 1−6, are observed with oxalic acid, malonic acid, and succinic acid. Isolation of the clusters followed by CID results mainly in sequential loss of disodium dicarboxylate moieties for the clusters of succinic acid, glutaric acid, and adipic acid. However, the dimer of sodiated hydrogen succinate, all malonate clusters, and all oxalate clusters, with the exception of the dimer, exhibit complex chemical reactions initiated by the collision of vibrationally excited clusters with water molecules. Generally, water molecules serve as proton donors for reacting dicarboxylate anions in the cluster, initiating dissociation pathways such as the decomposition of the malonate ion to yield an acetate ion and CO_2. The reactivity of several mixed dicarboxylate clusters is also reported. For example, malonate anion is shown to be more reactive than oxalate anion for decarboxylation when both are present in a cluster. The energetics of several representative cluster phase reactions are evaluated using computational modeling. The present results for cationic clusters are compared and contrasted to earlier studies of anionic sodiated dicarboxylic acid clusters.
© 2007 American Chemical Society. Received 25 January 2007. Published online 15 June 2007. Published in print 1 July 2007. The research described in this paper was carried out at the Beckman Institute and the Noyes Laboratory of Chemical Physics at the California Institute of Technology. We appreciate the support provided by the Mass Spectrometry Resource Center, Materials and Process Simulation Center in the Beckman Institute, and Planetary Science & Life Detection section, Jet Propulsion Laboratory, California Institute of Technology. Partial support was also provided by the National Science Foundation (NSF) under Grant No. CHE-0416381.