Reactivity and kinetics studies of organometallic cluster ions in the gas phase using Fourier-transform ion cyclotron resonance mass spectrometry

Kenneth Jerry Auberry, Purdue University

Abstract

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) is applied to the exploration of ionic clusters and metal-ligand systems. These include Ti8C12+, Nb6O 15+ and Nb7O17+, as well as C6F6 reacted with a wide variety of metal ions. Gas-phase Ti8C12+ ions are found to be reactive with NH3, H2O, CH3CN, CuH 6, and C2H4. Ti8C12 + is observed to rapidly add four molecules of NH3, H 2O and CH3CN, with four more additions occurring much more slowly, while C6H6 stops after four fast additions. This implies that there should be a connection between association reactivity and cluster structure, but insufficient hard evidence exists to make a final judgment on this front. The association kinetics of these molecular additions are examined in detail, revealing that there is a quantitative difference in association rates after the addition of a fourth ligand. We have also formulated a theoretical model which attempts to explain this shift in reactivity from the standpoint of both electrostatic attraction/repulsion and steric hindrance effects. Next, the collisional activation and gas-phase reactivity of Nb 6O15+ and Nb7O17 + with NH3 and H2O are examined. Nb6O 15+ is found to fragment under relatively high energy conditions to yield Nb6O14+ and O, as well as small amounts of Nb2O5+, Nb4O 10, NbO2+ and Nb5O13. Under similar conditions, Nb7O17+ fragments to give mostly Nb7O15+ and O2, with more significant amounts of NbO2+, Nb 6O15, Nb3O7+, and Nb 4O10. Reactivity studies conducted on Nb6O 15+ and Nb7O17+ with NH3 revealed four relatively rapid additions for both clusters, while reactions with H2O showed just three additions for both clusters. Rate constants for these sequential additions were carefully measured, and the results graphed and tabulated. Finally, the reactivity of C6Fu6 with a variety of first-, second-, and third-row transition metal ions is investigated. Three distinct reaction pathways are observed, including: multiple fluorine abstractions, dissociative charge exchange, and molecular addition of intact C6F 6 molecules. The number of fluorine abstractions that take place is closely linked with the number of available valence electrons present in a given metal's [+1] charge state, with a few minor exceptions. According to the product distribution observed, fluorine abstractions occur through a single-collision mechanism, rather than through multiple collisions with multiple C6F 6 molecules. Additionally, the dissociative CE pathway is always observed in conjunction with fluorine abstraction, suggesting that the two products are related by a common mechanism. Where possible, the rate constant for the primary reaction in each case is carefully measured. Results of this measurement show that fluorine abstraction by niobium occurs at nearly the collision rate for a single-collision reaction, while tantalum abstracts at nearly half that rate.

Degree

Ph.D.

Advisors

Freiser, Purdue University.

Subject Area

Analytical chemistry

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