Gas-Phase Ion Energetics

The experimental methods designed to investigate the energetics of gas-phase ions have been another important source of thermochemical data, particularly throughout the past two or three decades [9,10]. In this chapter, we discuss the main quantities that are measured experimentally and lead to reaction enthalpy values. 

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We have already mentioned the formation of solvated CpMg+ in the context of the solution phase reaction of magnesocene and thallocene. In this section are discussed aspects of the experimental gas-phase ion energetics of CpMg+, CpiMg and related species. 

Nist chemistry webbook, gas phase ion energetics data, available at http //webbook.nist.gov. 

Beynon, J.H. Gilbert, J.R. Energetics and Mechanisms of Unimolecular Reactions of Positive Ions Mass Spectrometric Methods, in Gas Phase Ion Chemistry, Bowcts, M.T., editor Academic Press New York, 1979 Vol. 2, Chap. 13, 153-179. 

These techniques use current beams that are in dynamic SIMS high enough to damage the surface. But they produce ions from the surface, as in static SIMS, because convection and diffusion inside the matrix continuously create a fresh layer from the surface for producing new ions. The energetic particles hit the sample solution, inducing a shock wave which ejects ions and molecules from the solution. Ions are accelerated by a potential difference towards the analyser. These techniques induce little or no ionization. They generally eject into the gas phase ions that were already present in the solution. 

Several studies have demonstrated the ability to observe a complete catalytic cycle in the gas-phase. Wallace and Whetten, and Woste and coworkers combined gas-phase experiments and theoretical calculations to elucidate the fuU catalytic cycle of CO oxidation including intermediate reaction steps [27-29]. Schwarz et al. have also demonstrated a full gas-phase catalytic cycle for the oxidation of CO in the presence of cationic platinum oxide clusters [30]. Furthermore, Armentrout and co-workers have studied the energetics of the individual steps in the overall catalytic cycles and produced a wealth of information on the thermochemistry, structure, and bond energies of transition metal clusters [31]. Clearly, the ability to probe the active sites and intermediates of complex catalytic reactions through gas-phase ion-molecule studies has yielded significant insight into the mechanisms of condensed-phase catalytic processes. 

The enthalpy of formation of gaseous p-xylylene of 210 20 kJ mol-1 was obtained from the energetics of gas-phase ion-molecule reactions from S. K. Pollack, B. C. Raine and W. J. Hehre, J. Am. Chem Soc., 103, 6308 (1981). Since gaseous p-xylylene polymerizes... 

The final area to be discussed here involves the study of thermal ion reactions by recoil methods. This represents one of the newest applications reported and potentially one of the most exciting. Studies to date are unique in that they incorporate a combination of radiochemical detection that allows investigation of submicroscopic ion concentrations and recoil energetics that often ensures escape from solvation. Thus, isolated ions can be studied in the liquid phase without the complications of ion pairing or solvation, and truly thermal processes involving gas phase ions that are diflBcult at best to observe in chemical accelerators due to space change limits can be investigated. 

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