Gas Phase Ion Chemistry. Volume 1


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Samenvatting

Fundamentals of Gas Phase Ion Chemistry

Faraday Discuss. Aten, J. E , 8, Carman, H. Compton, R. Hasegawa, A.


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Massey, H. Rudge, M. Weingold, E. McCarthy, I. Younger, S. Electron Impact Ionization; Mark, T. See also a Deutsch, H. Mass Spectrom. Ion Procs. Ref Data Ser. Lampe, F. D , 30, Ehrhardt, H. D , 1, 3. Brauner, M. B , 22, Jones, S. A , 48, R McGuire, E.

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Gas-phase ion chemistry of silyl cations obtained from hexamethyldisilazane

Bartmess, J. Vacuum , 33, Astrof , 2, Fisica , 27, See for example, Atkins, P. Physical Chemistry, Fifth ed. Faist, M. McMahon Io II.

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Unimolecular Dissociation of Cluster Ions. Metastable Ion Cyclotron Resonance Spectrometry. Infrared-Laser-Induced Thermal Dissociation. AH rights of reproduction in any form reserved. In addition a second, more weakly bound electrostatic complex is implicated as an entrance channel complex on this potential energy surface. Comparison with trajectory studies carried out by Hase shows good agreement with the experimental data. Data for a variety of deuterium-substituted variants of the acetone system reveal that structures other than the proton-bound dimer may play an important role.


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This understanding has advanced to an extent that quantitative predictions o f the rates of unimolecular dissociation are possible, given appropriate energetic and spectroscopic data. Current studies of unimolecular reactions can be broadly divided into three categories, based on different methods of activation of the decomposing species.

12 Organic gas-phase ion chemistry

The first, most classical, method is that of thermal activation o f the type first envisioned by Lindemann 7 to explain unimolecular dissociation phenomena brought about by heat energy. The second method involves chemical activation, 8 Unimolecular Dissociation of Gaseous Cluster ions 43 in which an exothermic bimolecular reaction results in the formation of transient species whose internal energy is the superposition of the exothermicity of the transient formation onto the Maxwell-Boltzmann distribution of thermal energies corresponding to the temperature at which the bimolecular reaction occurs.

In general, this chemical activation reaction efficiently deposits in the transient species considerably more energy than can be easily added via the thermal activation process. The final method is photoactivation, 9 in which a single-frequency radiation source, typically a laser, is matched to a resonant absorption by the species of interest to selectively deposit energy.

The subsequently induced unimolecular dissociation is then monitored. The speed and extent of energy randomization within the molecule can then also be frequently inferred. Metastable dissociations are typically carried out in reverse geometry double-focusing mass spectrometers. Collision-induced decomposition studies are similarly unable to characterize the unimolecular reaction quantitatively, because the energy deposition function resulting from the collision of a fast ion with a stationary, usually monoatomic, target is not well understood.

McMAHON reaction in the form of the unimolecular dissociation of the chemically activated intermediate ion--molecule complex, as in Equation 1. Each of the instruments was constructed, to a considerable degree, in-house at the University of Waterloo, and each contains features unique to its type of apparatus. The instruments in general and the unique features of the Waterloo apparatus in particular are described below. The Pulsed Ionization, High-Pressure, Reverse Geometry, Double-Focusing Mass Spectrometer The technique of high-pressure mass spectrometry was pioneered by Kebarle and has been described in some detail in a recent review.

This permits observation of metastable and collision-induced dissociation of wellthermalized cluster ions in the second field-free region FFR of the spectrometer. A schematic view of the instrument is shown in Figure 1. The mass spectrometer itself is a VG instrument whose geometry was reversed via extensive redesign of the ion optics between source and magnet entrance slit, incorporation of a differentially pumped collision cell in the second field-free region, and modified ion optics for the transition from second field-free region to the electrostatic analyzer ESA.

The sequence of dissociations, Equations 2a and 2b , inferred from these data strongly favor structure I for this cluster ion with a central hydronium ion core. Schematic of the reverse geometry double-focusing high-pressure mass spectrometer. The high-voltage ion optics permit the use of small apertures between pumping stages thus leading to a considerable saving in cost of pumps.

In addition, the deceleration assembly permits even very weakly bound cluster ions to be trapped in the FTICR cell and to survive collisions with inert gases, such as Ar, without any significant collisional dissociation occurring. In contrast, the reaction of hydroxide ion with water is found to give proton transfer at a half of the collision rate. In a related study, Dodd et al. These transition states do inhibit formation of the minimum-energy proton-bound dimer intermediate through which proton transfer occurs. Lim and Brauman 33 noted, in discussion of the possible reasons for a low proton transfer rate, that if the value for the methoxide-methanol association rate to form the proton transfer intermediate were incorrect, this would alter the conclusions reached regarding the efficiency of proton transfer.

Gas-phase ion chemistry in the 21st century

They concluded that, given the substantial depth of the well associated With formation of the proton-bound dimer, this species is probably formed on every collision, and the use of the ADO model will give this rate sufficiently accurately. In order to better understand the detailed dynamics of this system, an investigation of the unimolecular dissociation of the proton-bound methoxide dimer was undertaken. The data are readily obtained from high-pressure mass spectrometric determinations of the temperature dependence of the association equilibrium constant, coupled with measurements of the temperature dependence of the bimolecular rate constant for formation of the association adduct.

These latter measurements have been shown previously to be an excellent method for elucidating the details of potential energy surfaces that have intermediate barriers near the energy of separated reactants. Variation of relative ionic abundances with reaction time, in a high-pressure source at 5-tort CH4, for negative ions derived from deprotonation of methanol.


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  4. Typical data showing these kinetics are given in Figure 4. These data reveal that, contrary to expectation, the collisionally stabilized association adduct is not formed at the collision rate at this temperature. The variation of the association equilibrium constant, Keq, with reciprocal temperature is shown in Figure 6.

    This can be interpreted in terms of a transitionstate theory model. Interestingly, the enthalpy of activation derived is in excellent agreement with a value that would be obtained from the enthalpy of the forward association reaction and its negative enthalpy of activation, consistent with the absence of any substantial barrier above that of reactants on the potential energy surface. The entropy of activation, derived from the pre-exponential factor for the unimolecular dissociation rate constant, is somewhat less negative than the entropy of activation associated with the bimolecular reaction involving passage from the first electrostatic well to the deeper proton-bound dimer well on the surface.

    The value of only 4 cal tool-] K-] indicates that the transition state for passage from the proton-bound dimer well to the electrostatic well is only marginally less constrained than the proton-bound methoxide dimer itself. Evidently, the greater bond lengths in the transition state will be compensated to a certain extent by the tightness resulting from a four-center-type intermediate in the "isomerization.

    Van't Hoff plot [In Keq vs.

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    This model for the potential energy surface then leads naturally to the question of the detailed nature of the adduct initially formed between methoxide ion and methanol. As will be discussed in the following section, recent experiments from this laboratory have successfully determined the well depths for a number of SN2 electrostatic complexes between halide anions and alkyl halides, and these well depths are on the order of kcal mol-l.

    Gas Phase Ion Chemistry. Volume 1 Gas Phase Ion Chemistry. Volume 1
    Gas Phase Ion Chemistry. Volume 1 Gas Phase Ion Chemistry. Volume 1
    Gas Phase Ion Chemistry. Volume 1 Gas Phase Ion Chemistry. Volume 1
    Gas Phase Ion Chemistry. Volume 1 Gas Phase Ion Chemistry. Volume 1
    Gas Phase Ion Chemistry. Volume 1 Gas Phase Ion Chemistry. Volume 1
    Gas Phase Ion Chemistry. Volume 1 Gas Phase Ion Chemistry. Volume 1
    Gas Phase Ion Chemistry. Volume 1 Gas Phase Ion Chemistry. Volume 1
    Gas Phase Ion Chemistry. Volume 1 Gas Phase Ion Chemistry. Volume 1

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