Dissociation, Ions molecules

Precursor Ion Ion that reacts to form particular product ions. The reaction can be unimolecular dissociation, ion/molecule reaction, isomerization, or change in charge state. The term parent ion is deprecated (but still very much in use). 

When alkali metals or lead are added to flames containing hydrocarbons, ion concentrations have been observed to exceed both the equilibrium alkali metal ion concentration and the concentrations of hydrocarbon ions (Fig. 8). The enhanced ionization is presumably due to the rapid dissociative ion-molecule reaction,... 

Colorado, A. and Brodbelt, J. Class-selective collisionally activated dissociation/ion-molecule reactions of 4-Quinolone antibiotics. Anal. Chem. 66 2330-2335, 1994. 

GntoLDO, T. Riveros, J. M. Keto nol isomerization of gas-phase 2 -methylacetophenone molecular ions probed by high-temperature near-blackbody-induced dissociation, ion-molecule reactions, and ah initio calculations. J. Phys. Chem A 2002,106, 9930 9938. 

Collision-induced dissociation mass spectrum of tire proton-bound dimer of isopropanol [(CH2)2CHOH]2H. The mJz 121 ions were first isolated in the trap, followed by resonant excitation of their trajectories to produce CID. Fragment ions include water loss mJz 103), loss of isopropanol mJz 61) and loss of 42 anui mJz 79). (b) Ion-molecule reactions in an ion trap. In this example the mJz 103 ion was first isolated and then resonantly excited in the trap. Endothennic reaction with water inside the trap produces the proton-bound cluster at mJz 121, while CID produces the fragment with mJz 61. 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Charge-exchange (charge transfer) ionization. Occurs when an ion/atom or ion/molecule reaction takes place in which the chaise on the ion is transferred to the neutral species without any dissociation of either. 

The same ideas may be applied to the other processes of Fig. 1. The work required to dissociate a diatomic molecule into two electricallt/ neutral atoms may he quite small the dissociation energy of the bromine molecule Br2 in a vacuum, for example, is only 1.915 electron-volts. On the other hand, the work to dissociate a molecule into two atomic ions in a vacuum cannot be as small as this, since work must be done to set up the full electrostatic field of the positive ion, and the full electrostatic field of the negative ion and this must amount to at least a few electron-volts.1 In addition, the non-electrostatic forces may make a small or large contribution. 

Tn a double mass spectrometer several types of ion-molecule reactions - can be observed (a) charge exchange, A++B- A + B+, often followed by dissociation of B+ (b) transfer of part of A+ or B (e.g., proton transfer or hydride ion transfer) during the collisions (c) reactions at increased pressure in the collision chamber. 

The phase space theory in its present form suffers from the usual computational difficulties and from the fact it has thus far been developed only for treating three-body processes and a limited number of output channels. Further, to treat dissociation as occurring only through excitation of rotational levels beyond a critical value for bound vibrational states is rather artificial. Nevertheless, it is a useful framework for discussing ion-molecule reaction rates and a powerful incentive for further work. 

Here, a primary ion P+ formed by the radiation field reacts with a gas molecule M to give an intermediate complex [PM +] which can either dissociate to a secondary species S + and a neutral fragment N or react with another molecule to produce another complex [PM2 + ]. The latter then dissociates into a tertiary ion T+ or propagates the chain by forming a third intermediate [PM3 + ]. A quaternary ion Q+ may result from dissociation of [PM3 + ], or the chain may continue through reaction of [PM3 + ]. Wexler and Jesse (38), on the other hand, have suggested a model which states that reactive intermediate complexes are not involved in the propagation, but rather the polymerization proceeds by chains of simple consecutive and competitive ion-molecule reactions,... 

Phenomenological evidence for the participation of ionic precursors in radiolytic product formation and the applicability of mass spectral information on fragmentation patterns and ion-molecule reactions to radiolysis conditions are reviewed. Specific application of the methods in the ethylene system indicates the formation of the primary ions, C2H4+, C2i/3+, and C2H2+, with yields of ca. 1.5, 1.0, and 0.8 ions/100 e.v., respectively. The primary ions form intermediate collision complexes with ethylene. Intermediates [C4iZ8 + ] and [CJH7 + ] are stable (<dissociation rate constants <107 sec.-1) and form C6 intermediates which dissociate rate constants <109 sec. l). The transmission coefficient for the third-order ion-molecule reactions appears to be less than 0.02, and such inefficient steps are held responsible for the absence of ionic polymerization. 

Acetylene Ion. No evidence for the contribution of ion-molecule reactions originating with acetylene ion to product formation has been obtained to date. By analogy with the two preceding sections, we may assume that the third-order complex should dissociate at pressures below about 50 torr. Unfortunately, the nature of the dissociation products would make this process almost unrecognizable. The additional formation of hydrogen and hydrogen atoms would be hidden in the sizable excess of the production of these species in other primary acts while the methyl radical formation would probably be minor compared with that resulting from ethylene ion reactions. The fate of the acetylene ion remains an unanswered question in ethylene radiolysis. 

The rate constants for unimolecular dissociation of the intermediate ions suggested earlier indicate that all ions containing seven or more carbon atoms arise from reactions of the dissociation products of Steps 9, 13, and 17 when pressures are of the order of a few torr and of Step 20 and its analogues at pressures in excess of a few hundred torr. The product ions are generally quite complex, and the simple exothermicity rule given earlier will not apply. Thus, we may well expect that there will be inefficient ion-molecule reactions in the sequences originating with these ions as well. 

Consideration of work by Buchel nikova (4) on the dissociative attachment of electrons to HC1 leads to the conclusion that k4 is given approximately by k4 10-10 e 20 mtRT cm.3 molecule-1 sec.-1 Since K 4 2 X 10-3 el8 miRT at 2000°K., 4 10 13 cm.3 molecule-1 sec.-1 This is considerably smaller than rate constants for other exothermal ion-molecule reactions, which probably reflects the importance of participation of molecular vibrational energy in such reactions. Remember, however, that the uncertainty in 4 is probably at least an order of magnitude. 

Mass spectrometric studies of the ionic species which arrive at the cathode of both glow and corona discharges yield useful information regarding ion-molecule reactions which occur within these systems. Glow discharges have been used to study endothermic reactions, and their usefulness and limitations have been demonstrated by studies of the dissociative charge transfer reactions Ar+ + N2 N+ + N + Ar N2+ + N2 N+ + N + N2 N2+ + 02 0+ + O + N2. Exo-... 

Fragmentation occurs because the repeller voltage increases the kinetic energy of the ions, not only making collision-induced dissociation (CID) more likely but also allowing endothermic ion-molecule and solvent-switching reactions to occur. 

By contrast, addition-elimination mechanisms in their simplest form begin with formation of an addition complex resulting from a well on the PES, followed by dissociation of the complex, yielding products. Both the entrance to and exit from the well may be hindered by barriers on the PES. Addition mechanisms are uncommon in radical -b saturated closed-shell reactions due to the difficulty of bond formation with the saturated species (ion-molecule reactions are exceptions). By contrast, additions are more common in radical -b unsaturated closed-shell species, where the double or triple bond allows a low barrier or barrierless pathway for addition of the radical into the 7i-bond of the stable species, such as the reaction... 

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