Field ionization kinetics

Under field ionization kinetic conditions (FIK) the process of CH3 elimination from ionized 27 is not observed at t < 10-10 sec. This is a surprising result bearing in... 

Subsequent work confirmed this apparently abnormal behaviour. Deuteriation at remote sites (the S- or e-position) induces small inverse secondary isotope effects in a-cleavages occurring in the ion source, but normal isotope effects in the decomposition of metastable ions in the field-free regions94,95. The time dependence of the isotope effect was also studied by field ionization kinetics, which permit the analysis of fragmentations occurring after lifetimes as short as 10 12 s-1. It was found that the inverse isotope effect favouring loss of the deuteriated radical operates at times shorter than 10 9 s95. 

Example Secondary kinetic isotope effects on the a-cleavage of tertiary amine molecular ions occurred after deuterium labeling both adjacent to and remote from the bond cleaved (Chap. 6.2.5). They reduced the fragmentation rate relative to the nonlabeled chain by factors of 1.08-1.30 per D in case of metastable ion decompositions (Fig. 2.18), but the isotope effect vanished for ion source processes. [78] With the aid of field ionization kinetic measurements the reversal of these kinetic isotope effects for short-lived ions (lO -lO" s) could be demonstrated, i.e., then the deuterated species decomposed slightly faster than their nonlabeled isoto-pomers (Fig. 2.17). [66,76]... 

The field ionization kinetics (FIK) technique allows rates of ionic decompositions to be determined over a continuous range of time extending from picoseconds to microseconds [38, 41, 223]. The applications of the technique have been reviewed [45, 226, 745]. 

In field ionization kinetics (FIK), the isotope effects (dli/dt)/(dlu/dt) are obtained from the rates at anytime. The following discussion applies equally to these isotope effects since the rates have been determined by measuring the ion abundances / formed within a time interval At (rate = IIAt [223]). 

Early El studies by King suggested that 1,3-butadiene radical cations suffer isomerization and complete hydrogen scrambling prior to loss of H and C2H2. Later, Gross, Nibbering and coworkers- -" showed by field ionization kinetic (FIK) measurements that... 

Field Ionization Kinetics (FIK) Although limited to select research laboratories, FIK can also reveal the mechanism of ion dissociation [45], This technique studies ions with lifetimes in the range 10 to 10 s. Ions are generated under the influence of very high potential by field ionization (see Section 2.6). The data are used to obtain the relation of the rate constant function k t) to the reaction time f. In combination with isotope labeling, FIK has been successful in identifying the fragmentation mechanism of a variety of gas-phase ions. 

Isomerization of ionic structures is frequently observed. Several mass spectrometry techniques have been developed to identify the structure of the ion formed initially or its isomerized form. These methods are based on determination of the heat of formation, study of metastable ion spectra and CID spectra, experiments involving kinetic-energy-release measurements, isotope labeling, charge-permutation reactions, ion-molecule reactions, field ionization kinetics, and ab initio calculations. 

N. M. M. Nibbering, Mechanistic studies by field ionization kinetics. Mass Spectrom. Rev. 3, 445-477 (1984). 

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