Kinetic energy release measurements

Figure 2. Schematic view of reversed geometry double focusing mass spectrometer used for kinetic energy release measurements.

Despite its high heat of formation (AH = 224 kcalmoP ) the next higher homo-logue of 1, i. e. the cation radical of methylcyclopropane (107) appears to be formed as a substantial fraction of the C4H8 product ions generated from 1- and 2-chlorobutane (121, 122), 1-butanethiol (123), -butyl vinyl ether (124) and w-butyl formate (125), as evidenced by labelling studies, kinetic energy release measurements and the analysis of collision induced dissociations (Scheme 18). 

The differentiation of anomeric acyclonucleosides by FAB tandem mass spectrometry was reported (90MI1 91MI10). Regioisomeric compounds could be differentiated by kinetic energy release measurements [87-AQ(C)271 91RCM72]. Electronic structure, spectral properties, and acid-base and tautomeric equilibria of some adenosine acyclo derivatives were investigated (89MI5). 

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. 

An important example of the application of this method is seen for the case of ammonia. Referring to Figure 13, the measured average kinetic energy release of metastable (NH3)nH+ (n = 4-17) is seen to display a maximum value of... 

Figure 13. A plot of the measured average kinetic energy release < r> during the metastable unimolecular decomposition of (NH3)nH+, n = 4-17, as a function of cluster size. The technique involves use of the reflectron shown in Figure 2a. Taken with permission from ref. 2.

Figure 5. Kinetic energy release curve for 0+ atoms formed from (2 + 1) REM PI of 02 at 225 nm. The inset is a plot of the measured (FWHM) width of the stronger peaks versus the square root (sqrt) of their kinetic energy. Many of the measured peaks are actually two or more overlapped peaks, thus the width is often an upper limit. With this set of peaks the apparatus function W shows a 35-meV peak width at 1 eV, i.e., W = 35 /KE. With nonoverlapped peaks a value of 25V/KE is expected.

The El and CID mass spectra of CH2X2+ (X = Cl, Br and I) were also studied100. Large kinetic energy release (KER) values were measured for the fragmentation of the molecular ion to X2+ (X = Cl and Br), leading to the proposal that the CH2 elimination involves a bridged transition state (Scheme 17). 

Breakdown diagrams of CH4 and CD4 have been determined by a number of workers [131, 132, 797, 799, 877] and are explicable within the framework of QET. It was found [806] that there was no kinetic shift affecting the appearance energy of (CH3)+ from CH4. On the basis of translational energy releases measured with CH4 (and CD4), it has been suggested [798] that all rotational energy is available to assist the decomposition to (CH3)+ but that only 2 degrees of freedom contribute to formation of (CH2)t (from the molecular ion), cf. Sect. 2.3. 

Bv/B = mp/m[, the mass of the fragment can be determined. This method gives a better resolution than the MIKE method but does not allow measurement of the kinetic energy released. 

---