Metastable peaks

With 2-methyl- and 2,4-dimethylthiazole, the methyl thiirenium ion (m/e 72) is obtained, which can easily lose a hydrogen radical to give the ml ell ion (confirmed by the metastable peak). This latter can rearrange by ring expansion to give the thietenyl cation whose structure was confirmed in certain spectra by the presence of a metastable peak corresponding to the decomposition of the m/e 71 ion to give the thioformyl cation m/e 45, probably by elimination of acetylene. 

As in the case of thiazole and the alkylthiazoles, cleavage of the thiazole ring takes place at the 1,2 and 3,4 bonds, confirmed by a metastable peak. The other important peaks result from fragmentation of thiirenium ion, in the case of 4- and 5-phenylthiazole and of the phenyl ring. These latter are generally present in the spectra of all comptmnds with benzene-ring substituents, they occur at m/e 77, 76, 75, 51, 50, 39 (124). The ion m/e 45 (HCS" ) is always present. 

Metastable Peaks. If the mass spectrometer has a field-free region between the exit of the ion source and the entrance to the mass analyzer, metastable peaks m may appear as a weak, diffuse (often humped-shape) peak, usually at a nonintegral mass. The one-step decomposition process takes the general form ... 

For example, a metastable peak appeared at 147.9 mass units in a mass spectrum with prominent peaks at 65, 91, 92, 107, 108, 155, 172, and 200 mass units. Try all possible combinations in the above expression. The fit is given by... 

Although the conventional mass spectra of the five C- nitro derivatives of indazole are nearly identical, the corresponding metastable peak shapes associated with the loss of NO-can be used to differentiate the five isomers (790MS114). The protonation and ethylation occurring in a methane chemical ionization source have been studied for a variety of aromatic amines, including indazoles (80OMS144). As in solution (Section 4.04.2.1.3), the N-2 atom is the more basic and the more nucleophilic (Scheme 5). 

In magnetic-sector instruments, metastable ions are normally observed as small broad peaks. However, in GC/MS the analyst looks only at centrioded (processed) data thus, metastable peaks are not obvious and generally appear as part of the background. Metastable ions, when observed, can be used to link specific product and precursor ions. 

OH elimination from ortho substituted aldoximes 179 (X = CH2, NH, O) may be at least partially the result of a hydrogen migration/cyclization/elimination process, whereby the heterocycles 182 are formed72 (46). A metastable peak shape analysis, the investigation of 2H-labelled derivatives and the study of positional isomers indicate that in addition to 182 the protonated isocyanide 183 is formed via a mechanism which is not fully understood. However, it is known that the generation of 183 occurs without any detectable interaction with the XH ortho substituent. 

Metastable Peaks. If the mass spectrometer has a field-free region between the exit of... 

Registration of a metastable ion in the spectrum is rather useful, as it confirms realization of a certain fragmentation reaction. The fragmentation schemes are considered to be true if corresponding metastable peaks are detected. On the other hand, metastable peaks deteriorate spectral resolution. Depending on the amount of energy released, the forms of the metastable peaks may be quite different. These peaks are eliminated from the spectra as part of the computer deconvolution process. 

Holmes, J.T. Terlouw, J.K. The Scope of Metastable Peak Shape Observations. Org. Mass Spectrom. 1980,15, 383-396. 

Baldwin, M.A. Derrick, P.J. Morgan, R.P. Correction of Metastable Peak Shapes to Allow for Instrumental Broadening and the Translational Energy Spread of the Parent Ion. Org. Mass Spectrom. 1976,11, 440-442. 

Evers, E.A.I.M. Noest, A.J. Akkerman, O.S. Deconvolution of Composite Metastable Peaks a New Method for the Determination of Metastable Transitions Occurring in the First Field Free Region. Org. Mass Spectrom. 1977, 72, 419-420. 

Astonishingly, the study of the mechanism of formaldehyde loss from anisole revealed two different pathways for this process, one involving a four- and one a five-membered cyclic transition state (Fig. 6.37). [129] The four-membered transition state conserves aromaticity in the ionic product, which therefore has the lower heat of formation. Prompted by the observation of a composite metastable peak, this rather unusual behavior could be uncovered by deconvolution of two different values of kinetic energy release with the help of metastable peak shape analysis (Chap. 2.8). 

Fig. 6.37. Energetics of formaldehyde loss from anisole. The inset shows the composite metastable peak due to two different amounts of kinetic energy release. Adapted from Ref. [129] with permission. American Chemical Society, 1973.

Sequential cleavage of the initially formed fragment cation accounts for most of the other major peaks in the spectrum each of these processes is supported by the observation of metastable peaks. Alternatively, the molecular ion can break down by elimination of an ArCHj- fragment derived from C(2) this route generally gives rise to low-intensity peaks (<5%), but can be important in certain cases [e.g., for Ar = 1-naphthyl, (M-ArCHj) = miz 207 (41%)] (82MI1). 

Thiocoumarins characteristically lose carbon monoxide from their strong molecular ion in the mass spectrum, thus forming a benzo[Z>]thiophene cation, invariably accompanied by a large metastable peak. Similar observations are recorded for selenocoumarins. ... 

Figure 16.28—Metastable peaks. The figure shows the three masses involved in a metastable transition. The relationship between these three masses is given in the above text.

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