Metastable ion peak

The mass spectra of methyl 3-deoxy-p-v-tkreo-pentopyrano-side, methyl 4-deoxy-j3-T>-thieo-pentopyranoside, and 5-deoxy-fi-D-xylo-furanoside are discussed and compared fragmentation paths are sufficiently different to allow identification on the basis of their mass spectra. On the other hand, the mass spectra of methyl 2- and 3-deoxy-5-O-methyl-f3-i>-erythro-pentofuranosides do not exhibit fragmentation differences. The mass spectra of 3-deoxy-l,2 5,6-di-O-isopropylidene -d-xylo - hexofuranose, 5- deoxy -1,2-0-isopropylidene-D-xy o-hexofuranose, and 6-deoxy-l,2-0-iso-propylidene-D-glucofuranose show prominent differences, even between the 5- and 6-deoxy isomers. The interpretation of the spectra was aided by metastable-ion peaks, mass spectra of DzO-exchanged analogs, and the mass spectrum of an O-isopropylidene derivative prepared with acetone-d6. 

On the other hand, a metastable-ion peak at m/e 88.1 (calculated, 88.0) is present in the mass spectrum of 11 (Figure 8) for the formation of m/e 129 from m/e 189, by loss of acetic acid. In the mass spectrum of the D20-exchanged analog, m/e 129 partially shifts to m/e 130 and partially stays at m/e 129. Metastable-ion peaks are also present at m/e 154.8 (calculated, 154.7) and m/e 97.3 (calculated, 97.3) for the loss of water from m/e 189 followed by the loss of ketene, to give an ion at m/e 129. Since m/e 171 from the loss of water remains at m/e 171, the loss of water must involve the hydroxyl hydrogens. Scheme 3 is an attempt to summarize this in terms of structures which are entirely... 

Here, I designates the ion core (NHJ in the case of ammonia) and L the clustering ligand (e.g. NH3). The intensity and width of the metastable ion peaks carry information on the internal energy of the parent cluster ions. 

In mass spectra of borabenzene metal complexes, ions of type [M(C5H5B)]+ are commonly observed [Co(CsH5B)]+ decays via loss of an uncharged C5H5B fragment as evidenced by the observation of a metastable ion peak (7). 

Note This explains the occurence of diffuse peaks due to metastable ion dissociations at fractional m/z values in the B scan spectra of B and EB instruments (Chap. 2.7.1). [83,84] In turn, the mass spectra obtained from BE instruments do not show any metastable ion peaks in normal operation. 

The major pathways for the fragmentation of kojic acid (81,5-hydroxy-2-hydroxymethyl-pyran-4-one), are shown in Scheme 14 support for each route was provided by the appearance of metastable ion peaks (67MI22203). An RDA cleavage followed by loss of a CH2OH radical produces ion (81a), m/e 69, the structure of which was substantiated by deuteration experiments. The ion at m/e 97 arises by extrusion of CO from the molecular ion and loss of HO- from the side chain structures (81b) and (81c) were proposed. Decomposition of [M]t occurs to give ethylene and an HC=0 fragment. The initial stage involves loss of a -CHO radical from the hydroxymethyl substituent, a process which has... 

A mass spectral study of 2-methyl-, 3-methyl- and 2,3-dimethyl-chromone (136), (137) and (138) has been reported (790MS345). In each case the molecular ion appears as the base peak, together with ions which correspond to [M-CO]-, [M-CHO]t, [RDA]t, [RDA + H]+ and [RDA-CO]t. Metastable peaks confirmed that the formation of [M-CHO]- occurs in two steps from [M]t. The reaction pathway for (136) and (138) is given in equation (2). In compound (137), [M-CHO]t is an abundant fragment ion (60%, cf. 35% for 136). That its generation occurs by more than one route is suggested not only by its high abundance, but also by the appearance of appropriate metastable ion peaks two pathways are operative (Scheme 17). 

For compound (151 R = Me or Ph), the base peak arises from an [M-HI]t ion. The molecular ion from the parent compound is not observed. Metastable ion peaks aided the elucidation of the fragmentation pathways which are outlined in Scheme 25. The [M—HI] ion (151a) may possess either a methylene pyran or an oxepin structure. Further decomposition of this ion occurs by loss of a hydrogen radical. Expulsion of a methyl radical from (151a) generates (151b) which decomposes as shown (Scheme 25). 

The molecular ion peaks of all the compounds studied are the most intense ones, while the m/z 147 and 148 peak intensities are negligible. It has been shown [1337] that metastable ion peaks (m/z 133, [M-NO]+) can be of help in distinguishing the five nitroindazole isomers, but in general their spectra are much alike. The fragmentation route to [M-NO]+ for the 4-nitro isomer is as follows (Scheme 3.70) ... 

The very short-lived ions (< 10 s) will decompose in the source of the mass spectrometer and will not be detected as such, while long-lived ions (> 10 s) are accelerated through the mass spectrometer to reach the detector intact. The ions of intermediate lifetime (10 5 to 10 6) are accelerated from the source with mass m but before reaching the mass analyser decompose to give a smaller ion of mass m. In the case of a magnetic sector instrument a broad metastable ion peak (m ) is observed at a non-integral m/z value given by the relationship... 

Figure 16.32 Metastable ion peak. Theoretical aspect of the three peaks constituting the metastahle transition given as an example. For the fragmentation m/z= mlz=ll a metastahle ion peak at m/z = 56.7 is observed (calcd. 56.5).

Vinyl cation analogues with two silicon atoms were suggested112 as products formed (in low intensity) upon electron bombardment of linear organopolysilanes. For example, the disilaethene 196 (and 210 R = Me) undergoes loss of Me (evidenced by a metastable ion peak) to generate 228 (reaction 89). 

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