Electron ionization rearrangement ions

If a charge exchange process, A + + B- A -f- B +, occurs when the distance between the two particles is large, we expect that no transfer of translational energy takes place in the reaction and that the same selection rules govern the ionization as in spectroscopic transitions. This means that if the molecule B is in a singlet state before the ionization, the ion B + will be formed in a doublet state after ionization of one electron without rearrangements of any other electrons, at least for small molecules. 

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9]... 

Electron ionization (25 eV) mass spectra of picolinic esters from Go-octadenoic acid and ante-isononadecanoic acid. The fragment ion at mfz 151 corresponds to a McLafferty rearrangement product. Reproduced (modified) from Harvey D.J., Biomed. Mass Spectrom., 9,33,1982, with permission. 

Sample ionization. Requirements for sample ionization are much more severe in MS/MS than in GC/MS. For MS/MS, the ionization method should create one ion for each component, and the structure of the ion should be the same as that of the neutral surrogate. Electron ionization usually does not fulfill these requirements, since the ions formed often include those from rearrangement reactions, and the degree of fragmentation is excessive. Chemical ionization provides the requisite single ion for each component of the matrix in the form of the quasimolecular ion (MFH)+. 

The formation of thiopyrylium (2) as a rearrangement ion has been invoked in the electron impact mass spectra of 2- and 3-alkylthiophenes (59CCC1602 88IZV905). The tendency toward the formation of 2, which represents the most abundant species, grows as the side-chain increases in length. Cation 2 has been also detected in the reaction zone of a C Hg/ CS2/H2 flame, by flame ionization/mass spectroscopy (84AJC511). 

The maximum residence times in many electron-impact instruments is generally considered to be about 10 sec. (Chupka, 1959) but field-ionization mass spectra are usually obtained after residence times of about 10 sec. and these contain no rearranged ions (Beckey al, 1969). To explain these observations it was suggested that field ionization residence times were too short to allow the slower rearrangement processes to occur and only reactions with low entropies of activation were observed. 

Field ionization spectra appear to yield greatly reduced numbers of rearranged ions because of the short time (10" sec.) between ion formation and collection (Beckey et al., 1969). High-resolution dual field-ionization/electron-impact sources are available commercially, so that mass spectra under the two conditions may readily be compared (Chait et al., 1969 Schulze et al., 1969). 

FIGURE 9.32 Electron ionization mass spectrum of methyl 2-methylpentanoate and McLafferty rearrangement occurring from the molecular ion leading to ion at m/z 88. 

FIGURE 9.35 Electron ionization mass spectrum of butyl propanoate and McLafferty rearrangement with double transfer of hydrogen atoms from the molecular ion. 

A book (B-71MS) and a review by Nishiwaki (74H(2)473) contain much information about the behaviour of pyrazoles under electron impact. The Nishiwaki review covers mainly the hydrogen scramblings and the skeletal rearrangements which occur. One of the first conclusions reached was that pyrazoles, due to their aromatic character, are extremely stable under electron impact (67ZOR1540). In the dissociative ionization of pyrazole itself, the molecular ion contributes about 45% to the total ion current thus, the molecular ion is the most intense ion in the spectrum. 

Diphenylthiirene 1-oxide and several thiirene 1,1-dioxides show very weak molecular ions by electron impact mass spectrometry, but the molecular ions are much more abundant in chemical ionization mass spectrometry (75JHC21). The major fragmentation pathway is loss of sulfur monoxide or sulfur dioxide to give the alkynic ion. High resolution mass measurements identified minor fragment ions from 2,3-diphenylthiirene 1-oxide at mje 105 and 121 as PhCO" and PhCS, which are probably derived via rearrangement of the thiirene sulfoxide to monothiobenzil (Scheme 2). 

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the ewdo-brosylate was considered normal, because its reactivity was comparable to that of cyclohexyl brosylate. Elaborating on a suggestion made earlier concerning rearrangement of camphene Itydrochloride, Winstein proposed that ionization of the ero-brosylate was assisted by the C(l)—C(6) bonding electrons and led directly to the formation of a nonclassical ion as an intermediate. 

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