The molecular ions of fhoro-and chlorotoluenes are shown to have undergone a ring-expansion rearrangement prior to fragmentation ; in contrast, the bronio-and iodo-toluenes do not undergo such rearrangement prior to fragmentation.IN the mass spectrometer, C,H,+ ions generated from various different precursors (C,H,X) invariably lose C2H2.1These ions have been shown by deuterium1 and carbon-1329 labelling to have scrambled the hydrogen and carbon atoms prior to C,H, expulsion.We have compared the spectra of various fluoro-, chloro-, bromo-, and iodo-toluenes with the spectra of the corresponding benzyl halides and halogenobenzenes to obtain information concerning rearrangement (if any) in the molecular ions of these compounds prior to loss of the halogen radical. Ionisation and appearance potentials of various ions were also determined.The mass spectra of 2-, 3-, and 4-chloro-toluenes are similar to one another; the molecular ion decomposes via loss of a hydrogen or, more favourably, a chlorine atom (Table 1). If no rearrangement occurs in the molecular ' Ref. 1, p. 496. Ref. 1, p. 488. ch. 10.
From a study of various specifically deuterated aliphatic ketones, it has been shown that internal hydrogen rearrangement (H/D scrambling) occurs at a rate which is slow on the time scale of the mass spectrometer. Single-bond cleavage processes show little evidence of H/D scrambling at 70 eV because the rate of fragmen-(3) .
The C4HB+ cations generated from CH,*CH,*CH,*CH,X systems have been shown to undergo complete scrambling of both the carbon and hydrogen atoms before methane loss with a rate constant in the region 105-106 sec.-l.The C4H8.+ radical-cations, generated from the same types of system, have been shown to undergo complete or almost complete hydrogen scrambling before methyl radical loss with a rate constant in the region 1 05-l Os sec.l.The C,H9+ cations generated from n-, iso-, s-, and t-butyl structures have isomerized completely, or almost completely, to ths same structure, or mixture of structures, before loss of ethylene or methane with the rate constants given above.
CambridgeTHE unambiguous identification of metastable transitions focusing ' technique,1$2 greatly clarifies interpretation of occurring in the first field-free region of a double-focusing the mass spectra of specifically deuteriated compounds. mass spectrometer, by means of the ' metastable de-Thus, Jennings was able to demonstrate hydrogen
An investigation of competing metastable transitions in the mass spectra of ethylene ketals RsRLC(OCH,), (where R, is a larger n-alkyl group than R,) has established that in most cases R, is lost with a lower activation energy than R,. This technique has also been applied to ketones R,R,C=O, to show again that R, is usually lost with the lower activation energy (thus supporting earlier data based on relative daughter ion abundances at the threshold). In the classes of compounds so far investigated, although [M+ -R,] ions are formed with lower activation energies than [Mf -R,] ions, the ion yield of [M+ -R,] ions is anomalously low from ions of high internal energy. Factors which may influence the [Mk -R,]/[M+ -RL] ratio of daughter ion intensities are examined. It is suggested that at the threshold [M+ -R,] and [M+ -R,,] ions may be formed with rearrangement, or from an electronic state that cannot be effectively populated from molecular ions of high internal energies.
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