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It is dcnionstratcd in mass spcctromctry that alkylamincs substitutcd at C2 and containing weak internal cncrgy isorncrizc in thc gas phasc into intermediary ions composcd of ionizcd cyclopropanes co~nplexcd with an ammonia niolcculc. This process is induced by a C3 hydrogen shift on thc nitrogen atom.. Aftcr opening of thc cyclopropanc, thc dissociation of thcsc complcxcs lcads to rn/z 44, 58. and 72 ions having [CH3(CH2),,CH NHZ] structures. Thc nicclianisni of thcir formation is dcmonstratcd by thc MlKE spectra of I3C and deutcriurn labcllcd compounds. Thc cxpcrirncntal rcsults arc in good agrccment with thosc described by Gross e! ell.. who studied thc fragmcntation of thc complcxcs fornicd during thc reaction bctwccn substitutcd ionizcd cyclopropancs and ammonia in thc gas phasc.

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The metastable dissociation of the methoxymethyl cation and a number of its deuterium and 13C variants was examined using a reverse-geometry double-focusing mass spectrometer. The loss of methane from the methoxymethyl cation clearly showed a composite peak shape which, when deconvoluted, revealed a bimodal kinetic energy release distribution in the resulting formyl cations. Labelling experiments revealed that the two carbon atoms and all hydrogens become equivalent on the time-scale of the unimolecular dissociation lifetime of the decomposing ion. A small deuterium isotope effect was found which can be rationalized on the basis of zero point energy effects. The bimodal kinetic energy release distribution was shown, with the aid of a four-sector instrument, to be due to the production of both formyl cation (with a large kinetic energy release) and isoformyl cation (with a much smaller kinetic energy release). The methoxymethyl cation was also prepared with a precisely defined amount of internal energy in a Fourier transform ion cyclotron resonance (FTICR) spectrometer by the reaction of methyl cation with formaldehyde. Experiments with I3C and deuterium labelling revealed that the dissociation to formyl cation of the methoxymethyl cations formed in the low-pressure FTICR cell by reaction of methyl cation with formaldehyde is accompanied by complete scrambling of the carbons and incomplete scrambling of the hydrogens. Ab initio calculations were carried out which identified and characterized each of the stable minima and transition states for the appropriate reactions. The calculations were fully consistent with the mechanism deduced on the basis of the experimental data.

Radical cations derived from the ethers ROCH,CH,OR (R, R = H, CH, , C,HJ were studied, since Bdistonic oxonium ions are often prepared from ionized ethers of glycol. The first step in the fragmentation is a 1,Stransfer of an a-hydrogen to oxygen of a terminal alkoxy group leading to a Wistonic oxonium ion. This step is thermoneutral and reversible in the ROCH,CH,OH radical cations and exothermic and irreversible in the dialkyl ether radical cations. Depending on R and R', these Sdistonic oxonium ions fragment by three reactions: the loss of an alcohol or a water molecule, the formation of a /Idistonic oxonium ion 'CH2CH20(H)+R and a 1,QH migration between carbon atoms. Competition between these processes is discussed.

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La fragmentation en spectrométrie de masse par ionisation chimique de composés bifonctionnels peut conduire à la formation d'ions cycliques.