A. Maquestiau

Multi-step reaction pathways in the decomposition of ionised n-hexanoic acid

Abstract Metastable molecular ions of n-hexanoic acid ( 1 ) decompose unimolecularly to C2H5 and protonated methacrylic acid ( 5 -H+). Investigation of the mechanism reveals that 1) the branched cation radical 11 must be regarded as the essential intermediate in the course of the rearrangement/dissociation reaction and 2) the process commences with intramolecular hydrogen transfer from either C-3 or C-5 to the ionised carbonyl oxygen. Hydrogen transfer from C-4, which would correspond to the well-known McLafferty rearrangement, is of no importance in the C2 H5 elimination from 1 . The same conclusion applies for various alternative mechanisms, as for example a SR i type reaction, e.g. 1 + → 2 -H+. The gas phase chemistry of the cation radical of 1 , and in particular the hydrogen exchange processes between the methylene groups C -2 C -3 and C -5 C -6 , is in surprisingly close correspondence to the chemistry of alkyl radicals.

The resolution of charge-stripping peaks using charge exchange in a multi-sector mass spectrometer

Abstract The charge-stripping process, when studied in sector mass spectrometers, is often masked by the presence of interfering collision-induced fragmentation processes. This presents problems, for example, in interpreting E/2 spectra which are meant to be representative of the doubly charged ions formed by charge-stripping singly charged ions issuing from the ion source. A method is described for completely resolving the spectrum of doubly charged ions from interferences, which uses charge exchange and a further stage of energy(-to-charge) analysis. A simple modification to a double-focusing mass spectrometer is described for achieving this. Examples of fully “resolved” E/2 and charge-stripping spectra are given.

Identification of the transient benzonitrile n-sulfide by flash-vacuum pyrolysis and neutralization—reionization mass spectrometry

Abstract Using a combination of flash-vacuum pyrolysis and tandem mass spectrometry, it is shown that benzonitrile N -sulfide, 1 , is a stable molecule in the diluted gas phase. This result is confirmed by neutralization—reionization experiments performed on a beam of the corresponding radical cations.

Origine et structure d'ions C4H9O+ issus de la methylation de l'acetone par l'iodomethane ionise

Abstract Electron impact ionization of a mixture of acetone and iodomethane in a high-pressure ion source induces the protonation of acetone and also the methylation yielding C 4 H 9 O + ( m/z 73) ions. The analysis of the collisional activation spectra of these ions and appropriate deuterium labelled analogues indicates the formation of dimethylmethoxycarbenium ions, a . These ions are also formed by protonation of 2-methoxypropene; however, depending upon the experimental conditions, an isomeric methyl-2-propenyl-oxonium species, b , was also detected. Ion cyclotron resonance experiments show that the C 4 H 9 O + ions are formed by methyl cation transfer from the molecular ions of iodomethane; dimethyliodonium ions (CH 3 ) 2 I + were found to be unreactive. The gas-phase basicity of 2-methoxypropene has also been measured (206.2±0.2 kcal mol −1 ) and indicates preferential protonation of the double bond to give the carbenium ions a . These results are supported by ab initio molecular orbital calculations (3–21 G) which show that ion a is more stable than b by 18 kcal mol −1 . The proposed existence of hydrogen-bonded molecule pairs of the kind propyne-H + −methanol, c , is also supported by calculations.

A novel mass-spectroscopic technique for the investigation of vapour-phase tautomeric equilibria

Comparison of collision-induced dissociation–mass analysed ion kinetic energy (CID–MIKE) spectra of cations found by chemical ionisation ethylation with those of chemical ionisation protonation of fixed ethyl derivatives, allows identification of predominant tautomers in vapour phase equilibria.

Flash-vacuum pyrolysis of N-vinylbenzotriazoles: formation of N-phenylketenimines

A real-time analysis of the flash-vacuum pyrolysis products of 1-vinyl-, 1-(2-methylprop-1-enyl)-, and 1-styryl-benzotriazole (3), (16), and (18) has been performed by tandem mass spectrometry. In the 500–700 °C temperature range, these compounds lose nitrogen yielding the N-phenylketenimines (14), (17), and (19), respectively. At higher pyrolysis temperatures (3) gives indole (4)via isomerization of (14), whereas from (16) the secondary products of (17) are benzene and methacrylonitrile. In a preparative pyrolysis of (18) at 800 °C 2- and 3-phenylindole (9) and (10) respectively, have also been detected.

A new example of internal oxidation of remote groups in organic nitroaromatic ions

Intramolecular redox reactions occurring in ionized benzotriazoles and triazolopyridines bearing o-nitroaryl substituents on nitrogen produce ionized fulven-6-one and azafulven-6-ones.

Conversion of triazolides into oxazoles by flash-vacuum pyrolysis.

Abstract Flash-vacuum pyrolysis of azolides of 1,2,4-triazole affords 5-monosubstituted oxazoles.

Solution and gas-phase basicities of pyridines with bulky α-substituents

Steric hindrance to solvation of pyridinium cations is largest for two α-t-butyl substituents, less for one t-butyl and one phenyl, less for two α-phenyl groups, and least for one phenyl and one alkyl. The effect of α-phenyl groups is reduced by buttressing. Gas-phase proton affinities of the pyridine determined by dimer dissociation do not correlate with the solution pKa values. Steric accelerations of pyridinium dealkylation show fair correlation with solution pKa; reasons for discrepancies are discussed.

Activation collisionnelle de cations butyle formés par décomposition de divers precurseurs métastables

Abstract Collision-induced dissociation—mass-analysed ion kinetic energy (CID—MIKE) spectrometry has been applied to the characterisation of butyl cations produced by decomposition of some metastable precursor ions using a triple-sector ( E/B/E ) mass spectrometer. Significant differences are observed in the fragmentation when acylium ions are used to generate the isomeric butyl cations. The formation of various mixtures of t -butyl and s -butyl ions is proposed to explain these differences. The structure of butyl ions generated from metastable oxonium ions has also been investigated. The results indicate the predominant formation of t -butyl ions in this case.