Monique Barbieux-Flammang

Definitive characterization of some C5H5N·+ and C6H7N·+ radical cations by associative ion–molecule reactions

Abstract Isomeric C 5 H 5 N ·+ and C 6 H 7 N ·+ radical cations were allowed to interact with various neutral reagents in the quadrupole collision chamber of a six sector magnetic deflection type tandem mass spectrometer. Using CH 3 SSCH 3 , CH 3 OH, or H 2 O, iso-C 3 H 7 I, and tert-C 4 H 9 NC as substrates, specific associative ion–molecule reactions occur whose products were analyzed in a subsequent high energy collision induced dissociation experiment. Differentiation between ionized pyridine and its 1,2-H shift isomer is readily achieved with each of the four substrates as the latter C 5 H 5 N ·+ ion exhibits a radical-type reactivity characteristic of its distonic character. An unambiguous structure characterization of the C 6 H 7 N ·+ ions, namely ionized 2-methylpyridine, the N -methylene pyridinium ylide ion, the 2-methylene-1-hydropyridine ion, and the N -methyl-2-dehydropyridine radical cation, is realized when all of the above substrates are used in the analysis.

Formation of thiophenethiols by flash vacuum pyrolysis of1,6,6aλ4-trithiapentalenes

The thermal behaviour of 1,6,6aλ4-trithiapentalene 3 and some methyl-substituted derivatives 5–7 has been investigated using a combination of flash vacuum pyrolysis (FVP), tandem mass spectrometry (MS–MS) and matrix isolation IR spectroscopy. The main products of the fragmentation (losses of CS and/or CH2CS) are shown to be thiophene-3-thiones (or the thiol tautomers) which are also shown to be readily available by direct sulfuration of thiophenes in chemical ionization (CS2 reagent gas) conditions.

Ionized aniline and its distonic radical cation isomers

Abstract A multi-sector tandem mass spectrometer fitted with a radiofrequency (rf)-only quadrupole collision cell (Qcell) has allowed fast and unambiguous identification of dehydroanilinium ions, the distonic isomers of ionized aniline. These ions were prepared by collisional deiodonation of protonated iodoanilines previously generated by chemical ionization (CI) or even better by liquid secondary ion mass spectrometry (LSIMS) conditions. Ab initio quantum chemical calculations were also used to probe the protonation of aniline and halogeno-anilines, X–C6H4–NH2 ( X = F , Cl, Br and I) and the relative stabilities of ionized aniline and its distonic radical cation isomers. Proton affinities (PAs) of anilines at nitrogen and carbon sites were found rather dependent on the nature and the position of the substituents.

Distonic isomers of ionized benzaldehyde

Dissociative ionization of phthalaldehyde 1 and collisional deiodination of protonated isomeric iodobenzaldehydes 2–4 were attempted in searching for the possible production of distonic isomers (b–d) of ionized benzaldehyde a. These distonic species were initially produced in these experiments, but most of them readily isomerized to ionized benzaldehyde. Density functional theory B3LYP/6-31+G(d,p) calculations indicate that the distonic ions are surprisingly low energy species being 50–60 kJ mol −1 above a, but kinetically metastable with respect to b → a isomerization that has an energy barrier of only 70 kJ mol −1 . It is also suggested that low energy hydrogen migrations or a benzyl/tropylium isomerization likely precede collisional deiodination of protonated iodobenzaldehydes. (Int J Mass Spectrom 217 (2002) 65–73)

Ion-molecule reactions between ionized nitrile oxides and neutral nitrile

Abstract The ion–molecule reactions of ionized nitrile oxide, RCN + O , with several neutral nitriles have been studied using both tandem mass spectrometric techniques and ab initio molecular orbital calculations. Ionized oxygen atom transfer as well as a formal substitution of nitric oxide by the neutral reagent in the radical cation were the main processes. Whereas the former reaction yields the corresponding ionized nitrile oxide, the second process gives an even electron species tentatively ascribed, following high-kinetic energy collisional activation experiments, to an aromatic azirinyl cation. All the experimental data point to a two-step reaction sequence where the primarily formed intermediate ions competitively dissociate by the loss of nitrile or of nitric oxide respectively giving nitrile oxide ions and azirinyl ions. From a theoretical point of view, the mechanism of the simplest reaction HCNO + +HCN→ cyclo -HCCHN + +NO has been explored at the MP2/6-31G(d) level of theory. The most favorable reaction profile involves the formation of a CN bond between the positively charged carbon atom of HCNO + and the nitrogen atom of cyanhydric acid giving an HCNO + /HCN intermediate which isomerizes into an ionized nitrosoazirine before losing NO .

Collisional activation of protonated C-halogenopyrazoles

Abstract Collisional activation of protonated 3-halogenopyrazoles (X–Pz, X=Cl, Br and I) in the high or low translational energy regime induced an intense loss of X giving C 3 H 4 N 2 + radical cations whose structure depends on the nature of the halogen. Protonated 3-I–Pz generated thus ionized pyrazole a , whereas protonated 3-Cl–Pz was a precursor of an isomeric species ascribed to a dehydropyrazolium distonic structure b . A mixture of C 3 H 4 N 2 + ions was formed in protonated 3-Br–Pz. B3LYP/6-31++G(d,p) computations confirmed a regiospecific N 2 -protonation, and a low energy content of the distonic ions b or c (50 kJ mol −1 above a and lying in deep energy wells). Two competitive C–H and C–X bond cleavages were invoked to explain the contrasting behaviour of various protonated X–Pz under dehalogenation conditions.

Dehalogenation of protonated C-halogeno-1,2,4-triazoles: synthesis of new heterocyclic carbenic and ylid radical cations and contrasting behaviour of collision gases ☆

Abstract C(3,5)-halogeno-1,2,4-triazoles, protonated under chemical ionization conditions, are found to undergo easy dehalogenation upon 8 keV collisional activation conditions, provided the collision gas is oxygen, not helium. The ions produced under these reactions are demonstrated to be five-membered cyclic carbenic ions or ylid ions, isomers of more conventional molecular ions of 1,2,4-triazoles. The same unconventional radical cations can also be produced in the low kinetic energy regime (∼20–30 eV) if the halogen is bromine, not chlorine. These conclusions were derived from tandem mass spectrometric measurements (collisional activation, neutralization–reionization, and specific ion molecule reactions) performed on a hybrid tandem mass spectrometer of sector–quadrupole–sector configuration. Quantum chemical calculations using the density functional theory (DFT) at the B3LYP/6-31G(d,p) + ZPE level were also carried out on the protonated C-halogenated-1,2,4-triazoles in both singlet and triplet states and their fragmentation products (halogen = Cl and Br). Calculated results suggest that the dehalogenation, occurring when oxygen gas was employed, is likely to arise from an excitation of protonated species into their lowest-lying triplet state prior to dissociation. Ionization energies and proton affinities of triazoles were also evaluated.