Chrysostomos Wesdemiotis

An improved tandem double-focusing mass spectrometer for neutralization—reionization and collisional activation studies

Abstract A four-sector EBEB mass spectrometer is modified to conduct neutralization—reionization mass spectrometry (NRMS), MS/MS/MS, and MS/NRMS/CAD-MS experiments. Four separate collision chambers enable the instrument to perform, for example, neutralization, neutral dissociation, reionization, and collisionally activated dissociation (CAD). The independent differential pumping systems of the key collision chambers and the improved general pumping system reduce the background pressure by more than one order of magnitude, with > 95% of collisions occurring within the collision system. This increases the collision efficiency by ∼ 70% and minimizes the cross-contamination of collision gases between regions, and for neutral lifetime measurements insures that neutral dissociation or reionization by gas admitted to the second collision chamber occurs there. Redesign of the Hg neutralization chamber to include low-temperature Hg condensation provides further differential pumping and increases the reproducibility of neutralization at high Hg pressure. A field ionization chamber provides a capability for detecting neutrals in high Rydberg states.

Mass spectral evidence for the hydroxymethylene radical cation

Abstract Collisionally-activated decompositions show that ·CHO + H ions are stable, confirming theoretical predictions.

Massenspektrometrische untersuchungen zu dyotropen umlagerungen : III. Ether-spaltungen, silylen- und carben-eliminierung durch anchimere effekte von silylgruppen bei zerfällen ionisierter alkyl-(silylmethyl)-ether

Abstract Detailed mass spectroscopic investigation of a variety of structurally related alkyl silylmethyl ethers reveals some novel types of unimolecular decomposition of gaseous radical cations. Among these are ether cleavages induced by anchimeric assistance of silyl functions, elimination of divalent silicium species and some other unusual reaction pathways (e.g. carbene formation). An extensive comparison of thermally and electron impact induced processes is given and the influences of different structural parameters are discussed in detail. With the help of some recent techniques, as for instance collisional activation mass spectrometry, extensive defocusing experiments, low ionisation energy measurements (12 eV) and the investigation of deuterium labelled precursors ion structure and reaction mechanisms are established.

Carbon versus oxygen alkylation of gaseous nucleophiles

Abstract The reaction of gaseous ethyl cations with the ambident nucleophile 2,4-pentanedione produces o-alkylation, in agreement with the “hard and soft” acid-base principle.

Massenspektrometrische Untersuchungen zur Genese von [M—Me] [M-Et] [C7H7]+ und [C7H8]+ aus n-Butyl- und n-Pentylbenzol: Zeitabhängige Nachbargruppeneffekte, Isomerisierungen und kompetitive Wasserstoff-Umlagerungen / Mass Spectrometric Investigations on the Formation of [M-Me] [M-Et] [C7H7]+ und [C7H8]+ from w-Butyl and w-Pentyl Benzenes: Time Dependent Neighbouring Group Participation, Isomerization and Competitive Hydrogen Rearrangements

Abstract The details of the elimination of CH3· and C2H5· from the molecular ions of n-butyl and n-pentyl benzenes as well as the formation of C7H7+ and C7H7+ -have been established using a combination of different mass spectrometric techniques. Among these techniques are field ionization kinetics (FIK), collisional activation (CA), unimolecular decomposi-tion of metastable ions (MI), kinetic energy release determinations (Tkin)> appearance potential measurements (AP) as well as high resolution mass spectrometry. The appli-cation of these methods and the investigation of nine 13 carbon and eleven deuterium labelled n-butyl and n-pentyl benzenes clearly demonstrate that 1) the loss of methyl and ethyl from terminal positions of the alkyl chains are accompanied by an interaction with the phenyl ring and 2) the regioselectivity of these processes is time dependent. Contrary to previous conclusions it is shown that hydrogen transfer in the formation of C7H8+ -occurs via five-, six-and seven-membered transition states. The time dependence of the hydrogen exchange reaction preceding the alkene eliminations is discussed in detail.

Ringöffnung CO+- und COOCH3+·-substituierter Cyclopropane und CO-Eliminierung aus isomeren C3H5CO+-Ionen / Ring Opening of CO+ and COOCH3+· Substituted Cyclopropanes and CO Loss from Isomeric C3H5CO+ Ions

Abstract The non-decomposing molecular ions of methyl cyclopropanecarboxylate (14) are found to rearrange to ionised methyl but-3-enoate (15). For ions with sufficient internal energy to decompose, this isomerization is in competition with · OCH3 loss, via direct cleavage of the ester group. Collisional activation spectroscopy may be used to distinguish between the C3H5CO+ ions formed by · OCH3 loss from the molecular ions of 14, 15 and other isomeric precursors. Four distinct C3H5CO+ species (18-21) can be identified in this way; these C3H5CO+ ions may themselves decompose, via CO elimination. Consideration of the metastable peak shape for CO loss, in conjunction with collisional activation spectroscopy on the resulting C3H5+ -ions, leads to two main conclusions, (i) Two C3H5+ ions (22 and 27) exist in potential energy wells. The very narrow metastable peaks for CO loss from 19 and 21 (leading to 22 and 27, respectively) show that these processes are continuously endothermic. In contrast, CO loss from either 18 or 20 gives rise to much broader metastable peaks. This suggests that rate-determining rearrangement of the incipient C3H5+ cations, to a more stable isomer, occurs prior to decomposition, (ii) Elimination of CO from the [M- · OCH3]+ fragment of 14 gives rise to a composite metastable peak, thus indicating the occurrence of two competing channels for dissociation. These channels are assigned to CO loss from 18 (larger kinetic energy release) and CO loss from 19 (smaller kinetic energy release).

Elf stabile C5H9+-Kationen in der Gasphase. Zur dissoziativen Ionisierung von 31 isomeren C6H9Br-Verbindungen / Eleven Stable C5H9+ Cations in the Gas Phase. On the Dissociative Ionization of 31 Isomeric C5H9Br Compounds

Abstract Collisional activation mass spectrometry (CA) reveals the existence of 11 stable C5H9+ cations in the gas phase, e. g. the substituted allyl cations a, b, c, d,and e, the sub-stituted vinyl cations f, g, h, and i, the methyl cyclobutyl cation j and the cyclopentyl cation k, respectively. The ethyl substituted allyl cation a is formed via dissociative ionization of the isomeric precursors 1, 3, 4, 5, 18, 19, 20, 22, and 28 by means of various mechanistic processes, whereas the 1,3-dimethylallyl cation b is generated from both 2 (by allylic cleavage) and in part from the stereoisomeric cyclopropan derivatives 25, 26 and 27. 6+ -gives a mixture of the vinyl cations 1 and g. From 13 and 14 the main product generated is the 1,2-dimethylallyl cation d, which is formed directly from 11 and also by quite complicated processes from 13, 14 and to a certain extent from 25, 26 and 27. The dissociative ionization of 9, 15, 16, 21, 24 and (in part) 23 give rise to the formation of the substituted vinyl cation h. Decomposition of 23+ • results not only in formation of h but generates also the 1,1-dimethylallyl cation e. From 29+-and 30+-both the methylcyclo-butyl cation j and cyclopentyl cation k are produced, whereas the isomeric precursor 28 gives mainly the substituted allyl cation a and a second, as yet, unidentified C5H9 cation. In general, it can be stated that the gas phase chemistry of cation radicals of substituted cyclopropanes is characterised by multistep-reactions, commencing with spontaneous ring opening. The so formed intermediates undergo various rearrangements (including hydrogen and alkyl shifts) prior to expulsion of Br·. Direct elimination of Br· from intact cyclopropan-like structures, followed by ring opening of the intermediate cyclopropyl cation, cannot compete with the above-mentioned multistep-sequences.

Methyl-Eliminierung aus dem metastabilen Homoadamantan-Radikalkation / Methyl Loss from Metastable Homoadamantane Cation Radical

Abstract It is demonstrated that methyl loss from ionized homoadamantane (1) yields exclusively the 1-adamantyl cation (4); there is no experimental evidence for the formation of the secondary adamantyl cation (5). From both model calculations and the investigation of [4-13C]-homoadamantane (1a) and 1(13C-methyl)adamantane (2a) it is concluded, that 24% of the metastable homoadamantane cation radicals dissociate after one isomerization (1→2); the remaining 76% are able to undergo at least a second (degenerate) isomerization cycle (2→1→2) prior to methyl loss. 78% of metastable 1-methyl-adamantane cation radical, obtained upon direct ionization of the corresponding neutral hydrocarbon, dissociate directly, whereas the loss of methyl from the remaining 22% is preceded by an isomerization 2 →1 →2.

Experimental evidence for the existence of gaseous cyclobutyl cation

It is shown from kinetic energy release measurements and collisional activation studies that Br· loss from ionised homoally bromide, cyclopropylmethyl bromide, and cyclobutyl bromide in the gas phase leads to formation of cyclobutyl cation.