Seymour Meyerson

Organic Ions in the Gas Phase. IV. C7H7+ and C5H5+ Ions from Alkylbenzenes and Cycloheptatriene

Electron‐impact studies have led to the proposal that the C7H7+ ion derived from toluene is not benzyl but tropylium. The mass spectra of toluene‐α‐C13 and cycloheptatriene confirm this proposal. Energy considerations indicate that toluene and related aromatic hydrocarbons yield the same kind of C7H7+ ion as cycloheptatriene, and that formation of this ion from toluene and cycloheptatriene proceeds through the same intermediate. A metastable peak, labeling, and correlations indicate that C5H5+ ions from n‐alkylbenzenes are derived in part from a C7H8+ precursor, and not exclusively from C7H7+.

Correlations of Alkybenzene Structures with Mass Spectra

Correlations of alkylbenzene structures with mass spectra show clearly that structural features determine the products of dissociation by electron impact. The most-abundant ions derive from cleavage of bonds β to the benzene ring. Increase in length of side-chains promotes: (a) decomposition of the parent ion, (b) cleavage of carbon-carbon bonds in competition with carbon-hydrogen bonds, (c) cleavage of β bonds in competition with a bonds, and (d) hydrogen migration accompanying bond cleavage. Side-chain branching, number, and position on the benzene ring help fix the product distribution. The factors that govern dissociation of alkylbenzenes apply broadly to the dissociation of other arenes and related heterocyclic compounds. A scheme for the identification of alkylbenzenes, based upon the correlations, has proved effective in analyzing samples that did not yield readily to other techniques.

Multiply Charged Organic Ions in Mass Spectra

Reports and speculation about multiply charged organic ions in condensed phases have appeared in the recent literature. The occurrence and nature of such ions in the gas phase, as revealed in mass spectra, seem relevant. Triply charged ions have been detected in the spectra of 55 compounds; they are evidently far more common that has been generally recognized. Despite the known susceptibility of doubly charged ions to disruption to two singly charged ions, 10 instances are now known in which a metastable peak furnishes evidence for decomposition of a doubly charged ion to a neutral particle and a different doubly charged ion. Relatively stable structures are possible despite the presence of two or three charges in a molecule.

Effects of Isotopic Labeling and Molecular Structure on Probability of Ionization by Electron Impact

Isotope effects have been reported on probability of ionization of molecules by photons, by excited rare‐gas atoms, and by beta rays. Such effects furnish evidence that ionization occurs largely as a secondary process in competition with nonionizing processes following primary excitation to an energy level above the ionization potential. Measurements of total ion current produced in a mass‐spectrometer source show similar effects for the case of ionization by 70‐V electrons. Experimental support has been obtained also for the corollary that probability of ionization depends upon molecular structure and is not simply, as has been postulated, a constitutive property of the atoms.

Mechanisms of Formation and Reactions of arynes at high Temperatures

Publisher Summary This chapter reviews the mechanisms of formation and reactions of arynes at high temperatures. Arynes are commonly invoked as intermediates for a whole host of reactions in solution. The formation of arynes from aromatic anhydrides involves reactions of benzyne with benzene, reactions of benzyne with deuteriated benzenes, arynes from aromatic anhydrides other than phthalic, reactions with chlorinated benzenes, reactions with pyridine, reactions with thiophene and benzothiophene, reactions of tetraphenylbenzyne from tetraphenylphthalic anhydride. The formation and mechanism of benzyne from o-sulphobenzoic anhydride is also discussed. o -Sulphobenzoic anhydride possesses potentially two good leaving groups, sulphur dioxide and carbon dioxide, and could yield benzyne on pyrolysis. The chapter also describes the mechanism of benzyne from acetylene. The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state and polymerization of monomeric or dimeric acetylene biradicals.

Mass spectrometry of biosynthetically labeled ricinine

Abstract The high sensitivity and precision made possible by modern instrumentation enables mass spectrometry to be used effectively even at the low isotopic enrichment levels readily attainable in studies of biosynthesis. A particular virtue of mass spectrometry for studies of reaction paths in biosynthesis, as elsewhere, is the ability to locate a label within a molecule without prior wet-chemical degradation. However, the extraction of such information is contingent on prior knowledge of ionization-dissociation mechanisms induced by electron impact. Preparation of the specifically labeled compounds needed to define these mechanisms may be greatly simplified by employing biosynthesis if the pertinent biosynthetic pathways are well enough established. Thus, findings from studies of ricinine from labeled nicotinamide, formate, and acetate furnished the necessary basis for mass-spectral interpretation to permit use of label-retention measurements on an appropriate fragment ion to determine the position in the ricinine molecule of nitrogen administered as formamide- 15 N. Ricinine isolated 12 hr after injection of the formamide contained the label only in the ring. After 24 hr, the label was found in both the ring and the nitrile group in proportions of about 3:1.

Organic Ions in the Gas Phase. XV. Decomposition of n‐Alkanes under Electron Impact

Ionic decomposition reactions of n‐alkanes under electron impact have been explored by studying the mass spectra of multiply deuterated undecanes and dodecanes and of octane‐1‐d. Label retentions in fragment ions in 70‐V spectra were supplemented by voltage dependence measurements and by metastable peaks. Alkanes appear to break down mainly by initial nonselective cleavage of carbon—carbon bonds, followed by further stepwise decomposition of the primary products, with intervening energy‐releasing rearrangements to give a characteristic product distribution.

Reactions of Δ2-unsaturated esters and ketones and of 3-chloroalkanoates triggered by γ-hydrogen migration under electron impact: a photochemical analogy

Abstract Photolytic γ-hydrogen migration and β-cleavage in valerophenone, which together comprise the well-known Norrish Type-II dissociation, have recently been shown to occur in two distinct steps. In view of the apparently identical mechanisms with which carbonyl compounds undergo this reaction photochemically and in the mass spectrometer, this finding implies that the two events may also occur stepwise under electron impact. Reaction sequences initiated by γ-hydrogen migration, but not involving β-cleavage, can account plausibly for puzzling features of the mass spectra of Δ2-unsaturated esters and ketones and of esters of 3-chloroalkanoic acids.

Supplemental Measurements in Probing Electron Impact-Induced Decompositions via Mass Spectra of Labeled Compounds

The value of isotope labeling in probing the chemistry underlying mass spectra is well established. The usefulness of this approach and the reliability of conclusions reached thereby can be greatly increased by checking dependence of label retentions in fragment ions on operating conditions. For example, varying the ionizing voltage, source temperature, or repeller potential alters the relative contributions of the two competing processes by which methylcyclopentane loses a methyl radical to form isomeric C5H9+ ions. The ions resulting from the two processes are derived from different parts of the original molecule and so can be distinguished by appropriate labeling.

EFFECT OF ELECTRON ENERGY ON SOME ELECTRON-IMPACT PROCESSES

Decomposition processes induced by electron impact have been inferred from appearance potentials and from isotopic distributions of fragment ions in mass spectra of labeled compounds. Implicit in such inferences is an assumption that the paths by which the fragment ions are formed are the same near the appropriate appearance potentials as at the higher electron energies usually employed. For five ions that were the subjects of earlier studies—C7H7+ from two deuterated p‐xylenes and C5H5+ from two deuterated toluenes and a deuterated benzyl chloride—isotopic distribution has been found to be essentially independent of electron energy. These results support the assumption made in earlier studies.