Rearrangements in chemical ionization mass spectra

Rearrangements reported in the literature for positive ions formed by chemical ionization are briefly reviewed, with particular emphasis on illustrative examples of hydrogen and skeletal rearrangements.     

Stereochemical effects in mass spectrometry: Part 8-chemical ionization mass spectra of cyclic glycols and monosaccharides using formaldehyde dimethyl acetal as reagent gas

The chemical ionization mass spectra of cyclic glycols and monosaccharides using formaldehyde dimethyl acetal as reagent gas have been studied. In the gas phase, the stereospecific reaction of methylene acetal formation was observed. From the relative abundances of the characteristic ions the stereoisomers of these compounds may be definitely distinguished.     

Stereochemical effects in mass spectrometry. 3—Detection of chirality by chemical ionization mass spectrometry

It was found that the chemical ionization mass spectra of the enantiomers of some α-amino acids and α-hydroxy acids could be definitely distinguished when a chiral compound, l-amyl alcohol, was used as a component of the reagent gases. The relative abundances of some characteristic ions produced through ion-molecule reactions in the gas phase were much higher in the D isomers than in the L isomers. The results were highly reproducible. This approach proved to be a convenient way for the detection of the chirality of these compounds by chemical ionization mass spectrometry.     

Isobutane chemical ionization mass spectra of unsaturated alcohols

Analysis of the isobutane chemical ionization mass spectra of hexenols, cyclohexenols and various syn/anti pairs of bicyclic and tricyclic homoallylic alcohols shows that: (i) the spectra of the allylic alcohols are dominated by [M + H – H₂O]⁺ and [M + C₄H₉–H₂O]⁺ ions and contain traces of [M + H]⁺ ions; (ii) [M + H]⁺ ions are prominent in the spectra of acyclic and certain cyclic homoallylic alcohols; and (iii) [M + H]⁺ ions dominate the spectra of other acyclic unsaturated alcohols. The [M + H]⁺ ions may result from either: (a) protonation of the hydroxyl group, followed by a very rapid intramolecular proton transfer from the protonated hydroxyl group to the carbon–carbon double bond or internal solvation of the protonated hydroxyl group by the carbon–carbon double bond; and/or (b) direct protonation of the carbon–carbon double bond with significant internal solvation of the resulting carbocation by the hydroxyl group, which may lead to carbon–oxygen bond formation to give a protonated cyclic ether. The consequences of placing various geometric constraints on the possible intramolecular interactions between the hydroxyl group and the carbon–carbon double bond in unsaturated alcohols are explored.     

Direct exposure chemical ionization mass spectra of explosives

Mass spectra of explosives, including TNT, tetryl, nitroglycerin, PETN and RDX have been recorded by direct exposure chemical ionization with isobutane as reagent at source temperatures of 50–100°C. The mass spectra contain major [MH]⁺ ions, adduct ions and some fragment ions. The configuration of the relative abundances of these ions has been found to be a function of temperature and source pressure. Maximum [MH]⁺ ion abundance has been obtained at source pressures much lower than normal chemical ionization pressures.     

Stereochemical effects in mass spectrometry: 2—Chemical ionization mass spectra of some cyclic glycols and mono- and di-saccharides using trimethyl borate as reagent gas

The chemical ionization mass spectra of cyclic glycols and mono- and di-saccharides using trimethyl borate as reagent gas have been studied. In the gas phase, the trimethyl borate ions react stereospecifically with molecules of cis-cyclic glycols to form characteristic ions, from which the stereochemical isomers of 1,2-cyclopentanediols, 1,2-cyclohexanediols and mono- and di-saccharides can be definitely distinguished.     

Negative chemical ionization mass spectra of polycyclic chlorinated insecticides

Negative ion mass spectra obtained under chemical ionization conditions (NCI) employing methane, isobutane or methylene chloride as the enhancement gas are presented for a series of chlorinated polycyclic insecticides. All of the compounds examined except 1-hydroxychlordene yielded molecular anions of substantial relative abundance (6 to 39%). The most significant features of the spectra are the prominent peaks at masses greater than that of the molecule ion formed via ionmolecule association reactions. Peaks representing association of the parent molecule with ionic species such as H?, O?, OH?, Cl?, H₂OCl?, HCl₂?, ClO? and Cl₃? were observed in some cases. The base peak in all spectra was associated with the isotopic group of the [M + Cl]? on if contributions from other negative, even electron ions of low mass values present in high concentrations (Cl?, H₂OCl? Cl₂? and HCl₂?) are neglected. Fragmentation processes were limited to elimination reactions involving loss of combinations of the even electron neutral species H, Cl and HCl. In addition, fragmentation resulting from a nucleophilic radical displacement of Cl by O? from the parent molecule was observed in all cases except 1-hydroxychlordene when the source was modestly wet (methane as reactant gas). NCI mass spectra of polycyclic chlorinated pesticides are reproducible, intense, interpretable in terms of classical carbanion chemistry and thus may have important analytical utility, particularly when used in conjunction with positive electron-impact and chemical ionization mass spectral methods and selective use of different enhancement gases.     

Electron ionization and chemical ionization mass spectra of pyridinium and isoquinolinium ylides

Electron ionization (EI) spectra and both positive and negative chemical ionization (CI) spectra have been obtained for four isoquinolinium ylides and two pyridinium ylides. Electron transfer reactions dominate the CI mass specra. The base peak in negative chemical ionization is the [M]^?· ion, formed by electron capture. In the positive methane CI spectra the molecular ion, [M]^+·, is relatively more intense than [MH]⁺ showing electron transfer to be the main positive ionization process. In the positive ammonia CI spectra, proton transfer to give [MH]⁺ is the main ionization process, but electron transfer is also observed. The EI spectra show fragmentations in which the aromatic nitrogen moiety retains the charge and fragmentation is by loss of radicals or small neutral molecules from the side-chains. Radical driven reactions are proposed to explain these spectra.     

Positive chemical ionization mass spectra of polycyclic chlorinated pesticides

Methane chemical ionization (CI) mass spectra for a series of ten polycyclic chlorinated insecticides and metabolites have been examined. In all cases except heptachlor epoxide the base peak corresponded to elimination of Cl, or OH from the molecule ion. In the spectrum of heptachlor epoxide the [M + H]⁺ and [M ? Cl]⁺ clusters were of approximately equal intensity. The CI spectra were remarkably simple, invariably less complex than the corresponding electron-impact (EI) mass spectra and the intensity of the ions with high information content, e.g. [M ? CI]⁺ was uniformly high. All of these features are important to the analytical potential of these studies. Retro Diels-Alder (RDA) fragments were observed for the chlordanes, aldrin, isodrin, nonachlor and heptachlor epoxide. The reported preliminary data suggest that the relative intensity of RDA ions in CI mass spectra may be useful in establishing molecular configurations.     

Substituent effects in the field ionization mass spectra of acetophenones

Substituent effects in the electron-impact and field ionization mass spectra of acteophenones are compared. No Hammett correlation could be found for the fragments formed on the surface of the field anode. A straight line log z/z0 versus σ⁺ was observed, however, for the metastable decomposition of para-substituted acetophenones (and also some benzophenones). Meta-substituted compounds did not yield such straight lines.     

Stereochemical effects in the electron impact mass spectra of cis-trans isomeric 1,2,3,4-tetramethylcyclohexanes

The mass spectra of six cis-trans isomeric 1,2,3,4-tetramethylcyclohexanes are discussed. The intensity ratio of [M? CH₃]⁺/[M? C₂H₅]⁺ correlates with the strain energies of the stereoisomers. Therefore, the identification of cis-trans isomers is possible by means of their mass spectra. The mass spectra of deuterium labelled compounds demonstrate favoured fragmentation of the axial methyl groups and ring opening between the cis substituted carbon atoms of the cyclohexane.     

Stereochemical effects in mass spectrometry XV— chemical ionization of some substituted alkenes using dichlorocarbene as the reagent gas

Chemical ionization (CI) of trans- and cis-stilbene and of fumaric and maleic acid, using dichlorocarbene as reagent, was studied. In the CI ion source (CH₄), dichlorocarbene added stereospecifically to the double bond of the sample molecule to form the corresponding ion of the 1,1-dichloropropane derivative. From the different relative abundances of the characteristic fragment ions of these cyclopropanes, the geometric configuration of the original substituted olefins may be predicted.     

Atmospheric pressure chemical ionization negative mass spectra of the dinitrotoluene isomers

The atmospheric pressure chemical ionization of the dinitrotoluene isomers in ambient air was studied with a quadrupole mass spectrometer operating in the negative mode. The isomers can be grouped on the basis of the product ions: 2,5-, and 2,6- and 3,5-dinitrotoluene give the molecular anion with little fragmentation; 2,3- and the 3,4-dinitrotoluene behave similarly but with more extensive fragmentation; 2,4-dinitrotoluene gives the quasimolecular [DNT ? H]^? ion with little fragmentation. The results are discussed in terms of the molecular structure of the isomers.     

Stereochemical effects in electron impact mass spectra of cis-trans isomeric 1,2,4,5-tetramethylcyclohexanes

The mass spectrometric fragmentation of the five cis-trans isomeric 1,2,4,5-tetramethylcyclohexanes is influenced by their configuration. The thermochemically stable isomers show a higher abundance of the [M? CH₃]⁺ ions and a lower abundance of the [M? C₂H₅]⁺ ions than the strained isomers. The [M? CH₃]⁺/[M? C₂H₅]⁺ intensity ratio correlates with the enthalpy difference ΔH_isom and with the mean number of gauche arrangements Z_g^I of the stereoisomers.     

The electron impact and chemical ionization mass spectra of dimethyltrisulfide

Summary The mass spectra obtained by electron impact ionization (EI) of dimethyltrisulfide, both at constant sample pressure and during elution from a GC column, are essentially identical, with the molecular ion Me₂S ₃ ⁺ providing the basis peak. Masses heavier than the molecular ion are not observed. Chemical ionization, using nitrogen, methane or isobutane, gives rise to numerous ions of larger mass than that of the molecular ion. Particularly characteristic are sulfonium type structures Me₃S _n ⁺ , with n=3–6. In addition, radical cations of the type Me₃S_nCH ₂ ⁺ and protonated trisulfide, Me₂S₃H⁺, are observed, even with N₂ as ionizing gas, together with a variety of ions of lower hydrogen content. Further, a large number of ion types of lower mass than the parent molecule are formed. The mass distribution of ions in the spectrum is found to be highly dependent on the partial pressure of dimethyltrisulfide in the ion source. These phenomena were investigated and accounted for semiquantitatively.

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Die EIund CI-Massenspektren des Dimethyltrisulfids

Zusammenfassung Dimethyltrisulfid liefert nach Elektronenstoßionisation bei konstantem Probendruck und bei GC-Probeneinlaß im wesentlichen identische Massenspektren. Das Molekülion Me₂S ₃ ⁺ stellt dabei den Basispeak dar. Größere Massen als die des Molekülions wurden nicht beobachtet. Bei chemischer Ionisation unter Verwendung von Stickstoff, Methan oder Isobutan als Reaktantgas entstehen zahlreiche Ionen mit höherer Masse als der des Molekülions. Charakteristisch sind hierbei sulfoniumartige Strukturen Me₃S _n ⁺ mit n=3–6. Daneben beobachtet man Radikalkationen des Typs Me₂S_nCH ₂ ⁺ und protoniertes Trisulfid Me₂S₃H⁺ auch bei Ionisation mittels N₂ sowie Ionenarten mit einem geringeren Wasserstoffgehalt. Weiterhin wird eine größere Anzahl von Ionenarten gebildet, deren Massen kleiner sind als die des Stammoleküls. Die Massenverteilung der Ionen im Spektrum hängt außerordentlich stark vom Partialdruck des Dimethyltrisulfids in der CI-Quelle ab. Diese Abhängigkeiten wurden semiquantitativ untersucht und dargestellt.

     

Electron impact and chemical ionization mass spectra of alkyldiphenylphosphine oxides

In the 70 e V electron impact mass spectra of a series of alkyldiphenylphosphine oxides (R?₂PO, R = Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, t-Bu, neopentyl, n-decyl), molecular ions of low abundance are observed and [M + H]⁺ ions are formed to a small extent at high sample pressures. The major ions include [?₂PO]⁺, [?₂POH]⁺; [?₂CH₂PO]⁺ and [?₂CH₂POH]⁺ which are formed by rearrangement and cleavage processes. The chemical ionization mass spectra obtained with methane and isobutane reagents consist of [M + H]⁺ ions. The proton affinity of R?₂PO was found to be 219 ± 2.5 kcal mol^?1.     

Electron impact and chemical ionization mass spectra of aryl ureas

The electron impact and chemical ionization mass spectra of a series of N,N′, -diaryl ureas have been compared. The electron impact mass spectra indicate rearrangements leading to two pairs of aromatic amines and isocyanates, either as ions or molecules. The chemical ionization mass spectra showed the formation of protonated amines and isocyanates via rearrangement.     

Site of protonation in the chemical ionization mass spectra of olefinic methyl esters

The H₂ and CH₄ chemical ionization mass spectra of the olefinic esters methyl acrylate, methyl methacrylate, methyl crotonate, methyl 3-butenoate, methyl 2-methyl-2-butenoate, methyl 3-methyl-2-butenoate and methyl cinnamate have been determined. In addition to the expected loss of CH₃OH from [MH]⁺, in many cases the protonated molecules also show loss of CO or CH₂CO with methoxy group migration to the positive ion centre, indicative of protonation at the double bond. These rearrangement reactions, which have analogies in electron impact mass spectra, result in chemical ionization mass spectra of isomeric molecules which show more substantial differences than the electron impact mass spectra. In the case of methyl cinnamate, isotopic labelling experiments show considerable interchange of the added proton with the ortho and meta phenyl hydrogens prior to CH₃OH or CH₂CO loss, although the extent of interchange is not the same for both cases.     

Stereochemical effects in the electron impact mass spectra of some 2,6-dideoxy-3-c-methylhexopyranoses

The electron impact mass spectra of the methyl α-and β-glycosides of axenose, of a number of derivatives, and of mycarose, display substantial differences, characteristic of their stereochemistry. In order to interpret this behaviour the structures of the fragments and the main features of the fragmentation patterns have been established, and the methyl glycosides of some related monosaccharides (arcanose, cladinose and idose) have been examined. The experimental results allow the observed differences to be attributed to the presence of an intramolecular hydrogen bond between the hydroxyl group at C-3 and the oxygen atom of the methoxy group in the methyl α-glycosides of axenose and mycarose.