Identification of some isomeric [C8H6O]+˙ ions generated from phenyl substituted cyclic ketones using collisional activation

[C₈H₆O]⁺˙ ions with o-quinonoid ketene, benzocyclobutenone, phenyl ketene and benzofuran structures have been generated from various precursors. Their collisionally induced decompositions in both field free regions of a double focusing mass spectrometer with so-called reversed geometry have been studied using mass analysed ion kinetic energy scans and B/E linked scans. In both cases the abovementioned [C₈H₆O]⁺˙ structures can be distinguished–except the benzocyclobutenone ion which gives very similar spectra to the o-quinonoid ion–on the basis of the intensity ratios [m/z 77]/[m/z 76] and [m/z 104]/[m/z 102]. The stable [C₈H₆O]⁺˙ ions generated from the molecular ions of 7 -phenylbicyclo[3.1.1]heptan-6-one appear to have the phenyl ketene structure, as was suspected from previous kinetic energy release measurements.     

A mass spectrometry/mass spectrometry investigation of the nature of [C10H10]+˙, [C9H7]+ and [C10H8]+˙ gas phase ions

Additional evidence for the rearrangement of the 1- and 3-phenylcyclobutene radical cations, their corresponding ring-opened 1,3-butadiene ions and 1,2-dihydronaphthalene radical cations to methylindenetype ions has been obtained for the decomposing ions by mass analysed ion kinetic energy spectroscopy (MIKES). The nature of the [C₉H₇]⁺ and [C₁₀H₈]^+˙ daughter ions arising from the electron ionization induced fragmentation of these [C₁₀H₁₀]^+˙ precursors has been investigated by collisionally activated dissociation (CAD), collisional ionization and ion kinetic energy spectroscopy. The [C₉H₇]⁺ produced from the various C₁₀H₁₀ hydrocarbons are of identical structure or an identical mixture of interconverting structures. These ions are similar in nature to the [C₉H₇]⁺ generated from indene by low energy electron ionization. The [C₁₀H₈]^+˙ ions also possess a common structure, which is presumably that of the maphthalene radical cation.     

The [C5H8]+˙ radical cation: Structural studies by energy-resolved mass spectrometry

Control of the degree of collisional excitation of an ion can be achieved by varying the collision energy or the number of collisions. The former experiment yields a series of daughter spectra which is analogous to a breakdown curve and which serves to characterize the ion undergoing collisionally activated dissociation (CAD). Recognition of differences in the spectra between isomeric ions is facilitated if the curves obtained from the energy-resolved data are considered in pairs and their differences are plotted. This procedure serves to demonstrate that ionized 1,3- and 1,4-pentadiene are structurally distinct, even though they are not distinguishable in conventional CAD. 2-Pentyne and 3-octyne give [C₅H₈]⁺˙ fragment ions which are closely related to the 1,3- and 1,4-pentadiene structures, respectively. Any particular ion can be characterized by another form of difference spectrum: one comparing data at two collision pressures each taken over a range of collision energies. This procedure yields conclusions regarding ion structure which match those reached from the energy-resolved experiments.     

A tandem mass spectrometry study of the gas phase RSC6H5Cr+L (R = H,C6H5; L = arene) and C6H5SSC6H5Cr+ ions

Ion-molecule reactions of chromium containing ions with arylsulfides have been studied in the gas phase and their products have been characterized by tandem mass spectrometry. C₆H₅SH and (C₆H₅)₂S react as typical aromatic compounds and give rise to Cr⁺C₆H₅SR] and RC₆H₅Cr⁺QH₅SR′ [R = H, CH₃, CH(CH₃)₂; R′ = H, C₆H₅] ions. Metastable ion mass spectra of the latter species show that the metal is more strongly bound to diphenylsulfide than to alkylbenzenes. C₆H₅SSC₆H₅ reacts with chromium-containing ions to form only Cr⁺(C₆H₅SSC₆H₅). The decomposition characteristics of this ion and, in particular, the presence of a recovery signal in the neutralization-reionization mass spectrum are in keeping with the formation of a 1,2-dithia[2]cyclophane complex ion, which rearranges into a structurel(s) that contains Cr?S bond(s). No evidence was found for metal atom insertion into S?S, C?S, or S?H bonds.     

Structures of decomposing and non-decomposing gas-phase [C4H6O]+· radical cations,and their [C2H2O]+· and [C3H6]+· product ions

The structures of gas-phase [C₄H₆O]^+· radical cations and their daughter ions of composition [C₂H₂O]^+· and [C₃H₆]^+· were investigated by using collisionally activated dissociation, metastable ion measurement, kinetic energy release and collisional ionization tandem mass spectrometric techniques. Electron ionization (70 eV) of ethoxyacetylene, methyl vinyl ketone, crotonaldehyde and 1-methoxyallene yields stable [C₄H₆O]^+· ions, whereas the cyclic C₄H₆O compounds undergo ring opening to stable distonic ions. The structures of [C₂H₃O]^+· ions produced by 70-eV ionization of several C₄H₆O compounds are identical with that of the ketene radical cation. The [C₃H₆]^+· ions generated from crotonaldehyde, methacrylaldehyde, and cyclopropanecarboxaldehyde have structures similar to that of the propene radical cations, whereas those ions generated from the remainder of the [C₄H₆O]^+· ions studied here produced a mixed population of cyclopropane and propene radical cations.     

Differentiation of isomeric C8- and N 2-deoxyguanosine adducts of 2-acetylaminofluorene by fast-atom bombardment and tandem mass spectrometry

Product-ion studies of source-produced ions corresponding to acetylated and nonacetylated N ²- and C8-substituted aminofluorene adducts of deoxyguanosine were conducted to identify specific fragmentation pathways differentiating the isomers and to determine the influence of the acetyl group on the fragmentation of the arylamide modified deoxyguanosine adducts. The collision-induced dissociation spectra of the BHZ ₂ ⁺ ion and other significant source-produced ions showed no evidence to suggest that ketene loss (deacetylation) resulted in significant alteration of the structure of the adducts. However, other significant ion formation processes, particularly loss of water from the N ²-substituted arylamide did appear to require rearrangement, likely involving bond formation between the carcinogen moiety (acetyl group) and the N1 or N2 position of the guanine base. The combined loss of ketene and water constitute a fragmentation pattern specific for the N ²-arylamide, 3-(deoxyguanosin-N ²-yl)-2-acetylaminofluorene.     

Structure of [C3H5O]+ ions produced from unimolecular decomposition of several [C4H8O]+˙ metastable ions

It is demonstrated by means of collisionally activated decomposition (CAD) that [C₃H₅O]⁺ originating from metastable [C₄H₈O]^+˙ ions are either acylium [C₂H₅CO]⁺ (a) or hydroxycarbenium [CH₂CHCHOH]⁺ (b). Butanone gives exclusively a but 2-methyl-2-propen-1-ol, 2-buten-1-ol, 3-buten-1-ol, butanal and 2-methylpropanal lead to ion b. Both structures a and b are produced from 3-buten-2-ol. These results are discussed in conjunction with experimental and calculated (MINDO/3) thermodynamic data.     

Quantitation of isomeric ion mixtures using collisional activation mass spectra

Recently Bass and Bowers have criticized methods for calculation of isomeric composition of gaseous ions based on mass spectra from collisionally activated dissociations (CAD). They fault values from this laboratory on the proportion of benzyl and tropylium ions calculated by two methods from product ion abundances. However, a careful remeasurement of the relevant yields of CAD product ions shows that neither calculation method affected the result outside experimental error, as the ion yields for at least 14 higher energy dissociations are insensitive to both isomeric identity and internal energy. Reducing the [C₇H₇]⁺ internal energy by 1.5 eV does give a 25% reduction in the ion yield for the lowest energy process, supporting the accepted recommendation that these be omitted for quantitative spectral comparisons. A calculation error was made in one of our previous reports on [C₇H₇]⁺, and corrected isomeric composition values are presented. The disparate values from recalculation by Bass and Bowers of other data from this laboratory on [C₂H₄O]^+˙ ions are shown here to be consistent with the large experimental error of those measurements; the medians of their values are actually near the values originally reported. In direct contrast to their assertions, we find no evidence that previous calculation methods have produced misleading conclusions, or that the assumption of linear superposition of CAD spectra in ion mixtures has yielded unreliable results.     

A field ionization and collisionally activated dissociation/charge stripping study of some [C9H10]+˙ ions

The extent of isomerization of [C₉H₁₀]^+˙ ions, with lifetimes of approximately 10^?11 and 10^?6 s has been investigated using field ionization, collisionally activated dissociation and charge stripping techniques. The [C₉H₁₀]^+˙ ions which were investigated included the molecular ions of α-methylstyrene, β-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, indan, cyclopropylbenzene, allylbenzene and the product of water loss from 3-phenylpropanol. The field ionization spectra of all the C₉H₁₀ hydrocarbons were different indicating that isomerization to a common ion structure had not occurred to a measurable extent for ions with lifetimes of approximately 10^?11 s. Collisionally activated dissociation and charge stripping results indicated that most of the [C₉H₁₀]^+˙ ions continued to maintain unique ion structures (or mixtures of structures) at ion lifetimes of 10^?6 s. Possible exceptions are the [C₉H₁₀]^+˙ ions from allylbenzene and cyclopropylbenzene which gave indistinguishable collisionally activated dissociation and charge stripping spectra.     

Structure determination of some C4H5N+˙ Ions from substituted pyridines by collisionally activated dissociation mass spectrometry

A standard procedure for recording and correcting collisionally activated dissociation mass spectra is proposed, and used to distinguish between the C₄H₅N^+˙ ions formed from hydroxy- and amino-pyridines after loss of CO and HCN, respectively. It is concluded that these ions are cyclic. From the 4-isomers the 3H-pyrrole ion is formed whereas from 2-hydroxypyridine the 1H-pyrrole ion is formed. In the other cases, mixtures of 2H- and either 1H- or 3H-pyrrole ions are generated, depending on the nature of the precursor.     

Structural identification of fatty acid methyl esters by collision spectra of their [M ? H]− species

Negative ion chemical ionization linked to collisionally activated decomposition (CAD) experiments has proved to be an efficient analytical tool in mass spectrometric characterization of fatty acids. The CAD mass-analysed ion kinetic energy spectra of [M ? H]^? species, obtained by OH^? reaction with a selection of six C₁₈ fatty acid methyl esters, reveal useful correlations with the original structure of the neutrals, giving evidence of both chain branching and double bond positions.     

A study of [C7H8]+˙ and [C7H8]2+ ions formed from different precursor molecules

On the basis of unimolecular and collisionally activated decompositions, as well as their charge stripping behaviour, [C₇H₈]⁺˙ and [C₇H₈]²⁺ ions from a variety of precursors have been studied. In particular, structural characteristics of molecular ions of toluene, cycloheptatriene, norborna-2,5-diene and quadricyclane have been compared to those of [C₇H₈]⁺˙ and [C₇H₈]²⁺ rearrangement fragment ions obtained from n-butylbenzene, 2-phenylethanol and n-pentylbenzene. Severe interferences from [C₇H₇]^2+˙ ion fragmentations have been observed and rationalized.     

Ion chemistry of phthalamic acids. 2—mass spectral retrosynthesis of phthalanilic acids and structure analysis of [C8H6NO2]+ ions from N-cyclohexylphthalamic acid and [MH]+ of N-cyclohexylphthalimide

Under electron impact o-phthalanilic acids show the retrosynthetic reaction previously described for other phthalamic acids. As primary amine derivatives they undergo thermal and electron impact induced water loss. Like the molecular ions of the related phthalimides, their [M? H₂O]^+˙ do not give [C₈H₆NO₂]⁺ fragments, which are obtained from the N-cyclohexyl derivative. The structure of such fragments is investigated by collisionally activated mass analysed ion kinetic energy spectra, and compared with the [MH]⁺ of phthalimide, obtained by chemical ionization with CH₄ or NH₃ and assumed to be possible models.     

Low–energy,low-temperature mass spectra. 8—The McLafferty rearrangement

The McLafferty rearrangement giving rise to the McLafferty ion, the complementary ion and a protonated molecule is discussed in the cases of the alkenes, alkanals, alkanones, alkanoic acids and alkanoic esters, in terms of the low-energy, low-temperature mass spectra. Based on the behaviour of the alkanals, which show a series of peaks corresponding to [M ? C_nH_2n]⁺˙, it is suggested that these also involve McLafferty rearrangements from rearranged distonic ions. The same processes are held to be responsible for similar series of peaks in the case of the alkenes and that of the amines. Where possible the energetics of the processes concerned are discussed. The low-energy mass spectra of the linear alkenes propene to octene and those of the octenes are reported.     

High resolution mass spectrometric studies of some bicyclic γ-lactones

The mass spectra of three bicyclic γ-lactones have been studied, and the fragmentation pathways have been proposed with the aid of accurate mass measurements and metastable transitions. An unusually low [M ? CH₃] ion and the presence of an abundant [M ? C₅H₉] ion in the mass spectrum of dihydroactinidiolide were interpreted as a possible rearrangement involving a methyl migration. The eliminations of methyl radical, carbon monoxide and ketene are important processes in many cases.     

Characterization of C8H10 alkylbenzenes by negative ion mass spectrometry

The O^?˙ chemical ionization mass spectrri of the C₈H₁₀ alkylbenzenes, o-, m-. andp -xylene and ethylbenzene, show formation of [M ? H + O]^?, [M ? H]^?, [M ? H₂]^?˙ and, for the xylenes, [M ? CH₃ + O]^? as primary reaction products; the relative importance of these products depends on the isomer. However, [OH]^? is a primary product from reaction of O^?˙ with both the C₈H₁₀ isomers and hydrogen-containing impurities; [OH]^? reacts further with the alkylbenzenes to produce [M ? H]^? with the result that the chemical ionization mass spectra depend on experimental conditions such as sample size and the presence of impurities. The collision-induced charge inversion mass spectra of the [M ? H + O]^? and [M ? H]^? products allow only distinction of ethylbenzene from the xylenes. However, the collision-induced charge inversion mass spectra of the [M ? H₂]^?˙ ions show differences which allow identification of each isomer.     

Gaseous [C2H3O]+ ions

[C₂H₃O]⁺ ions with the initial structures [CH₃CO]⁺, and [CH₂CHO]⁺ cannot be distinguished on the basis of their collisional activation spectra, demonstrating that these isomers interconvert at energies below their threshold for decomposition. Self-protonation of ketene leads to the [CH₃CO]⁺ ion, while the [C₂H₃O]⁺ ion generated from glycerol most probably has the structure of an oxygen protonated ketene [CH₂?C?OH]⁺.     

The structure of [C3H4N2]+· and [C5H5N2]+ ions formed from vinylimidazoles,studied by collisionally activated dissociation mass spectrometry

Mass spectra of the three isomeric vinylimidazoles have been compared and the structures of the fragment ions [C₃H₄N₂]^+· and [C₅H₅N₂]⁺ have been investigated by collisionally activated dissociation mass spectrometry. The greater part of the non-decomposing ions m/z 68 from 2-vinylimidazole and from 2-imidazolecarboxylic acid methyl ester, and a minor part of this ion formed from the free acid, all have the same structure: the imidazole ring system, with hydrogens at both nitrogen atoms but none at C(2). An analogous structure, with an ethyl group at C(2), is proposed for the m/z 93 ion from 2-vinylimidazole.     

Charge stripping mass spectra: A method for identifying isomeric [C5H8]+˙ and [C3H6]+˙ ions

Charge stripping (collisional ionization) mass spectra are reported for isomeric [C₅H₈]⁺˙ and [C₃H₆]⁺˙ ions. The results provide the first method for adequately quantitatively determining the structures and abundances of these species when they are generated as daughter ions. Thus, loss of H₂O from the molecular ions of cyclopentanol and pentanal is shown to produce mixtures of ionized penta-1,3- and -1,4-dienes. Pent-1-en-3-ol generates [penta-1,3-diene]⁺˙. [C₃H₆]⁺˙ ions from ionized butane, methylpropane and 2-methylpropan-1-ol are shown to have the [propene]⁺˙ structure, whereas [cyclopropane]⁺˙ is produced from ionized tetrahydrofuran, penta-1,3-diene and pent-1-yne.     

Electron-impact-induced rearrangements of organic ions VIUnimolecular reactions of isomeric[C8H9]+ions

Metastable ion spectra and deuterium labelling have been used to investigate a series of gaseous [C₈H₉]+ ions of isomeric structures. The similarity of the intensities of their metastable loss of hydrogen, acetylene and ethylene molecules and metastable reactions of specifically labelled ions, suggests that the [C₈H₉]+ reacting ions, formed initially with different structures, isomerise to a common structure or mixture of structures via deep-seated rearrangement reactions which render all hydrogen atoms equivalent. The isomerisation process involved is controlled by a conversion of a vinyl bond into an allyl-type bond, thus destroying the aromatic moiety.