The structures and formation of [C4H6N]+ ions 2—ion structures derived from 2-methylpropanenitrile

The loss of a hydrogen atom from ionized 2-methylpropanenitrile is preceded by a drastic rearrangement of the molecular ion. The result of this fragmentation is the generation of two stable structurally different [C₄H₆N]⁺ ions, formed via different pathways. Their structures can be established by a careful comparison of the metastable ion spectra, collision activation spectra, and charge stripping spectra from the compound and its three deuterium labeled analogues and from [C₄H₆N]⁺ ions generated from reference compounds via electron impact ionization or in selected ion/molecule reactions.     

Gas phase reactivity of cation radicals and protonated species from isomeric C4H8S thioethers

Four isomeric thioethers, 2,3-dimethylthiirane ( 1 ), 2-methylthietane ( 2 ), tetrahydrothiophene ( 3 ), and allyl methyl thioether ( 4 ), have been subjected to mass spectrometric analysis in the gas phase, under electron impact (El) and chemical ionization (CI) conditions. The metastable molecular ions M⁺′ generated from 1-4 under EI (70 eV) conditions give distinct patterns of unimolecular fragmentation, thus indicating that isomer interconversion reactions are slower than dissociation (a possible exception, to some extent, is the case of [M₂]⁺′ and [M₂]⁺′). The change of the relative intensities of some prominent peaks with increasing ion lifetime (decomposition within the ion source, the first, and the second field-free regions of the mass spectrometer) is pointed out. Metastable [MH]⁺ ions, generated from 1-4 in chemical ionization experiments with CH₄, all eliminate H₂ and H₂S, although in different relative proportions. In addition to these processes protonated 4 also undergoes loss of C₂H₄ and C₃H₆, likely from a C-protonated structure.     

Chemical Ionization—A Mass-Spectrometric Analytical Procedure of Rapidly Increasing Importance

The mode of ionization of a molecule has a strong influence on its behavior in the mass spectrometer and thus on the information that can be obtained from its mass spectrum. In chemical ionization a reagent gas, e.g. methane, is first ionized by electron impact. The ions formed in ion-molecule reactions, in particular [CH₅]⁺, [C₂H₅]⁺, and [C₃H₅]⁺, then react “chemically” with the substrate M in fast acid/base type reactions to form ions of the type [MH]⁺, [M(C₂H₅)]⁺, etc., which subsequently fragment to various extents. Alternatively, chemical ionization can be effected by charge exchange, in that ions of a reagent gas, e.g. [He]^+?, react with the substrate M to form molecular ions [M]^+·. Chemical ionization can thus be conducted in a more or less mild fashion and the extent of the fragmentation can be controlled over a very wide range.     

A study of the electron impact induced retro-diels-alder process with selected 5,6,6a, 7,12,12a-hexahydrobenzo[a]anthracenes

The mass spectra of 5,6,6a,7,12,12a-hexahydrobenzo[a]anthracene and 2-methoxy, 3-methoxy-, 4-methoxy and 1-methyl-4-methoxy derivatives are reported. Among the fragment ions observed under electron impact ionization, [C₈H₈]^+· and [M? C₈H₈]^+· can be generated by a retro-Diels-Alder process. Studies of metastable ion reactions show these ions to be formed by fragmentation directly from the molecular ion. The CA spectra of the [C₈H₈]^+· ions from the subject compounds were compared with spectra from ions of the same composition from various sources. From these data, kinetic energy release measurements and stereochemical considerations, it is concluded that these ions are formed by a stepwise, rather than a concerted mechanism.     

Mass spectral behavior of n-alkynes

The 70 e V-electron impact mass spectra of the C₇–C₁₀ n-alkynes have been determined as well as the metastable ion spectra of the molecular ions and the [CS₂]⁺ and [N₂O]⁺ charge exchange mass spectra of the C₇-C₉ n-alkynes. The metastable ion mass spectra provide only a limited opportunity to distinguish between isomers; however, the 70-eV EI mass spectra of isomeric compounds permit a ready distinction between isomers. The [CS₂]⁺ charge exchange mass spectra of isomeric compounds also show substantial differences. The [N₂O]⁺ charge exchange mass spectra do not show the enhancement of β-fission fragments observed in field ionization experiments, despite representing ions of similar internal energy, and it is concluded that field dissociation is responsible for the β-fission fragments in the field ionization experiments.     

Interference peaks in collisional activation mass spectra: The loss of CH3˙ from [C13H9] ions

It was concluded from earlier metastable ion and collisional activation experiments that [C₁₃H₉]⁺ ions generated by dissociative ionization of stilbenes and diphenylmethane consist of a mixture of isomeric structures. It is shown in this paper that the methyl loss upon which this conclusion rests originates to a large extent from metastable molecular ions decomposing in the magnetic sector field (interference signal). Our results do not show, nor do they require, that more than one isomeric [C₁₃H₉]⁺ species is present. The importance of recognizing such interference signals is stressed.     

A structural investigation of [C6H6]2+ and [C6H6]+˙ ions involved in charge exchange reactions

Mass-analysed ion kinetic energy spectra for collisional activation (CA) of [C₆H₆]⁺˙ formed via electron capture by [C₆H₆]²⁺ ions in collision with neutral benzene molecules have been compared for the C₆H₆ isomers benzene, 1,5-hexadiyne and 2,4-hexadiyne. Comparisons of fragment abundance and total CA fragment yields were also made for [C₆H₆]⁺˙ ions generated by electron ionization (EI). CA conditions of ion velocity and collision gas pressure were identical in these comparisons. In general the fragment abundance patterns for the ions formed by charge exchange were very similar to those for singly charged benzene ions generated by EI. However, significant variations in CA fragment yield (the ratio of the total CA fragment ion abundance to the abundance of the incident unfragmented ions) were observed. It is not clear from the results whether these variations reflect structurally different ions or ions of different internal energies. The CA spectra of [C₆H₆]⁺˙ ions derived from charge exchange reactions between the benzene dication and the target gases He, Ne, Ar, Kr and Xe have also been recorded and, once again, very similar fragment abundance patterns were observed along with large variations in total CA fragment yields. Charge exchange efficiency measurements are reported for reactions between the benzene dication and the targets He, Ne, Ar, Kr, Xe and C₆H₆ (benzene) and also for the doubly charged ions derived from the linear C₆H₆ isomers. In the latter case Xe and benzene targets were used. The energetics and efficiency measurements for the former reactions suggest that for targets such as He and Ne the processes probably involve excited states of the doubly charged ions. The efficiencies measured for the latter reactions were distinctly different for the three C₆H₆ isomers and may indicate a strong dependence of charge exchange cross-section on doubly charged ion structure.     

Low-energy mass spectra of some aliphatic ketones

The charge exchange mass spectra of a selection of C₅-C₇ ketones have been measured using [CS₂]⁺˙, [COS]⁺˙ and [N₂O]_+. as reagent ions. The low energy charge exchange with [CS₂]⁺˙ or [COS]⁺˙ provides simple primary ion mass spectra, which readily permit structure elucidation in contrast to metastable ion spectra. In several cases, isomer distinction is easier from the charge exchange mass spectra than from the electron impact mass spectra. The energy transfer from [N₂O]⁺˙ is sufficiently high for complex spectra resembling electron impact mass spectra to be obtained.     

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.     

Energy dependence of the fragmentation of some isomeric [C6H12]+˙ ions

The charge exchange mass spectra of 14 C₆H₁₂ isomers have been determined using [CS₂]^+˙, [COS]^+˙, [Xe]^+˙, [CO]^+˙, [N₂]^+˙ and [Ar]^+˙ as the major reactant ions covering the recombination energy range from ∼10.2 eV to ∼15.8 eV. From the charge exchange data breakdown graphs have been constructed expressing the energy dependence of the fragmentation of the isomeric [C₆H₁₂]^+˙ molecular ions. The electron impact mass spectra are discussed in relation to these breakdown graphs and approximate internal energy distribution functions derived from photoelectron spectra.     

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.     

Structure determination of [C3H3N2]+ ions,formed from monosubstituted pyrazoles C3H3N2R,by collision-induced dissociation mass spectrometry

[C₃H₃N₂]⁺ ions have been generated under electron impact conditions from some monosubstituted pyrazoles C₃H₃N₂R. Collision-induced dissociation (CID) mass spectra of deuterium-labelled precursors suggest that the majority of the [C₃H₃N₂]⁺ ions formed from 1-nitro- and 4-bromo-pyrazole retain their cyclic structure, whereas the ions from 3(5)-bromopyrazole are mainly linear. This is confirmed by the relative values observed for the overall cross-sections for CID and for ion loss. An isotope effect of the order of 1.5–1.9 has been found for the collision-induced loss of H^˙ from [C₃H₃N₂]⁺, generated from 3(5)- and 4-bromopyrazole.     

Mass spectrometric decomposition processes of 2-methyl-1-hexene

Unimolecular decompositions of 2-methyl-1-hexene and several labelled analogues were studied following 70 eV electron impact (normal and metastable spectra) and field ionization (field ionization kinetic measurements). Molecules labelled with ¹³C in the 1-position and the methyl position were found to behave essentially identically. This is attributed to rapid transfer of a hydrogen atom mainly from C-5 to C-1 (γ-hydrogen shift). Loss of ethene, propene or propenyl do not involve loss of the methyl carbon or C-1. All three reactions are better than 90% specific in this respect under all conditions studied. At shorter times, C₃H₆ loss is the dominant reaction, while at longer times C₂H₆ loss accounts for >90% of the ion current. It is proposed that at least two distinct pathways for C₂H₄ loss operate in linear 1-alkenes, one of which (loss of carbons 1 and 2) is blocked by a 2-methyl substituent. The [C₆H₁₁]^+˙ and [C₅H₁₀]^+˙ ions formed from ¹³C labelled 2-methyl-1-hexenes fragment in an essentially statistical fashion.     

Structure of gas phase [C5H6]+˙ ions

Charge-stripping spectra have been used to differentiate ionized cyclopentadiene from its acyclic isomers. The minimum amounts of translational energy lost during the charge-stripping processes and the relative charge-stripping efficiencies, which are also structurally important parameters, have been measured for these ionic species. [C₅H₆]⁺˙ ions, formed by dissociative ionization of various precursors in the ion source are found, usually, to be a mixture of cyclic and acyclic ions. In contrast, [C₅H₆]⁺˙ ions, derived from the dissociation of metastable molecular ions from a series of organic compounds, have the cyclopentadienyl structure. This structure was confirmed by collision-induced dissociation of ions formed in the first field-free region of a triple sector mass spectrometer.     

Characterization of five [C4H7O]+ Ion structures. Fragmentation of the 2-pentanone molecular ion

The [C₄H₇0]⁺ ions [CH₂?CH? C(?OH)CH₃]⁺ (1), [CH₃CH?CH? C(?OH)H]⁺ (2), [CH₂?C(CH₃)C(?OH)H]⁺ (3), [Ch₃CH₂CH₂C?O]⁺ (4) and [(CH₃)₂CHC?O]⁺ (5) have been characterized by their collision-induced dissociation (CID) mass spectra and charge stripping mass spectra. The ions 1–3 were prepared by gas phase protonation of the relevant carbonyl compounds while 4 and 5 were prepared by dissociative electron impact ionization of the appropriate carbonyl compounds. Only 2 and 3 give similar spectra and are difficult to distinguish from each other; the remaining ions can be readily characterized by either their CID mass spectra or their charge stripping mass spectra. The 2-pentanone molecular ion fragments by loss of the C(1) methyl and the C(5) methyl in the ratio 60:40 for metastable ions; at higher internal energies loss of the C(1) methyl becomes more favoured. Metastable ion characteristics, CID mass spectra and charge stripping mass spectra all show that loss of the C(1) methyl leads to formation of the acyl ion 4, while loss of the C(5) methyl leads to formation of protonated vinyl methyl ketone (1). These results are in agreement with the previously proposed potential energy diagram for the [C₅H₁₀O]⁺˙ system.     

The origins of the base peak in the electron impact spectrum of limonene

The origins and nature of the [C₅H₈]⁺? ions which form the base peak in the electron impact spectrum of limonene, at nominal electron energies greater than 11 eV, have been investigated. Linked scan techniques were used to study unimolecular and collision induced fragmentation reactions. No fragmentation pathway leading to [C₅H₈]⁺? could be found. Measurement of ionization efficiency curves indicated that the threshold for formation of C₅H₈[⁺?] lies above the range of internal energies deposited in incident ions by collisional activation. By a combination of comparisons of collisionally activated spectra and energetic considerations, the [C₅H₈]⁺? ions formed from limonene were shown to resemble those of the molecular ion of isoprene, while the neutral fragment is most likely isoprene also. Deuterium labelling experiments yielded evidence of extensive scrambling prior to fragmentation. The most probable mechanism of formation of [C₅H₈]⁺? appears to involve a retro Diels–Alder reaction of a structurally intact molecular ion of limonene.     

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.     

Elimination reactions of ions in the gas phase. IV—Comparative investigation of water elimination in odd and even electron hydroxyl cations of 6-undecanol

Reactivity differences between odd ([M]⁺) and even electron ions (α-cleavage product) were studied by comparing water elimination mechanisms in 6-undecanol. The compounds specifically labelled with deuterium in positions 6, 5 + 7, 4 + 8 and 3 + 9 were made, and a detailed investigation of tghe metastable ion transitions carried out. A highly specific 1,4 elimination of water without preceding intramolecular hydrogen exchange occurs from [M]⁺, but equal amounts of 1,3 and 1,4 elimination of water preceded by specific hydrogen exchange between -OH and the hydrocarbon chain occurs from the α-cleavage ion [M – C₅H₁₁]⁺ . To make such distinctions a thorough examination of metastable ions is essential.     

Hydrogen rearrangement in molecular ions of alkyl benzenes: Mechanism and time dependence of hydrogen migrations in molecular ions of 1, 3-diphenylpropane and deuterated analogues

Hydrogen migrations in the molecular ions of 1,3-diphenylpropane, preceding the fragmentations to [C₇H₇]⁺ and [C₇H₈]⁺ ions, have been investigated by use of deuterated derivatives. By comparing the distribution of deuterium labels in the [C₇(H, D)₈]⁺ products from metastable molecular ions with the distribution patterns calculated for various exchange models, it is shown that the H migrations occur by two processes linked by a common intermediate: (i) exchange between hydrogen isotopes at the γ-methylene group and at the ortho positions of the phenyl group: (ii) exchange between hydrogen isotopes at the ortho and orthó positions in the intermediate. In these mechanisms the eight hydrogen isotopes at both benzylic positions and both the ortho and orthó positions of 1,3-diphenylpropane participate in a mutual exchange. A statistical equipartition of the hydrogen isotopes at these eight positions is not reached in metastable molecular ions, however. The distribution pattern of [C₇(H, D)₈]⁺ ions from the deuterium labelled compounds as a function of the mean number n of exchange cycles has been calculated according to this reaction model and compared with experimental results for unstable molecular ions, generated by 70 eV and 12 eV electrons, respectively, and metastable molecular ions. Good agreement is obtained for all compounds and n = 0.4–0.8 for unstable molecular ions and n = 5–8 for metastable ions. Therefore, the hydrogen exchange in the molecular ion of 1,3-diphenylpropane is a rather slow process. These results firmly establish the isomerization reaction involving the conversion of the molecular ion of 1,3-diphenylmethane to the intermediate and hence to the molecular ion of 7-(2-phenylethyl)-5-methylene cyclohexa-1,3-diene and preceding the fragmentations. The postulated intermediate is a true one which corresponds to a s?-complex type ion and which fragments to [C₇H₈]⁺ ions. Surprisingly, no isomerizations of the intermediate by hydrogen shifts within the protonated aromatic system (‘ring walks’) are observed.     

Ion structural studies by ion kinetic energy spectrometry: [C7H7]+, [C7H8]+˙, [C7H7OCH3]+˙

The use of kinetic energy release measurements in the structural characterization of ions formed in the mass spectrometer and in the determination of fragmentation mechanisms is demonstrated. In combination with information on the mode of energy partitioning in some of these reactions this allows the following conclusions: (i) The metastable [C₇H₈]⁸˙ ions formed from toluene, cyclohepatatriene, n-butylbenzene, the three methyl anisoles, methyl tropyl ether and benzyl methyl ether all undergo loss of H˙ from a common structure. (ii) The metastable [C₇H₇]⁺ ions generated from the same sources and from benzyl bromide, benzyl alcohol, p-xylene and ethylbenzene appear to undergo loss of acetylene from both the benzylic and the tropylium structures. (iii) The metastable [C₇H₇OCH₃]⁺˙ ether molecular ions undergo loss of CH₃˙ by two types of mechanism, simple cleavage to give the aryloxy cation (not observed for benzyl methyl ether) and a rearrangement process which appears to lead to protonated tropone as the product. (iv) Loss of formaldehyde from the metastable [C₇H₇OCH₃]⁺˙ molecular ions involves hydrogen transfer via competitive 4- and 5-membered cyclic transition states in the case of the anisoles and in the case of methyl tropyl ether, while for benzyl methyl ether, hydrogen transfer in the nonisomerized molecular ion occurs via a 4-membered cyclic transition state to yield the cycloheptatriene molecular ion.