The electron impact-induced cyclization of o-carboxy- and o-carboxamidocyclopropylbenzenes

The isomerization of M^{+ ˙} of o-carboxy- and o-carboxamidocyclopropylbenzenes into five- and six-membered heterocycles due to the ortho effect has been demonstrated by means of electron impact mass spectra, high-resolution mass spectrometry data and metastable ion spectra. The criteria for the quantitative estimation of the degree of cyclopropane cyclization into these isomeric cycles were formulated. The nature and quantity of heterocyclic ions proved to be in good accord with the results for cyclization of the same cyclopropanes in the strong acid solutions.     

The [M?OH]+ ions of m- and p-ethylnitrobenzene: A common structure?

The structures of the [M? OH]⁺ ions of m- and pethylnitrobenzene have been compared by measurements of metastable ion spectra, collisional activation spectra, kinetic energy releases and critical energies for the formation of these ions and their subsequent decomposition. Normalized rates of fragmentation of metastable molecular ions and metastable [M? OH]⁺ ions have been compared for ion lifetimes up to 30 μs. The energy measurements fail to distinguish between the structures of the [M? OH]⁺ ions, but the normalized fragmentation rates and the collisional activation spectra show their structures to be different.     

The mass spectra of alkyl- and phenyl-4-imidazolin-2-ones

The mass spectra of a variety of alkyl- and aryl-4-imidazolin-2-ones have been determined and the fragmentation mechanisms have been analyzed by deuterium labelling, high resolution and metastable transitions allowing certain differentiations of positional isomers. In contrast to the benzoid systems the mass spectra of isomeric alkyl-4-imidazolin-2-ones are distinctive. The influence of the position of substituents is demonstrated by phenyl-4-imidazolin-2-ones establishing an exact prediction of fragmentation pathways. Fragment ions (e.g. [M-HNCO].⁺) which are the result of rearrangement processes were excluded for structure determinations. The ion structures involved were elucidated by collisional activation comparing model ions. Alkyl-phenyl-4-imidazolin-2-ones give almost identical mass spectra, but the positional isomers can easily be distinguished by different fragmentation patterns in both metastable and collisional activation spectra of the molecular ions.     

7H-benz [kl]acridin-7-ylium closed-shell ion produced from 1,8-bis(phenylamino)naphthalenes under electron impact: Tandem mass spectrometric structure elucidation

Not only strongly basic aromatic amines such as ‘proton sponges’ show characteristic [M – Me₂NH – H]⁺ peaks corresponding to cyclization to stable heterocyclic ions under electron impact. The fragmentation of the title compounds, which are weak bases, leads to [M – RNH₂ – C₆H₅]⁺ heteroaromatic ions. These are the 7-phenyl-7H-benz[kl]acridine radical cation and the delocalized stable 7H-benz[kl]acridin-7-ylium ion as shown by unimolecular metastable ion spectra, collision-induced decomposition (CID) tandem mass spectrometry (MS/MS) and accurate mass measurements. The high-collision-energy CID tandem mass spectra of these ions are perfectly superimposable on those of the ions produced by the reference substance, 7-phenyl-7H-benz[kl]acridine obtained as a by-product in the Ullmann phenylation of 1,8-diaminonaphthalene. This combination of MS/MS experiments also provides strong support for the gas-phase reaction mechanism leading to the benz[kl]acridine ions and for the structures of these ions.     

Electron impact fragmentation of isomeric 2-diazo-2-cyanoacetamides and 4-cyano-5-hydroxy-l,2,3-triazoles

On the basis of electron impact mass spectra, high-resolution mass spectra and metastable ion spectra, the isomerization of the molecular ions of N-aryl-2-diazo-2-cyanoacetamides and 1-aryl-4-cyano-5-hydroxy-l,2,3-triazoles into each other or into the common structure has been demonstrated. Corresponding N-cyclohexyl derivatives are unable to undergo such a transformation.     

Electron impact studies—LXV: Negative-ion mass spectrometry of functional groups 2-aryl-1,3-dithianes

The negative-ion mass spectra of 2-aryl-1,3-dithianes contain pronounced molecular anions together with fragment ions which are produced by both simple and complex cleavage reactions. These spectra contain further examples of specific hydrogen scrambling processes in negative ions. Nitroaryldithianes give intense negative-ion spectra, and that of the o-nitro derivative exhibits an unusual proximity effect. The fragmentation patterns have been examined using both ²H and ¹³C labelling and the metastable defocusing technique.     

Consecutive reactions in the vitamin D series. A New insight into the mechanism of vitamin D and provitamin D isomerizations in the mass spectrometer

The individual steps of the consecutive reactions arising from metastable molecular ions, derived from vitamin D₃, vitamin D₂ and their respective provitamins (7-dehydrocholesterol, ergosterol), were examined in different field-free regions of a triple-sector mass spectrometer of B/E/E geometry. The comparison of the translational energy release (T) and the metastable peak shapes corresponding to these reactions, as well as unimolecular and collision-induced dissociation mass-analysed ion kinetic energy spectra, showed that there are probably two structures of the [M – H₂O]⁺˙ and [M – CH₃˙]⁺ ions depending upon whether the respective ions are formed in the ion source through high-energy reactions, or from the fragmentation of metastable molecular ions through slow, low-energy processes which occur in the first field-free region.     

Ortho effects in schiff bases of diaminodicyanoethene

In the electron impact mass spectra of azomethines derived from various substituted aromatic aldehydes and diarninodicyanoethene the superposition of two ortho effects concurring with the azomethine group is apparent: one involving the amino group of the diaminodicyanoethene part accounts for the cyclization to [C₅H₃N₄]⁺ ions and the other involving ortho substituents of the benzylidene part which can interact with the azomethine moiety is responsible for specific fragment ions, suppressing the typical fragmentations of azomethines. The ortho effect was studied for the o-nitro derivative by labelling experiments, analysis of metastable transitions and collisional activation comparing model ions, demonstrating that the specific [M-H₂O]⁺˙ and [C₇H₅NO₂]⁺˙ ions are the result of cyclization processes.     

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.     

Mass spectrometric studies of structural isomers—II: Mono-and bicyclic C6H10 molecules

The electron-impact-induced ionization and fragmentation of six C₆H₁₀ structural isomers have been studied in order to determine the effect of isomerism upon their mass spectrometric behavior. The 70 eV mass spectra, metastable transitions and appearance potentials of the principal ions are reported. Significant differences between the mass spectra of the six isomers were observed; however, metastable transition and appearance potential data indicate that the fragmentation path-ways are the same for all the C₆H₁₀ molecules. Experimentally determined ionization potentials for the structural isomers are presented and compared to ionization potentials calculated by the bond orbital method. Utilizing fragmentation pathways deduced from general features in the mass spectra and from observed metastable transitions, we calculated heats of formation (ΔH_f) for the observed principal ions and compared these values to ΔH_f values for isomeric ions from other molecules.     

Fragmentation and rearrangement reactions in certain 1,2,3-triaryl-2-propen-1-ones following electron impact and chemical ionization

The electron impact (EI) and chemical ionization (CI) mass spectra of certain 1,2,3-triaryl-2-propen-1-ones (TAPs) have been studied in detail with the help of exact mass measurements, deuterium labelling and metastable data. The E- and Z-isomeric pairs do not show any difference in their behaviour under EI or CH₄ CI conditions. EI-induced rearangement reactions in the TAPs include aryl migration to carbonium ion centres. A study of the metastable transitions reveals aryl group interchange in the molecular ions prior to fragmentation. Under EI conditions loss of arene involves either C(2) or C(3) aryl groups while under CI conditions the C(1) aryl is lost as a neutral arene molecule. Mechanisms for the different fragmentation modes are given.     

Mass spectra of 1-(2′-hydroxy-5′-alkylphenyl)-1-alkanone (E)-oximes

The mass spectra of 1-(2′-hydroxy-5′-alkylphenyl)-1-ethanone (E)-oximes 1–6 and 1-(2′-hydroxy-5′-methylphenyl)-1-alkanone (E)-oximes 7–12 are given and the major fragmentation pathways discussed. The simultaneous loss of water and alkyl moieties from the molecular ion indicates that a skeletal rearrangement take place and a cycloheptatrienyloheterocyclic system is formed. The McLafferty rearrangement, γ-fission in the side aliphatic chain and oxygen expulsion are discussed with evidence being drawn from accurate mass measurements, metastable ions and comparison with mass spectral data of related compounds.     

Mass spectral studies of some 1,2,4-oxa-(thia)diazol-5(4H)-ones(thiones)

Electron ionization mass spectra of some 3,4-disubstituted 1,2,4-oxadiazole-5(4H)-thiones, thiadiazol-5(4H)-ones and thiadiazole-5(4H)-thiones are reported and fragmentation pathways of their molecular ions are studied in view of metastable ion experiments and accurate mass measurements. The main fragmentation route of the compounds under investigation is retro 1,3-dipolar cycloaddition.     

Mass spectrometry of β-keto aldehydes: Novel hydroxyl transfer reactions

The mass spectral fragmentation of 2,3-diaryl-3-oxo-1-propanals has been investigated by low electron energy, metastable defocusing and shift techniques. The o-nitro group strongly interacts with α-cleavage reactions of the carbonyl group. The cyclization/elimination processes, accompanied by oxygen and hydroxyl transfer in the mass spectra of o-nitro-β-keto aldehydes, are preceded by hydrogen abstraction by the nitro group.     

Synthesis and electron impact mass spectra of 3-substituted 2-acylaminoindazoles

3-Substituted-2-acylaminoindazoles 2 were prepared via oxidative cyclization of o-aminoaryl ketone acylhydrazones 1 with iodosobenzene diacetate. Their electron ionization mass spectra were recorded and in addition to the molecular ions show common fragmentation pathways corresponding to the [M-N₂]⁺, [M-NHCOX]⁺ and [M-COX]⁺ ions, with some influence on the skeletal fragmentation by different substituents.     

Metastable ion and induced stable ion fragmentation of isomeric 5-methyl-3-tolyl-1,2,4-oxadiazoles

The electron ionization fragmentation patterns of 5-methyl-3-(o-, m- and p-tolyl)-1,2,4-oxadiazoles (1a—c) have been examined by metastable ion and high resolution mass spectrometry. The o-tolyl isomer loses CO and C₂H₂O from the metastable molecular ion whereas the m- and p-tolyl isomers lose only CH₃CN thus indicating a strong ortho effect in directing the fragmentation in 1a. Slight differences between o-, m- and p-tolyl isomers in the collisional activation fragmentation of stable [C₇H₆N]⁺ ions suggest that structural differences exist even after a series of extensive rearrangements of the molecular ions. Metastable ion kinetic energy (MIKE) and collisional activation (CA) spectra were very helpful in providing valuable information about many fragments.     

N‐Methylimidazolidin‐4‐one organocatalysts: gas‐phase fragmentations of radical cations by experiment and theory

Electron ionisation mass spectra of N‐methylimidazolidin‐4‐one organocatalysts were studied by experimental and theoretical means. The molecular ions mostly undergo alpha cleavages of exocyclic substituents that leave the five‐membered ring intact. The type of substituent strongly dominates the appearance of the spectra. Fragmentation cascades are corroborated by metastable ion mass spectra. Quantum Chemistry Electron Ionisation Mass Spectra calculations correlate reasonably well with the experimental electron ionisation spectra and reveal mechanistic details of fragmentation pathways. The drawbacks and benefits of such calculations are discussed. Copyright © 2017 John Wiley & Sons, Ltd.     

Massenspektrometrie kondensierter N-Heterocyclen. 1–Elektronenstoßinduzierte Bildung von Cyclaziniumionen aus 5-Aryl(Hetaryl)-9-methyl-s-triazolo[4,3-c]tetrazolo-[1,5-a]pyrimidinen

The electron impact mass spectra of 5-aryl(hetaryl)-9-methyl-s-triazolo[4,3-c]tetrazolo[1,5- a]pyrimidines have been investigated. There is a general fragmentation mechanism, which forms stable cyclazinium ions by cyclization. In the case of the 5-hetaryl derivatives the proposed structures were confirmed by means of a comparison of mass analysed ion kinetic energy spectra with suitable model compounds.     

Isomerization of hydrocarbon ions. VIII—The electron impact induced decomposition of n-dodecane

The literature on the mass spectrometry of ²H and ¹³C labelled higher alkanes is reviewed and the decomposition behaviour of both the molecular and the fragment ions of n-dodecane, n-dodecane-1, 12-[¹³C₂] and n-dodecane-1,1,1,12,12,12-[²H₆] studied with special emphasis on metastable decompositions. It is shown that the elimination of alkane molecules and alkyl radicals from the n-dodecane molecular ion occurs primarily by simple splitting of the C? C bond. In addition, both small alkane molecule and alkyl radicals are eliminated with low probability from centreal parts of the molecular ion. The alkane elimination is less specific than the alkyl elimination. The methyl elimination shows an exceptionally high non-specificity, but is of negligible abundance in the 70 e V electron impact spectrum. The metastable ion spectra suggest, but do not prove unambiguously, that those small alkyl ions (with up to four carbon atoms) originating directly from the molecular ion, may be formed both by direct cleavage of the terminal groups and from central parts of the molecular ion. However, the majority of the small alkyl fragment ions in the 70 eV spectrum are formed by secondary decomposition explaining their apparent non-specific formation. The strikingly different fragmentation behaviour of even electron, [C_nH_2n+1]⁺, and odd electron fragment ions, results from differences in the product stabilities. Using collisional activation and metastable ion spectra it is shown that the odd electron fragments have the structure of the linear alkene (most probably the 1-alkene) molecular ion. In contrast to the molecular ions, alkyl fragment ions decompose with complicated skeletal rearrangements, which lead to substantial, but not complete, carbon randomization. The terminal hydrogen atoms, however, show little scrambling.     

Origin of the [C4H9O]+ ions in the mass spectrum of n-butyl ethyl ether

The mechanisms of formation of m/z 73 ions in the mass spectrum of the ionized title compound were investigated by deuterium substitution and by examining the decompositions of metastable ions. Two routes to the [C₄H₉O]⁺ ions were found in the normal spectrum. The ethyl lost by the major pathway contains the α- and β-hydrogens and a γ-hydrogen from the butyl group. The minor route involves the loss of ethylene from the [M? H]⁺ ion. There were metastable peaks for losses of ethyl, ethanol and methyl from the molecular ion. The ethyl contains the α- and β-methylenes and a γ-hydrogen, while the methyl is the δ-methyl of the butyl group. The labeling data rule out a previous mechanistic proposal for the loss of ethyl and support a mechanism involving stepwise isomerization to the sec-butyl ethyl ether molecular ion. However, the metastable ion chemistries of the molecular ions from the n- and sec-butyl ethyl ethers are highly dissimilar, perhaps due to decompositions from different electronic states. The n-pentyl methyl ether ions loses both ethyl and propyl, apparently following rearrangements to the 3-pentyl and 2-pentyl ether ions. Di n-butyl and n-butyl methyl ethers also give metastable peaks for loss of methyl, ethyl and the shorter chain alcohol.