The loss of SO2H from an imido-sulfite under electron impact: Structural elucidation by collision activation of the resulting fragment

Electron impact mass spectrometry of an imido-sulfite 5,6-benzo-2-imino-N-(2,4,6-trichlorophenyl)-1,3-dioxa-2-thiacycloheptane shows an intense fragment ion corresponding to the expulsion of SO₂H from the molecular ion. A mechanism that requires a rearrangement of the molecular ion is proposed. Structural elucidation of the [M? SO₂H]⁺ ion was obtained by recording its metastable ion and collisional activation spectra. Comparison of these spectra with similar spectra recorded from a precursor ion of known structure demonstrated that they were identical. Thus, the results support the proposed structure which derives from the expulsion of SO₂H from the molecular ion of the compound 5,6-benzo-2-imino-N-(2,4,6-trichlorophenyl)-1,3-dioxa-2-thiacycloheptane.     

Mechanistic study of the decomposition reactions of azobenzene

The electron impact-induced fragmentation of azobenzenes and its d₁, d₂, d₅, d₁₀, and ¹⁵N analogues was studied by mass Spectrometry and ion kinetic energy spectroscopy. The main fragment ions found in the mass spectrum of azobenzene are due to two parallel stepwise processes from the molecular ion: the expulsion of N₂ and two hydrogen radicals producing an ion at m/z 152 having possibly a biphenylene radical cation structure and loss of C₆H₅? and N₂. Except in the elimination of two hydrogen atoms from [M ? N₂]^+· ions, hydrogen scrambling between the phenyl rings does not feature in azobenzene upon electron impact.     

Elektronenstoß-induzierte Fragmentierung von 1,2,3,-Thiadiazolen

The mass spectra of some alkyl-,aryl-, acyl-and aroyl-substituted 1,2,3-thiadiazoles and ofbenzothiadiazole are reported and interpreted. After the elimination of N₂ from the molecular ion a rearrangement of the [M—N₂]⁺.-ion occurs producing a thioketenion-radical. Some of the subsequent fragmentation processes demand this structure. Additionally expulsion of S and ·SH respectively from the non-rearranged [M—N₂]⁺.-ion is discussed.     

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.     

The elimination of metal(I) hydride in the mass spectra of some metal(II) compounds

Metal(I) hydrides are eliminated as neutral species in the electron impact ionization mass spectra of copper(II) and palladium(II) complexes of ethylene-N,N′-3-benzoylprop-2-en-2-amine. Deuterium labelling shows that the hydrogen atom of the metal(I) hydride is derived predominantly from the ethylene bridge both for ion source reactions and for metastable ion transitions. Evidence supporting the proposed rationalization for elimination of metal(I) hydride is provided by the observation of an analogous reaction in the mass spectrum of (ethylene-N,N′-salicylaldiminato)copper(II). The mass spectrum of ethylene-d₄-N,N′-3-benzoylprop-2-en-2-amine shows an unusual rearrangement to give [C₇H₅D₂]⁺ ions involving a formal phenyl-to-methylene transfer.     

Positive electron impact and chemical ionization mass spectra of some nitramine nitrates

The positive electron impact (EI) and isobutane chemical ionization (CI) mass spectra of six nitramine nitrates were studied with the aid of some accurate mass measurements. In the EI spectra, β fission relative to both the nitramine and nitrate ester is important. In the CI spectra a major ion occurs at [MH – 45]⁺ and was found to be mainly due to [M + 2H ? NO₂]⁺. All of the compounds except N-(2 hydroxyethyl)-N-(2′,4′,6′-trinitrophenyl)nitramine nitrate gave an [MH]⁺ ion. The [MH – 45]⁺ ion in the isobutane CI mass spectra of tetryl is also due to [M + 2H ? NO₂]⁺.     

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.     

Mass spectrometry of substituted 1,3-dihydro-2H-imidazole-2-thiones

The electron impact mass spectra of the 4-formyl-1, 3-dihydro-2H-imidazole-2-thione, its six 1-methyl(n-propyl, n-hexyl)-3-methyl(phenyl)-disubstptuted derivatives, and the 1,3-dihydro-1-phenyl-2H-imidazole-2-thiome are discussed. The fragmentation pattern is strongly influenced by the alkyl or phenyl N-substituents, as well as by the length of the alkyl chain. The odd-electron ions containing an N-phenyl substituent, but not a propyl or hexyl group, eject a hydrogen atom from the phenyl ring, while the presence of a long alkyl chain greatly enhances the loss of the sulphyhydryl radical and facilitates the expulsion of several alkenes, and alkyl and alkenyl radicals.     

Ortho effects in organic molecules on electron impact. 12—Mechanism of the H2O loss from the molecular ion of 2-(phenylamino)benzoic acid

The mass spectrum of 2-(phenylamino)benzoic acid is characterized by the presence of the base peak at m/z 195, formed by the expulsion of H₂O from the molecular ion. A mechanism for the water loss, involving the ? COOH and ? NH functions followed by cyclization leading to the molecular ion of acridone, is proposed based on the study of the substituted derivatives and MIKE spectra.     

Stereochemical applications of mass spectrometry: 1—The utility of electron impact and chemical ionization mass spectrometry in the differentiation of stereoisomeric benzoin oximes and phenylhydrazones

A study of the electron impact and chemical ionization (H₂, CH₄, and iso-C₄H₁₀) mass spectra of stereoisomeric benzoin oximes and phenylhydrazones indicates that while the former can be distinguished only by their chemical ionization mass spectra the latter are readily distinguishable by both their electron impact and chemical ionization mass spectra. The electron impact mass spectra of the isomeric oximes are practically identical; however, the chemical ionization spectra show that the E isomer forms more stable [MH]⁺ and [MH? H₂O]⁺ ions than the Z isomer for which both the [MH]⁺ and [MH? H₂O]⁺ ions are relatively unstable. In electron impact the Z-phenylhydrazone shows a lower [M]⁺˙ ion abundance and more facile loss of H₂O than does the E isomer. This more facile H₂O loss also is observed for the [MH]⁺ ion of the Z isomer under chemical ionization conditions.     

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.     

Mass spectrometry in structural and stereochemical problems—CCXXXV : A hidden hydrogen rearrangement in long chained anilides

The mass spectra of deuterium labeled hexananilides show an unexpected rearrangement of a hydrogen atom from C-4 with formation of a C₆H₇N⁺ ion. The decomposition of the McLafferty rearrangement ion in these compounds is shown to involve expulsion of the elements of ethynyl alcohol rather than ketene.     

Characterization of N‐methylated amino acids by GC‐MS after ethyl chloroformate derivatization

Methylation is an essential metabolic process in the biological systems, and it is significant for several biological reactions in living organisms. Methylated compounds are known to be involved in most of the bodily functions, and some of them serve as biomarkers. Theoretically, all α‐amino acids can be methylated, and it is possible to encounter them in most animal/plant samples. But the analytical data, especially the mass spectral data, are available only for a few of the methylated amino acids. Thus, it is essential to generate mass spectral data and to develop mass spectrometry methods for the identification of all possible methylated amino acids for future metabolomic studies. In this study, all N‐methyl and N,N‐dimethyl amino acids were synthesized by the methylation of α‐amino acids and characterized by a GC‐MS method. The methylated amino acids were derivatized with ethyl chloroformate and analyzed by GC‐MS under EI and methane/CI conditions. The EI mass spectra of ethyl chloroformate derivatives of N‐methyl ( 1–18 ) and N,N‐dimethyl amino acids ( 19–35 ) showed abundant [M‐COOC₂H₅]⁺ ions. The fragment ions due to loss of C₂H₄, CO₂, (CO₂ + C₂H₄) from [M‐COOC₂H₅]⁺ were of structure indicative for 1–18 . The EI spectra of 19–35 showed less number of fragment ions when compared with those of 1–18 . The side chain group (R) caused specific fragment ions characteristic to its structure. The methane/CI spectra of the studied compounds showed [M + H]⁺ ions to substantiate their molecular weights. The detected EI fragment ions were characteristic of the structure that made easy identification of the studied compounds, including isomeric/isobaric compounds. Fragmentation patterns of the studied compounds ( 1–35 ) were confirmed by high‐resolution mass spectra data and further substantiated by the data obtained from ¹³C₂‐labeled glycines and N‐ethoxycarbonyl methoxy esters. The method was applied to human plasma samples for the identification of amino acids and methylated amino acids. Copyright © 2016 John Wiley & Sons, Ltd.     

What is the Structure of b2 Ions Generated from Doubly Protonated Tryptic Peptides?

A recent statistical study (Savitski, M. M.; Falth, M.; Eva Fung, Y. M.; Adams, C. M.; Zubarev, R. A. J. Am. Soc. for Mass Spectrom. doi: 10.1016/j.jasms.2008.08.003) of a large spectral database indicated that the product ion spectra of doubly protonated tryptic peptides fall into two distinct classes. The main factor distinguishing the two classes is the relative abundance of the y _N-2 fragment: for Class I spectra y _N-2 is the most abundant y fragment while for Class II other y ions dominate the corresponding spectra. To explain the dominance of y _N-2 for Class I spectra formation of a nontraditional b ₂ ion with a diketopiperazine (6-membered cyclic peptide) rather than an oxazolone structure was proposed. Here we present evidence from tandem mass spectrometry, hydrogen/deuterium exchange, and density functional calculations that do not support this proposal. Namely, that CID of doubly protonated YIGSR, YGGFLR, and YIYGSFK produce Class I product ion spectra, yet the b ₂ fragment is shown to have the traditional oxazolone structure.     

Collision-induced dissociation study of cyclization of α-diazo-ω-arylsulphonylaminoalkan-2-ones

The collision-induced dissociation mass-analysed ion kinetic energy (CID MIKE) spectra (electron impact and chemical ionization) of five α-diazo-ω-arylsulphonylaminoalkan-2-ones and corresponding N-arylsulphonylazetidin-3-ones and N-arylsulphonylpyrrolidin-3-ones were studied. The [M ? N₂]⁺˙ and [MH ? N₂]⁺ ions of two types of the diazo ketones provide CID MIKE spectra similar to those of the corresponding M⁺˙ and MH⁺ of the heterocyclic compounds, i.e. a cyclization analogous to that in solution takes place. For the other three types of diazo compounds the Wolff rearrangement prevails in both the gas and liquid phases. The effect of the substituents on the cyclization process was studied. The data obtained permit the results of acid-catalysed cyclization of similar diazo ketones to be predicted on the basis of their CID MIKE spectra. Chemical ionization provides a closer similarity with reactions in solution than electron impact ionization, which can be rationalized by the protonation of the diazo ketone molecule being the driving force of the cyclization reaction either in solution or in the ion source of a mass spectrometer.     

Mass spectrometry: XI—Electron impact induced fragmentation of some amides of F-acids and their deuterated homologues

The mass spectrometry of perfluoro compounds (F-alkyl compounds) has seldom been the subject of systematic studies. Fluorocarbons excepted, only a few mass spectra of such compounds have been analysed and corresponding fragmentations correlated. In this paper we report the mass spectra of 19 amides of perfluoro acids (R_FCONHR) with R=benzyl, 2-phenylethyl, 2-phenylpropyl, 3-phenylpropyl, 2-(N-phenylamino)ethyl, and R_F=F-methyl, n-perfluoropropyl, n-perfluoropentyl and n-perfluoroheptyl. These compounds exhibit quite different behaviour from their hydrocarbon homologues under electron impact (for instance no [R_FCO]⁺ fragment was found). Specific deuterium labelling and high resolution measurements have been used to show typical rearrangements and to establish the fragmentation routes.     

Mass spectral fragmentation pattern of 2,3-dimethyl- and 2-methyl-3-phenyl-4-arylazoisoxazol-5-ones

2,3-Dimethyl-4-arylazoisoxazol-5-ones and 2-methyl-3-phenyl-4-aryiazoisoxazol-5-ones undergo considerable fragmentation on electron impact. The base peaks in the mass spectra are at mass 56 and 118 respectively attributed to the N-methylacetonitrile cation and the N-methylbenzonitrile cation.     

A tandem mass spectrometric study of the N-oxides, quinoline N-oxide, carbadox, and olaquindox, carried out at high mass accuracy using electrospray ionization

A mass spectrometric study of three N-oxides, quinoline N-oxide, and the synthetic antibiotics carbadox and olaquindox, was carried out with a hybrid quadrupole/time-of-flight (TOF) mass spectrometer coupled with electrospray (ES) and atmospheric pressure chemical ionization (APCI) sources. The full scan mass spectra of the N-oxides obtained with ES are similar to those obtained with APCI, and the characteristic fragment ions corresponding to [M+H−O]⁺√ were observed in the full scan mass spectrum of each N-oxide examined. The protonated molecule of each N-oxide was subjected to collision-induced dissociation (CID) and accurate mass measurements were made of each fragment ion so as to determine its elemental composition. Fragment ions generated at enhanced cone voltages upstream of the first mass-resolving element were subjected to CID so as to identify the direct product ion–precursor ion relationship. Plausible structures have been proposed for most of the fragment ions observed. Elimination of OH√ radicals generated from the N→O functional group is a characteristic fragmentation pathway of the N-oxides. The expulsion of radicals and small stable molecules is accompanied by formation and subsequent contraction of heterocyclic rings.     

Mass spectral fragmentation of metal dithiocarbamate complex salts

A summary of the mass spectra of metal dithiocarbamate complex salts (ML₂ and ML₃) is presented. Only divalent metal dithiocarbamate ions without an electronic configuration containing an inert s-orbital electron pair exhibited both expulsion of a ligand radical (L) and the neutral even electron species (L–H) generated from the ligand via hydrogen transfer to the metal-containing fragment ion. Divalent metal dithiocarbamate ions can be generated either by direct electron impact ionization of gas phase ML₂ molecules or ionization of ML₃ molecules followed by loss of a ligand radical. A highly stable sp² hybridized, gas phase ion of a monobidentate lead dithiocarbamate complex is proposed.     

Retrosynthetic reactions in mass spectrometry. Deacylation of ortho phthalamic acids under electron impact

Ortho Phthalamic acids under electron impact show a retrosynthetic reaction leading to both phthalic anhydride and amine complementary ions, the corresponding neutrals of which are the usual synthetic precursors of the original compounds. The single case of a primary amine derivative is examined, which shows the formation of [M? H₂O]^+˙ ions having the structure of the related N-substituted imide, by a process which parallels a well known thermal reaction. It also gives the species [C₈H₆NO₂]⁺ (of the same nominal mass as phthalic anhydride), the structure of which is still under study. Ionic structures are supported by collision induced mass analyzed ion kinetic energy spectra.