An electron impact study of HCN elimination from indole by use of 13C labelling

The study of the loss of HCN from the molecular ions of [2-¹³C]indole and [3-¹³C]indole shows that, to a good approximation, only the two carbon atoms of the pentagonal ring are involved in this fragmentation process, contrary to the behaviour of the H atoms; the C-2 atom is eliminated predominantly, chiefly in the ion source (85–90%) and a little less in the metastable energy range (75–80%). The losses of ¹³CCH₃^˙ and C₂H₃^˙ from the [M? H¹²CN]^+˙ ions of the two compounds suggest the occurrence of different structures, providing evidence for several mechanisms of HCN elimination.     

An electron impact study of HCN elimination from indolizine by use of 13C labelling

The study of the loss of HCN from the molecular ions of [1-¹³C]-, [2-¹³C]- and [3-¹³C]-indolizine shows that, if the C-3 atom is eliminated predominantly, as may be expected, the C-2 atom, and (a) carbon atom(s) of the hexagonal ring are also involved. The losses of ¹³CCH₃^. and C₂H₃^. from the [M? H¹²CN]^+˙ ions of the three compounds point to the interference of distinct mechanisms of HCN elimination, leading to different structures for the [C₇H₆]^+˙ ions.     

The loss of CO and HCN in the fragmentation of isatin and the methyl isatins

3-¹³C-Isatin has been shown to eliminate the 2-carbonyl group as the initial fragment, followed predominantly by loss of HCN. In 4-, 5-and 6-methyl-isatin the loss of the second carbonyl group competes evenly with HCN loss, whilst the 7-methyl isomer fragments almost exclusively by successive loss of two molecules of CO from the molecular ion.     

Identification of two separate mechanisms for HCN loss from gas phase quinoline cations

Critical energy measurements on ¹³C-labelled quinolines show that the energy requirement for elimination of H(C-2)N from metastable ions is lower than that for H(C-3)N elimination, the critical energies being 4.03 ± 0.05 eV and 4.17 ± 0.05 eV, respectively. It is also shown that involvement of C-2 and C-3 in unimolecular HCN elimination from ions in the second field-free region is greater, by factors of approximately 3 and 2 respectively, than would be anticipated for elimination following complete carbon randomization.     

The fragmentation of organic molecules under electron-IMPACT—II. The mass spectra of tetronic acid and some of its derivatives

The mass spectra of tetronic acid and of a number of its derivatives are recorded and discussed. Where necessary, the composition of the fragment ions has been checked by high resolution mass measurements. Fragmentation of the molecular ion is explained in terms of the rupture of a bond attached to C-4, and the predominant routes for many of the compounds can be explained on the basis of two schemes, the first involving preliminary ketonisation of the molecular ion, followed by elimination of carbon monoxide, the second involving rupture of the C-4? R³ bond prior to elimination of carbon monoxide. Special structural features, however, provide alternative routes.     

Fragmentation behaviour and ring expansion of 1-methylimidazole and 1-methylpyrazole upon electron-impact

The relative losses of unlabelled vs. labelled HCN from the [M]⁺˙ and [M – 1]⁺ ions of a number of specifically labelled 1-methylimidazoles (I) and 1-methylpyrazoles (II) have been determined. Hydrogen randomisation in the molecular ions prior to fragmentation is insignificant. Expulsion of HCN follows two distinct pathways: elimination involving positions 2 and 3 (predominant in I) and elimination involving the methyl group and the nitrogen atom at position 1 (predominant in II). The molecular ions eject H˙ from the methyl groups to a high degree of specificity. In both cases some contribution by position 5 is observed. The resultant [M – 1]⁺ ions exhibit extensive, but incomplete hydrogen randomisation. Loss of HCN from these ions is consistent with intermediacy of ring-expanded ions, but notably in II a proportion of the HCN is generated from the group. A mechanism for this observation is presented.     

The Mass Spectrometric Fragmentation of 1-Alkyl Ions derived from Halides

The mass-spectrometric fragmentation of n-pentyl, n-hexyl, n-octyl and n-nonyl ions has been studied using ¹³C- and D-labelling. The ions were produced from the corresponding halide ions. The loss of an olefin as a neutral fragment is the main reaction. The elimination of this fragment must be by a complex mechanism, since the terminal carbon atoms have the smallest probability of being lost with the neutral fragment. On chains with five to six carbon atoms, hydrogen scrambling seems to preceed the fragmentation; this is not true for hydrogen on terminal positions of longer chains. Ring formation prior to the fragmentation could explain some of the results; but no reasonable conclusion could be reached.     

Fragmentation de la Méthyl-2 Indolizine sous Impact Electronique: perte de H˙ à partir de l'Ion Moléculaire; perte de HCN à partir de l'Ion [M?H˙]+

The origin of the hydrogen radical lost in the ionization chamber from the molecular ion of 2-methylindolizine has been studdied by examination of the spectra of four specifically deuterated species. Hydrogen loss involves preferentially a hydrogen from the methyl substituent but also one of the hydrogens of either ring, especially those of the 5-membered ring. The HCN elimination from the metastable [M? H^˙]⁺ ions was studied using a linked scan method; the results are consistent with loss of identity of all the hydrogen atoms of the precursor ion, which implies an extensive reorganization prior to fragmentation.     

Mass spectra of 1,2,4-triazoles—II: Alkyl 1,2,4-triazoles

The mass spectra of monomethyl 1,2,4-triazoles contain fragment ions produced by specific cleavage of the heterocyclic ring. A major fragmentation from many molecular ions involves the elimination of HCN, but loss of N₂ is either very small or completely absent. No N or H scrambling occurs within the triazole ring system, as evidenced by labelling studies. The loss of a hydrogen atom from the molecular ions of 3-alkyl-1,2,4-triazoles (alkyl ? C₂H₅) originates from hydrogens attached to the β carbon and nitrogen atoms.     

Zur Lokalisierung Funktioneller Gruppen mit Hilfe der Massenspektrometrie. XV—Massenspektren von androstanen mit Hydroxygruppen in den Positionen 3, 16 und 17

Compared with other isomeric androstane triols the mass spectra of those androstane triols with hydroxy groups in positions 3, 16 and 17 are characterized by intense molecular ions. Important fragment ions show the loss of C-15, C-16 and C-17 in the form of a fragment comprising 75 mass units. This ion shows facile loss of the hydroxy group in position 3 in the form of a water molecule producing a [M ? 93]⁺ ion. Further key ions are at m/e 60, 84 and 110. With the aid of three deuterated androstane trioles and high resolution it was shown that these ions contain parts of the D-ring system.     

Loss of hydrogen cyanide from monocyanopyridines upon electron impact: Dewar pyridine structures and ring opening—Ring closure reactions revealed by 13C and 15N Labelling

Extensive ¹³C and ¹⁵N labelling has shown that the molecular ions of 2-, 3- and 4-cyanopyridine with lifetimes up to 10^?6 s eliminate hydrogen cyanide originating predominantly from the ring (?65%). Moreover, this hydrogen cyanide loss occurs after an equilibrated positional interchange of the ring carbon atoms at positions interchange of the ring carbon atoms at positions 2, 4 and 6 via Dewar pyridine structures. In molecular ions with lifetimes of 10^?6–10^?5 s skeletal rearrangements have taken place in such a way that both nitrogen atoms have become equivalent prior to the loss of hydrogen cyanide. Arguments are put forward that this equivalence of nitrogen atoms is caused by the intermediacy of ions with a 1,4-dicyanobuta-1,3-diene structure. About 60% of these intermediate ions eliminate hydrogen cyanide in a fast process. The remaining 40% of these ions undergo ring closure again to a pyridine ring in which the carbon atoms of positions 2, 4 and 6 are positionally interchanged rapidly via Dewar pyridine structures followed by ring opening again and eventual loss of hydrogen cyanide. This interpretation of the ¹³C and ¹⁵N labelling results is further corroborated by a study of the loss of hydrogen cyanide from molecular ions of 1,4-dicyanobuta-1,3-diene labelled with ¹³C in both cyano groups.     

Cyclization processes in the fragmentation of the molecular ions of N-(aza-9-fluorenylidene)amines

The dissociative ionization of 17 Schiff bases obtained from 2(4)-azafluorenones and linear benzo-1,4-diazafluorenone was investigated. The intensities of the [M-H]⁺ and [M-CH₃]¹ ion peaks depend on the structures of the ketone and imine parts of the molecules and are determined by the possibility of the occurrence of cyclization processes with the participation of their structural elements. The fragmentation of the investigated azomethines is also accompanied by the elimination of an NR particle and a hydrocarbon R radical by the molecular ions. This process takes place most easily when a cyclohexyl substituent is present in the imine fragment. In contrast to previously investigated azomethines, the loss of an HCN molecule by the M⁺ ion occurs without participation of the exocyclic nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 102–109, January, 1981.     

Stereochemical applications of mass spectrometry 4. Energy-resolved study of the fragmentation of esters of maleic and fumaric acids

The fragmentation of the dimethyl and diethyl esters of maleic and fumaric acids have been studied as a function of the internal energy of the molecular ions using charge exchange techniques and metastable ion studies in combination with isotopic labelling. The dimethyl ester molecular ions show distinctive behaviours at both low and high internal energies, indicating that interconversion of the molecular ions does not occur. The fumarate molecular ion fragments by elimination of CH₂O and (CO₂ + CH₃) in the metastable ion time-frame, while the maleate ester fragments primarily by loss of CH₃O. At higher internal energies both molecular ions fragment primarily by loss of CH₃O but the fragment ion from the maleate ester shows a greater stability, presumably because it assumes the cyclic cationated maleic anhydride structure. The diethyl maleate and diethyl fumarate molecular ions show identical metastable ion characteristics; in addition the [COS]⁺· charge exchange mass spectra are very similar. These results indicate that low-energy molecular ions interconvert. At higher internal energies interconversion does not occur, and, although both moiecular ions fragment by loss of C₂H₅O, the resultsint fragment ions show different stabilities and fragmentation reactions.     

Massenspektrometrie von Silylpyrazolen—I: Isomere Trimethylsilylpyrazole

By a comparison of the mass spectra of mono-, bis- and tris(trimethylsilyl)pyrazole isomers, characteristic influences of the different TMS-groups are revealed. An N-TMS-group forms an ion of mass 72 in the spectrum of N-TMS-pyrazole and with more than one TMS-group present, leads to the expulsion of a neutral fragment C₃H₈Si. Decomposition of the 4-TMS-group is more facile than the others, giving rise to a small molecular ion of intensity (approx.) 5%Σ₄₀. In N-silylated pyrazoles without a 4-TMS-group, intensive elimination of HCN from the [M ? CH₃˙]⁺ ion can be assigned to an unsubstituted C-3 position. In the case of unsymmetrically substituted pyrazoles without and N-TMS-group, silylation by an N-TMS acetamide may help to identify the predominating tautomer.     

Iron carbonyl complexes containing an azomethylene moiety III. Mass-spectrometric study of diiron hexacarbonyl complexes from azomethynes and azines

The electron impact mass spectra of diironhexacarbonyl complexes prepared from benzylidene- and α-naphthylidene-anilines, benzalazine and ketazines of acetophenone, p-bromoacetophenone and benzophenone are studied. The main fragmentation of the ligands occurs only after complete decarbonylation of the molecular ions and involves rupture of metal-ligand bonds, elimination of a part of the central ligand in the form of HCN and RCN, elimination of a neutral aromatic fragment or elimination of a part of the ligand.     

Collision-induced dissociation of uracil and its derivatives

The collision-induced dissociation of protonated uracil has been studied by tandem mass spectrometry using models extensively labeled with stable isotopes, and derivatives of the kinds found in nucleic acids. Following collisional activation at 30 eV translational energy, protonated uracil dissociates through two principal pathways which do not occur in electron ionization mass spectra: (1) elimination of NH3 almost entirely from N-3, followed by loss of CO from C-4, 0⁴; (2) loss of H2O, equally from 0² and 0⁴. Elimination of HNCO, also the principal dissociation process from odd-electron molecular ions, proceeds primarily by loss of N-3, C-Z, O² and 10% from N-l, C-Z, 0². Several secondary dissociation products are formed with quantitative site specificity of skeletal atoms: C,HO+ (4-C0, C-5, C-6); H2CN+ (N-l, C-6); C2H2+ (N-l, C-5, C-6). First-step dissociation reactions are interpreted in terms of pyrimidine ring opening at likely sites of protonation after collisional activation of MH+. Collision-induced dissociation mass spectra of uracils with structural themes common to nucleic acids (methylation, replacement of 0 by S, C-5 substitution) follow analogous reaction paths which permit assignment of sites of substitution, and exhibit ion abundance changes attributed to differences in substituent basicity and electron density.     

The long-lived molecular ion of propionitrile: An ion cyclotron resonance study

Evidence has been reported that primary loss of H and of HCN from the molecular ions of propionitrile, isobutyronitrile and butyronitrile in the mass spectrometer is preferentially preceded by hydrogen migration from C-2 to C-1. Ion cyclotron double resonance spectra of proton (or deuteron-) transfer products derived from propionitrile-2-d2 and -3-d3 and a series of bases provide evidence that such migration occurs also in long-lived propionitrile molecular ions.     

Mass spectra and structures of isomeric phenylaminopyrazoles

The processes involved in the dissociative ionization of isomeric phenylaminopyrazoles under the influence of electron impact were studied. The pathways of fragmentation of the molecular ion (M⁺) were proved rigorously by means of the spectra of the metastable ions. The empirical compositions of the fragment ions were confirmed by the high-resolution mass spectra. It was established on the basis of the mass spectra of the amino-group-deuterated analogs that M⁺ exists exclusively in the amide from. A rearrangement leading to the formation of benzodiazepine cation radicals precedes fragmentation of M⁺. The elimination of an HCN particle in the first step of the fragmentation of M⁺ does not involve the amino group. The pK_a values are presented for all of the investigated phenylaminopyrazoles.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1381–1388, October, 1978.     

Electron-impact studies—XLVII: The mass spectra of 2-aryl-1,3-dithianes and -1,3-dithiolanes

The fragmentation patterns in the spectra of 2-aryl-1,3-dithianes and -1,3-dithiolanes have been elucidated by deuterium labelling studies. Ortho effects are observed in the spectra of 2(o-alkoxyphenyl)-1,3-dithianes. The molecular ions of the 1,3-dithianes eliminate S2H· and metastable kpeaks substantiate these eliminations. The hydrogen involved in this elimination randomises (C-4, 5 and 6) Prior to elimination in the spectrum of 2-phenyl-1,3-dithiane, but originates mainly from C-5 in that of the bis propane-1,3-dithioacetal of terephthaldehyde.     

Isomerization of hydrocarbon ions. VI—parameters determining the isomerization of aliphatic hydrocarbon ions

The radical or non-radical character of aliphatic hydrocarbon ions determines the extent to which these ions equilibrate to a mixture of interconverting structures prior to decomposition. It is suggested that the radical (odd electron) ions have both lower thresholds for decomposition and higher barriers for isomerization that non-radical (even electron) ions, thus explaining their reduced tendency for isomerization. Moreover, the molecular size seems to be a major influencing factor for the isomerization of unsaturated hydrocarbon molecular ions. With decreasing molecular size isomerization prior to decomposition becomes more pronounced. Collisional activation spectra of fragment ions (formed by loss of H₂O from the corresponding alcohols) and of [C₅H₁₀]⁺ and [C₄H₈]⁺ molecular ions are reported in support of these conclusions.