Stereochemical effects in electron impact mass spectra of cis-trans isomeric 1,2,4,5-tetramethylcyclohexanes

The mass spectrometric fragmentation of the five cis-trans isomeric 1,2,4,5-tetramethylcyclohexanes is influenced by their configuration. The thermochemically stable isomers show a higher abundance of the [M? CH₃]⁺ ions and a lower abundance of the [M? C₂H₅]⁺ ions than the strained isomers. The [M? CH₃]⁺/[M? C₂H₅]⁺ intensity ratio correlates with the enthalpy difference ΔH_isom and with the mean number of gauche arrangements Z_g^I of the stereoisomers.     

Stereochemical effects in the electron impact mass spectra of cis–trans isomeric 1,2,3,5-tetramethylcyclohexanes and some trimethylcyclohexanes

The mass spectra of 10 isomeric trimethylcyclohexanes and six cis–trans isomeric 1,2,3,5-tetramethylcyclohexanes are discussed. The thermochemically stable isomers show a higher abundance of the [M? CH₃]⁺ ions and a lower abundance of the [M? C₂H₅]⁺ ions than the strained isomers. The log of the intensity ratio for loss of methyl and ethyl from the molecular ions correlates well with the strain energies and with the mean number of gauche arrangements of the stereoisomers.     

Stereochemical effects in the electron impact mass spectra of some 2,6-dideoxy-3-c-methylhexopyranoses

The electron impact mass spectra of the methyl α-and β-glycosides of axenose, of a number of derivatives, and of mycarose, display substantial differences, characteristic of their stereochemistry. In order to interpret this behaviour the structures of the fragments and the main features of the fragmentation patterns have been established, and the methyl glycosides of some related monosaccharides (arcanose, cladinose and idose) have been examined. The experimental results allow the observed differences to be attributed to the presence of an intramolecular hydrogen bond between the hydroxyl group at C-3 and the oxygen atom of the methoxy group in the methyl α-glycosides of axenose and mycarose.     

Differentiation of isomeric cyclic diamides by electron impact mass spectra

The behaviour under electron impact of two series of isomeric cyclopentane- and cyclohexane-1, 2- and -1, 3-dicarboxylic acid dipiperidides was studied. Diagnostic fragmentation pathways were found to differentiate the isomeric diamides. Additional evidence was obtained from the metastable transitions.     

Stereochemical effects in the chemical ionization mass spectra of cyclic diols

The chemical ionization mass spectra of configurational isomers of many cyclic diols give substantial differences which are characteristic of their stereochemistry. For the cis-isomer of 1,3- and 1,4-cyclohexanediols, formation of a stable intramolecular proton bridge involving the OH groups gives rise to dominant MH⁺ peaks, suppressing the (M-H)⁺ peaks found in the spectra of the trans-isomers and monoalcohols. The stability of the proton bridge in cis-1,3-cyclohexanediol structures is decreased by a sterically interfering cis-5-methyl substituent, but increased by a cis-5-OH group due to additional proton solvation. cis-Stereochemistry also gives increased formation of the dimeric M₂H⁺ ions, but decreased formation of trimers, at higher diol concentrations, for the 1,3- and 1,4-diols. The similarity of the CI spectra of cis- and trans-1,2-cyclohexanediol are explicable in terms of the similarities of the most stable proton-bridged conformers; the reduced ring flexibility in cis- and trans-1,2-cyclopentanediols makes such similar structures unfavorable, as shown by the substantial differences between their CI spectra. The substantial, but expected, variations in behavior with temperature and reagent gas are useful for maximizing stereochemical effects on CI spectra; conditions of lowest energy are usually, but not always, the most useful.     

Proximity effects in the electron impact mass spectra of aurones and related compounds

The principal ions in the electron impact mass spectra of a series of 6-methoxyaurones have been shown to be due to four separate reactions associated with proximity effects involving the phenyl group and the coumaran-one residue. A detailed study with labelled derivatives has been supplemented by a study of the vinylogue 2-cinnamylidene-6-methoxycoumaran-3-one and compounds in which the aurone phenyl group has been replaced by α-naphthyl, β-naphthyl and 9-anthryl.     

Stereochemical applications of mass spectrometry. II—Chemical ionization mass spectra of isomeric dicarboxylic acids and derivatives

The H₂ and CH₄, chemical ionization mass spectra of the cis dicarboxylic acids, maleic and citraconic acid, show much more extensive loss of H₂O from [MH]⁺ than the trans isomers, fumaric acid and mesaconic acid. Similarly, esters of maleic acid show a much more facile loss of ROH (R=alkyl or phenyl) from [MH]⁺ than do esters of fumaric acid. Similar differences are observed in the chemical ionization mass spectra of the isomeric phthalic and isophthalic acids and derivatives, where the ortho isomers show more extensive fragmentation of [MH]⁺ than the meta isomers. The facile fragmentation of [MH]⁺ for the cis and ortho isomers is attributed to ROH elimination involving interaction between the two carboxylate functions and forming the stable cyclic anhydride structure for the fragment ion. By contrast ROH elimination from [MH]⁺ for the trans and metu isomers requires a symmetry-forbidden [1,3]-H migration in the carboxyl protonated species and cannot lead to the cyclic anhydride structure. The chemical ionization mass spectra of cis and trans cyclohexane-1,2-dicarboxylic acids are essentially identical and show extensive fragmentation of the [IMH]⁺ ion. Experiments using deuterium labelling show extensive carboxyl group interactions for both isomers. The chemical ionization mass spectra of maleanilic and phthalanilic acids and of the related anhydrides and imides also are reported, as are the electron impact mass spectra of diphenyl maleate, diphenyl fumarate, diphenyl phthalate, maleanilic acid and phthalanilic acid.     

Differences in the mass spectra of isomeric benzolactams

It is shown that 3- and 4-substituted dihydro-2-quinolones can be distinguished from the isomeric dihydro-1-isoquinolones by mass spectrometry. The [M - CO]⁺ ion is characteristic for the mass spectra of dihydroquinolone derivatives, whereas retrodiene fragmentation of the molecular ion is characteristic for dihydroisoquinolone derivatives. The intense [M - R]⁺ and [M - R, - H₂O]⁺ ion constitute evidence that the substituent is located in the 3 (for dihydroisoquinolones) or 4 (for dihydroquinolones) position. The processes that occur in the fragmentation were confirmed by data from the high-resolution mass spectra, an analysis of the observed metastable ions, and an analysis of the mass spectra of 3-methyl-3,4-dihydro-1-isoquinolone and 4-methyl-3,4-dihydro-2-quinolone containing deuterium attached to the nitrogen atoms.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 246–249, February, 1979.     

Fragmentation pathways of isomeric dodecenols studied by electron impact mass spectrometry

Electron impact-induced fragmentations of isomeric dodecen-1-ols were investigated using mass-analysed ion kinetic energy (MIKE) spectrometry and high-resolution data. The principal fragmentation pathways of the positional isomers studied are dominated by the C?C double bond and involve migration and radical-site rearrangements along the carbon chain with the aid of consecutive rearrangements of hydrogen atoms.     

Field ionization and electron impact mass spectra of cycloolefins

The field ionization mass spectra of monosubstituted cyclopentenes and cyclohexenes with C₁-C₇ n-alkyl and C₄-C₅ isoalkyl substituents in positions 1 and 3 have been investigated and compared with the previously reported electron impact mass spectra of these compounds. The cleavage of the C? C bond β to the double bond in the non-isomerized molecular ion was found to be a typical degradation reaction of the higher homologues in the strong electric field. So, by means of the field ionization mass spectra, the >C₁ alkyl substituent can be readily located in the parent molecule. The electron impact mass spectra exhibit a less specific fragment ion distribution for positional isomers due to the extensive molecular ion isomerization prior to decomposition, but provide useful information on the ring size. For structure determination it is appropriate to use both ionization techniques.     

The electron impact mass spectra of some rotenoid derivatives

The fragmentation behaviour of several rotenoids functionalized in the alicyclic portions of the molecule is described.     

The electron impact mass spectra of some cevine orthoacetates

The most important fragmentation in the electron impact mass spectra of dihydrocevine orthoacetate and some analogues involves loss of an acetoxyl radical from the molecular ion, and is associated with strain in the orthoacetate group bridging a six-membered ring. The process is much less important in the less-strained dihydrocevine isoorthoacetate, which gives the ion m/z 112, characteristic of cevine alkaloids, as the most abundant. A further important difference concerns an ion involving the loss of C(20) and its substituents together with a hydrogen atom from M⁺˙, also relatively unimportant in the isoorthoacetate.     

质谱学中的立体化学效应-公桕立体异构体的质谱研究

本文考察了十三个鬼桕毒及其衍生物的立体异构体的质谱性能,借助高分辨质谱裂解方式及特征离子相对丰度的影响,并提出了合理的解释。     

Stereochemical effects in the mass spectra of 2-hydroxy-, 5-hydroxy- and 2,5-dihydroxyprotoadamantanes

Electron ionization and methane and isobutane chemical ionization mass spectra have been obtained for the four configurationally isomeric 2,5-protoadamantanediols and the corresponding model monoalcohols. Although the electron impact (both 70 and 15 eV) and methane chemical ionization mass spectra of the 2,5-protoadamantanediols are distinct, the variations are not large and in general they are difficult to rationalize in terms of specific structural features. In contrast, the isobutane chemical ionization mass spectra of these diols show significant differences which can easily be correlated with sterochemical orientations of the substituents.     

Fragmentation selectivity in electron impact ionization mass spectra of substituted dimethoxybenzenes

Several 1-X-sabstitirted-3-methoxy-4-trideuteromethoxybenzens were synthesized and their electron impact ionization mass spectra were measured with an ionizing energy of 20 eV. From the peak intensity ratio of [M ? CD₃ ] and [M ? CH₃] the fragmentation-directing ability of the substituent X was evaluated. The most powerful group was found to be NH₂, which expelled a methoxy methyl group only from its para position. The CH₃ group and four halogen atoms, F, Cl, Br and I, exerted a moderate effect Electron-withdrawing groups such as NO₂, CHO and CN had only a little influence on the fragmentation selectivity. These results were interpreted in terms of the effect of X on the distribution of both the unpaired electron and the positive charge in the molecular ion.     

The electron impact and chemical ionization mass spectra of dimethyltrisulfide

Summary The mass spectra obtained by electron impact ionization (EI) of dimethyltrisulfide, both at constant sample pressure and during elution from a GC column, are essentially identical, with the molecular ion Me₂S ₃ ⁺ providing the basis peak. Masses heavier than the molecular ion are not observed. Chemical ionization, using nitrogen, methane or isobutane, gives rise to numerous ions of larger mass than that of the molecular ion. Particularly characteristic are sulfonium type structures Me₃S _n ⁺ , with n=3–6. In addition, radical cations of the type Me₃S_nCH ₂ ⁺ and protonated trisulfide, Me₂S₃H⁺, are observed, even with N₂ as ionizing gas, together with a variety of ions of lower hydrogen content. Further, a large number of ion types of lower mass than the parent molecule are formed. The mass distribution of ions in the spectrum is found to be highly dependent on the partial pressure of dimethyltrisulfide in the ion source. These phenomena were investigated and accounted for semiquantitatively.

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Die EIund CI-Massenspektren des Dimethyltrisulfids

Zusammenfassung Dimethyltrisulfid liefert nach Elektronenstoßionisation bei konstantem Probendruck und bei GC-Probeneinlaß im wesentlichen identische Massenspektren. Das Molekülion Me₂S ₃ ⁺ stellt dabei den Basispeak dar. Größere Massen als die des Molekülions wurden nicht beobachtet. Bei chemischer Ionisation unter Verwendung von Stickstoff, Methan oder Isobutan als Reaktantgas entstehen zahlreiche Ionen mit höherer Masse als der des Molekülions. Charakteristisch sind hierbei sulfoniumartige Strukturen Me₃S _n ⁺ mit n=3–6. Daneben beobachtet man Radikalkationen des Typs Me₂S_nCH ₂ ⁺ und protoniertes Trisulfid Me₂S₃H⁺ auch bei Ionisation mittels N₂ sowie Ionenarten mit einem geringeren Wasserstoffgehalt. Weiterhin wird eine größere Anzahl von Ionenarten gebildet, deren Massen kleiner sind als die des Stammoleküls. Die Massenverteilung der Ionen im Spektrum hängt außerordentlich stark vom Partialdruck des Dimethyltrisulfids in der CI-Quelle ab. Diese Abhängigkeiten wurden semiquantitativ untersucht und dargestellt.

     

Oxygen equilibration in the electron impact mass spectra of aryl esters and carbamates

The positional integrity of an ¹⁸O label is partially lost in the molecular ion in the electron impact mass spectra of 4-methylphenyl benzoate, 4-methylphenyl 4-nitrobenzoate and 4-methylphenyl N,N-dimethylcarbamate.