Isomerization of linear octene cations in the gas phase

Borchers, Friedrich; Levsen, K.; Schwarz, Helmut; Wesdemiotis, Chrysostomos; Winkler, Hanna

doi:10.1021/ja00461a031

Cited by 54 publications

(22 citation statements)

References 2 publications

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“…molecular ions, and their [C,H,]+ daughter thus been identified among molecular ions of C,H8 ions are presented in Table 1. together with the (3, C5Hlo (lo), C,H,, ( l l ) , C6H12 (lo), C7Hl, (2), kinetic energy releases evaluated from the halfand C8Hl, (9). From the above it is evident that the height widths of the metastable peaks for CHIe loss.…”

Section: Introductionmentioning

confidence: 88%

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On the formation of cyclopentenium cations from all C₆H₁₀^+• molecular ion structures

Wolkoffs¹,

Holmes²,

Lossing³

1980

Can. J. Chem.

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PEDER WOLKOFF, J O H~ L. HOLMES, and F. P. LOSSING. Can. J. Chem. 58.251 (1980). The structures of [C,H,]+ ions produced by methyl loss from thirty isomeric C6Hl, molecular ions have been assessed from measurements of ionic heats of formation and the kinetic energy release associated with the fragmentation. It is proposed that the thern~odynamically most stable [C,H,]' ions [cyclopentenium]' are formed a t the dissociation threshold irrespective of the initial molecular ion structure. Mechanistic aspects of the fragmentations were studied by mean of deuterium labelling.Observations of metastable peak shapes for labelled compounds permitted the identification of reaction pathways having different energy requirements. The lowest energy routes involved little or no positional mixing of H and D prior to fragmentation. It is proposed that the molecular ions with sufficient energy to fragment on the ps time scale undergo successive (and reversible) isomerization by allylic 1,3-hydrogen shifts to their isomeric isolated diene molecular ions which, by an allylic 1,2-hydrogen shift, may isomerize to a methylcyclopentene molecular ion prior to loss of methyl. The sequence of isomerization of the molecular ions is alkyne -t allene 1 ; conjugated diene 1 ; isolated diene. The de-conjugation step has an activation energy of ca. 10 kcal m o l l . We suggest. however. that allene molecular ions rearrange directly to isolated diene molecular ions. PEDER WOLKOFF. JOHN L. HOLMES et F. P. L o s s~s c .Can. J. Chern. 58.251 (1980). O n a determine la structure des ions [C,H,]' provenant de la pel-te d'un methyle par trente ions moleculaires isomeres C6H,,. a partir des mesures de chaleur ionique de formation et de l'energie cinetique associees a la fragmentation. On propose que les ions [cyclopentenium]+ [C,H,]+ les plus stables thermodynamiquement, sont formes au debut de la dissociation quelle que soit la structure de I'ion moleculaire initial. On a etudie le mecanisme de ces fragmentations par marquage isotopique au deuterium. L'observation de la forme des pics mCtastables des composes marques a perrnis d'identifier des mecanismes reactionnels requerant des energies differentes. La voie exigeant la plus faible energie implique peu ou pas de melange de position de l'hydrogene ou d u deuterium avant la fragmentation. On propose egalement que les ions moleculaires ayant suffisamment d'energie pour se fragmenter dans des temps de I'ordre de la ps subissent des isomerisations successives. (et reversibles) par transpositions 1,3 d'hydrogenes allyliques conduisant a leurs ions moleculaires isomeres comportant des dienes non conjugues, qui par une transposition allylique-1,2 peuvent s'isomeriser en un ion moleculaire methylcyclopentene avant la perte d'un methyle. Lesions moleculaires s'isomerisent suivant la sequence: alcyne + allene + diene conjugue + diene non conjugue. L'etape de deconjugaison a u n e energie d'activation de I'ordre de 10 kcal m o l l . Nous suggerons cependant que I'ion moleculaire allenique se transpose en ion mol...

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Section: Introductionmentioning

confidence: 88%

“…Incomplete atom randomization prior to fragmentation on the ps time scale only becomes prevalent in ions containing six or more C atoms, e.g., in dimethylpentenes (9, heptenes (6,7), C,H,, isomers (8), and octenes (9).…”

Section: Introductionmentioning

confidence: 99%

On the formation of cyclopentenium cations from all C₆H₁₀^+• molecular ion structures

Wolkoffs¹,

Holmes²,

Lossing³

1980

Can. J. Chem.

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“…In a recent study of the isomeric octenes (16), it was concluded that the molecular ions had isomerised isotopic labelling ( 6 ) , have led to the identification of to a common structure (or structures) in less than reacting configurations of fragmenting ions, daughter s. However, at times shorter than 10-1° s, the ion structures, and, in some cases, where good decomposing molecular ions retained their structural thermochemical data are available, to the identifica-integrity and decomposed by specific mechanisms. tion of the neutral fragment's structure as well (7).…”

Section: Introductionmentioning

confidence: 99%

Isomeric cyclic [C₆H₁₀]^+• ions. The energy barrier to ring opening

Wolkoff¹,

Holmes²

1979

Can. J. Chem.

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Appearance energies and metastable peak shapes for methyl loss from the molecular ions of cyclohexene, methylcyclopentenes, methylenecyclopentane, bicyclo[3.1.0]hexane, and 2-methyl-1,4-pentadiene indicate that they have a common reaction pathway which produces [cyclopentenium]+ as the daughter ion at threshold.From measurements of appearance energies and from relative peak abundances and kinetic energy releases for the metastable losses of CH3• and labelled methyl from the deuterium labelled cyclic [C6H10]+• molecular ions it was concluded that: (a) H/D mixing in [cyclohexene]+• occurs in the intact ring prior to a concerted methyl extrusion; (b) [methylcyclopentenes]+• loses methyl without any H/D mixing at or near to threshold by a straight C—C cleavage, possibly via the 3-isomer. Methyl losses involving H/D mixing are preceded by ring opening which has an activation energy of ca. 1 eV. However, [cyclopentenium]+• is again the daughter ion; (c) methylenecyclopentane and bicyclo[3.1.0]hexane molecular ions isomerize to methylcyclopentenes prior to methyl loss.H/D mixing (prior to methyl loss) in the molecular ion 1,1-2H2-2-methyl-1,4-pentadiene takes place before cyclisation to methylcyclopentene.

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“…' This and corresponding statements6 which are in conflict with our conclusion reached from collisional activation spectra of isomeric octanes that alkane molecular ions retain their original structure prior to decomp~sition,~ prompted us to review the original t Author to whom correspondence should be addressed $For Part VII, see Ref. 32. Ltd, 1911 mass spectrometric literature on labelled n-alkanes and to carry out this additional study on I3C and 2H labelled n-dodecane.…”

Section: Introductionmentioning

confidence: 97%

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

Levsen

Heimbach

Shaw

et al. 1977

Org. Mass Spectrom.

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The literature on the mass spectrometry of ' H and "C 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-['H61 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 central 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 eV 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, [CnH2,,+J+, and odd electron [C,H2,]? 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.

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Isomerization of linear octene cations in the gas phase

Cited by 54 publications

References 2 publications

On the formation of cyclopentenium cations from all C₆H₁₀^+• molecular ion structures

On the formation of cyclopentenium cations from all C₆H₁₀^+• molecular ion structures

Isomeric cyclic [C₆H₁₀]^+• ions. The energy barrier to ring opening

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

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Isomerization of linear octene cations in the gas phase

Cited by 54 publications

References 2 publications

On the formation of cyclopentenium cations from all C6H10+• molecular ion structures

On the formation of cyclopentenium cations from all C6H10+• molecular ion structures

Isomeric cyclic [C6H10]+• ions. The energy barrier to ring opening

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

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On the formation of cyclopentenium cations from all C₆H₁₀^+• molecular ion structures

On the formation of cyclopentenium cations from all C₆H₁₀^+• molecular ion structures

Isomeric cyclic [C₆H₁₀]^+• ions. The energy barrier to ring opening