A mass spectrometry/mass spectrometry investigation of the nature of [C10H10]+˙, [C9H7]+ and [C10H8]+˙ gas phase ions

Additional evidence for the rearrangement of the 1- and 3-phenylcyclobutene radical cations, their corresponding ring-opened 1,3-butadiene ions and 1,2-dihydronaphthalene radical cations to methylindenetype ions has been obtained for the decomposing ions by mass analysed ion kinetic energy spectroscopy (MIKES). The nature of the [C₉H₇]⁺ and [C₁₀H₈]^+˙ daughter ions arising from the electron ionization induced fragmentation of these [C₁₀H₁₀]^+˙ precursors has been investigated by collisionally activated dissociation (CAD), collisional ionization and ion kinetic energy spectroscopy. The [C₉H₇]⁺ produced from the various C₁₀H₁₀ hydrocarbons are of identical structure or an identical mixture of interconverting structures. These ions are similar in nature to the [C₉H₇]⁺ generated from indene by low energy electron ionization. The [C₁₀H₈]^+˙ ions also possess a common structure, which is presumably that of the maphthalene radical cation.     

Charge stripping mass spectra: A method for identifying isomeric [C5H8]+˙ and [C3H6]+˙ ions

Charge stripping (collisional ionization) mass spectra are reported for isomeric [C₅H₈]⁺˙ and [C₃H₆]⁺˙ ions. The results provide the first method for adequately quantitatively determining the structures and abundances of these species when they are generated as daughter ions. Thus, loss of H₂O from the molecular ions of cyclopentanol and pentanal is shown to produce mixtures of ionized penta-1,3- and -1,4-dienes. Pent-1-en-3-ol generates [penta-1,3-diene]⁺˙. [C₃H₆]⁺˙ ions from ionized butane, methylpropane and 2-methylpropan-1-ol are shown to have the [propene]⁺˙ structure, whereas [cyclopropane]⁺˙ is produced from ionized tetrahydrofuran, penta-1,3-diene and pent-1-yne.     

1,2-Carbon shifts in [C4H8O]+˙ and [C5H10O]+˙ ions

Present results demonstrate that α,β-shifts of the functional group carbon strongly dominate β,α-methyl shifts in [C₄H₈O]⁺˙ and [C₅H₁₀O]⁺˙ ions, paralleling observations of others on methyl isobutyrate ions.     

Photodissociation of gaseous [C5H8]+˙ ions

Photodissociation permits the distinction of four isomeric [C₅H₈]^+˙ ions (ionized 2-pentyne, 1,2-pentadiene, 1,3-pentadiene and cyclopentene) which cannot be identified via collisional activation spectrometry. Both the relative cross-section for photodissociation and the relative abundance of the photodissociated fragments can be used to characterize the ion structure. Furthermore, upper and lower limits for the barrier for interconversion between 1,3-pentadiene and the other isomers can be determined.     

Metastable [C4H8O]+˙ ions: Additional decompositions via the [2-butanone]+˙ ion

The losses of methyl and ethyl through the intermediacy of the [2-butanone]⁺˙ ion are shown to be the dominant metastable decomposition of 14 of 19 [C₄H₈O]⁺˙ ions examined. The ions that decompose via the [2-butanone]⁺˙ structure include ionized aldehydes, unsaturated and cyclic alcohols and enolic ions. [Cyclic ether]⁺˙ [cyclopropylmethanol]⁺˙ and [2-methyl-1-propen-1-ol]⁺˙ ions do not decompose through ionized 2-butanone. The rearrangements of various [C₄H₈O]⁺˙ ions the the 2-butanone ion were investigated by means of deuterium labeling. Those pathways involve up to eight steps. Ions with the oxygen on the end carbon rearrange to a common structure or mixture of structures. Those ions which ultimately rearrange to the [2-butanone]⁺˙ ion then undergo oxygen shifts from the terminal to the second and third carbons at about equal rates. However, this oxygen shift does not precede the losses of water and ethylene. Losses of water and ethylene were unimportant for ions with the oxygen initially on the second carbon. Ionized n-butanal and cyclobutanol, but not other [C₄H₈O]⁺˙ ions, undergo reversible hydrogen exchange between the oxygen and the terminal carbon. Rearrangement of ionized n-butanal to the [cyclobutanol]⁺˙ ion is postulated.     

The decompositions of metastable [C4H8O]+˙ ions and the [C4H8O]+˙ potential surface

Collisionally activated decomposition (CA) spectra of [C₄H₈O]⁺˙ ions and the products of their metastable decompositions are used to refine a previously presented picture of the reactions of [C₄H₈O]⁺˙ ions. Metastable [C₄H₈O]⁺˙ isomers predominantly rearrange to the 2-butanone ion and decompose by loss of methyl and ethyl, although up to 38% of the methyl losses take place by other pathways to form \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{CH}}_{\rm{2}} = {\rm{CHCH = }}\mathop {\rm{O}}\limits^{\rm{ + }} {\rm{H}}{\rm{.}} $\end{document} . The CA spectra of many of the [C₄H₈O]⁺˙ ions with the oxygen on the first carbon are very similar, consistent with those ions isomerizing largely to common structures before or after collision. However, several of these ions have unique CA spectra, so they must remain structurally distinct from the majority of the [C₄H₈O]⁺˙ ions below energies required for decomposition. The CA spectra of ions with the oxygen on the second carbon are distinct from those of ions with the oxygen on the first carbon, so there is limited interconversion of the non-decomposing forms of the two types of ions. A potential energy diagram for the reactions of metastable [C₄H₈O]⁺˙ ions is constructed from appearance energy measurements. As would be expected, the relative importances of most of the [C₄H₈O]⁺˙ isomerizations seem to be inversely related to the activation energies for those processes. Some parallels between the isomerizations of [C₄H₈O]⁺˙ ions and those of related ions are pointed out.     

Photodissociation of gaseous [C4H5N]+˙ ions

The photodissociation of [C₄H₅N]⁺˙ ions generated by ionization of pyrrole (1), allyl cyanide (2), crotonitrile (3), cyclopropyl cyanide (4) and methacrylonitrile (5) has been studied using ion beam techniques. At least four different stable ion structures have been distinguished, which is in contrast to earlier CAD studies. In addition it has been shown that [C₂H₃N]⁺˙ fragment ions formed by dissociative ionization of the same precursors have structures which are distinct from that of ionized acetonitrile.     

Structure of gas phase [C5H6]+˙ ions

Charge-stripping spectra have been used to differentiate ionized cyclopentadiene from its acyclic isomers. The minimum amounts of translational energy lost during the charge-stripping processes and the relative charge-stripping efficiencies, which are also structurally important parameters, have been measured for these ionic species. [C₅H₆]⁺˙ ions, formed by dissociative ionization of various precursors in the ion source are found, usually, to be a mixture of cyclic and acyclic ions. In contrast, [C₅H₆]⁺˙ ions, derived from the dissociation of metastable molecular ions from a series of organic compounds, have the cyclopentadienyl structure. This structure was confirmed by collision-induced dissociation of ions formed in the first field-free region of a triple sector mass spectrometer.     

A structural investigation of [C6H6]2+ and [C6H6]+˙ ions involved in charge exchange reactions

Mass-analysed ion kinetic energy spectra for collisional activation (CA) of [C₆H₆]⁺˙ formed via electron capture by [C₆H₆]²⁺ ions in collision with neutral benzene molecules have been compared for the C₆H₆ isomers benzene, 1,5-hexadiyne and 2,4-hexadiyne. Comparisons of fragment abundance and total CA fragment yields were also made for [C₆H₆]⁺˙ ions generated by electron ionization (EI). CA conditions of ion velocity and collision gas pressure were identical in these comparisons. In general the fragment abundance patterns for the ions formed by charge exchange were very similar to those for singly charged benzene ions generated by EI. However, significant variations in CA fragment yield (the ratio of the total CA fragment ion abundance to the abundance of the incident unfragmented ions) were observed. It is not clear from the results whether these variations reflect structurally different ions or ions of different internal energies. The CA spectra of [C₆H₆]⁺˙ ions derived from charge exchange reactions between the benzene dication and the target gases He, Ne, Ar, Kr and Xe have also been recorded and, once again, very similar fragment abundance patterns were observed along with large variations in total CA fragment yields. Charge exchange efficiency measurements are reported for reactions between the benzene dication and the targets He, Ne, Ar, Kr, Xe and C₆H₆ (benzene) and also for the doubly charged ions derived from the linear C₆H₆ isomers. In the latter case Xe and benzene targets were used. The energetics and efficiency measurements for the former reactions suggest that for targets such as He and Ne the processes probably involve excited states of the doubly charged ions. The efficiencies measured for the latter reactions were distinctly different for the three C₆H₆ isomers and may indicate a strong dependence of charge exchange cross-section on doubly charged ion structure.     

Structures and stabilities of [C3H2]+ ˙ and [C3H2]2+ ions

Ab initio molecular orbital theory using basis sets up to 6-311G^{* *}, with electron correlation incorporated via configuration interaction calculations with single and double substitutions, has been used to study the structures and energies of the C₃H₂ monocation and dication. In agreement with recent experimental observations, we find evidence for stable cyclic and linear isomers of [C₃H₂]^{+ ˙}. The cyclic structure (, a) represents the global minimum on the [C₃H₂]^{+ ˙} potential energy surface. The linear isomer (, b) lies somewhat higher in energy, 53 kJ mol^?1 above a. The calculated heat of formation for [HCCCH]^{+ ˙} (1369 kJ mol^?1) is in good agreement with a recent experimental value (1377 kJ mol^?1). For the [C₃H₂]²⁺ dication, the lowest energy isomer corresponds to the linear [HCCCH]²⁺ singlet (h). Other singlet and triplet isomers are found not to be competitive in energy. The [HCCCH]²⁺ dication (h) is calculated to be thermodynamically stable with respect to deprotonation and with respect to C? C cleavage into CCH⁺ + CH⁺. The predicted stability is consistent with the frequent observation of [C₃H₂]²⁺ in mass spectrometric experiments. Comparison of our calculated ionization energies for the process [C₃H₂]^{+ ˙} → [C₃H₂]²⁺ with the Q_min values derived from charge-stripping experiments suggests that the ionization is accompanied by a significant change in structure.     

The structure of [C3H4N2]+· and [C5H5N2]+ ions formed from vinylimidazoles,studied by collisionally activated dissociation mass spectrometry

Mass spectra of the three isomeric vinylimidazoles have been compared and the structures of the fragment ions [C₃H₄N₂]^+· and [C₅H₅N₂]⁺ have been investigated by collisionally activated dissociation mass spectrometry. The greater part of the non-decomposing ions m/z 68 from 2-vinylimidazole and from 2-imidazolecarboxylic acid methyl ester, and a minor part of this ion formed from the free acid, all have the same structure: the imidazole ring system, with hydrogens at both nitrogen atoms but none at C(2). An analogous structure, with an ethyl group at C(2), is proposed for the m/z 93 ion from 2-vinylimidazole.     

Characterization of isomeric [CH5P]+˙ and [C2H7P]+˙ radical cations and some [C2H6P]+ phosphonium ions in the gas phase by their collisional activation spectra

Collisional activation spectra were used to characterize isomeric ion structures for [CH₅P]^+˙ and [C₂H₇P]^+˙ radical cations and [C₂H₆P]⁺ even-electron ions. Apart from ionized methylphosphane, [CH₃PH₂]^+˙, ions of structure [CH₂PH₃]^+˙ appear to be stable in the gas phase. Among the isomeric [C₂H₇P]^+˙ ions stable ion structures [CH₂PH₂CH₃]^+˙ and [CH₂CH₂PH₃]^+˙/[CH₃CHPH₃]^+˙ are proposed as being generated by appropriate dissociative ionization reactions of alkyl phosphanes. At least three isomeric [C₂H₆]⁺ ions appear to exist, of which \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} - \mathop {\rm P}\limits^{\rm + } {\rm H = CH}_{\rm 2} $\end{document} could be identified positively.     

A Study of C5H10 radical cations by collisionally activated dissociation,charge stripping,and low energy ion-molecule reactions

C₅H₁₀ radical cations generated from a variety of olefins and cycloalkanes were investigated by collisionally activated dissociation, charge stripping, and low energy ion-molecule reactions. It has been determined that all of the isomers studied can be distinguished by charge stripping, whereas collisionally activated dissociation and the ion-molecule reactions are less informative. The radical cations from cyclopentane, methylcyclobutane and substituted cyclopropanes retain their cyclic structures for at least a fraction of the population having lifetimes in the microsecond range.     

A study of [C7H8]+˙ and [C7H8]2+ ions formed from different precursor molecules

On the basis of unimolecular and collisionally activated decompositions, as well as their charge stripping behaviour, [C₇H₈]⁺˙ and [C₇H₈]²⁺ ions from a variety of precursors have been studied. In particular, structural characteristics of molecular ions of toluene, cycloheptatriene, norborna-2,5-diene and quadricyclane have been compared to those of [C₇H₈]⁺˙ and [C₇H₈]²⁺ rearrangement fragment ions obtained from n-butylbenzene, 2-phenylethanol and n-pentylbenzene. Severe interferences from [C₇H₇]^2+˙ ion fragmentations have been observed and rationalized.     

Propanoyl ion formation from metastable [C3H6O]+˙ ions

Collisionally activated spectra demonstrate that CH₃CH₂C?O⁺ rather than \documentclass{article}\pagestyle{empty}\begin{document}${\rm CH}_{\rm 2} = {\rm CHCH = }\mathop {\rm O}\limits^{\rm + } {\rm H}$\end{document} is formed in the metastable losses of hydrogen from [C₃H₆O]^+˙ ions with the oxygen on the first carbon. This provides another example of formation of an acyl ion following ‘ketonization’ prior to metastable decomposition.     

Structure determination of some C4H5N+˙ Ions from substituted pyridines by collisionally activated dissociation mass spectrometry

A standard procedure for recording and correcting collisionally activated dissociation mass spectra is proposed, and used to distinguish between the C₄H₅N^+˙ ions formed from hydroxy- and amino-pyridines after loss of CO and HCN, respectively. It is concluded that these ions are cyclic. From the 4-isomers the 3H-pyrrole ion is formed whereas from 2-hydroxypyridine the 1H-pyrrole ion is formed. In the other cases, mixtures of 2H- and either 1H- or 3H-pyrrole ions are generated, depending on the nature of the precursor.     

Structure of [C3H5O]+ ions produced from unimolecular decomposition of several [C4H8O]+˙ metastable ions

It is demonstrated by means of collisionally activated decomposition (CAD) that [C₃H₅O]⁺ originating from metastable [C₄H₈O]^+˙ ions are either acylium [C₂H₅CO]⁺ (a) or hydroxycarbenium [CH₂CHCHOH]⁺ (b). Butanone gives exclusively a but 2-methyl-2-propen-1-ol, 2-buten-1-ol, 3-buten-1-ol, butanal and 2-methylpropanal lead to ion b. Both structures a and b are produced from 3-buten-2-ol. These results are discussed in conjunction with experimental and calculated (MINDO/3) thermodynamic data.     

Water loss from [C5H10O]+˙ isomers: A reassessment

From a detailed analysis of metastable peak shapes it is proposed, contrary to earlier conclusions, that the loss of H₂O from the molecular ions of the C₅H₁₀O isomers cyclopentanol, pentanal and pent-1-en-3-ol yields only [penta-1,3-diene]^+˙ in the metastable time frame. Therefore the composite metastable peak for this process arises from two competing reaction channels yielding a common daughter ion. The observation that the first two isomers also produce [penta-1,4-diene]^+˙ as a daughter ion is attributed to a high energy (ion source) reaction which contributes negligibly to first field free region fragmentations.