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.     

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.     

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 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.     

[M + 11]+˙ and [M + 11]−˙ ions in the nitrogen positive and negative ion chemical ionization mass spectra of aromatic amines

The N₂ negative ion chemical ionization (NICI) mass spectra of aniline, aminonaphthalenes, aminobiphenyls and aminoanthracenes show an unexpected addition appearing at [M + 11]^?˙. This addition is also observed in the N₂ positive chemical ionization (PCI) mass spectra. An ion at [M – 15]^? is found in the NICI spectra of aminoaromatics such as aniline, 1- and 2-aminonaphthalene and 1- and 2-aminoanthracene. Ion formation was studied using labeled reagents, variation of ion source pressure and temperature and examination of ion chromatograms. These experiments indicate that the [M + 11]^?˙, [M – 15]^?˙ and [M + 11]^+˙ ions result from the ionization of analytes altered by surface-assisted reactions. Experiments with ¹⁵N₂, [¹⁵N] aniline, [2,3,4,5,6-²H₅] aniline and [¹³C₆] aniline show that the [M + 11]^?˙ ion corresponds to [M + N – 3H]^?˙. The added nitrogen originates from the N₂ buffer gas and the addition occurs with loss of one ring and two amino group hydrogens. Fragmentation patterns in the N₂ PCI mass spectrum of aniline suggest that the neutral product of the surface-assisted reaction is 1,4-dicyanobuta-1,3-diene. Experiments with diamino-substituted aromatics show analogous reactions resulting in the formation of [M – 4H]^?˙ ions for aromatics with ortho-amino groups. Experiments with methylsubstituted aminoaromatics indicate that unsubstituted sites ortho to the amino group facilitate nitrogen addition, and that methyl groups provide additional sites for nitrogen addition.     

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.     

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.     

Structural characterization of isomeric triazolocoumarins by high- and low-energy collision spectrometry of M+˙, [M + H]+ and [M − H]− species

Two monometayl- and four dimethyl-triazolocoumarin isomers were characterized by their electron impact mass spectra and by low-energy collision experiments performed on molecular ions M⁺˙ and other fragment ions with an ion-trap mass spectrometer. High-energy collision-activated dissociation measurements were performed on the protonated [M + H]⁺ and deprotonated [M ? H]^? molecular ion obtained by fast atom bombardment and M⁺˙ species produced by electron impact ionization on a double-focusing, reverse-geometry instrument. The data obtained allowed unequivocal structural identification of all the compounds investigated.     

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.     

Photodissociation and structures of [C6H4]+ ˙ ions generated from cyanobenzene and other precursors

Previous work on the electron impact induced loss of hydrogen cyanide from the radical cations of cyanobenzene has revealed that ring opening is important in the formation of the corresponding [C₆H₄]^{+ ˙} ions. Photodissociation experiments now show that these [C₆H₄]^{+ ˙} ions and those generated from 2-ethynylpyridine, 1,3-hexadiyn-6-nitrile and 1,2-diiodobenzene all photodissociate in the visible region to [C₄H₂]^{+ ˙}. The corresponding photodissociation spectra are all the same and have a maximum at about 370 nm, in agreement with spectra of ions with three conjugated double or triple bonds. Owing to the high reactivity, the low photodissociation rate and, possibly, the internal energy of the ions, the photodissociation kinetics are too complicated to be solved. The experiments nevertheless show that at least a major fraction of the [C₆H₄]^{+ ˙} ions has a ring-opened structure. This conclusion is supported by MNDO calculations, which indicate that the heats of formation of the possible acyclic structures are about 150 kJ mol^?1 lower than those of the o-, m- and p-benzyne structures.     

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.     

Competitive formation of M+˙ and [M + H]+ ions under fast atom bombardment conditions

The competitive formation of molecular ions M^+˙ and protonated molecules [M + H]⁺ under fast atom bombardment (FAB) conditions was examined using various kinds of organic compounds. The use of protic/hydrophilic matrices such as thioglycerol and glycerol resulted in relatively large values of the peak intensity ratio I([M + H]⁺)/I(M^+˙) compared with the use of relatively aprotic/hydrophobic matrices such as m-nitrobenzyl alcohol and o-nitrophenyl octyl ether. The change of matrix from thiol-containing such as thioglycerol and dithiothreitol to alcoholic such as glycerol and pentamethylene glycol increased the I([M + H]⁺)/I(M^+˙) ratio. Furthermore, the change of matrix increased the peak intensity ratio of the doubly charged ion [M + 2H]²⁺ to [M + H]⁺ in the FAB mass spectra of angiotensin I and gramicidin S. The addition of acids to the matrix solution increased the I([M + H]⁺)/I(M^+˙) ratio, although such an effect did not always occur. The acetylation of simple aniline compounds markedly increased the I([M + H]⁺)/I(M^+˙) ratio. It was concluded from these results that the hydrogen bonding interaction between hydroxyl groups(s) of the matrix and basic site(s) of analyte molecules in solution acts advantageously as a quasi-preformed state for [M + H]⁺ formation, and that the presence of significant proton acceptor(s) such as carbonyl group in analytes hinder the M^+˙ formation which may generally occur under FAB conditions. The formation of M^+˙ and [M + H]⁺ ions seemed to occur competitively, reflecting or according to the interaction or solvation states between the analyte and matrix molecules in solution and the structural characteristics of the analytes.     

The correlation of the intensities of the M+˙ and [M − H]+ ions in electron impact mass spectra of cyclic acetals and thioacetals containing a phenylalkoxyester group

The correlation between the intensities of the M^+˙ and [M ? H]⁺ ions of 47 cyclic acetals and thioacetals of alkyl formylphenoxyacetates and propionates as well as the fragmentation patterns in the vicinity of the molecular ion are discussed. The characteristic differences between the spectra of 1,3-dioxolanes, 1,3-oxathiolanes, 1,3-dithiolanes and 1,3-dioxanes are presented.     

Collisional activation spectra. Gas phase [C12H10O]+˙ and [C12H9O]+ ions from dephenyl ether,diphenyl carbonate,and o-, m- and p-phenylphenol

[C₁₂H₁₀O]⁺˙ ions in the mass spectra of diphenyl ether and diphenyl carbonate have the same structure, but one which differs from that of these ions from phenylphenol. [C₁₂H₉O]⁺ ions from all three compounds have isomerized to a common structure.     

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.     

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.