Rearrangements and fragmentations of [C2H5S]+ ions

From deuterium labelling experiments it was concluded that metastable molecular ions of ethyl methyl sulfide lose a methyl radical with the formation of both [CH₃S?CH₂]⁺ amd [CH₃CH?SH]⁺˙ The fragmentation reactions of metastable ions generated with these structure are losses of C₂H₂, H₂S and CH₄. These reactoins and the preceding isomerizations have also been studied by means of deuterium labelling. From the results it is concluded that the three fragmentation reactions most probably occur from ions with a C? C? S skeleton. Appearance energy measurements for ions generated with the two structures above and all give rise to the same ΔH_f value for these three isomeric forms. Ab initio molecular orbitals calculations confirm that these three ions fortuitously have very similar heats of formation. A potential energy diagram rationalizing the isomerizations and the principal fragmentation reaction is presented.     

Rearrangements and fragmentations of metastable [C13H9S]+, [C14H11]+, [C13H11]+ and [C8H7S]+ ions

[C₁₃H₉S]⁺, [C₁₄H₁₁]⁺, [C₁₃H₁₁]⁺ and [C₈H₇S]⁺ ions with unknown structures were generated from two [C₁₄H₁₂S]^+·precursor ions by fragmentation reactions that must be preceded by extensive rearrangements. Ions with the same compositions, each with several initial structures, were prepared by simple bond-breaking reactions. Metastable characteristics were compared for each of the four types of ions. It was found than in all cases fast isomerization reactions occur prior to fragmentation, so that no information about the unknown ion structures could be obtained by comparison of the observed fragmentations of metastable ions.     

Structure and fragmentations of [C3H7S]+ ions

The principal fragmentation reactions of metastable [C₃H₇S]⁺ ions are loss of H₂S and C₂H₄. These reactions and the preceding isomerizations of [C₃H₇S]⁺ ions with six different initial structures were studied by means of labelling with ¹³C or D. From the results it is concluded that the loss of H₂S and C₂H₄ both occur at least mainly from ions with the structure [CH₃CH₂CH? SH]⁺ or from ions with the same carbon sulfur skeleton, with the exception of the ions with the initial structure [CH₃CH₂S? CH₂]⁺, which partly lose C₂H₄ without a preceding isomerization. For all ions, more than one reaction route leads to [CH₃CH₂CH?SH]⁺. It is concluded that the loss of H₂S is at least mainly a 1,3-elimination from the [CH₃CH₂CH?SH]⁺ ions. Both decomposition reactions are preceded by extensive but incomplete hydrogen exchange.     

The ions [CH3S]+, [C2H5S]+ and [CH3O]+ formed by electron-impact

The mass spectra of deuterated species shows that both the isomeric ions [CH₂?SH]⁺ and [CH₃? S]⁺ are formed in the ratio 2:1 from CH₃SH; the ions [CH₃CH?SH]⁺ and [CH₃CH₂S]⁺ in the ratio 0·8:1 from CH₃CH₂SH; and [CH₂?OH]⁺ and [CH₃? O]⁺ in the ratio 6·7:1 from methanol. The heats of formation of [CH₃S]⁺ and [C₂H₅S]⁺ are of the order of 222 and 203 Kcal.mole^?1 respectively. The isomeric ions cannot be distinguished on thermodynamic grounds.     

Gaseous [C2H3O]+ ions

[C₂H₃O]⁺ ions with the initial structures [CH₃CO]⁺, and [CH₂CHO]⁺ cannot be distinguished on the basis of their collisional activation spectra, demonstrating that these isomers interconvert at energies below their threshold for decomposition. Self-protonation of ketene leads to the [CH₃CO]⁺ ion, while the [C₂H₃O]⁺ ion generated from glycerol most probably has the structure of an oxygen protonated ketene [CH₂?C?OH]⁺.     

Electron impact studies. CXII—The properties of positive ions produced by charge stripping from negative ions. [C6H5O]+, [C6H5S]+ and [C7H7S]+

The dissociative spectrum of the [C₆H₅S]⁺ ion derived by charge inversion from [C₆H₅S]^?, shows a variety of fragmentations including the competitive losses of H?, C₃H₄ and the formation of [CHS]⁺. The spectrum of a deuteriated derivative shows that these three processes are preceded or accompanied by H/D scrambling. The corresponding [C₆H₅O]⁺ species also undergoes hydrogen scrambling prior to fragmentation. In marked contrast, the ion [p-MeC₆H₄S]⁺ does not undergo hydrogen randomization between the methyl and aryl groups, and positional integrity is retained during fragmentation. These results are compared with the properties of the same ions produced by conventional ionization.     

[C7H7]+ and [C6H5]+ fragment ions produced from methylphenol isomers by electron impact

Appearance energies for [C₇H₇]⁺ and [C₆H₅]⁺ fragment ions obtained from methylphenol isomers were measured at the threshold using the electron impact technique. Different processes for the formation of the ions are suggested and discussed. Metastable peaks were detected and the kinetic energies released were determined. The results indicate that [C₇H₇]⁺ ions are formed from metbylpbenois with both benzyl and tropylium structures, whereas [C₆H₅]⁺ ions are formed with the phenyl structure at the detected thresholds. Kinetic energies released on fragmentation of reactive [ C₇H₇]⁺ and [C₆H₅]⁺ ions were used as a probe for the structure of the ions at 70 eV.     

Structure and fragmentation mechanism of [C3H7S]+ ions

From a comparison of the metastable ion bundance ratios for loss of C₂H₄ and H₂S from [C₃H₇S]⁺ ions in a series of alkyl thio ethers and thiols it was concluded that in most compunds these ion s isomerize to a common structure prior to decomposition in the first field free region. The mechanism for C₂H₄ loss from the [C₃H₇S]⁺ ion gen erated from CH₃SCH₂CH₃ was investigated in detail using ¹³C and ²H labelling. A rearrangement with formation of a cyclic intermediate prior to the decompistion reaction is proposed. The fragmentation is preceded by extensive hydrogen scrabling. The carbon atoms of the expelled C₂H₄ molecule are those of the CH₂?CH₃ moiety.     

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.     

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.     

Assigning structures to [C2H3O]+ ions

The problem of assigning structures to [C₂H₃O]⁺ ions produced from a wide variety of precursor molecules has been readdressed. The identification of the acetyl cation, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{CH}}_{\rm{3}} \mathop {\rm{C}}\limits^{\rm{ + }} = {\rm{O}} $\end{document}, from metastable peak characteristics and collisional activation mass spectra appears to be straightforward. The structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{CH}}_{\rm{2}} = \mathop {\rm{C}}\limits^{\rm{ + }} - {\rm{OH}} $\end{document} is also known to exist as a stable ion. A third ion, whose structure may be represented as \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm{C}}\limits^{\rm{ + }} {\rm{H}}_{\rm{2}} {\rm{CHO}} $\end{document} or has also been characterized.     

Isomerisation of hydrocarbon ions III–[C8H8]+·, [C8H8]2+, [C6H6]+· AND [C6H5]+ IONS

Collisional activation spectra of [C₈H₈]⁺·, [C₈H₈]²⁺, [C₆H₆]⁺· and [C₆H₅]⁺ ions from fifteen different sources are reported. Decomposing [C₈H₈]⁺· ions of ten of these precursors isomerise to a mixture of mainly the cyclooctatetraene and, to a smaller extent, the styrene structure. Three additional structures are observed with [C₈H₈]⁺· ions from the remaining precursors. [C₈H₈]^2+., [C₈H₈]⁺·, [C₆H₆]⁺· and [C₆H₅]⁺· ions mostly decompose from common structures although some exceptions are reported.     

The heats of formation of some protonated olefinic carbonyl compounds: [C5H9O]+ and [C4H7O2]+ ions

The proton transfer equilibrium reactions involving 3-penten-2-one, 3-methyl-3-buten-2-one, crotonic acid and methacrylic acid were carried out in an ion cyclotron resonance (ICR) spectrometer. The semiempirical method MNDO, used to estimate the heats of formation for 14 protonated [C₅H₉O]⁺ and [C₄H₇O₂]⁺ ions and the energetic aspect of the fragmentations of metastable [C₆H₁₂O]^+. and [C₆H₁₂O₂]^+. ions, leads to the conclusion that the ions corresponding to protonation at the carbonyl oxygen are the most stable. Thus the experimentally determined heats of formation of protonated olefinic carbonyl compounds can be attributed to the following structures: [CH₃COHCHCHCH₃]⁺ (δH_f = 490 KJ mol^?1), [CH₃COHC(CH₃)CH₂]⁺ (δH_f = 502 KJ mol^?1), [HOCOHCHCHCH₃]⁺ (δH_f = 330 KJ mol^?1) and [HOCOHC(CH₃)CH₂]⁺ (δH_f = 336 KJ mol^?1).     

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.     

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.     

The isomerization mechanism of alkylamines: Structure of [C2H6N]+ and [C3H8N]+ fragment ions

The MIKE spectra of amines RCH₂NH₂ containing more than five carbon atoms exhibit m/z 44 and m/z 58 peaks. The structures of these [C₂H₆N]⁺ and [C₃H₈N]⁺ ions have been established by collisional activation spectra. The results are in agreement with the fragmentation mechanisms previously proposed.     

Rationalization of scrambling processes among [C2H3O]+ ions

Scrambling data for the three observed [C₂H₃O]⁺ isomers, namely [CH₃CO]⁺ (a), [CH₂COH]⁺ (b) and (c), are rationalized by using ab initio molecular orbital calculations. For ions a and c, processes leading to scrambling of the carbon atoms require substantially more energy than the threshold for decomposition to [CH₃]⁺ + CO. Accordingly, little or no carbon scrambling is predicted nor is any observed in the metastable dissociation of a and c. The observed carbon scrambling in b prior to metastable dissociation to [CH₃]⁺ + CO has previously been explained in terms of a mechanism involving the oxiranyl cation (c). However, this mechanism is shown to be unlikely because of the high energies involved. An alternative lower-energy pathway involving the intermediacy of protonated oxirene (h) is proposed. Such a mechanism is fully compatible with the experimental data.