Potential energy profiles for the unimolecular reactions of [C3H7S]+ Ions

Appearance energies were measured for several types of [C₃H₇S]⁺ ions. From these appearance energy values the heats of formation of the ions were calculated. For the isomeric ions that could not be generated in the mass spectrometer, the heats of formation were estimated by means of the isodesmic substitution method. Transition state energies for the decomposition to C₂H₄ and [CH₃S]⁺ along two pathways were determined from the appearance energies of these ions. Using the energy values, potential energy diagrams were constructed for the rearrangements and decompositions of the [C₃H₇S]⁺ ions. A supplementary ¹³C Clabelling experiment is described for the determination of the rearrangement pathway of [CH₃CH₂CH?SH]⁺ ions prior to 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.     

Electron impact mass spectrometry of [C7H8N]+ and [C6H7]+ and [C6H7]+ fragment ions produced from o-toluidine and N-methylaniline

An energetic study of the production of [C₇H₈N]⁺ and [C₆H₇]⁺ fragment ions from o-toluidine and N-methylaniline is reported. The mechanisms for the formation of the ions are suggested. Metastable peaks associated with the formation and fragmentation of reactive [C₇H₈N]⁺ and [C₆H₇]⁺ ions were detected and kinetic energy released were determined. The results indicate that the [C₇H₈N]⁺ ion is formed at threshold from o-toluidine with an aminotropylium structure whereas for N-methylaniline the ion is formed with anN-phenylmethaniminium structure. [C₆H₇]⁺ ions are believed to be formed at threshold from the two precursors with a protonated benzene structure.     

Electron-impact-induced rearrangements of organic ions VIUnimolecular reactions of isomeric[C8H9]+ions

Metastable ion spectra and deuterium labelling have been used to investigate a series of gaseous [C₈H₉]+ ions of isomeric structures. The similarity of the intensities of their metastable loss of hydrogen, acetylene and ethylene molecules and metastable reactions of specifically labelled ions, suggests that the [C₈H₉]+ reacting ions, formed initially with different structures, isomerise to a common structure or mixture of structures via deep-seated rearrangement reactions which render all hydrogen atoms equivalent. The isomerisation process involved is controlled by a conversion of a vinyl bond into an allyl-type bond, thus destroying the aromatic moiety.     

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.     

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]⁺.     

Internal energy effects on metastable characteristics. The structure of [C3H7O]+ ions

The effect of changes in the internal energy distribution of the fragmenting ion on the ratio of metastable ion intensities for two competing fragmentation reactions has been investigated both theoretically and experimentally. Model calculations have shown that if the competing reactions have significantly different activation energies the metastable intensity ratio does depend on the internal energy distribution although large changes are necessary before the ratio changes by more than a factor of two. Experimentally the metastable characteristics of [C₃H₇O]⁺ ions of nominal structures [CH₃CH₂O⁺?CH₂] (I), [(CH₃)₂C?O⁺H] (II), [CH₃CH₂CH?O⁺H] (III) and [CH₃O⁺?CHCH₃] (IV) have been examined. For each structure the metastable characteristics are found to be distinctive and independent of changes in the internal energy distribution of the fragmenting ion where these changes result from altering the precursor of the [C₃H₇O]⁺ ions. It is suggested that these internal energy changes can be estimated from the fraction of [C₃H₇O]⁺ ions which fragment in the ion-source. It is concluded that structures I to IV represent stable and distinct ionic structures.     

Structures and relative energies of gas phase [C3H7O]+ ions

Ab initio molecular orbital calculations have been carried out for 17 possible isomeric [C₃H₇O]⁺ structures. Optimized geometries have been obtained with a split-valence basis set and improved relative energies determined with polarization basis sets and with incorporation of electron correlation. The results agree well with available experimental data. In particular, (CH₃)₂COH⁺, CH₃CH₂CHOH⁺, CH₃CHOCH₃⁺, CH₃CH₂OCH₂⁺, and have been confirmed as low-energy isomers. Six additional structures appear to be energetically accessible and to offer a reasonable prospect for experimental observation. These are CH₂CHCH₂OH₂⁺, CH₂C(CH₃)OH₂⁺, CH₃CHCHOH₂⁺, CH₂CHOHCH₃⁺, and .     

The energy surface for isomeric [C2H3O]+ ions: Further experimental evidence

Mass spectra from collisionally activated dissociation (CAD) of [C₂H₃O]⁺ ions, including isotopically labeled analogs, provide further information on the isomers [CH₃C?O⁺] (a), [CH₂?C?O⁺H] (b), [⁺CH₂CH?O] (c) and (d). Our data generally support the recent conclusions from theory by Radom and coworkers and from experiment by Terlouw, Holmes and coworkers. Most acetyl-containing molecular ions form a ions in high purity only at low energies, consistent with isomerization of higher energy molecular ions to form the more stable enol which dissociates to b. Isomer d, prepared from (CICH₂)₂CHOH, undergoes facile hydrogen scrambling, presumably through a degenerate 1,2-hydrogen shift. Theory suggests that c undergoes spontaneous isomerization to a and d; although [C₂H₃O]⁺ ions from BrCH₂CHO appear to consist of a and ～15% d, the latter are formed without substantial hydrogen scrambling.     

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.     

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.     

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.     

Structure and fragmentation of [C3H7O]+ ions formed by chemical ionization

The unimolecular metastable and collision-induced fragmentation reactions of [C₃H₇O]⁺ ions produced by gas-phase protonation of acetone, propanal, propylene oxide, oxetan and allyl alcohol have been studied. The CID studies show that protonation of acetone and allyl alcohol yield different stable ions with distinct structures while protonation of propanal or propylene oxide yield [C₃H₇O]⁺ ions of the same structure. Protonated oxetan rearranges less readily to give the same structure(s) as protonated propanal and propylene oxide. The [C₃H₇O]⁺ ions fragmenting as metastable ions after formation by CI have a higher internal energy than the same ions fragmenting after formation by EI. Deuteronation of the C₃H₆O isomers using CD₄ reagent gas shows that loss of C₂H₃D proceeds by a different mechanism than loss of C₂H₄. The results are discussed in terms of potential energy profile for the [C₃H₇O]⁺˙ system proposed earlier.     

Structures of decomposing and non-decomposing gas-phase [C4H6O]+· radical cations,and their [C2H2O]+· and [C3H6]+· product ions

The structures of gas-phase [C₄H₆O]^+· radical cations and their daughter ions of composition [C₂H₂O]^+· and [C₃H₆]^+· were investigated by using collisionally activated dissociation, metastable ion measurement, kinetic energy release and collisional ionization tandem mass spectrometric techniques. Electron ionization (70 eV) of ethoxyacetylene, methyl vinyl ketone, crotonaldehyde and 1-methoxyallene yields stable [C₄H₆O]^+· ions, whereas the cyclic C₄H₆O compounds undergo ring opening to stable distonic ions. The structures of [C₂H₃O]^+· ions produced by 70-eV ionization of several C₄H₆O compounds are identical with that of the ketene radical cation. The [C₃H₆]^+· ions generated from crotonaldehyde, methacrylaldehyde, and cyclopropanecarboxaldehyde have structures similar to that of the propene radical cations, whereas those ions generated from the remainder of the [C₄H₆O]^+· ions studied here produced a mixed population of cyclopropane and propene radical cations.     

[C3H3O]+ ions; reacting and non-reacting configurations

Three [C₃H₃O]⁺ ion structures have been characterized. The most stable of these is \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 2} = {\rm CH} - \mathop {\rm C}\limits^ + = {\rm O} $\end{document} its heat of formation ΔH_f was measured as 749±5 kJ mol^?1. In the μs time frame this ion fragments exclusively by loss of CO, a process which also dominates its collisional activation mass spectrum. The other stable [C₃H₃O]⁺ structures, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}\equiv \mathop {\rm C}\limits^ + - {\rm CHOH} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 2} = {\rm C} = \mathop {\rm C}\limits^{\rm + } - {\rm OH}, $\end{document}, were generated from some acetylenic and allenic precursor ions; their heats of formation were estimated to be 830 and 880 kJ mol^?1 respectively. The former ion was also produced by the gas phase protonation of propynal. These ions show loss of C₂H₂ and CO in both their metastable ion and collisional activation mass spectra. The broad Gaussian-type metastable peak for the loss of CO was shown to consist of two components corresponding to gragmentations having different activation energies.     

Collisional activation spectra. Behavior of [C3H7O]+ and other [CnH2n+1O]+ ions

Collisional activation spectra have identified (i) as Stable ion structures. Evidence is presented for a variety of pathway for their formation, including anchimeric assistance and hydrogen migration in less stable isomers. The fragmentation behavior of a number of [C_nH_2n+1O]⁺ isomers of n = 2 to 5 shows that extensive rearrangements are common, but that some reactions appear to be useful for ion structure elucidation. One reaction identified is unusual in that it represents the decomposition of an even-electron ion to yied an odd-electron ion product in significant abundance.     

Structure and formation of gaseous [C3H4N]+ ions

gas phase has been established. This conclusion could be derived from a careful study of their collisional activation spectra, which show minor but characteristic differences. The ions studied were generated from various precursor ions in single or multiple fragmentation processes as well as via ion-molecule reactions. Their heats of formation vary from 925 to 1085 kJ mol^?1 according to MINDO/3 or from 925 to 1050 kJ mol^?1 according to MNDO calculations.     

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.     

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).     

Concerning the structure and fragmentation of [C3H7O]+ ions derived from alcohols

The formation of [CH2OH]^+. by fragmentation of [C3H7O]^+. ions in the electron-impact mass spectra of 2-methyl-2-propanol and 2-propanol has been investigated using 13C labeling, deuterium labeling and metastable studies. The similar fragmentation reaction in the chemical ionization mass spectrum of acetone has been studied. It is concluded that the fragmentation reaction does not involve complete randomization of the carbon atoms and therefore does not proceed through formation of a hydroxylated cyclopropane intermediate. Alternative mechanisms are discussed.