The methylamine radical cation [CH3NH2]+˙ and its stable isomer the methylenammonium radical cation [CH2NH3]+˙

The potential energy surface for the [CH₅N]^+˙ system has been investigated using ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence-electron correlation. Two [CH₅N]^+˙ isomers can be distinguished: the well known methylamine radical cation, [CH₃NH₂]^+˙, and the less familiar methylenammonium radical cation, [CH₂NH₃]^+˙. The latter is calculated to lie 8 kJ mol^?1 lower in energy. A substantial barrier (176 kJ mol^?1) is predicted for rearrangement of [CH₂NH₃]^+˙ to [CH₃NH₂]^+˙. In addition, a large barrier (202 kJ mol^?1) is found for loss of a hydrogen radical from [CH₂NH₃]^+˙ via direct N—H bond cleavage to give the aminomethyl cation [CH₂NH₂]⁺. These results are consistent with the existence of the methylenammonium ion [CH₂NH₃]^+˙ as a stable observable species. The barrier to loss of a hydrogen radical from [CH₃NH₂]^+˙ is calculated to be 140 kJ mol^?1.     

Four isomers of [C3H8O]+˙ distinguished by collisional activation

Collisional activation of the molecular ions of 1-propanol, 2-propanol and methyl ethyl ether, and of the m/z 60 ion from 1,2-dimethoxyethane provides evidence for four distinct forms of [C₃H₈O]^+˙. Collision induced decompositions may be explained either by simple cleavages, by cyclic processes involving adjacent substituents, or by bicyclic processes of adjacent substituents. Evidence for a form of [C₃H₈O]^+˙ in which charge and radical site are separate is assembled from the spectra.     

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.     

The potential energy surface for the [C2H2O]+˙ system: The ketene radical cation [CH2CO]+˙ and its isomers

Ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence electron correlation have been employed to examine the [C₂H₂O]^+˙ potential energy surface. Four [C₂H₂O]^+˙ isomers have been identified as potentially stable, observable ions. These are the experimentally well-known ketene radical cation, [CH₂?C?O]^+˙ (a), and the presently unknown ethynol radical cation, [CH₂?C? OH]^+˙ (b), the oxirene radical cation (c) and an ion resembling a complex of CO with [CH₂]^+˙, (d). The calculated energies of b, c and d relative to a are 189, 257 and 259 kJ mol^?1, respectively. Dissociation of ions a and d is found to occur without reverse activation energy.     

Endothermic proton transfer reactions from three [C6H6]+˙ isomers

A triple quadrupole mass spectrometer was used to establish the proton affinities of phenyl, CH₃C?CC?CCH₃ and HC?CCH₂C?C?CH radicals as 870±29, 824±25, and 757±21 kJmol^?1, respectively, from the kinetic energy of benzene, 2,4-hexadiyne, and 1.5-hexadiyne molecular ions at which the onset of proton transfer to less basic species occurs in the second rod assembly. These values were confirmed by other triple quadrupole experiments involving bracketing of exothermic proton transfers.     

Kinetic energy release and ion structure: [C6H6O]+˙

It is shown that, contrary to a recent report, the kinetic energy release for CO loss from phenol does not decrease as the ion accelerating voltage is decreased. In fact, an increase is observed which is attributed, at least in part, to greater discrimination against off-axis ions at low voltage. The kinetic energy release data do not demand the tautomerization of ionized phenol before decarbonylation.     

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.     

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.     

Structures and relative energies of gas-phase [C2H7N]+˙ radical cations

Ab initio molecular orbital calculations with split-valence plus polarization basis sets and incorporating electron correlation and zero-point energy corrections have been used to examine possible equilibrium structures on the [C₂H₇N]⁺˙ surface. In addition to the radical cations of ethylamine and dimethylamine, three other isomers were found which have comparable energy, but which have no stable neutral counterparts. These are \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm .} {\rm H}_{\rm 2} {\rm CH}_{\rm 2} \mathop {\rm N}\limits^{\rm + } {\rm H}_{\rm 3} $\end{document}, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} \mathop {\rm C}\limits^{\rm .} {\rm H}\mathop {\rm N}\limits^{\rm + } {\rm H}_{\rm 3} $\end{document}and\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} \mathop {\rm N}\limits^{\rm + } {\rm H}_{\rm 2} \mathop {\rm C}\limits^. {\rm H}_{\rm 2} {\rm }, $\end{document} with calculated energies relative to the ethylamine radical cation of ?33, ?28 and 4 kJ mol^?1, respectively. Substantial barriers for rearrangement among the various isomers and significant binding energies with respect to possible fragmentation products are found. The predictions for \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^. {\rm H}_{\rm 2} {\rm CH}_{\rm 2} \mathop {\rm N}\limits^ + {\rm H}_{\rm 3} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} \mathop {\rm C}\limits^{\rm .} {\rm H}\mathop {\rm N}\limits^{\rm + } {\rm H}_{\rm 3}$\end{document} are consistent with their recent observation in the gas phase. The remaining isomer, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} \mathop {\rm N}\limits^{\rm + } {\rm H}_{\rm 2} \mathop {\rm C}\limits^{\rm .} {\rm H}_{\rm 2} {\rm },$\end{document}is also predicted to be experimentally observable.     

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.     

Unimolecular reactions of [C5H10O]+ ˙ radical cations derived from4-penten-1-ol

The reactions of metastable [C₅H₁₀O]^{+ ˙} radical cations produced by ionization of 4-penten-1-ol are reported and discussed. These [C₅H₁₀O]^{+ ˙} species undergo mainly ethyl radical loss, with smaller contributions of methyl radical and water expulsion. ²H-Labelling studies reveal different specificities of hydrogen selection in these three fragmentations. The behaviour of these [C₅H₁₀O]^{+ ˙} ions is compared to those reported previously for isomeric radical cations containing linear alkenyl chains and a terminal hydroxyl group.     

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.     

Ion structural studies by ion kinetic energy spectrometry: [C7H7]+, [C7H8]+˙, [C7H7OCH3]+˙

The use of kinetic energy release measurements in the structural characterization of ions formed in the mass spectrometer and in the determination of fragmentation mechanisms is demonstrated. In combination with information on the mode of energy partitioning in some of these reactions this allows the following conclusions: (i) The metastable [C₇H₈]⁸˙ ions formed from toluene, cyclohepatatriene, n-butylbenzene, the three methyl anisoles, methyl tropyl ether and benzyl methyl ether all undergo loss of H˙ from a common structure. (ii) The metastable [C₇H₇]⁺ ions generated from the same sources and from benzyl bromide, benzyl alcohol, p-xylene and ethylbenzene appear to undergo loss of acetylene from both the benzylic and the tropylium structures. (iii) The metastable [C₇H₇OCH₃]⁺˙ ether molecular ions undergo loss of CH₃˙ by two types of mechanism, simple cleavage to give the aryloxy cation (not observed for benzyl methyl ether) and a rearrangement process which appears to lead to protonated tropone as the product. (iv) Loss of formaldehyde from the metastable [C₇H₇OCH₃]⁺˙ molecular ions involves hydrogen transfer via competitive 4- and 5-membered cyclic transition states in the case of the anisoles and in the case of methyl tropyl ether, while for benzyl methyl ether, hydrogen transfer in the nonisomerized molecular ion occurs via a 4-membered cyclic transition state to yield the cycloheptatriene molecular ion.     

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.     

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.     

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.     

The elimination of carbon monoxide from molecular ions of phenol and other [C6H6O]+˙ isomers

Phenol and five acyclic isomeric compounds have been investigated using electron impact and field ionization techniques, mass analysed ion kinetic energy spectrometry and collision induced dissociation mass analysed ion kinetic energy spectrometry. The kinetic energy release data corresponding to the elimination of carbon monoxide from the molecular ions show that at least two structures of the reactant ion are involved. The electron impact and field ionization collision induced dissociation mass analysed ion kinetic energy spectra indicate that under electron impact conditions, the phenol ion partially isomerizes to another ion structure. An isomerization of about 40% to the structure of cis-2-hexen-4-yn-1-al is in good accordance with the spectral data.     

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.     

On the shift of H2O between the carbons of the radical cation [CH2CH2OH2]+˙

Computations predict that H₂O will shift rapidly between the carbons of \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm .} {\rm H}_2 {\rm CH}_2 \mathop {\rm O}\limits^ + {\rm H}_2 $\end{document} over most of the energy range between the threshold for formation of the ion and the onset of its decomposition. This prediction is important to understanding the relationships and contrasts between the chemistries of free radicals and carbonium ions. We present experimental evidence that the theoretical predictions are correct.