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.     

Competition in the formation of ion–neutral complexes. Expulsion of methane from the molecular ion of acetamide

An ion–neutral complex is a non-covalently bonded aggregate of an ion with one or more neutral molecules in which at least one of the partners rotates freely (or nearly so) in all directions. A density-of-states model is described, which calculates the proportion of ion–neutral complex formation that ought to accompany simple bond cleavages of molecular ions. Application of this model to the published mass spectrum of acetamide predicts the occurrence of ions that have not hitherto been reported. Relative intensities on the order of 0.1 (where the abundance of the most intense fragment ion = 1) ere predicted for [M – HO]⁺ and [M – CH₄]⁺˙ ions, which have the same nominal masses as the prominent [M – NH₃]⁺˙ and [M – NH₂]⁺ fragments. High-resolution mass spectrometric experiments confirm the presence of the predicted fragment ions. The [M – HO]⁺ and [M – CH₄]⁺˙ fragments were observed with relative abundances of 0.02 and 0.04, respectively. Differences between theory and experiment may be ascribed to effects of competing distonic ion pathways.     

Dissociative charge exchange of C3F6

Charge exchange of neutral C₃F₆ by a variety of atomic and molecular ions in the 1 to 25 eV range of collision energies is used to characterize the energies associated with formation of [C₃F₆]⁺˙. The internal energy of the nascent [C₃F₆]⁺˙ ion, assessed by observing the degree to which it fragments, increases with the recombination energy of the charge-exchange reagent. The existence of excited states of the reagent ions is identified from the fragmentation behaviour of [C₃F₆]⁺˙ in the cases of [CS₂]⁺˙, NO⁺, O₂⁺˙, [NH₃]⁺˙ and possibly [CH₄]⁺˙. In addition, the data confirm that the [C₃F₆]⁺˙ parent ion fragments from both the ground state and a long-lived isolated electronic state. The latter is populated by near-resonant charge transfer. Translational excitation contributes relatively little to internal excitation of the charge-exchanged product ion and even less in the case of the isolated state.     

Gas-phase protonation and methyl cation transfer from methyl halide ions

The ion-molecule reactions between [CH₃X]⁺˙ [CH₃XH] ⁺, [CH₃XCH₃]⁺ ions (X = F, Cl, Br, I) and a number of nucleophiles have been studied by ion cyclotron resonance techniques. Protonation of the nucleophiles is observed to occur from both the molecular ions [CH₃]X⁺˙ and protonated species [CH₃XH]⁺ whereas dimethylhalonium ions [CH₃XCH₃]⁺ react principally by methyl cation transfer. A notable exception occurs in methyl iodide where the molecular ions [CH₃I]⁺˙ act both as proton and methyl cation donors, whereas dimethyliodonium ions are found unreactive. The results are discussed with reference to the use of alkyl halides as reagent gases in chemical ionization experiments.     

The [CH5NO]+ ˙ potential energy surface: Distonic ions,lon-dipole complexes and hydrogen-bridged radical cations

By combining results from a variety of mass spectrometric techniques (metastable ion, collisional activation, collision-induced dissociative ionization, neutralization-reionization spectrometry, ²H, ¹³C and ¹⁸O isotopic labelling and appearance energy measurements) and high-level ab initio molecular orbital calculations, the potential energy surface of the [CH₅NO]^{+ ˙} system has been explored. The calculations show that at least nine stable isomers exist. These include the conventional species [CH₃ONH₂]^{+ ˙} and [HO? CH₂? NH₂]^{+ ˙}, the distonic ions [O? CH₂? NH₃]^{+ ˙}, [O? NH₂? CH₃]^{+ ˙}, [CH₂? O(H)? NH₂]^{+ ˙}, [HO? NH₂? CH₂]^{+ ˙}, and the ion-dipole complex CH₂?NH₂⁺ …? OH˙. Surprisingly the distonic ion [CH₂? O? NH₃]^{+ ˙} was found not to be a stable species but to dissociate spontaneously to CH₂?O + NH₃^{+ ˙}. The most stable isomer is the hydrogen-bridged radical cation [H? C?O …? H …? NH₃]^{+ ˙} which is best viewed as an immonium cation interacting with the formyl dipole. The related species [CH₂?O …? H …? NH₂]^{+ ˙}, in which an ammonium radical cation interacts with the formaldehyde dipole is also a very stable ion. It is generated by loss of CO from ionized methyl carbamate, H₂N? C(?O)? OCH₃ and the proposed mechanism involves a 1,4-H shift followed by intramolecular ‘dictation’ and CO extrusion. The [CH₂?O …? H …? NH₂]^{+ ˙} product ions fragment exothermically, but via a barrier, to NH₄^{+ ˙} HCO^…? and to H₃N? C(H)?O^{+ ˙} H˙. Metastable ions [CH₃ONH₂]^+…? dissociate, via a large barrier, to CH₂?O + NH₃⁺ + and to [CH₂NH₂]⁺ + OH˙ but not to CH₂?O^{+ ˙} + NH₃. The former reaction proceeds via a 1,3-H shift after which dissociation takes place immediately. Loss of OH˙ proceeds formally via a 1,2-CH₃ shift to produce excited [O? NH₂? CH₃]^{+ ˙}, which rearranges to excited [HO? NH₂? CH₂]^{+ ˙} via a 1,3-H shift after which dissociation follows.     

Isomeric [C,H3,N,O]+˙ ions and their neutral counterparts

The isomeric ions [H₂NC(H)O]⁺˙, [H₂NCOH]⁺˙, [H₃CNO]⁺˙ and [H₂CNOH]⁺˙ were examined in the gas phase by mass spectrometry. Ab initio molecular orbital theory was used to calculate the relative stabilities of [H₂NC(H)O]⁺˙, [H₂NCOH]⁺˙, [H₃NCO]⁺˙ and their neutral counterparts. Theory predicted [H₂NC(H)O]⁺˙ to be the most stable ion. [H₂NCOH]⁺˙ ions were generated via a 1,4-hydrogen transfer in [H₂NC(O)OCH₃]⁺˙, [H₂NC(O)C(O)OH]⁺˙ and [H₂NC(O)CH₂CH₃]⁺˙. Its metastable dissociation takes place via [H₃NCO]⁺˙ with the isomerization as the rate-determining step. [H₂CNOH]⁺˙ undergoes a rate-determining isomerization into [H₃CNO]⁺˙ prior to metastable fragmentation. Neutralization-reionization mass spectrometry was used to identify the neutral counterparts of these [H₃,C,N,O]⁺˙ ions as stable species in the gas phase. The ion [H₃NCO]⁺˙ was not independently generated in these experiments; its neutral counterpart was predicted by theory to be only weakly bound.     

The significance of field ionisation kinetics to ion structure: Isomerisation and decompostition of [C4H8]+· radical ions

The kinetics of formation of [C₃H₅]⁺[M ? CH₃]⁺, [C₃H₄]⁺·[M ? CH₄]⁺· and [C₂H₄]⁺·[M ? C₂H₄]⁺· from but-1-ene, cis- and trans-but-2-ene, 2-methylpropene, cyclobutane and methylcyclopropance following field ionisation have been determined as a function of time 20 (or 30) picoseconds to 1 nanosecond and at two points in the microsecond time-frame. The results are consistent with the supposition that at the shortest accessible times (20 to 30 picoseconds) the structure of the [C₄H₈]⁺· molecular ion qualitatively resembles that of its neutral precursor, but suggest that prior to decomposition within nanoseconds the various molecular ions (excepting cyclobutane where the processes are slower) attain a common structure or mixture of structures. Reaction pathways of the presumed known ion structures are delineated from the nature of decompostion at the shortest times.     

Structure and formation of gaseous [C4H6O]+˙ ions: 1—The enolic ions [CH2C(OH)?CHCH2]+˙ and [CH2CH?CHCH(OH)]+˙ and their relationship with their keto counterparts

The [C₄H₆O]^+˙ ion of structure [CH₂?CHCH?CHOH]^+˙ (a) is generated by loss of C₄H₈ from ionized 6,6-dimethyl-2-cyclohexen-1-ol. The heat of formation ΔH_f of [CH₂?CHCH?CHOH]^+˙ was estimated to be 736 kJ mol^?1. The isomeric ion [CH₂?C(OH)CH?CH₂]^+˙ (b) was shown to have ΔH_f, ? 761 kJ mol^?1, 54 kJ mol^?1 less than that of its keto analogue [CH₃COCH?CH₂]^+˙. Ion [CH₂?C(OH)CH?CH₂]^+˙ may be generated by loss of C₂H₄ from ionized hex-1-en-3-one or by loss of C₄H₈ from ionized 4,4-dimethyl-2-cyclohexen-1-ol. The [C₄H₆O]^+˙ ion generated by loss of C₂H₄ from ionized 2-cyclohexen-1-ol was shown to consist of a mixture of the above enol ions by comparing the metastable ion and collisional activation mass spectra of [CH₂?CHCH?CHOH]^+˙ and [CH₂?C(OH)CH?CH₂]^+˙ ions with that of the above daughter ion. It is further concluded that prior to their major fragmentations by loss of CH₃˙ and CO, [CH₂?CHCH?CHOH]⁺˙ and [CH₂?C(OH)CH?CH₂]^+˙ do not rearrange to their keto counterparts. The metastable ion and collisional activation characteristics of the isomeric allenic [C₄H₆O]^+˙ ion [CH₂?C?CHCH₂OH]^+˙ are also reported.     

Studies in chemical ionization mass spectrometry 10—CI(CH3NH2) spectra of olefins

Both alkenes and cyloalkenes react with [CH₃NH₂]⁺˙. The possibilities and limitations of CI(CH₃NH₂) for the identification of structural isomers and for the determination of double bond positions are discussed. The quasi-molecular ions [M+CH₃NH₂]⁺˙ are shown to fragment in a manner observed for amines under MIKE conditions rather than at 70 eVelectron impact, which suggests that one is dealing with long-lived low-energy species.     

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 effect of vibrational energy content on the neutralization–reionization mass spectra of [H2]+˙ ions

Neutralization-reionization experiments were performed on beams of [H²]⁺˙ ions of different, known vibrational energy content using a variety of neutralization target gases (Xe, H₂, Ne) and reionization gases (He, O₂). The recovery of [H₂]⁺˙ ions was found to be only weakly dependent on the vibrational energy of the original [H₂]⁺˙ ions. The ion kinetic energy spectra of H⁺ fragments from the neutralization-reionization experiments were independent of the collision gas; the processes by which they were generated were identified.     

Chemical ionization and collisional activation spectra of α,β-unsaturated nitriles

A study of the chemical ionization (CI) and collisional activation (CA) spectra of a number of α, β-unsaturated nitriles has revealed that the even-electron ions such as [MH]⁺ and [MNH₄]⁺ produced under chemical ionization undergo decomposition by radical losses also. This results in the formation of M ⁺˙ ions from both [MH]⁺ and [MNH₄]⁺ ions. In the halogenated molecules losses of X˙ and HX compete with losses of H˙ and HCN. Elimination of X˙ from [MH]⁺ is highly favoured in the bromoderivative. The dinitriles undergo a substitution reaction in which one of the CN groups is replaced with a hydrogen radical and the resulting mononitrile is ionized leading to [M ? CN + 2H]⁺ under CI(CH₄) or [M ? CN + H + NH₄] and [M ? CN + H + N₂H₇]⁺ under CI(NH₃) conditions.     

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.     

Collisionally induced dissociative ionization of neutral products from unimolecular ion fragmentations. 1—Neutral product structures

A method is described for the investigation of the structure of neutral products from the unimolecular (metastable) dissociative ionizations of mass selected ions, by means of the collisionally induced dissociative ionization of the neutral species themselves. The neutral species, with kilovolt translational energies, enter a positively charged collision cell situated in the second field free region of a standard ZAB-2F mass spectrometer. Dissociative ionization of the neutrals results therein from their collisions with He target gas. The resulting ions are analysed by means of the electric sector and the relative ion abundances are shown to be structure characteristic. For such experiments the neutral flux should be c. ≥ 0.5% of the selected precursor ion flux; the collision gas pressure must be insufficient to cause significant precursor ion fragmentation in the field free region preceding the collision cell. It was shown that HNC is generated in the fragmentation of aniline molecular ions, whereas HCN is the neutral product in the dissociative ionizations of pyridine, benzonitrile and benzyl cyanide. The neutral radical [C, H₃, O˙] formed together with [CH₃CO]⁺ from ionized methyl acetate has the structure ˙CH₂OH, but that from the analogous fragmentation of the methyl propanoate molecular ion has the structure CH₃O˙. Allyl radicals were shown to be generated from [(CH₃)₂CHCH₂OH]⁺˙ together with [CH₃OH₂]⁺ ions.     

Losses of isobaric neutral species from molecular ions of isomeric nitroanisoles

Alternative losses of the isobaric neutral species CH₂O and NO˙ have been assessed for molecular ions of isomeric nitroanisoles fragmenting in the ion source and the first field free region of a double focusing mass spectrometer. Mass analyses of the primary fragment ions indicate that specific loss of CH₂O occurs from molecular ions of 2-nitroanisole, while specific loss of NO˙ occurs from molecular ions of 3-nitroanisole. Although the peak due to [M? NO]⁺ ions is negligible in the mass spectrum of 2-nitroanisole, evidence is presented to show that they are transient intermediates in the consecutive fragmentation for loss of the elements of CNO₂ from the molecular ions.     

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.     

A mass spectral study of 1-(aryldimethylsilyl)-2-propanones and their isomeric 2-(aryldimethylsiloxy)-1-propenes

The mass spectra of a series of β-ketosilanes, p-Y? C₆H₄Me₂SiCH₂C(O)Me and their isomeric silyl enol ethers, p-Y? C₆H₄Me₂SiOC(CH₃)?CH₂, where Y = H, Me, MeO, Cl, F and CF₃, have been recorded. The fragmentation patterns for the β-ketosilanes are very similar to those of their silyl enol ether counterparts. The seven major primary fragment ions are [M? Me·]⁺, [M? C₆H₄Y·]⁺, [M? Me₂SiO]⁺˙, [M? C₃H₄]⁺˙, [M? HC?CCF₃]⁺˙, [Me₂SiOH]⁺˙ and [C₃H₆O]⁺˙ Apparently, upon electron bombardment the β-ketosilanes must undergo rearrangement to an ion structure very similar to that of the ionized silyl enol ethers followed by unimolecular ion decompositions. Substitutions on the benzene ring show a significant effect on the formation of the ions [M? Me₂SiO]⁺˙ and [Me₂SiOH]⁺˙, electron donating groups favoring the former and electron withdrawing groups favoring the latter. The mass spectral fragmentation pathways were identified by observing metastable peaks, metastable ion mass spectra and ion kinetic energy spectra.     

Three new isomers of [C2H6O]+˙: The radical cations [CH3O(H)CH2]+˙, [CH3CHOH2]+˙ and a low-energy isomer of unassigned structure

Three new [C₂H₆O]⁺˙ ions have been generated in the gas phase by appropriate dissociative ionizations and characterized by means of their metastable and collisionally induced fragmentations. The heats of formation, ΔH_f⁰, of the two ions which were assigned the structures [CH₃O(H)CH₂]⁺˙ and [CH₃CHOH₂]⁺˙ could not be measured. The third isomer, to which the structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 2} = \mathop {\rm C}\limits^{\rm .} {\rm H} \cdot \cdot \cdot \mathop {\rm H}\limits^ + \cdot \cdot \cdot {\rm OH}_{\rm 2} $\end{document} is tentatively assigned, was measured to have ΔH_f⁰ = 732±5 kJ mol^?1, making it the [C₂H₆O]⁺˙ isomer of lowest experimental heat of formation. It was found that the exothermic ion–radical recombinations [CH₂OH]⁺+CH₃˙→[CH₃O(H)CH₂]⁺˙ and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} \mathop {\rm C}\limits^{\rm + } {\rm HOH + H}^{\rm .} $\end{document}→[CH₃CHOH₂]⁺˙ have large energy barriers, 1.4 and ?0.9 eV, respectively, whereas the recombinations yielding [CH₃CH₂OH]⁺˙ have little or none.     

The ethyl halides: Stable neutral and radical cation isomers [C2, H2, X] where X = F,Cl, Br,I

The following isomers of the ethyl halide molecular ions have all been shown to be stable species in the gas phase: [CH₂CH₂FH]⁺˙; [CH₃ClCH₂]⁺˙ (ΔH_f° = 1012 kJ mol^?1); [CH₃CHClH]⁺˙ (ΔH_f° = 971 kJ mol^?1); [CH₂CH₂ClH]⁺˙; [CH₃BrCH₂]⁺˙ (ΔH_f° = 1058 KJ mol^?1); [CH₃CHBrH]⁺˙ (ΔH_f° = 995 kJ mol^?1) and [CH₂CH₂BrH]⁺˙. Neutralization–reionization mass spectrometry, employing Xe as the electron transfer target gas and O₂ as the target gas for reionization, indicated that the ylides CH₃ClCH₂ and CH₃BrCH₂ could not be generated by such means. However, the species CH₃CHClH, CH₂CH₂ClH and CH₂CH₂BrH (and possibly CH₃CHBrH) were unambiguously identified.     

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.