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.     

The unimolecular chemistry of [1,2-propanediol]+˙: a rationale in terms of hydrogen-bridged radical cations

By combining results from a variety of mass spectrometric techniques (metastatle ion, collisional activation, collision-induced dissociative ionization, neutralization–reionization spectrometry and appearance energy measurements) and the classical method of isotopic labelling, a unified mechanism is proposed for the complex unimolecular chemistry of ionized 1,2-propanediol. The key intermediates involved are the stable hydrogen-bridged radical cations [CH₂?C(H)? H…?O…?O(H)CH₃]⁺˙, which were generated independently from [4-methoxy, 1-butanol]⁺˙ (loss of C₂H₄) and [1-methoxyglycerol]⁺˙ (loss of CH₂O), [CH₃? C?O…?H…?O(H)CH₃]⁺˙ and the related ion-dipole complex [CH₂?C(OH)CH₃/H₂O]⁺˙. The latter species serves as the precursor for the loss of CH³˙ and in this reaction the same non-ergodic behaviour is observed as in the loss of CH³˙ from the ionized enol of acetone.     

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.     

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.     

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 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.     

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.     

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.     

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.     

Further evidence for the stability of methylenechloronium ylide,H2CCIH. A neutralization-reionization ion-dissociation experiment

Recent theoretical and experimental studies are in conflict with our previous neutralization–reionization evidence for the surprising stability of simple ylides such as H₂CCIH. Neutralization of [H₂CCIH]⁺˙ ions from CICH₂COOH with Hg followed by reionization of the resulting neutral molecules with O₂ produces intense [CCIH₃]⁺˙ parent ions. Collisionally activated dissociation of these mass-selected ions shows that 47 ± 5% are the [H₂CCIH]⁺˙ isomer. This demonstrates that stable H₂CCIH molecules can be formed by neutralization, surviving intact for the lifetime of the experiment (10^?6 s). Field ionization conditions effective in ionizing excited argon atoms do not produce appreciable ionization of the neutrals from [H₂CCIH]⁺˙, suggesting that the stable H₂CCIH neutrals are not Rydberg state molecules.     

Loss of methyl from ionized 2-hydroxy-1,3-butadiene

The loss of methyl from unstable, metastable and collisionally activated [CH₂?CH? C(OH)?CH₂]⁺˙ ions (1⁺˙) was examined by means of deuterium and ¹³C labelling, appearance energy measurements and product identification. High-energy, short-lived 1⁺˙ lose methyl groups incorporating the original enolic methene (C(1)) and the hydroxyl hydrogen atom (H(0)). The eliminations of C(1)H(1)H(1)H(4) and C(4)H(4)H(4)H(0) are less frequent in high-energy ions. Metastable 1⁺˙ eliminate mainly C(1)H(1)H(1)H(4), the elimination being accompanied by incomplete randomization of the five carbon-bound hydrogen atoms. The resulting [C₃H₃O]⁺ ions have been identified as the most stable CH₂?CH? CO⁺ species. The appearance energy for the loss of methyl from 1 was measured as AE[C₃H₃O]⁺ = 10.47 ± 0.05 eV. The critical energy for 1⁺˙ → [C₃H₃O]⁺ + CH₃˙ is assessed as E_c ? 173 kJ mol^?1. Reaction mechanisms are proposed and discussed.     

Unimolecular reactions of ionized methyl acetate and its hydrogen-rearranged isomers

The proposed formation of [CH₃C(OH)OCH₂]⁺˙ (b) as the intermediate in the isomerization [CH₂?C(OH)OCH₃]⁺˙ (c)?b?[CH₃COOCH₃]⁺˙ (c has been confirmed by preparation of b from CH₃COOCH₂OCH₃. For the three isomers a–c the dominant metastable ion (MI) dissociation, CH₃O˙ loss, involves identical kinetic energy release values. The kinetic barriers for a?b and b?c must be nearly as high as that for CH₃O˙ loss from c, as shown by the insensitivity of the mass spectra from collisionally activated dissociation (CAD) of a–c to ionizing electron energy. The H/D scrambling of metastable [CH₂?C(OD)OCH₃]⁺˙ and c–D₃ ions confirm this, indicating that the barrier for a?b is slightly below that for b?c. Minor low-energy dissociations include losses of CH₄ and CH₃OH from a and losses of ˙CHO and CH₂O from b. Comparison of MI and CAD spectra of a–c with those from [CH₃(OH)CH₂O]⁺˙ (d) and [CH₃COCH₂OH]⁺˙ (e) give no evidence for skeletal rearrangement of a–c to d or e.     

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.     

Substituent effects in the mass spectra of diethyl phenyl phosphates

The mass spectra of diethyl phenyl phosphates show substituent effects with electron-donating groups favouring the molecular ion M⁺˙, and the [M? C₂H₄]⁺˙, [M – 2C₂H₄]⁺˙ and [XPhOH]⁺˙ ions. The [PO₃C₂H₆]⁺ (m/z 109) and [PO₃H₂]⁺ (m/z 81) ions are favoured by electron-withdrawing groups. Results suggest that the formation of the [XPhC₂H₃]⁺˙ ion involves rearrangement of C₂H₃ to the position ortho to the phosphate group. Ortho effects are also observed.     

Ion-neutral complex-mediated hydrogen exchange in ionized n-butanol: A mechanism for ‘non-specific’ hydrogen migrations

Water elimination from ionized n-butanol reflects near randomization of all hydrogens in ions decomposing after ～10^?5s. This probably takes place in ion-neutral complexes by formation of a cyclobutane ion–H₂O complex and/or rearrangement within [C₄H₈]⁺˙ in open-chain [C₄H₈⁺˙? H₂O] complexes, in either case accompanied by hydrogen exchange between water and open-chain hydrocarbon moieties. Extensive hydrogen rearrangements in which restraints on conventional transition-state ring size have little apparent influence may generally be ion–neutral complex-mediated processes.     

Loss of methyl from [H2CC(OH)?CH3]+˙ ions prepared by electron impact ionization of unstable 2-hydroxypropene

Unstable 2-hydroxpropene was prepared by retro-Diels-Alder decomposition of 5-exo-methyl-5-norbornenol at 800°C/2 × 10^?6 Torr. The ionization energy of 2-hydroxypropene was measured as 8.67±0.05 eV. Formation of [C₂H₃O]⁺ and [CH₃]⁺ ions originating from different parts of the parent ion was examined by means of ¹³C and deuterium labelling. Threshold-energy [H₂C?C(OH)? CH₃]^+˙ ions decompose to CH₃CO⁺+CH₃^˙ with appearance energy AE(CH₃CO⁺) = 11.03 ± 0.03 eV. Higher energy ions also form CH₂?C?OH⁺ + CH₃ with appearance energy AE(CH₂?C?OH⁺) = 12.2–12.3 eV. The fragmentation competes with hydrogen migration between C(1) and C(3) in the parent ion. [C₂H₃O]⁺ ions containing the original methyl group and [CH₃]⁺ ions incorporating the former methylene and the hydroxyl hydrogen atom are formed preferentially, compared with their corresponding counterparts. This behaviour is due to rate-determining isomerization [H₂C?C(OH)? CH₃]^+˙ →[CH₃COCH₃]^+˙, followed by asymmetrical fragmentation of the latter ions. Effects of internal energy and isotope substitution are discussed.     

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.     

The elusive methene ylides CH2CIH,CH2FH and CH2OH2

The possible existence of the ylides CH₂CIH, CH₂₅FH and CH₂OH₂ as stable neutral species in the gas phase has been investigated by the neutralization–reionization (NR) mass spectrometry of their radical cations using a double-focusing mass spectrometer of reversed geometry. The experiments were, for the most part, performed under single-collision conditions with Xe as the neutralization target gas and He and O₂ as reionization agents. For each ylidion a peak was observed in their NR mass spectrum which indicated that the neutral ylide had apparently been produced. However for CH₂FH⁺˙ and CH₂OH₂⁺˙ the m/z 34 and m/z 32 peaks, respectively, were attribut-able to interferences from the natural isotopic abundance of ions of lower mass. For CH₂CIH⁺˙, the NR recovery signal was found to arise from the presence of CH₃CI⁺˙ as an impurity in the ylidion flux. This was proved by examination of the collisional activation mass spectra of the [C, H₃, CI]⁺˙ ions produced in the NR mass spectra of the conventional ions and ylidions, an experiment performed using a triple-sector mass spectrometer.     

Stereoelectronic effects on the retro-Diels-Alder fragmentation of ionized bicyclo[4.3.0]nona-3,7-dienes

The electron impact and collision-induced dissociation mass spectra of cis- and trans-annulated bicyclo[4.3.0]nona-3,7-dienes differ in their relative abundances of [C₅H₆]⁺˙ fragments formed by the retro-Diels-Alder decomposition. The formation of [C₅H₆]⁺˙ is not preceded by hydrogen migration in the short-lived and long-lived molecular ions. The appearance energy of [C₅H₆]⁺˙ from both annulation isomers is identical within experimental error: AE_cis([C₅H₆]⁺˙)=10.56±0.10 eV and AE_trans([C₅H₆]⁺˙)=10.54±0.15 eV. The barrier to the retro-Diels-Alder fragmentation lies 68–76 kJ mol^?1 above the thermo-chemical threshold corresponding to [C₅H₆]⁺˙ + C₄H₆. Investigation of the two-dimensional reaction coordinate by the Topological Molecular Orbital treatment shows that the lowest energy path for the retro-Diels-Alder reaction involves a two-step dissociation of the C(5)? C(6) and C(1)? C(2) bonds in the molecular ion, the latter step overcoming a barrier, calculated as 80 kJ mol^?1 above the thermochemical threshold. The stereochemical difference between the geometric isomers is due to stereoelectronic assistance of the π orbitals of the cis-annulated isomer in the cleavage of the C(5)? C(6) bond. Other mechanisms of the retro-Diels–Alder reaction are discussed.     

Alkane elimination from ionized alkanols

The energetics, metastable characteristics and daughter ion structures for the loss of small alkane molecules from ionized 2-propanol, 2-butanol and 3-pentanol have been examined in detail. [2-Propanol]^+˙ ions lose CH₄ to generate the keto and enol forms of [C₂H₄O]^+˙ and the same daughter ions are produced by loss of C₂H₆ from [2-butanol]^+˙. Ionized 3-pentanol does not lose CH₄ but readily eliminates C₂H₆ to produce the enol ion [CH₃CH?CHOH]^+˙. The last reaction was shown to proceed by a simple 1,2 elimination mechanism in the μs time-frame; isotope effects are also discussed.