Charge reversal of the conjugate base of formamide

Charge reversal collisional activation mass spectremetry of negative ions has been used in conjunction with positive ion collisional activation to investigate several isomeric [H₂, C, N, O]⁺ ions. Generation of m/z 44 ions from formamide, acetamide, JV-methylformamide, acetaldoxime and by charge reversal of the [M–1]^? ion formed from formamide yields several different isomeric structures. Charge reversal of the conjugate base of formamide appears to yield a mixture of singlet and triplet H₂NC?O⁺ ions; experiments with deuterium-labeled compounds have been used to support this. Ab initio molecular orbital calculations indicate that the triplet ion is a stable structure, existing in a potential minimum 390.6 kJ mol^?1 above the ground state singlet.     

Quantitation of isomeric ion mixtures using collisional activation mass spectra

Recently Bass and Bowers have criticized methods for calculation of isomeric composition of gaseous ions based on mass spectra from collisionally activated dissociations (CAD). They fault values from this laboratory on the proportion of benzyl and tropylium ions calculated by two methods from product ion abundances. However, a careful remeasurement of the relevant yields of CAD product ions shows that neither calculation method affected the result outside experimental error, as the ion yields for at least 14 higher energy dissociations are insensitive to both isomeric identity and internal energy. Reducing the [C₇H₇]⁺ internal energy by 1.5 eV does give a 25% reduction in the ion yield for the lowest energy process, supporting the accepted recommendation that these be omitted for quantitative spectral comparisons. A calculation error was made in one of our previous reports on [C₇H₇]⁺, and corrected isomeric composition values are presented. The disparate values from recalculation by Bass and Bowers of other data from this laboratory on [C₂H₄O]^+˙ ions are shown here to be consistent with the large experimental error of those measurements; the medians of their values are actually near the values originally reported. In direct contrast to their assertions, we find no evidence that previous calculation methods have produced misleading conclusions, or that the assumption of linear superposition of CAD spectra in ion mixtures has yielded unreliable results.     

Dissociation dynamics of halotoluene ions,production of tolyl,benzyl and tropylium ([C7H7]+)ions

The dissociation rates and energetics of the loss of halogen atoms from energy-selected halotoluene ions were investigated by photoelectron photoion coincidence (PEPICO) and collisional activation (CA) mass spectrometric experiments. Dissociation onsets, determined from the dissociation rates measured as a function of the internal energy of the parent ion, revealed the formation of three [C₇H₇]⁺ isomers, which were identified, on the basis of the CA data, as the tolyl, benzyl and tropylium ions. All of the ions investigated produced a mixture of isomeric ions. Only iodotoluene ions produced any tolyl product ions by a direct bond cleavage. The bromo- and chlorotoiuene ions produced mixtures of benzyl and tropyl ions. The observed two-component decay rates of the iodotoluene ions revealed the participation of a lower energy [C₇H₇I]^{+ ˙} isomer in the dissociation process. The identity of this isomer is not known but it probably does not have the cycloheptatriene ion structure because considerable kinetic energy was released in this dissociation.     

Further examples of skeletal rearrangements of the Wagner-Meerwein type in chemical ionization mass spectrometry: The case of [C6H9]+ ions

The [C₆H₉]⁺ ions produced either via unimolecular H₂O loss from 13 [C₆H₁₁O]⁺ precursors or direct protonation of 1,3- and 1,4-cyclohexadiene have identical collisional activation mass spectra. The kinetic energy release data for the process [C₆H₁₁O]⁺→[C₆H₉]⁺+H₂O are also very similar (on average T_0.5=24 meV) irrespective of the constitution of the precursor. From the proton affinities of 1,3-cyclohexadiene (PA=837.2 kJ mol^?1) using ion cyclotron resonance mass spectrometry the heat of formation of the [C₆H₉]⁺ ion is determined to 804.6 kJ mol^?1. This value taken together with the results of molecular orbital calculations (MNDO) and the structure indicative losses of CH₃^. and C₂H₄ upon collisional activation suggest that the [C₆H₉]⁺ ion has the structure of the 1-methylcyclopentenylium ion f and not that of the slightly less stable cyclohexenylium ion g. The generator of an easily interconverting system of isomeric [C₆H₉]⁺ ions is unlikely to be due to the high barrier separating the various isomers.     

Mass spectrometric investigation of phenylthiovinyl chlorides

Three isomeric phenylthiovinyl chlorides (β-, E and Z, and α-) have been examined under EI and FI conditions with particular attention paid to the fragmentation leading to [C₈H₇S]⁺ ions via loss of Cl˙. Kinetic energy release data, MIKE and CAD MIKE spectra obtained with these precursors, as well as with model compounds, suggest the thioacylium ion PhCH₂CS⁺ as the predominant structure observed for the [C₈H₇S]⁺ species in all cases examined. A comparison is made with the solution behaviour of incipient β-thiovinyl carbenium ions, which are known to rearrange to the S-bridged thiirenium structure.     

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.     

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.     

A relation between ion structures and experimental overall cross-sections for collisionally activated decomposition

Collisionally activated dissociation (CAD) mass spectra sometimes appear to be identical in spite of the fact that the precursor ion structures are known to differ. It is shown that determination of the experimental overall cross-section for collisionally activated decomposition yields valuable extra information. After applying it to examples of known structure, [C₄H₅N]^+·, [C₅H₅N]^+· and [C₅H₅O]⁺, it is used to study a more complex problem, that of [C₆H₆]^+· ions from four isomeric precursors.     

The structures of [C5H5O]+ ions formed from isomers of nitrophenol in the gaseous phase

Translational energy release measurments on metastable ions are used in the comparison of the structures of isomeric ions. Metastable ions, m₂⁺, formed from m₁⁺ ions as the result of a high energy process in the ion source are compared with isomeric metastable ions formed as daughters from fragmentation of metastable m₁⁺ ions in a field. In the case of o-, m- and p-nitrophenol the structure of the [C₅H₅O]⁺ ions formed from [C₆H₅O]⁺ ions by these two independent methods is different as verified by comparison of the behaviour of [C₅H₅O]⁺ ions formed from several other compounds.     

Interference peaks in collisional activation mass spectra: The loss of CH3˙ from [C13H9] ions

It was concluded from earlier metastable ion and collisional activation experiments that [C₁₃H₉]⁺ ions generated by dissociative ionization of stilbenes and diphenylmethane consist of a mixture of isomeric structures. It is shown in this paper that the methyl loss upon which this conclusion rests originates to a large extent from metastable molecular ions decomposing in the magnetic sector field (interference signal). Our results do not show, nor do they require, that more than one isomeric [C₁₃H₉]⁺ species is present. The importance of recognizing such interference signals is stressed.     

Oxidation reactions in triterpenes under chemical ionization conditions

From a collisional activation spectral study it has been found that certain triterpene alcohols with an ursane or oleanane skeleton undergo oxidation to the corresponding ketones under chemical ionization (NH₃) conditions giving rise to abundant [M + NH₄ ? 2]⁺ ions. Mass-analysed ion kinetic energy and B²/E scan results indicate that both [M + NH₄]⁺ and [M + N₂H₇ ? 2]⁺ ions contribute to the formation of the [M + NH₄ ? 2]⁺ ion.     

The [C5H8]+˙ radical cation: Structural studies by energy-resolved mass spectrometry

Control of the degree of collisional excitation of an ion can be achieved by varying the collision energy or the number of collisions. The former experiment yields a series of daughter spectra which is analogous to a breakdown curve and which serves to characterize the ion undergoing collisionally activated dissociation (CAD). Recognition of differences in the spectra between isomeric ions is facilitated if the curves obtained from the energy-resolved data are considered in pairs and their differences are plotted. This procedure serves to demonstrate that ionized 1,3- and 1,4-pentadiene are structurally distinct, even though they are not distinguishable in conventional CAD. 2-Pentyne and 3-octyne give [C₅H₈]⁺˙ fragment ions which are closely related to the 1,3- and 1,4-pentadiene structures, respectively. Any particular ion can be characterized by another form of difference spectrum: one comparing data at two collision pressures each taken over a range of collision energies. This procedure yields conclusions regarding ion structure which match those reached from the energy-resolved experiments.     

Ion/molecule reactions of ammonia and methylamine with chloro- and nitrobenzene. A comparison of chemical ionization and ion cyclotron resonance experiments

It is shown by ion cyclotron resonance measurements that ion/molecule reactions, leading to substitution or reduction product ions from chloro- and nitrobenzene with the title amines, are those between the molecular ions [RNH₂]⁺ or [C₆H₅X]⁺˙ and their respective counterparts C₆H₅X or RNH₂. The protonated reagent gas ions [RNH₃]⁺ are not involved in these reactions. In the case of nitrobenzene, adduct ions [C₆H₅NO₂·RNH₃]⁺ do not decompose within the time scale of the measurements. The results obtained are compared with those found under chemical ionization conditions.     

O−• chemical ionization of carbonyl compounds

The negative ion chemical ionization mass spectra of twentyeight C₄ to C₇ carbonyl compounds were recorded using the oxide radical anion O^?? as reagent ion. As noted earlier, the reactions occurring include H⁺ abstraction, H ₂ ^+? abstraction, H? atom displacement, and alkyl radical displacement. In addition, the [M?2H]^? ions fragment further by alkyl radical elimination. The relative importance of these reactions depends strongly on molecular structure, with the result that isomer distinction frequently is possible. Where this is not possible, as for isomeric aldehydes, the collisional charge inversion mass spectra of common product ions provides isomer distinction. The H ₂ ^+? abstraction reaction is shown to involve abstraction not only of two hydrogens from the same α-carbon but also, in part, abstraction of one hydrogen from each α-carbon.     

Structural determination of [C7H7O]+ ions in the gas phase by ion cyclotron resonance spectrometry

The [C₇H₇O]⁺ ions from a series of compounds have been studied using ion cyclotron resonance spectrometry. The techniques employed in this gas phase ion structure determination of [C₇H₇O]⁺ were photodissociation, ion-molecule reactions, and collisionally activated dissociation using a Fourier transform mass spectrometer (FTMS-CAD). In addition to the low energy FTMS-CAD results, high energy CAD data obtained with a sector mass spectrometer is also provided. Evidence was found for five unique [C₇H₇O]⁺ structures, including the hydroxybenzyl ion, the hydroxytropylium ion, the protonated benzaldehyde ion, the methylaryloxy ion and the phenyl methylene ether ion. Ion-molecule reactions, invovling both proton transfer and methylene transfer, provided the most unambiguous results and yielded qualitative and quantitative evidence for the five structures. However, a combined approach using the three techniques was necessary to identify all of the structures. The tropylium form of [C₇H₇O]⁺ was found to absorb strongly at 305 nm, while the protonated benzaldehyde ion was found to have a strong absorption band at 305 nm and a weak band at 370 nm. The proton affinity of 2,4,6-cycloheptatrienone was determined to be 918±8 kJ mol^?1, which is considerably lower than a previously reported value. In addition, deprotonation reactions of the methylaryloxy ion yielded a proton affinity of 871±14 kJ mol^?1 for 4-methylenecyclohexa-2,6-diene-1-one.     

Isomerization and fragmentation of methylfuran ions and pyran ions in the gas phase

The mutual interconversion of the molecular ions [C₅H₆O]⁺ of 2-methylfuran (1), 3-methylfuran (2) and 4H-pyran (3) before fragmentation to [C₅H₅O]⁺ ions has been studied by collisional activation spectrometry, by deuterium labelling, by the kinetic energy release during the fragmentation, by appearance energles and by a MNDO calculation of the minimum energy reaction path. The electron impact and collisional activation mass spectra show clearly that the molecular ions of 1–3 do not equilibrate prior to fragmentation, but that mostly pyrylium ions [C₅H₅O]⁺ arise by the loss of a H atom. This implies an irreversible isomerization of methylfuran ions 1 and 2 into pyran ions before fragmentation, in contrast to the isomerization of the related systems toluene ions/cycloheptatriene ions. Complete H/D scrambling is observed in deuterated methylfuran ions prior to the H/D loss that is associated with an iostope effect k_H/k_D = 1.67–2.16 for metastable ions. In contrast, no H/D scrambling has been observed in deuterated 4H-pyran ions. However, the loss of a H atom from all metastable [C₅H₅O]⁺ ions gives rise to a flat-topped peak in the mass-analysed ion kinetic energy spectrum and a kinetic energy release (T₅₀) of 26 ± 1.5 kJ mol^?1. The MNDO calculation of the minimum energy reaction path reveals that methylfuran ions 1 and 2 favour a rearrangement into pyran ions before fragmentation into furfuryl ions, but that the energy barrier of the first rearrangement step is at least of the same height as the barrier for the dissociation of pyran ions into pyrylium ions. This agrees with the experimental results.     

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.     

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.     

Internal energy effects in collision induced dissociation spectra

The effect of the precursor ion internal energy on the branching ratios obtained from collision induced dissociation fragmentation patterns was examined for [NH₃]⁺ and [C₂H₄N]⁺. The ion internal energy was changed by varying both the chemical ionization reagent gas and the ion source pressure. Effects observed in the collision induced dissociation fragmentation patterns as a function of the ion source pressure are explained by the reaction exothermicities and by collisional deactivation of internally excited ions (at high pressure).     

Determination of the isomeric composition of an ionic mixture using collision cross-sections

A theory has been developed for the determination of the isomeric composition of an ionic mixture. The probability theory describing the collisionally activated decomposition (CAD) which was developed previously provided a general foundation for the present theory. Quantification was based on the cross-sections for the collisional production of daughter ions as observed in the mass-analysed ion kinetic energy Spectrometry (MIKES). Experimental details for the implementation of the theory have also been developed. The method has been applied to the case of C₂H₄O^+· generated from butane-l,3-diol, which is known to be a mixture of more than one isomeric structure. The results seem to be far superior to those based on previous methods, as evidenced by the excellent internal consistency.