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.     

Charge reversal of the enolate ions of acetaldehyde and acetone

Collisionally activated charge reversal of HCOCH₂^? and CH₃COCH₂^? produces positive ions whose fragments differ from those of other [C₂H₃O]⁺ ions formed by fragmentation of positive molecular ions, including the [C₂H₃O]⁺ ions from 2,2-dichlorethanol, considered formerly to have the HCOCH₂⁺ structure. The fragmentations of the charge reversed ions are concordant with the RCOCH₂⁺ structures. Least-squares correlations of the collisional activation spectra [C₂H₃O]⁺ are probed as a useful guide to claiming similarity or dissimilarity of ionic structure.     

The [M?OH]+ ions of m- and p-ethylnitrobenzene: A common structure?

The structures of the [M? OH]⁺ ions of m- and pethylnitrobenzene have been compared by measurements of metastable ion spectra, collisional activation spectra, kinetic energy releases and critical energies for the formation of these ions and their subsequent decomposition. Normalized rates of fragmentation of metastable molecular ions and metastable [M? OH]⁺ ions have been compared for ion lifetimes up to 30 μs. The energy measurements fail to distinguish between the structures of the [M? OH]⁺ ions, but the normalized fragmentation rates and the collisional activation spectra show their structures to be different.     

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]⁺.     

Formation and structure of [C8H8O]+˙ ions,generated from gas phase ions of phenyl-cyclopropylcarbinol and 1-phenyl-1-(hydroxymethyl)cyclopropane

The structure and formation of [C₈H₈O]^+. ions generated from phenylcyclopropylcarbinol and 1-phenyl-1-hydroxymethylcyclopropane upon electron impact, have been studied using kinetic energy release measurements, by determination of ionization and appearance energies and by collisional activation. It is shown that the non-decomposing [C₈H₈O]^+˙ ions have exclusively the structure of the enol ion of phenylacetaldehyde, although it is less stable than the enol ion of acetophenone by about 45 kJ mol^?1. This has been interpreted as an indication that the [C₈H₈O]^+˙ ions from phenylcyclopropylcarbinol are formed by an attack of either the phenyl ring or the hydroxyl group upon the C-1? C-2 (or C-1? C-3) bond of the cyclopropane ring under a simultaneous expulsion of ethene and migration of the attacking group to the C-1 position. The [C₈H₈O]^+˙ ion from 1-phenyl-1-(hydroxymethyl)cyclopropane is formed by opening of the cyclopropane ring via a benzylic cleavage. A kinetically controlled hydrogen shift in the resulting ring opened ion prior to or during ethene loss then leads to the formation of [C₈H₈O]^+˙ ions which have the structure of the enol ion of phenylacetaldehyde.     

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.     

The structures and formation of [C4H6N]+ ions 2—ion structures derived from 2-methylpropanenitrile

The loss of a hydrogen atom from ionized 2-methylpropanenitrile is preceded by a drastic rearrangement of the molecular ion. The result of this fragmentation is the generation of two stable structurally different [C₄H₆N]⁺ ions, formed via different pathways. Their structures can be established by a careful comparison of the metastable ion spectra, collision activation spectra, and charge stripping spectra from the compound and its three deuterium labeled analogues and from [C₄H₆N]⁺ ions generated from reference compounds via electron impact ionization or in selected ion/molecule reactions.     

Formation and stability of gaseous tolyl ions

Collisional activation mass spectra confirm that tolyl ions can be produced from a variety of CH₃C₆H₄Y compounds. High purity o-, m- and p-tolyl ions are prepared by chemical ionization of the corresponding fluorides (Y=F) as proposed by Harrison. In electron ionization of CH₃C₆H₄Y formation of the more stable tropylium and benzyl ionic isomers usually accompanies that of the o-, m- and p-tolyl ions. Isomerization of low energy [CH₃C₆H₄Y]⁺? to [Y–methylenecyclohexadiene]⁺? is proposed to account for most [benzyl]⁺ formation, while the tropylium ion appears to arise from the isomerization of tolyl ions formed with higher internal energies, [o-, m-, p-tolyl]⁺→ [benzyl]⁺→ [tropylium]⁺, consistent with Dewar's predictions from MINDO/3 calculations.     

Structure and formation of [C4H6N]+ ions: 1—Ion structures derived from aliphatic nitriles

Structures and formation of the [C₄H₆N]⁺ ions present in the mass spectra of eleven α-substituted and eleven α-unsubstituted nitriles have been investigated from collisional activation and metastable ion spectra. Collisional activation spectra lead to identification of six structures. The [C₄H₆N]⁺ ions from some branched compounds prove to be mixtures. This, as well as the identity of all metastable ion parameters and certain spectral data, shows that energy differences between all structures are small. This is corroborated by MINDO/3 calculations showing a spread from 724 to 891 kJ mol^?1 over the structures. Earlier proposals for two different [C₄H₆N]⁺ ion structures, based on the mass spectra of deuterium labelled compounds, appeared to be correct. A computer program to calculate the contribution of standard spectra in a measured spectrum has been developed.     

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.     

Double collision experiments

A variety of double collision experiments, whereby fast species undergo collisional interactions in two distinct regions of a mass spectrometer, are described. These include two-stage charge reversal of negative ions, two-stage double electron transfer from targets to cations, neutralization-reionization experiments as well as delayed analysis of organic cations formed in a one-step charge reversal of anions. Experiments have been performed on a number of systems of current interest in gas-phase ion chemistry. It is concluded that autoelectron detachment of benzyl anions leads to benzyl radicals, whereas the collisionally induced electron detachment produces a mixture of benzyl and tropyl radicals. By contrast, electron detachment from [H₃CNH]^? is not a metastable process and occurs only after excitation to produce H₃CNH˙ radicals, which do not rearrange into the thermodynamically more stable H₂CNH₂˙. It is shown that in the double electron transfer reactions H⁺ + Xe→H˙ + Xe⁺˙ and H˙ + Xe→H^? + Xe⁺˙, excited states are produced. From double collision experiments on methyl formate ions, it is concluded that the non-decomposing ions have undergone rearrangement on the time-scale of 10 μs into the distonic isomer, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm H} - \mathop {\rm C}\limits^ + ({\rm OH}){\rm O}\mathop {\rm C}\limits^. {\rm H}_2 $\end{document}. Finally, it is shown that short-lived (<0.2 μs) [H₂, C, N]⁺ ions generated by charge-reversal of [H₂CN]^? have the [H₂CN]⁺ structure, whereas most of the long-lived (10 μs) ions have the [HCNH]⁺ structure.     

Comparison of collisional activation spectra of some positive ions produced both by charge reversal of negative ions and by decomposition of positive ions

The charge reversal collision induced decomposition mass analyzed ion kinetic energy spectrum of allyl anion has been compared with the collision induced dissociation mass analyzed ion kinetic energy spectrum of allyl cation and found to be identical except for the presence of +2 ions formed by charge stripping in the spectrum of the [C₃H₅]⁺ ion. Likewise, the collision induced dissociation mass analyzed ion kinetic energy charge reversal spectrum of [CH₃Se]^? has been compared with the collision induced dissociation mass analyzed ion kinetic energy spectrum of [CH₃Se]⁺ and found to be identical. A study of the pressure dependence of the collision induced dissociation mass analyzed ion kinetic energy spectrum of [C₃H₅]⁺ and [C₃H₅]^? showed increasing fragmentation with increasing collision gas pressure, and suggests that a greater mean number of collisions converts more energy to internal modes in the collision induced dissociation mass analyzed ion kinetic energy experiment even at low pressures.     

Isomeric [C3H4]+· ions: Their identification and generation in dissociative ionizations

The collisional activation (CA) and charge stripping (CS) mass spectra of the three [C₃H₄]^+· isomers, allene, propyne and cyclopropene, are reported. The extent of isomerization among these ions prior to collisional excitation depends on their internal energy content, but is small. Each [C₃H₄]^+· ion structure also can uniquely be generated via appropriate dissociative ionizations. Analysis of mixtures of [C₃H₄]^+· (daughter) ion structures is, in general, not possible from CA and CS mass spectra alone but may be aided by appearance energy measurements.     

Metastable ions in chemical ionization

The ion [C₃H₅]⁺ generated in a chemical ionization source by a variety of methods, including protonation and charge exchange, exhibits a metastable peak for H₂ loss which is two orders of magnitude weaker than that formed in an electron impact source. The stable [C₃H₅]⁺ ions generated by electron impact and chemical ionization undergo collision-induced dissociation to a comparable extent, both losing H₂ by only one of the two competitive mechanisms observed for metastable ions. In contrast to the behavior of [C₃H₅]⁺, the molecular ions of p-substituted nitrobenzene, generated by charge exchange at high source pressure, yield composite metastable peaks for NO loss which are very similar in shape and intensity to those generated by electron impact. The contrasting behavior of the metastable ions extracted from high pressure ion sources in the two systems may be due to differences in the efficiencies of quenching of the ionic states responsible for fragmentation as metastable ions. It is noteworthy that the NO loss reactions require considerably lower activation energies than does the H₂ loss reaction.     

Collisional activation of ions formed by [Li]+ ion attachment

Since no unimolecular fragmentation is observed with [M+Li]⁺ ions under normal operating conditions the collisional activation method was used to study the fragmentation behaviour of these ions. It was found that the liberation of the [Li]⁺ ion is a dominant process only with smaller molecules. In addition, direct bond cleavages and new types of rearrangement reactions lead to fragment ions in which the lithium is normally retained. The decomposition behaviour of [M+Li]⁺ ions represents an intermediate case between that of [M]+ ions and excited neutral molecules and is quite different from that of [M+H]⁺ ions.     

Internal energy distributions deposited in doubly and singly charged tungsten hexacarbonyl ions generated by charge stripping,electron impact,and charge exchange

The distribution Pε of internal energies deposited in W(CO)₆ ^+?. ions upon charge stripping (that is, electron detachment to yield the doubly charged ion in the course of a single kiloelec-tronvolt energy collision) was estimated by a thermochemical method from the measured relative abundances of the doubly charged fragment ions produced. The thermochemical information needed to estimate P/ge was obtained by measuring the threshold translational energy losses associated with charge stripping of the singly charged fragment ions, W(CO) _n ⁺ (n = 0-5). The P(/ge) curve falls exponentially with increasing internal energy. The average energy transferred to W(CO)₆ ^+? upon a 7.8-keV collision with O₂ is 19 eV, yielding W(CO)₆ ^2? ions with an average of 4 eV of internal energy. In its general appearance, the P(ε) distribution associated with charge stripping is similar to the curves obtained from simple collisional activation of either W(CO) ₆ ^+?. or W(CO)₆ ^2+? in kiloelectronvolt energy gaseous collisions. Given that charge stripping occurs by way of an electronic excitation process, this similarity in the energy deposition function is taken to indicate that electronic excitation is also the major mechanism for simple collisional activation in this system at zero scattering angle in the kiloelectronvolt energy regime. The internal energy distribution associated with a related charge-stripping process, charge inversion from the metal carbonyl anions to yield the corresponding cations, was also recorded. This reaction shows a large (～7 eV) average internal energy deposition with a distribution that indicates near-zero probability of formation of unexcited ions. These data are tentatively interpreted in terms of vibrationalelectron detachment. The internal energy distribution associated with an exothermic process, charge exchange [W(CO)₆ ^2+? + O₂ → W(CO) ₊ ^6?+O₂ ^+?], was also characterized. Unexpectedly strong coupling of translational to internal energy is observed, and there is a large probability of depositing internal energies in excess of 10 eV, even though the exothermicity is only 3 eV. Finally, the internal energy distributions associated with the formation of doubly charged W(CO)₆ ^2+? ions by electron ionization have been measured. Unlike the distribution for charge stripping, but like that for singly charged ions generated by electron impact, this distribution shows considerable structure, presumably due to Franck-Condon factors.     

Charge remote fragmentation of fatty acids cationized with alkaline earth metal ions

Fatty acids can be collisionally activated as [M ? H + Cat]⁺, where Cat is an alkaline earth metal, by using tandem mass spectrometry. High-energy collisional activation induces charge remote fragmentation to give structural information. In the full scan mass spectra molecular ions are easily identified, particularly when barium is used as a cationizing agent; ions are shifted to a higher mass, lower chemical noise region of the mass spectrum. Moreover, the isotopic pattern of barium is characteristic, and the high mass defect of barium allows an easy separation of the cationized analyte from any remaining interfering ions (chemical noise), provided medium mass-resolving power is available. An additional advantage is that most of the ion current is localized in [M ? H + Cat]⁺ species. Structural analysis of fatty acids can be performed when the sample size is as low as 1 ng.     

Energy dependence of the fragmentation of ester enolate ions

The energy dependence of the fragmentation of a selection of ester enolate ions has been studied by variable, low-energy collision-induced dissociation experiments in the quadrupole collision cell of a hybrid BEQQ mass spectrometer. The dominant fragmentation reactions observed are where ΔH₁ ? ΔH₂=PA([RCCO]^?) ? PA([?O]^?) (PA=proton affinity). The anion of lowest proton affinity is formed preferentially at low internal energies with the yield of the anion of higher proton affinity increasing with increasing internal energy. The [CH₃OCOCOCH₂]^? anion derived from methyl pyruvate forms [CH₃OCO]^? by reaction (2); this anion readily fragments to [CH₃G]^?+ CO consistent with a structure represented by a dipole-stabilized cluster of [CH₃O]^? and CO. Comparison of the 8-keV with the 50-eV collision-induced dissociation mass spectra indicated that the average internal energy of the fragmenting ions is considerably lower in the high-energy collisional experiments than it is in the low-energy collisional experiments.     

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 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.