A comparison of three experimental techniques for ion structure studies via collision-induced reactions: The [C5H8]+˙ example

Structure differentiation between [C₅H₈]⁺˙ ions, formed by electron ionization of various precursors, has been used as a test case for comparison of three experimental techniques involving collision-induced dissociation (CID). Low-energy CID in an rf-only quadrupole collision cell has been studied in the range 1–150 eV laboratory collision energy. These data have been compared with those obtained using mass-analyzed ion kinetic energy spectroscopy at 8 keV energy, and with results from dissociative charge-stripping (DCS) coupled with a second electron capture collision (EC) in order to remove intense interferences (DCS/EC). The greatest degree of structure differentiation was possible using the DCS/EC technique. The other two methods were comparable in this regard, although effects of pre-collision internal energy was apparent for collision energies much below 30 eV. Day-to-day reproducibility of spectra was most difficult to obtain for the low-energy CID technique. Of the [C₅H₈]⁺˙ ions thus tested, the isoprene molecular ion was clearly the best match to the fragment ion formed from limonene.     

A study of the electron impact induced retro-diels-alder process with selected 5,6,6a, 7,12,12a-hexahydrobenzo[a]anthracenes

The mass spectra of 5,6,6a,7,12,12a-hexahydrobenzo[a]anthracene and 2-methoxy, 3-methoxy-, 4-methoxy and 1-methyl-4-methoxy derivatives are reported. Among the fragment ions observed under electron impact ionization, [C₈H₈]^+· and [M? C₈H₈]^+· can be generated by a retro-Diels-Alder process. Studies of metastable ion reactions show these ions to be formed by fragmentation directly from the molecular ion. The CA spectra of the [C₈H₈]^+· ions from the subject compounds were compared with spectra from ions of the same composition from various sources. From these data, kinetic energy release measurements and stereochemical considerations, it is concluded that these ions are formed by a stepwise, rather than a concerted mechanism.     

Hydrogen rearrangement in molecular ions of alkyl benzenes: Appearance potentials and substituent effects on the formation of [C7H8]+ ions

The mechanism of the formation of [C₇H₈]⁺ ions by hydrogen rearrangement in the molecular ions of 1-phenylpropane and 1,3-diphenylpropane has been investigated by looking at the effects of CH₃O and CF₃ substituents in the meta and para positions on the relative abundances of the corresponding ions and on the appearance energies. The formation of [C₇H₈]⁺ ions from 1,3-diphenylpropane is much enhanced at the expense of the formation of [C₇H₇]⁺ ions by benzylic cleavage, due to the localized activation of the migrating hydrogen atom by the γ phenyl group. A methoxy substituent in the 1,3-diphenylpropane, exerts a site-specific influence on the hydrogen rearrangement, which is much more distinct than in 1-phenylpropane and related 1-phenylalkanes, the rearrangement reaction being favoured by a meta methoxy group. The mass spectrum of 1-(3-methoxyphenyl)-3-(4-trideuteromethoxyphenyl)-propane shows that this effect is even stronger than the effect of para methoxy groups on the benzylic cleavage. From measurements of appearance potentials it is concluded that the substituent effect is not due to a stabilization of the [C₇H₇X]⁺ product ions. Whereas the [C₇H₇]⁺ ions are formed directly from molecular ions of 1-phenylpropane and 1,3-diphenylpropane, the [C₇H₈]⁺ ions arise by a two-step mechanism in which the s? complex type ion intermediate can either return to the molecular ion or fragment to [C₇H₈]⁺ by allylic bond cleavage. Obviously the formation of this s? complex type ion, is influenced by electron donating substituents in specific positions at the phenyl group. This is borne out by a calculation of the ΔH_f values of the various species by thermochemical data. Thus, the relative abundances of the fragment ions are determined by an isomerization equilibrium of the molecular ions, preceding the fragmentation reaction.     

[C7H7]+ and [C6H5]+ fragment ions produced from methylphenol isomers by electron impact

Appearance energies for [C₇H₇]⁺ and [C₆H₅]⁺ fragment ions obtained from methylphenol isomers were measured at the threshold using the electron impact technique. Different processes for the formation of the ions are suggested and discussed. Metastable peaks were detected and the kinetic energies released were determined. The results indicate that [C₇H₇]⁺ ions are formed from metbylpbenois with both benzyl and tropylium structures, whereas [C₆H₅]⁺ ions are formed with the phenyl structure at the detected thresholds. Kinetic energies released on fragmentation of reactive [ C₇H₇]⁺ and [C₆H₅]⁺ ions were used as a probe for the structure of the ions at 70 eV.     

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.     

The mass spectra of isomeric hydrocarbons—II: The C5H8S isomers,spiropentane, cyclopentene, 1,3-pentadiene and isoprene; The mechanisms and energetics of their fragmentations

The mass spectra of spiropentane, cyclopentene, 1,3-pentadiene (piperylene), isoprene and some deuterium labelled analogues have been studied. The heats of formation of the molecular ions and the major daughter ions [C₅H₇]⁺ and [C₄H₅]⁺ have been estimated. Fragmentation of these molecules mostly proceeds via common intermediates in which hydrogen atoms have lost their positional identity. Only spiropentane displays any specific fragmentation behaviour related to its ground state geometry.     

Photoionization of the C-1?C-4 monosubstituted alkyl benzenes: Thermochemistry of [C7H7]+ and [C8H9]+ formation

The appearance energies for the [C₇H₇]⁺ and [C₈H₉]⁺ fragment ions produced in the fragmentation of the C-1? C-4 monosubstituted alkyl benzenes have been measured by photon impact. The mean heat of formation calculated for [C₇H₇]⁺ is 205.3 ± 1.9 kcal mol^?1 which is consistent with a threshold tropylium structure. For [C₈H₉]⁺ the mean heat of formation is calculated to be 199.2 ± 1.3 kcal mol^?1 which can be equated with either a methyl tropylium or α-phenylethyl structure at threshold. Some evidence is provided for the existence of the α-phenylethyl ion.     

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.     

Metastable ion studies. IV—thermochemistry and ion structures among fragmenting [C3H6]+· ions

Metastable ion peak shapes, dimensions and relative abundances have been measured for the three fragmentations [C₃H₆]⁺· → [C₃H₄]⁺· + H₂, [C₃H₆]⁺· → [C₃H₅]⁺ + H· and [C₃H₆]⁺· → [C₃H₃]⁺ + H₂ + H·. [C₃H₆]⁺· ions were derived from propene, cyclopropane, tetrahydrofuran, cyclohexanone, 2-methyl but-1-ene and cis-pent-2-ene. Activation energies for these fragmentations have been evaluated. Three daughter ion dissociations ([C₃H₅]⁺ → [C₃H₃]⁺ + H₂, [C₃H₅]⁺ → [C₃H₄]⁺· + H· and [C₃H₄]⁺· → [C₃H₃]⁺ + H·) have been similarly examined. Ion structures have been determined and the metastable energy releases have been correlated with the thermochemical data. It is concluded that the molecular ions of propene and cyclopropane become structurally indistinguishable prior to fragmentation and that differences in their metastable ion characteristics can be ascribed wholly to internal energy differences; the latter can be correlated with the photoelectron spectra of the isomers. The pathway for the consecutive fragmentation which generates the metastable ion peak (m/e 42 → m/e.39) has been shown to be It is likewise concluded that fragmentating [C₃H₆]⁺· ions generated from the various precursor molecules are also structurally indistinguishable and cannot be classified with either molecular ion of the isomeric C₃H₆ hydrocarbons.     

An energy-resolved study of the fragmentation of ionized 1-Penten-3-ol

A detailed energy-resolved study of the fragmentation of CH₂?CHCH(OH)CD₂CD₃ (1-d₅) has been carried out using metastable ion studies and charge exchange techniques, combined with collision-induced dissociation studies to establish the structures of fragment ions. At low internal energies (metastable ions) the molecular ion of 1-d₅ rearranges to the 3-pentanone structure and fragments by loss of C₂H₅ or C₂D₅ leading to the acyl structure, [CH₃CH₂C?O]⁺ or [CD₃CD₂C?O]⁺, for the fragment ion. However, with increasing internal energy of the molecular ion this rearrangement process decreases rapidly in importance and loss of C₂D₅ by direct cleavage, leading to [CH₂?CHCH?OH]⁺, becomes the dominant fragmentation reaction. As a result the [C₃H₅O]⁺ ion seen in the electron impact mass spectrum of 1-penten-3-ol has predominantly the protonated acrolein structure.     

Energy and substituent effects on the mass spectra of substituted benzophenones

For two competing decompositions of the same molecular ion to give products [A₁⁺] and [A₂⁺], the ratio [A₁⁺]/[A₂⁺], is equal to the ratio of rate constants for the formation of the stable ions. Thr ratios, [Y C₇H₄O⁺]/[C₇H₅O⁺], were determined for several benzophenones for electron energies from 15 to 70 eV. Plots of log [Y C₇H₄O⁺]/[C₇H₅O⁺] vs.[ω⁺] gave good straight lines at all energies. Similar correlations have been reported for log [Y C₇H₆⁺]/[C₇H₇⁺] from substituted diphenyl ethanes and are also true for substituted acetophenones, log [YøCO⁺]/[CH₃CO⁺]. A few charge exchange data were obtained which showed the same general trend as the electron-impact data and emphasize the contribution of low energy ions in the 70 eV mass spectra. Relatively poor correlations were obtained for the [Y C₆H₄⁺] and [C₆H₅⁺] ions that are formed by both one-step and two-step decompositions.     

Metastable ion studies. III—composite metastable peaks in fragmentations of some small hydrocarbon cations

Composite metastable peaks are generated in the unimolecular fragmentations (i) [C₃H₅]⁺ → [C₃H₃]⁺ + H₂ (flat-top upon flat-top) and (ii) [C₄H₉]⁺ → [C₃H₅]⁺ + CH₄ (flat-top and gaussian). The measurement of appearance potentials and kinetic energy releases lead us to conclude, in agreement with earlier proposals, that in (i) the components can arise from the generation of the isomeric cyclopropenium and propargyl daughter cations. In (ii) the components are proposed to arise from the fragmentation of tert- and sec-butyl cations yielding allyl as the common daughter ion. The composite peak observed in the fragmentation (iii) [C₃H₄]⁺· → [C₃H₃]⁺ + H· is shown to be present only if the decomposing molecular ion is large enough to also produce [C₆H₈]²⁺ ions. The second component in (iii) then arises from the reaction [C₆H₈]²⁺ → [C₆H₆]²⁺ + H₂.     

Laser-ion beam photodissociation studies of C4H4 radical cations

The laser-ion beam photodissociation for [C₄H₄]⁺˙ ions produced from a variety of precursors has been studied. Based on the data it is apparent that two structurally distinct forms of the [C₄H₄]⁺˙ ion are produced by fragmentation of larger systems. The relative population of the various structural forms is very dependent on the internal energy of the fragmenting ion, with 1-buten-3-yne [C₄H₄]⁺˙ ions being favored at low internal energies. As the internal energy of the reactant ion is increased, the relative population of butatriene [C₄H₄]⁺˙ ions increases. The laser-ion beam photodissociation technique is able to selectively sample these two structural forms.     

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.     

The mass spectra of isomeric hydrocarbons—I: Norbornene and nortricyclene; The mechanisms and energetics of their fragmentations

The mass spectra of norbornene, nortricyclene and deuterium labeled derivatives thereof have been studied. The appearance potentials of the ions [C₇H₁₀]^+·, [C₇H₉]⁺, [C₆H₇]⁺ and [C₅H₆]^+· have been determined for both compounds and heats of formation of the hydrocarbons have been estimated. Detailed fragmentation schemes are proposed for the molecular ions and it is concluded that they dissociate by essentially different mechanisms which do not involve common intermediates. The structures and energy contents of the primary fragment ions are discussed in detail by comparing energetics, labeling experiments and metastable ion abundances.     

Skeletal and hydrogen rearrangements in the electron impact mass spectrum of propene

The mass spectrum of propene-2-[¹³C] shows 81% retention of C-2 in the [C₂H₃]⁺ fragment ion at 70 eV electron energy, decreasing to 75% C-2 retention at low electron energies. The mass spectra of propene-2-d₁, propene-1,1,3,3,3-d₅, propene-1,1,2-d₃ and propene-3,3,3-d₃ also have been examined at a resolution sufficient to resolve H₂-D doublets. The results at 70 eV electron energy show complete H/D randomization prior to fragmentation to form [C₃(H, D)₅]⁺ but, in agreement with the ¹³C labelling data, incomplete H/D interchange prior to fragmentation to form [C₂(H, D)₃]⁺. The results are interpreted in terms of a reversible isomerization of the propene molecular ion to a cyclopropane structure in competition with fragmentation.     

Positive chemical ionization triple‐quadrupole mass spectrometry and ab initio computational studies of the multi‐pathway fragmentation of phthalates

We report the first positive chemical ionization (PCI) fragmentation mechanisms of phthalates using triple‐quadrupole mass spectrometry and ab initio computational studies using density functional theories (DFT). Methane PCI spectra showed abundant [M + H]⁺, together with [M + C₂H₅]⁺ and [M + C₃H₅]⁺. Fragmentation of [M + H]⁺, [M + C₂H₅]⁺ and [M + C₃H₅]⁺ involved characteristic ions at m/z 149, 177 and 189, assigned as protonated phthalic anhydride and an adduct of phthalic anhydride with C₂H₅⁺ and C₃H₅⁺, respectively. Fragmentation of these ions provided more structural information from the PCI spectra. A multi‐pathway fragmentation was proposed for these ions leading to the protonated phthalic anhydride. DFT methods were used to calculate relative free energies and to determine structures of intermediate ions for these pathways. The first step of the fragmentation of [M + C₂H₅]⁺ and [M + C₃H₅]⁺ is the elimination of [R? H] from an ester group. The second ester group undergoes either a McLafferty rearrangement route or a neutral loss elimination of ROH. DFT calculations (B3LYP, B3PW91 and BPW91) using 6‐311G(d,p) basis sets showed that McLafferty rearrangement of dibutyl, di(‐n‐octyl) and di(2‐ethyl‐n‐hexyl) phthalates is an energetically more favorable pathway than loss of an alcohol moiety. Prominent ions in these pathways were confirmed with deuterium labeled phthalates. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Four-centred rearrangements in the mass spectra of aliphatic ethers

The [CH₃O?CHCH₃]⁺ ions observed in the mass spectra of ethers of formula CH₃OCH (CH₃)R(R = H or alkyl) undergo two rearrangement fragmentation reactions to form [C₂H₅]⁺ and [CH₂OH]⁺. The scope of the rearrangements has been investigated and it is shown that enlargement of the alkyl group on either side of the ether linkage leads to alternative fragmentation routes. From a study of metastable intensities it is concluded that the fragmentations probably occur directly from the [CH₃O?CHCH₃]⁺ structure through four centred rearrangements rather than through the intermediacy of the [C₂H₅O?CH₂]⁺ ion.     

Molecular fragmentation on collision with protons for freon and propane molecules

In ab initio calculations, we determined the most probable routes of decomposition of the [CF₃Cl]⁺, [CF₂Cl₂]⁺, [CFCl₃]⁺, [CCl₄]⁺ molecular ions of freons and [C₃H₈]⁺ ions of hydrocarbons formed by collision of neutral molecules with protons with energies of the order of 10 keV. The calculated potential surface sections are compared on a qualitative level with the probability of various ion fragments in experiments on fragmentation. The role of the charge transfer dynamics between the proton and the molecule is discussed.