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.     

Structure determination of [C3H3N2]+ ions,formed from monosubstituted pyrazoles C3H3N2R,by collision-induced dissociation mass spectrometry

[C₃H₃N₂]⁺ ions have been generated under electron impact conditions from some monosubstituted pyrazoles C₃H₃N₂R. Collision-induced dissociation (CID) mass spectra of deuterium-labelled precursors suggest that the majority of the [C₃H₃N₂]⁺ ions formed from 1-nitro- and 4-bromo-pyrazole retain their cyclic structure, whereas the ions from 3(5)-bromopyrazole are mainly linear. This is confirmed by the relative values observed for the overall cross-sections for CID and for ion loss. An isotope effect of the order of 1.5–1.9 has been found for the collision-induced loss of H^˙ from [C₃H₃N₂]⁺, generated from 3(5)- and 4-bromopyrazole.     

Chemical ionization mass spectra of alkylphenol and linear alcohol polyethoxylates

The chemical ionization mass spectra of some tert-octylphenol ethoxylates and linear alcohol ethoxylates have been determined using methane and ammonia as reactant gases. Proton, [C₂H₅]⁺ and [C₃H₅]⁺ adducts were observed with attachment on the ring as well as on the oxyethylene chain for the tertiary octylphenol ethoxylates. Proton and [NH₄]⁺ addition reactions were also observed. Alkyl ion and olefin displacement reactions were also important fragmentations. Hydride abstraction and proton, [C₂H₅]⁺, [C₃H₅]⁺ and [NH₄]⁺ attachment were the most important reactions occurring for the linear alcohol ethoxylates.     

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.     

Electrophilic aromatic substitution under chemical ionization conditions. Methylamine and ammonia as reagent gases

For compounds C₆H₅X (X?Cl, Br, I) under chemical ionization conditions, methylamine causes ipso substitution of X by [NH₂CH₃]⁺ and by [NH₂]⁺˙. C₆H₅F is less reactive; it gives some [C₆H₅NH₂]⁺˙. Nitrobenzene gives an adduct ion [M+CH₃NH₃]⁺, a reduction product ion [C₆H₅NO₂]⁺˙, and an ion at m/z93, probably a substitution product [C₆H₅NH₂]⁺˙, but no [C₆H₅NH₂CH₃]⁺. It is also shown that the ion m/z94, formed from nitrobenzene with ammonia as reagent gas, is a substitution product rather than a reduction product ion. Carbonyl compounds C₆H₅. CO. X give adduct ions and some substitution, mainly [C₆H₅NH₂]⁺˙.     

Isomerisation of hydrocarbon ions III–[C8H8]+·, [C8H8]2+, [C6H6]+· AND [C6H5]+ IONS

Collisional activation spectra of [C₈H₈]⁺·, [C₈H₈]²⁺, [C₆H₆]⁺· and [C₆H₅]⁺ ions from fifteen different sources are reported. Decomposing [C₈H₈]⁺· ions of ten of these precursors isomerise to a mixture of mainly the cyclooctatetraene and, to a smaller extent, the styrene structure. Three additional structures are observed with [C₈H₈]⁺· ions from the remaining precursors. [C₈H₈]^2+., [C₈H₈]⁺·, [C₆H₆]⁺· and [C₆H₅]⁺· ions mostly decompose from common structures although some exceptions are reported.     

Thermal Ethane Activation by Bare [V2O5]+ and [Nb2O5]+ Cluster Cations: on the Origin of Their Different Reactivities

The gas‐phase reactivity of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane has been investigated by means of mass spectrometry and density functional theory (DFT) calculations. The two metal oxides give rise to the formation of quite different reaction products; for example, the direct room‐temperature conversions C₂H₆→C₂H₅OH or C₂H₆→CH₃CHO are brought about solely by [V₂O₅]⁺. In distinct contrast, for the couple [Nb₂O₅]⁺/C₂H₆, one observes only single and double hydrogen‐atom abstraction from the hydrocarbon. DFT calculations reveal that different modes of attack in the initial phase of C?H bond activation together with quite different bond‐dissociation energies of the M?O bonds cause the rather varying reactivities of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane. The gas‐phase generation of acetaldehyde from ethane by bare [V₂O₅]⁺ may provide mechanistic insight in the related vanadium‐catalyzed large‐scale process.     

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.     

Charge separation reactions of doubly charged benzene ions

Photoion-photoion coincidence spectra of benzene and benzene-d₆ photoionized by He(II) light and synchrotron radiation show the existence of six major and eight minor charge-separation reactions of the [C₆H₆]²⁺ ion. Three main groups of ion pairs are related to [C₃H₃]⁺ + [C₃H₃]⁺, [C₂H₃]⁺ + [C₄H₃]⁺ and [CH₃]⁺ + [C₅H₃]⁺, with appearance energies of 32.2 ± 0.5 eV, 31.3 ± 0.5 eV and 28.4 ± 0.3 eV. The kinetic energy release is the same for all pairs within a group, irrespective of hydrogen number, but differs from group to group. Results are interpreted in terms of fast, direct charge separation of [C₆H₆]²⁺, and subsequent hydrogen loss by the singly charged fragments.     

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.     

Carbon skeletal rearrangements in 2-phenylthiophene and 2,5-diphenylthiophene under conditions of electron-impact

The mass spectra of ¹³C-labelled 2-phenylthiophenes and 2,5-diphenylthiophenes were studied. The label distributions for the [HCS]⁺, [C₂H₂S]^+·, [C₈H₆]^+·, [C₉H₇]⁺ and [C₇H₅]S⁺ ions from 2-phenylthiophene and the [HCS]⁺, [C₉H₇]⁺, [C₇H₅S]^+·, and [C₁₅H₁₁]⁺ ions from 2,5-diphenylthiophene were interpreted in terms of both carbon skeletal rearrangements in the thiophene ring and migration of the phenyl substituent. The degree of carbon scrambling in the thiophene ring appeared to be almost independent of the electron beam energy. The formation of some of the fragment ions studied seems to be so fast that no carbon scrambling could be detected at all; in neither case was complete scrambling of the carbon atoms of the thiophene ring observed.     

The decomposition of a triply-charged metastable positive ion of biphenyl

The transition, [C12H10]⁺³→[C₁₁H₇]⁺² + [CH₃]⁺, has been detected both in the ion kinetic energy spectrum and in the mass spectrum of biphenyl. The width of a resulting ‘metastable peak’ has been measured by setting the magnetic field to accept [C₁₁H₇]⁺² ions and scanning the high voltage at fixed electric sector voltage. The kinetic energy released in the decomposition, calculated from the peak width, amounted to 4.5 eV. With the assumption that this energy release is due entirely to charge separation, the charge distribution in [C₁₂H₁₀]⁺³ is discussed. The derivation of the equations used to calculate the energy released is given in the Appendix.     

Structure and mechanism of fragmentation of [C2H5O]+

The decomposition reactions of [C₂H₅O]⁺ ions produced by dissociative electron-impact ionization of 2-propanol have been studied, using ¹³C and deuterium labeling coupled with metastable intensity studies. In addition, the fragmentation reactions following protonation of appropriately labeled acetaldehydes and ethylene oxides with [H₃]⁺ or [D₃]⁺ have been investigated. In both studies particular attention has been paid to the reactions leading to [CHO]⁺, [C₂H₃]⁺ and [H₃O]⁺. In both the electron-impact-induced reactions and the chemical ionization systems the fragmentation of [C₂H₅O]⁺ to both [H₃O]⁺ and [C₂H₃]⁺ proceeds by a single mechanism. For each case the reaction involves a mechanism in which the hydrogen originally bonded to oxygen is retained in the oxygen containing fragment while the four hydrogens originally bonded to carbon become indistinguishable. The fragmentation of [C₂H₅O]⁺ to produce [CHO]⁺ proceeds by a number of mechanisms. The lowest energy route involves complete retention of the α carbon and hydrogen while a higher energy route proceeds by a mechanism in which the carbons and the attached hydrogens become indistinguishable. A third distinct mechanism, observed in the electron-impact spectra only, proceeds with retention of the hydroxylic hydrogen in the product ion. Detailed fragmentation mechanisms are proposed to explain the results. It is suggested that the [C₂H₅O]⁺ ions formed by protonation of acetaldehyde or ionization of 2-propanol are produced initially with the structure [CH₃CH?\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^ + $\end{document}H] (a), but isomerize to [CH₂?CH? \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^ + $\end{document}H₂] (e) prior to decomposition to [C₂H₃]⁺ or [H₃O]⁺. The results indicate that the isomerization a → e does not proceed directly, possibly because it is symmetry forbidden, but by two consecutive [1,2] hydrogen shifts. A more general study of the electron-impact mass spectrum of 2-propanol has been made and the fragmentation reactions proceeding from the molecular ion have been identified.     

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.     

Elektronenstossinduzierte Fragmentierung von Acetylenverbindungen—VI: Struktur und Bildungsenthalpie der Ionen [C11H9]+ und [C9H7]+

The appearance potentials for [C₁₁H₉]⁺ from 1, e.g. 2-(chloromethyl)-naphthaline and 1-chloro-phenyl-(5)-penten-(2)-yne-(4), and for [C₉H₇]⁺ from indene, phenyl-propyne and phenyl-(5) penten-(2)-yne-(4)-ol-(1), have been correlated with thermochemical data to obtain the enthalpy of formation of the fragment-ions. A comparison between these enthalpies of formation and the values of enthalpy of hypothetical model structures shows that the [C₁₁H₉]⁺ ion is probably best represented as a benztropylium-ion and the [C₉H₇]⁺ ion as an ethynyl-tropylium-ion or as a phenylcyclopropenyl-cation. Open chain structures can be eliminated in all cases investigated.     

Photodissociation of protonated β-diketones in the gas phase

The gas phase photodissociation spectra of four protonated β-diketones were obtained and compared with the absorption spectra of the corresponding ions in solution. Protonated 2,4-pentanedione was observed to undergo the photodissociation process [C₅H₉O₂]⁺ +hν → [CH₃CO]⁺ +C₃H₆O with a λmax at 276±10 nm compared with a solution absorption maximum at 286 nm. Protonated 2,4-hexanedione was observed to undergo the photodissociation processes [C₆H₁₁O₂]⁺ +hν → [CH₃CO]⁺ +C₄H₈O and [C₆H₁₁O₂]⁺ +hν → [C₂H₅CO]⁺ +C₃H₆O with a λmax at 279±10 nm compared with a solution absorption maximum at 288 nm. Protonated 3-methyl-2,4-pentanedione was observed to undergo the photodissociation process [C₆H₁₁O₂]⁺ +hν → [CH₃CO]⁺ +C₄H₈O with a λmax at 295±10 nm compared with a solution absorption maximum at 305 nm. Protonated 1,1,1-trifluoro-2,4-pentanedione was observed to undergo the photodissociation process [C₅H₆F₃O₂]⁺ +hν → CF₃H+[C₄H₅O₂]⁺ with a λmax at 273±10 nm compared with a solution absorption maximum at 288 nm. The [CH₃CO]⁺ and [C₂H₅CO]⁺ produced photochemically with the first three ions react to regenerate the protonated β-diketone leading to a photostationary state. Photodissociation of the protonated alkyl β-diketones is believed to occur from the protonated keto form, whereas photodissociation of protonated 1,1,1-trifluoro-2,4-pentanedione is believed to occur from the protonated enol form. Mechanisms for the observed photodissociation processes are proposed and comparisons with results from related techniques are presented.     

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 formation of the anilinium ion from the ammonium ion in the ammonia chemical ionization of substituted benzenes

Compounds C₆H₅X(X ? F, Cl, Br, NO₂, CN, OCH₃) have been studied under chemical ionization conditions with ammonia as reagent gas. A pulsed electron beam and time resolved ion collection has allowed the determination of the reaction leading to the formation of [C₆H₅NH₃]⁺ (m/z 94). [NH₄]⁺ reacts with C₆H₅X(X ? F, Cl, Br) to yield m/z 94 but C₆H₅X (X ? CN, NO₂) forms this ion only by reactions involving either [NH₃]⁺ or [C₆H₅X]⁺. C₆H₅OCH₃ does not form m/z 94.     

[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.     

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.