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.     

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.     

Hydrogen rearrangement in molecular ions of alkyl benzenes: Mechanism and time dependence of hydrogen migrations in molecular ions of 1, 3-diphenylpropane and deuterated analogues

Hydrogen migrations in the molecular ions of 1,3-diphenylpropane, preceding the fragmentations to [C₇H₇]⁺ and [C₇H₈]⁺ ions, have been investigated by use of deuterated derivatives. By comparing the distribution of deuterium labels in the [C₇(H, D)₈]⁺ products from metastable molecular ions with the distribution patterns calculated for various exchange models, it is shown that the H migrations occur by two processes linked by a common intermediate: (i) exchange between hydrogen isotopes at the γ-methylene group and at the ortho positions of the phenyl group: (ii) exchange between hydrogen isotopes at the ortho and orthó positions in the intermediate. In these mechanisms the eight hydrogen isotopes at both benzylic positions and both the ortho and orthó positions of 1,3-diphenylpropane participate in a mutual exchange. A statistical equipartition of the hydrogen isotopes at these eight positions is not reached in metastable molecular ions, however. The distribution pattern of [C₇(H, D)₈]⁺ ions from the deuterium labelled compounds as a function of the mean number n of exchange cycles has been calculated according to this reaction model and compared with experimental results for unstable molecular ions, generated by 70 eV and 12 eV electrons, respectively, and metastable molecular ions. Good agreement is obtained for all compounds and n = 0.4–0.8 for unstable molecular ions and n = 5–8 for metastable ions. Therefore, the hydrogen exchange in the molecular ion of 1,3-diphenylpropane is a rather slow process. These results firmly establish the isomerization reaction involving the conversion of the molecular ion of 1,3-diphenylmethane to the intermediate and hence to the molecular ion of 7-(2-phenylethyl)-5-methylene cyclohexa-1,3-diene and preceding the fragmentations. The postulated intermediate is a true one which corresponds to a s?-complex type ion and which fragments to [C₇H₈]⁺ ions. Surprisingly, no isomerizations of the intermediate by hydrogen shifts within the protonated aromatic system (‘ring walks’) are observed.     

Evidence for long-lived excited states of [CnH2]2+ carbodications

The energetics, structures, stabilities and reactivities of[C_nH₂]²⁺ ions have been investigated using computational methods and experimental mass spectrometric techniques. Spontaneous decompositions of [C_nH₂]²⁺ into [C_nH]⁺ + H⁺ products, observed for ions with odd-n values, have been explained by invoking the formation of excited triplet states. Even-n [C_nH]⁺ ions possess triplet ground states with low-lying excited states, whereas odd-n ions have triplet states with energies several eV above ground singlet states. Radiationless transitions of vibrationally excited long-lived triplet state ions into singlet state continua are suggested as possible mechanisms for spontaneous deprotonation processes of odd-n [C_nH₂]²⁺ ions. Evidence for these long-lived excited states has been obtained in bimolecular single electron transfer reactions.     

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.     

[C6H6, C3H7 +]and [C6H7 +, C3H6] ion-neutral complexes intermediate in the reactions of protonated propylbenzenes

From the mass-analysed ion kinetic energy spectra of labelled ions, kinetic energy releases and thermodynamic data, it is proved that protonated n-propylbenzene (1) isomerizes into protonated isopropyl benzene (2). It is also shown that the dissociation of the less energetic metastable ions of (2), leading to [iso-C₃H₇]⁺ and [C₆H₇]⁺ product ions, is preceded by H exchange. This H exchange involves two interconverting ion-neutral complexes [C₆H₆, iso-C₃H₇⁺] (2π) and [C₆H₇⁺, C₃H₆] (2α).     

A comparative study on the behaviour of 1,3-diheterocyclopentanes (X = O,O; S,S; O,S) under electron impact. The formation of thioacetyl and thiiranyl cations

The electron-impact-induced mass spectra of 1,3-dioxolane (la), 1,3-dithiolane (2a) and 1,3-oxatbiolane (3a) and their 2-methyl (1b–3b) and 2,2-dimethyl [(CH₃)₂: 1c–3c or (CD₃)₂: 1d–3d] derivatives have been studied in detail to gain further insight into their ion structures and competing reaction pathways with low-resolution, high-resolution, metastable and collision-induced dissociation (CID) techniques. For compounds 1a–1d the most significant reaction is loss of H^˙ and CH₃^˙ by α-cleavage and a subsequent formation of CHO⁺ and C₂H₃O⁺ ions. The [M ? H]⁺ ions from 1a and 1b give a C₂H₃O⁺ ion which does not have the acyl cation structure as shown by their CID spectra. In compounds 3a–3d the sulphur-containing ions predominate, the C₂H₃O⁺ now having the acyl cation structure. 1,3-Dithiolanes (2a–2d) exhibit the most complicated fragmentation patterns. Furthermore the [M ? H]⁺ ion from 2a and [M ? CH₃]⁺ ion from 2b have different structures as well as the [M ? H]⁺ ion from 2b and [M ? CH₃]⁺ ion from 2c, as shown by their CID spectra. This can be utilized to explain why 3a–3c and 2a give principally a thiiranyl cation, whereas 2b gives a mixture of this and the thioacyl cation and 2c practically only the open-chain thioacetyl cation.     

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.     

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.     

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.     

Fragmentation of protonated O,O-diethyl O-aryl phosphorothionates in tandem mass spectral analysis

The gas-phase ion chemistry of protonated O,O-diethyl O-aryl phosphorothionates was studied with tandem mass spectrometric and ab initio theoretical methods. Collision-activated dissociation (CAD) experiments were performed for the [M+H]⁺ ions on a triple quadrupole mass spectrometer. Various amounts of internal energy were deposited into the ions upon CAD by variation of the collision energy and collision gas pressure. In addition to isobutane, deuterated isobutane C₄D₁₀ also was used as reagent gas in chemical ionization. The daughter ions [M+H?C₂H₄]⁺ and [M+H?2C₂H₄]⁺ dominate the CAD spectra. These fragments arise via various pathways, each of which involves γ-proton migration. Formation of the terminal ions [M+H?2C₂H₄?H₂O]⁺, [M+H?2C₂H₄?H₂S]⁺, [ZPhOH₂]⁺, [ZPhSH₂]⁺, and [ZPhS]⁺ [Z = substituent(s) on the benzene ring] suggests that (1) the fragmenting [M+H]⁺ ions of O,O-diethyl O-aryl phosphorothionates have protons attached on the oxygen of an ethoxy group and on the oxygen of the phenoxy group; (2) thiono-thiolo rearrangement by aryl migration to sulfur occurs; (3) the fragmenting rear-ranged [M+H]⁺ ions have protons attached on the oxygen of an ethoxy group and on the sulfur of the thiophenoxy group. To get additional support for our interpretation of the mass spectrometric results, some characteristics of three protomers of O,O-diethyl O-phenyl phosphorothionate were investigated by carrying out ab initio molecular orbital calculations at the RHF/3–21G* level of theory.     

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.     

[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.     

Photodissociation and structures of [C6H4]+ ˙ ions generated from cyanobenzene and other precursors

Previous work on the electron impact induced loss of hydrogen cyanide from the radical cations of cyanobenzene has revealed that ring opening is important in the formation of the corresponding [C₆H₄]^{+ ˙} ions. Photodissociation experiments now show that these [C₆H₄]^{+ ˙} ions and those generated from 2-ethynylpyridine, 1,3-hexadiyn-6-nitrile and 1,2-diiodobenzene all photodissociate in the visible region to [C₄H₂]^{+ ˙}. The corresponding photodissociation spectra are all the same and have a maximum at about 370 nm, in agreement with spectra of ions with three conjugated double or triple bonds. Owing to the high reactivity, the low photodissociation rate and, possibly, the internal energy of the ions, the photodissociation kinetics are too complicated to be solved. The experiments nevertheless show that at least a major fraction of the [C₆H₄]^{+ ˙} ions has a ring-opened structure. This conclusion is supported by MNDO calculations, which indicate that the heats of formation of the possible acyclic structures are about 150 kJ mol^?1 lower than those of the o-, m- and p-benzyne structures.     

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.     

Repeller-Induced dissociation of alkyl alcohols in thermospray mass spectrometry

Six alkyl alcohols were studied using thermospray mass Spectrometry. Whereas the dominant ion in the spectrum up to a repeller potential of 120 V was [M + NH₄]⁺, above that potential [M + H]⁺ and fragment ions appeared. The fragments observed were largely due to hydrogen release from alkyl ions ([C_nH_2n+1]⁺ – H2 → [C_nH_2n-1]⁺) and loss of water or some other stable molecule from the same species. The results are compared with those from ionization of the same alcohols under electron impact and photoionization conditions and with results obtained for methanol under thermospray conditions.     

An investigation of the decomposition of the intermediate ions produced by electron-impact. II—[C7H7]+ ions from several halogenotoluenes

The structure and decomposition of the [C₇H₇]⁺ ions produced by electron-impact from o-, m- and p-chlorotoluene, o-, m- and p-bromotoluence, and p-iodotoluence, have been investigated. By determining the relative abundance of normal and metastable ions, these [C₇H₇]⁺ ions at electron energy of 20 eV are shown to be so-called ‘tropylium ions’. The amount of the internal energy of the [C₇H₇]⁺ ion estimated by the relative ion abundance ratios, ? [C₅H₅]⁺/[C₇H₇]⁺ and m*/[C₇H₇]⁺ for the decomposition \documentclass{article}\pagestyle{empty}\begin{document}$ [{\rm C}_{\rm 7} {\rm H}_{\rm 7}]^ + \mathop \to \limits^{m^* } [{\rm C}_{\rm 5} {\rm H}_{\rm 5}]^ + + {\rm C}_{\rm 2} {\rm H}_{\rm 2} $\end{document}, is in the order iodotoluene > bromotoluene > chlorotoluene. The heats of formation of the activated complexes for the reaction \documentclass{article}\pagestyle{empty}\begin{document}$ [{\rm C}_{\rm 7} {\rm H}_{\rm 7}]^ + \mathop \to \limits^{m^* } [{\rm C}_{\rm 5} {\rm H}_{\rm 5}]^ + + {\rm C}_{\rm 2} {\rm H}_{\rm 2} $\end{document} were estimated. The values suggest that the decomposing [C₇H₇]⁺ ions from various halogenotoluenes are identical in structure.     

Isomerisation of hydrocarbon ions:I—Isomeric octanes: Acollisional actiation study

The molecular ions of isomeric octanes retain their structural identity, while their alkyl fragments [C_nH_2n+1]⁺ (n = 3 to 7) isomerise to common structures prior to decomposition. Structures for [C₆H₁₃]⁺ and [C₇H₁₅]⁺ ions are proposed.     

On the unimolecular loss of acetylene from metastable C11H9]+ ions

Extensive ¹³C labelling experiments demonstrate that loss of acetylene from metastable [C₁₁H₉]⁺ ions is a complex process, which can be described quantitatively in terms of a four-parameter model. The major reaction path (77.8%) involves scrambling of all 11 carbon atoms. Insight into the reaction details is provided neither by the kinetic energy release associated with the reaction [C₁₁H₉]⁺ → [C₉H₇]⁺ + C₂H₂ nor by the analysis of the collisional activation mass spectra of the resulting [C₉H₇]⁺ ions.     

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.