Benzene as a selective chemical ionization reagent gas

Dilute mixtures of C₆H₆ or C₆D₆ in He provide abundant [C₆H₆]^+· or [C₆D₆]^+· ions and small amounts of [C₆H₇]⁺ or [C₆D₇]⁺ ions as chemical ionization (CI) reagent ions. The C₆H₆ or C₆D₆ CI spectra of alkylbenzenes and alkylanilines contain predominantly M^+· ions from reactions of [C₆H₆]^+· or [C₆D₆]^+· and small amounts of MH⁺ or MD⁺ ions from reactions of [C₆H₇]⁺ or [C₆D₇]⁺. Benzene CI spectra of aliphatic amines contain M^+·, fragment ions and sample-size-dependent MH⁺ ions from sample ion-sample molecules reactions. The C₆D₆ CI spectra of substituted pyridines contain M^+· and MD⁺ ions in different ratios depending on the substituent (which alters the ionization energy of the substituted pyridine), as well as sample-size-dependent MH⁺ ions from sample ion-sample molecule reactions. Two mechanisms are observed for the formation of MD⁺ ions: proton transfer from [C₆D₆]^+· or charge transfer from [C₆D₆]^+· to give M^+·, followed by deuteron transfer from C₆D₆ to M^+·. The mechanisms of reactions were established by ion cyclotron resonance (ICR) experiments. Proton transfer from [C₆H₆]^+· or [C₆D₆]^+· is rapid only for compounds for which proton transfer is exothermic and charge transfer is endothermic. For compounds for which both charge transfer and proton transfer are exothermic, charge transfer is the almost exclusive reaction.     

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.     

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.     

Molecule/radical reactions prior to ionization under negative chemical ionization conditions

It is shown that alkyl radical species present in CH₄ or iso-C₄H₁₀ plasma can react with substrate molecules to give [M+C_nH_2n] species. These species become evident especially in negative chemical ionization as [M+C_nH_2n]^?˙ and, less obviously, in positive chemical ionization as [M+C_nH_2n+1]⁺ ions which, for example in natural products chemistry, may be mistaken for a series of homologous compounds present in the sample.     

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.     

The doubly-charged ion mass spectra of hydrocarbons

The doubly-charged ion mass spectra of some hydrocarbons, including a variety of structural types, have been obtained by a new technique in which doubly-charged ions are charge exchanged with neutral molecules and so separated from singly-charged ions. The spectra show strong similarities, independent of hydrocarbon structure; characteristic ions include [C_mH₂]⁺⁺ (m = 2 to 5), [C_nH₆]⁺⁺(n > 6), [C₁₀H₈]⁺⁺, [C₁₂H₈]⁺⁺, [C₁₁H₁₀]⁺⁺, [C₇H₇]⁺⁺·, [C₉H₇]⁺⁺· and [C₁₃H₁₁]⁺⁺·. The fragmentation pattern of 2-phenylnaphthalene has been reconstructed, based on observed reactions of metastable doubly-charged ions to give fragment doubly-charged ions. In addition, we examined metastable ion fragmentations leading to two singly-charged ions for some of the characteristic ions, using several compounds. The value of doubly-charged ion mass spectra of hydrocarbons appears to lie in the information they provide on ion structures; this information was sufficient to permit the proposal of structures for the major ions encountered in this study.     

Reaction of ammonia with accelerated benzoyl ions under multiple-collison conditions in a triple quadrupole instrument

The reaction of benzoyl ion with ammonia in multiple-collision conditions in the second quadrupole assembly of a triple quadrupole mass spectrometer at (laboratory) ion kinetic energies from 0 to 20 eV produced the even-electron ions [C₆H₅]⁺, [C₆H₅NH₃·(NH₃)_m]⁺ (m = 0, 1) and [C₆H₅CONH₃·(NH₃)_n]⁺ (n = 0, 1, 2, 3) and the odd-electron ions [C₆H₄NH₃·(NH₃)_p]⁺· (p = 0, 1). Thermochemical information could not be obtained under multiple-collision conditions: both exothermic and endothermic reactions were observed, with no translational-energy onset measurable for the endothermic processes, nor decrease in the yield of the exothermic processes at high energies. The behaviour of cluster-ion intensities as pressure varied was qualitatively as expected. There are pressure and energy regions where spectra change little; if this feature were to be general, it would point to some utility for these conditions in qualitative analysis.     

A comparison of the unimolecular decomposition pathways of some C7 and C8 ions in the gas phase

[CnH_2n?3]⁺ and [C_nH_2n?4]⁺·(n = 7, 8) ions have been generated in the mass spectrometer from C_nH_2n?3 Br (n = 7, 8) precursors and from two steroids. The relative abundances of competing ‘metastable transitionss’ indicate (partial) isomerization to a common structure (or mixture of structures) prior to decomposition in most examples of all four types of ions. In contrast, [C₈H₁₀O]⁺· and [C₈H₁₂O]⁺· ions, generated from different sources as molecular ions and by fragmentation of steroids, do not decompose through common-intermediates.     

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.     

Chemical Ionization—A Mass-Spectrometric Analytical Procedure of Rapidly Increasing Importance

The mode of ionization of a molecule has a strong influence on its behavior in the mass spectrometer and thus on the information that can be obtained from its mass spectrum. In chemical ionization a reagent gas, e.g. methane, is first ionized by electron impact. The ions formed in ion-molecule reactions, in particular [CH₅]⁺, [C₂H₅]⁺, and [C₃H₅]⁺, then react “chemically” with the substrate M in fast acid/base type reactions to form ions of the type [MH]⁺, [M(C₂H₅)]⁺, etc., which subsequently fragment to various extents. Alternatively, chemical ionization can be effected by charge exchange, in that ions of a reagent gas, e.g. [He]^+?, react with the substrate M to form molecular ions [M]^+·. Chemical ionization can thus be conducted in a more or less mild fashion and the extent of the fragmentation can be controlled over a very wide range.     

Adduct ion formation by metal complexes under methane negative chemical ionization conditions

Negative chemical ionization mass spectrometry is used as a probe to examine reactions between hydrocarbon radicals and metal complexes in the gas phase. The methane negative chemical ionization mass spectra of 27 complexes of cobalt(II ), nickel(II ) and copper(II ) in the presence of O₄, O₂N₂ and N₄ donor atom sets are characterized by two dominant series of adduct ions of the form [M + C_nH_2n]^? and [M + C_nH_2n+1]^? at m/z values above the molecular ion, [M]^?. Insertion of the CH radical into the ligand followed by radical/radical recombination and electron capture is proposed as the major mechanism leading to the formation of [M + C_nH_2n]^? adduct ions. A second pathway involves ligand substitution by C_nH_2n+1 radicals concomitant with H elimination and electron capture. Oxidative addition at the metal followed by ionization is suggested as the principal pathway for the formation of [M + C_nH_2n+1]^? adduct ions.     

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.     

Fragmentation of pyrazolecarbaldehyde thio- and dithioacetals under electron impact and chemical ionization

The mass spectra of 1-substituted 3,5-dimethyl-1H-pyrazole-4-carbaldehyde bis(2-hydroxyethyl) dithioacetals and thioacetals were studied for the first time. The main fragmentation pathways of their molecular ions generated under electron impact and chemical ionization were similar. Primary decomposition of the molecular ions of bis(2-hydroxyethyl) dithioacetals involves elimination of 2-sulfanylethanol molecule with formation of the corresponding 1,3-oxathiolane radical cation. Fragmentation of the molecular ions [M]⁺ · and [M + H]⁺ derived from 2-(3,5-dimethyl-1H-pyrazol-4-yl)-1,4,6-oxadithiocanes includes cleavage of the eight-membered heteroring and elimination of C₄H₉OS ·. Substituents in the heteroring of pyrazolecarbaldehydes inhibit decomposition processes related to the aldehyde group.     

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.     

Mass Spectra of New Heterocycles. XVII. Main Fragmentation Routes of Molecular Ions of 4-Alkoxy-5-amino-3-methylthiophene-2-carbonitriles under Electron and Chemical Ionization

For the first time decomposition was investigated of 4-alkoxy-5-amino-3-methylthiophene-2-carbonitriles under the conditions of electronic (70 eV) and chemical (reagent gas methane) ionization. At the electronic ionization the compounds under study [except for 4-(1-ethoxyethoxy) and 4-(ferrocenylmethoxy) derivatives] form stable molecular ions that decompose mainly by the cleavage of an alkyl radical from the alkoxy-substituent. Further fragmentation of the arising ion [M–Alk]⁺ depends on the substituent nature in the amino group. In the mass spectrum of 4-(ferrocenylmethoxy)-substituted thiophene peaks of the ion [FcCH₂]⁺ and its fragmentation products prevail. In the mass spectra of chemical ionization predominant peaks belong to ions M^+·, [M + H]⁺ and [M + C₂H₅]⁺, and fragment ions are absent.     

Mass spectral behavior of n-alkynes

The 70 e V-electron impact mass spectra of the C₇–C₁₀ n-alkynes have been determined as well as the metastable ion spectra of the molecular ions and the [CS₂]⁺ and [N₂O]⁺ charge exchange mass spectra of the C₇-C₉ n-alkynes. The metastable ion mass spectra provide only a limited opportunity to distinguish between isomers; however, the 70-eV EI mass spectra of isomeric compounds permit a ready distinction between isomers. The [CS₂]⁺ charge exchange mass spectra of isomeric compounds also show substantial differences. The [N₂O]⁺ charge exchange mass spectra do not show the enhancement of β-fission fragments observed in field ionization experiments, despite representing ions of similar internal energy, and it is concluded that field dissociation is responsible for the β-fission fragments in the field ionization experiments.     

Mass spectrometry of homoadamantane derivatives

Homoadamantane derivatives can be divided into two groups according to their mass spectra. To the first group belong compounds with electron attracting substituents (COOH, CI, COOCH₃, Br); compounds with electron releasing substituents (OCH₃, OH, NH₃, NHCOCH₃) constitute the second group. The most characteristic feature of the first group compounds is the splitting off of the substituent. The hydrocarbon fragment [C₁₁H₁₇]⁺ thus formed then loses olefin molecules with the formation of corresponding ionic species C_11?nH_17?2n. The 3-substituted compounds of this group undergo thermal Wagner-Meerwein type rearrangements into adamantane derivatives, resulting in the [C₁₀H₁₅]⁺ (m/e 135) ion formation; this is the main difference between 1- and 3-substituted homoadamantanes. The series of [C_nH_2n?6X]⁺ ions (where X = OCH₃, OH, NH₂, NHCOCH₃, n = 6 to 10) are characteristic of the mass spectra of the second group compounds, the ion [C₆H₆X]⁺, [M ? C₅H₁₁]⁺ being the most abundant. The intensity ratio of [M ? C₅H₁₁]⁺ to [M ? C₄H₉]⁺ ions is 10:1 for 1-substituted and 3:1 for 3-substituted compounds of this group, allowing the location of the substituent. Some individual features of the spectra are also reported.     

A mass spectrometry study of alkanes in air plasma at atmospheric pressure

The positive APCI-mass spectra in air of linear (n-pentane, n-hexane, n-heptane, n-octane), branched [2,4-dimethylpentane, 2,2-dimethylpentane and 2,2,4-trimethylpentane (i-octane)], and cyclic (cyclohexane) alkanes were analyzed at different mixing ratios and temperatures. The effect of air humidity was also investigated. Complex ion chemistry is observed as a result of the interplay of several different reagent ions, including atmospheric ions O₂^+•, NO⁺, H₃O⁺, and their hydrates, but also alkyl fragment ions derived from the alkanes. Some of these reactions are known from previous selected ion/molecule reaction studies; others are so far unreported. The major ion formed from most alkanes (M) is the species [M − H]⁺, which is accompanied by M^+• only in the case of n-octane. Ionic fragments of C _n H_2n ^+1/+ composition are also observed, particularly with branched alkanes: the relative abundance of such fragments with respect to that of [M − H]⁺ decreases with increasing concentration of M, thus suggesting that they react with M via hydride abstraction. The branched C₇ and C₈ alkanes react with NO⁺ to form a C₄H₁₀NO⁺ ion product, which upon collisional activation dissociates via HNO elimination. The structure of t-Bu⁺(HNO) is proposed for such species, which is reasonably formed from the original NO⁺(M) ion/molecule complex via hydride transfer and olefin elimination. Finally, linear alkanes C₅–C₈ give a product ion corresponding to C₄H₇⁺(M), which we suggest is attributed to addition of [M − H]⁺ to C₄H₈ olefin formed in the charge-transfer-induced fragmentation of M. The results are relevant to applications of nonthermal plasma processes in the fields of air depuration and combustion enhancement.     

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.     

Plenary lecture: Ion enthalpies and their application in mass spectrometry

The experimental determination of ionization and appearance energies is discussed, together with the calculation of heats of formation of ions. Results are presented for the ions [C_nH_2n+1]⁺, [C_nH_2n+2N]⁺ and [C_nH_2n+1O]⁺. The low temperature (～350K), low energy (12.1eV) mass spectra of some alkanes, amines and alcohols are presented and discussed.