Dechlorination of polychlorinated biphenyls in electron-capture negative-ion chemical ionization mass spectrometry

The electron-capture negative-ion chemical Ionization mass spectra of 36 ³⁵Cl mono- and di-labelled penta- and hexachlorobiphenyls were studied. Under the conditions of the analyses, the dechlorination of these congeners probably proceeds through electrophilic hydrogen atom attachment followed by rapid loss of the chlorine atom at the same position. These dechlorination reactions occur regioselectively. When present, ortho-chlorines are lost preferentially to meta- and para-chlorines. The extent of the dechlorination reaction, as seen by the relative abundance of the [MH ? Cl]^? ions, decreases with increasing number of ortho-chlorines.     

The ortho effect in the mass spectra of ortho/para isomers of bisphenol a derivatives and related compounds

New examples of the ortho effect in bisphenol A derivatives including interaction of the hydrogen of the ortho-hydroxy group with the neighbouring aromatic ring have been observed. The characteristic ions [M ? PhOH]^+middot; (m/z = 134) and [M ? CH₃ ? PhOH]⁺ (m/z = 119) were shown to form through the hydrogen transfer from hydroxy and isopropyl groups, respectively. The spectra of cyclic derivatives having ortho-hydroxy functions show [M ? 43]⁺, [M ? C₈H₉O]⁺, m/z = 147, m/z = 135 and [M ? C₉H₁₀O]⁺ ions. The proposed mechanims of the corresponding transformations were supported by mass spectra of deuterated analogues, methyl and trimethyl silyl ethers.     

Chemical ionization mass spectra of mononitroarenes

The H₂ and CH₄ chemical ionization mass spectra of a selection of substituted nitrobenzenes have been determined. It is shown that reduction of the nitro group to the amine is favoured by high source temperatures and the presence of water in the ion source. The H₂ chemical ionization mass spectra are much more useful for distinguishing between isomeric compounds than the CH₄ CI mass spectra because of the more extensive fragmentation. For ortho substituents bearing a labile hydrogen abundant [MH ? H₂O]⁺ fragments are observed. When the substituent is electron-releasing both ortho and para substituted nitrobenzenes show abundant [MH? OH]⁺ fragment ions while meta substituted compounds show abundant loss of NO and NO₂ from [MH]⁺. The latter fragmentation is interpreted in terms of protonation para to the substituent or ortho to the vitro function, while the first two fragmentation routes arise from protonation at the nitro group. When the substituent is electron-attracting the chemical ionization mass spectra of isomers are very similar except for the H₂O loss reaction for ortho compounds.     

Direct analysis in real time mass spectrometry (DART‐MS) of highly non‐polar low molecular weight polyisobutylenes

Low molecular weight polyisobutylenes (PIB) with chlorine, olefin and succinic acid end‐groups were studied using direct analysis in real time mass spectrometry (DART‐MS). To facilitate the adduct ion formation under DART conditions, NH₄Cl as an auxiliary reagent was deposited onto the PIB surface. It was found that chlorinated adduct ions of olefin and chlorine telechelic PIBs, i.e. [M + Cl]^? up to m/z 1100, and the deprotonated polyisobutylene succinic acid [M? H]^? were formed as observed in the negative ion mode. In the positive ion mode formation of [M + NH₄]⁺, adduct ions were detected. In the tandem mass (MS/MS) spectra of [M + Cl]^?, product ions were absent, suggesting a simple dissociation of the precursor [M + Cl]^? into a Cl^? ion and a neutral M without fragmentation of the PIB backbones. However, structurally important product ions were produced from the corresponding [M + NH₄]⁺ ions, allowing us to obtain valuable information on the arm‐length distributions of the PIBs containing aromatic initiator moiety. In addition, a model was developed to interpret the oligomer distributions and the number average molecular weights observed in DART‐MS for PIBs and other polymers of low molecular weight. Copyright © 2015 John Wiley & Sons, Ltd.     

Ortho effects in mass spectrometric fragmentation processes

The mass spectra of the phenylhydrazones and 2,4-dinitrophenylhydrazones of ortho substituted benzaldehydes and acetophenones (X = I, Br, Cl, OCH₃, OH) show characteristic [M ? X]⁺ ions which allow the ortho derivatives to be distinguished from their meta and para isomers.     

The Chemistry of C6H6O radical cations: A study of rearrangement reactions of halogen substituted ethyl phenyl ethers

New experimental data on the rearrangement reaction of various phenoxyethyl halides to give [C₆H₆O]^+˙ are presented and compared with previous studies so that a coherent picture of this process can be developed. By examining the metastable kinetic energy release for low energy decomposing molecular ions of the phenoxyethyl halides, it has been concluded that formation of [C₆H₆O] occurs by competitive 1,2 and 1,3 hydrogen shifts from the alkyl carbons to oxygen followed by a rate determining C? O bond cleavage. This is substantiated by the absence of a primary hydrogen isotope effect. For more highly activated molecular ions, a new mechanism comes into play as evidenced by the appearance of a small hydrogen isotope effect. It is postulated that this third mechanism involves transfer of the alkyl hydrogen to the ortho position of the ring by a rate determining 1,5 shift, followed by a 1,3 hydrogen shift from the ortho methylene group to oxygen and rapid C? O bond cleavage. This 1,3 hydrogen shift to oxygen appears to be ‘catalysed’ by the halogen atoms yielding phenol ions. No indications have been found for the formation of tautomeric 2,4-cyclohexadienone ions. Furthermore, highly activated molecular ions produce [C₆H₆O]^+˙ which can undergo metastable decomposition to lose carbon monoxide. Kinetic energy release measurements for the latter reaction show that the majority of these [C₆H₆O]^+˙ions have been formed as phenol ions as well. These arguments are supported by energetic measurements and by comparisons with previous ion cyclotron resonance and collisional activation studies.     

Electron impact mass spectra of chlorinated methyl propanoates

The mass spectral fragmentations of all eleven chlorinated methyl propanoates have been studied. Deuterium labelling and metastable ion analysis were used to elucidate the fragmentation mechanism. The molecular ion peaks of all compounds are small, except methyl 3,3-dichloropanoate (38%). In most cases α-cleavage gives the base peak [COOCH₃]⁺, and the loss of a chlorine atom from the molecular ion is characteristic of the 3-chloro, 3,3-dichloro and 3,3,3-trichloro compounds. Metastable ions showed the losses of small neutral molecules such as CH₃OH, CH₂CO, CO₂ and CO from the [M? Cl]⁺ ion. α-Cleavage and the loss of Cl^˙ gives an intense [M? COOCH₃? Cl]^+˙ peak, which is the base peak in the spectra of the 2,3-dichloro and 2,3,3-trichloro compounds.     

The Loss of a hydroxyl group from the molecular ions of alkylnitrobenzenes

It is confirmed that the loss of HO˙ from the molecular ion of o-nitrotoluene involves exclusively a hydrogen from the methyl group. However, in higher homologues hydrogen atoms from non-benzylic sites are also implicated. With such compounds this fragmentation mode is shown not only by the ortho but, to a lesser extent, by the meta and para isomers as well. The proportion of the total ion current borne by the [M – 17]⁺ ion follows the order ortho > meta > para, which is attributed to substituent migration around the ring with a hydroxyl radical only being lost when the groups are on adjacent ring atoms. Other ions present in the spectra point to interaction between substituents to form a new heterocyclic ring.     

[M + 11]+˙ and [M + 11]−˙ ions in the nitrogen positive and negative ion chemical ionization mass spectra of aromatic amines

The N₂ negative ion chemical ionization (NICI) mass spectra of aniline, aminonaphthalenes, aminobiphenyls and aminoanthracenes show an unexpected addition appearing at [M + 11]^?˙. This addition is also observed in the N₂ positive chemical ionization (PCI) mass spectra. An ion at [M – 15]^? is found in the NICI spectra of aminoaromatics such as aniline, 1- and 2-aminonaphthalene and 1- and 2-aminoanthracene. Ion formation was studied using labeled reagents, variation of ion source pressure and temperature and examination of ion chromatograms. These experiments indicate that the [M + 11]^?˙, [M – 15]^?˙ and [M + 11]^+˙ ions result from the ionization of analytes altered by surface-assisted reactions. Experiments with ¹⁵N₂, [¹⁵N] aniline, [2,3,4,5,6-²H₅] aniline and [¹³C₆] aniline show that the [M + 11]^?˙ ion corresponds to [M + N – 3H]^?˙. The added nitrogen originates from the N₂ buffer gas and the addition occurs with loss of one ring and two amino group hydrogens. Fragmentation patterns in the N₂ PCI mass spectrum of aniline suggest that the neutral product of the surface-assisted reaction is 1,4-dicyanobuta-1,3-diene. Experiments with diamino-substituted aromatics show analogous reactions resulting in the formation of [M – 4H]^?˙ ions for aromatics with ortho-amino groups. Experiments with methylsubstituted aminoaromatics indicate that unsubstituted sites ortho to the amino group facilitate nitrogen addition, and that methyl groups provide additional sites for nitrogen addition.     

Ion chemistry of phthalamic acids. III. The origin of [M ? 1]+ ions under electron impact

Electron impact mass spectra and collisional activation/mass-analysed ion kinetic energy spectra of some phthalamic acids and their deuterium labelled analogues suggested that the genesis of [M ? 1]⁺ ions is due to the loss of an aromatic hydrogen ortho to the amidic group, as for aromatic amides and thioamides.     

Mass spectra of chlorinated aromatics formed in pulp bleaching 2—Chlorinated guaiacols

The fragmentation of chlorinated guaiacols (2-methoxyphenols) on electron impact has been studied. The most common fragmentation processes are interpreted and in some cases the small differences between spectra of positional isomers are explained. In addition to the well-known alkyl-oxygen fission (loss of methyl radical), metastable ion studies and deuterium labelling have indicated several new fragmentation pathways. The most characteristic are the formation of [M? CH₃? HCl]⁺ and [M? CH₃? Cl]^+· ions. In general, however, the spectra of positional isomers are shown to be very similar.     

Effect of collision gas pressure and collision energy on reactions between oxygen and the negative molecular ion of tetrachlorodibenzo-p-dioxins in the collision cell of a triple -quadrupole mass spectrometer

Low-energy reactive collisions between the negative molecular ion of a tetrachlorodibenzo-p-dioxin (TCDD) and oxygen inside the collision cell of a triple-stage quadrupole mass spectrometer produce a substitution ion [M ? Cl + O]^?, a phenoxide ion [C₆H_4-nO₂Cl_n]^?·, [M ? HCl]^?·, and Cl^? by which 1,2,3,4-, 1,2,3,6/1,2,3,7- and 2,3,7,8-TCDD isomers can be distinguished either directly or on the basis of intensity ratios. The collision conditions have an important effect on the relative abundances. Energy- and pressure-resolved curves show that the ions formed by a collisionally activated reaction (CAR) process, i.e. [M ? Cl + O]^? and [C₆H_4-n,O₂Cl_n]^?·, are favoured by a high pressure of oxygen (3-6 mTorr) (1 Torr = 133.3 Pa) and a low collision energy (0.1-7 eV), whereas the ions formed by a collisionally activated dissociation (CAD) process, i.e. [M ? HCl]^?· and Cl^?, are favoured by high pressure and high energy. By choosing a relatively low collision energy (5 eV) and high pressure (4 mTorr), the CAR and CAD ions can be clearly detected.     

Studies in negative ion mass spectrometry: XIV—Electron attachment reactions of a series of η6-arenetricarbonylchromium(O) complexes

Electron attachment reactions of a series of (η⁶-arene)tricarbonylchromium(O) complexes have been examined in the gas phase. The electron capture process has been shown to be influenced by the structure of the η⁶-arene ligand and its substituents. Whereas (η⁶-benzene)- and (η⁶-mesitylene)tricarbonylchromium(O) undergo dissocative electron capture, or reductive decarbonylation, yielding [M? CO]^?˙ ions of highest abundance in their negative ion mass spectra, [η⁶-(2,2-dimethylindan-1,3-dione)]tricarbonylchromium(O) forms a molecular negative ion which undergoes sequential CO eliminations and finally a demetallation to give the arene radical ion. A localization of charge on the coordinated arene ligand is proposed for the formation of [M]^?˙ in this case. (η⁶-Methylbenzoate)tricarbonylchromium(O) also forms a molecular negative ion by secondary electron attachment which decomposes by simultaneous and consecutive eliminations of up to four CO molecules. The elucidation of a mechanism and sequence for these CO eliminations has been achieved by synthesizing and examining the negative ion mass spectrum of [η⁶-(C₆H₅·¹³CO₂Me)]Cr(CO)₃. The first CO loss in the principal fragmentation pathway occurs solely from the –Cr(CO)₃ group of [M]^?˙. The effect of para substituent groups on the stabilities of molecular negative ions and their fragmentations has been ascertained using a series of para-substituted (η⁶-methylbenzoate)tricarbonylchromium(O) compounds containing the groups NH₂, OH, OCH₃, CL and COOMe. The stabilities of the [M]^?˙ ions have been rationalized in terms of the Hammett and Taft parameters σ_P, σ_I, σ_R^P, σ_P^O and σ_R^O. The overall electronic substituent effect transmitted to the carbonyl groups of the –Cr(CO)₃ unit involves both resonance and inductive components. In this series of compounds the stability of [M]^?˙ decreases as the electron withdrawing capacities of the para substituents increase.     

The fragmentation of ortho-methoxyphenyl-2-propanone oximes and related compounds–further examples of an ortho-effect

An [M ? 31]⁺ ion was a prominent fragment in the mass spectra of three ortho-methoxy-phenyl-2-propanone oximes and is shown to be due to the expulsion of a methoxyl radical from the molecular ion as a result of an ortho-effect. In contrast, an [M ? 31]⁺ ion was absent from the spectra of a structurally related ketone and a hydroxylamine, and was not observed in the spectra of meta- and para-methoxyphenyl-2-propanone oximes.     

Fragmentation of certain substituted azadispiro(5.1.5.2)pentadec-9-ene and azadispiro(4.1.4.2)tridec-8-ene derivatives under electron ionization

A study of the electron ionization mass spectra of certain azadispiro(5.1.5.2)pentadec-9-ene-7,15-diones and azadispiro(4.1.4.2)tridec-8-ene-6,13-diones and their derivatives has revealed that these molecules undergo fragmentation primarily by two routes, viz. loss of CO and elimination of the substituent on the pyrrolidine nitrogen. Under positive ionization conditions loss of CO is the predominant process in the diones as it releases the ring strain, while in the 6- or 7-ols loss of the substituent on nitrogen is the favoured pathway. The further decomposition pathways of these primary fragments [M ? CO]⁺˙ and [M ? OR³]⁺ have been delineated with the help of high-resolution mass measurements, D₂O exchange and metastable spectra, These compounds give very simple negative ion spectra showing only [M ? OR³]^? and [NCO]^? ions except the N-hydroxy compounds which show [M ? H]^? ions as well.     

The unusual fragmentation of long‐chain feruloyl esters under negative ion electrospray conditions

Long‐chain ferulic acid esters, such as eicosyl ferulate ( 1 ), show a complex and analytically valuable fragmentation behavior under negative ion electrospay collision‐induced dissociation ((?)‐ESI‐CID) mass spectrometry, as studied by use of a high‐resolution (Orbitrap) mass spectrometer. In a strong contrast to the very simple fragmentation of the [M + H]⁺ ion, which is discussed briefly, the deprotonated molecule, [M – H]^?, exhibits a rich secondary fragmentation chemistry. It first loses a methyl radical (MS²) and the ortho‐quinoid [M – H – Me]^‐? radical anion thus formed then dissociates by loss of an extended series of neutral radicals, C_nH_2n + 1^? (n = 0–16) from the long alkyl chain, in competition with the expulsion of CO and CO₂ (MS³). The further fragmentation (MS⁴) of the [M – H – Me – C₃H₇]^? ion, discussed as an example, and the highly specific losses of alkyl radicals from the [M – H – Me – CO]^‐? and [M – H – Me – CO₂]^‐? ions provide some mechanistic and structural insights.     

Ortho effects in organic molecules on electron impact: XI—the interesting ortho effect of the methoxy group in o-methoxy aromatic thioamides

It has been noticed that the major part of the loss of ?H from the molecular ion of most of the o-methoxythioamides results from an ortho effect of the methoxy group. Comparison of the MIKE spectra of the [M? SH]⁺ of 1-(2-methoxyphenylthioxomethyl)piperidine and 1-(2-methoxyphenylthioxomethyl)pyrrolidine with the MIKE spectra of [M? SH]⁺ of the corresponding unsubstituted compounds, reported earlier, indicated two parallel pathways for the formation of [M? SH]⁺ in the o-methoxy compounds. In the first pathway, as has been noticed in thioamides in general, the loss of ?H involves the migration of either the α-hydrogen in the amine moiety or the hydrogen attached to nitrogen. In the second pathway, the migration of a hydrogen from the o-methoxy group to the sulphur atom followed by ejection of SH from the molecular ion leads to a stable cyclized ion. Interesting secondary fragmentations as a consequence of this ortho effect have also been noticed.     

Characterization of C8H10 alkylbenzenes by negative ion mass spectrometry

The O^?˙ chemical ionization mass spectrri of the C₈H₁₀ alkylbenzenes, o-, m-. andp -xylene and ethylbenzene, show formation of [M ? H + O]^?, [M ? H]^?, [M ? H₂]^?˙ and, for the xylenes, [M ? CH₃ + O]^? as primary reaction products; the relative importance of these products depends on the isomer. However, [OH]^? is a primary product from reaction of O^?˙ with both the C₈H₁₀ isomers and hydrogen-containing impurities; [OH]^? reacts further with the alkylbenzenes to produce [M ? H]^? with the result that the chemical ionization mass spectra depend on experimental conditions such as sample size and the presence of impurities. The collision-induced charge inversion mass spectra of the [M ? H + O]^? and [M ? H]^? products allow only distinction of ethylbenzene from the xylenes. However, the collision-induced charge inversion mass spectra of the [M ? H₂]^?˙ ions show differences which allow identification of each isomer.     

Gas-phase nazarov cyclization of protonated 2-methoxy and 2-hydroxychalcone: An example of intramolecular proton-transport catalysis

Upon CA, ESI generated [M + H]⁺ ions of chalcone (benzalacetophenone) and 3-phenyl-indanone both undergo losses of H₂O, CO, and the elements of benzene. CA of the [M + H]⁺ ions of 2-methoxy and 2-hydroxychalcone, however, prompts instead a dominant loss of ketene. In addition, CA of the [M + H]⁺ ions of 2-methoxy-β-methylchalcone produces an analogous loss of methylketene instead. Furthermore, the [M + D]⁺ ion of 2-methoxychalcone upon CA eliminates only unlabeled ketene, and the resultant product, the [M + D − ketene]⁺ ion, yields only the benzyl-d ₁ cation upon CA. We propose that the 2-methoxy and 2-hydroxy (ortho) substituents facilitate a Nazarov cyclization to the corresponding protonated 3-aryl-indanones by mediating a critical proton transfer. The resultant protonated indanones then undergo a second proton transport catalysis facilitated by the same ortho substituents producing intermediates that eliminate ketene to yield 2-methoxy- or 2-hydroxyphenyl-phenyl-methylcarbocations, respectively. The basicity of the ortho substituent is important; for example, replacement of the ortho function with a chloro substituent does not provide an efficient catalyst for the proton transports. The Nazarov cyclization must compete with an alternate cyclization, driven by the protonated carbonyl group of the chalcone that results in losses of H₂O and CO. The assisted proton transfer mediated by the ortho substituent shifts the competition in favor of the Nazarov cyclization. The proposed mechanisms for cyclization and fragmentation are supported by high-mass resolving power data, tandem mass spectra, deuterium labeling, and molecular orbital calculations.     

Reactivity in the gas phase. Behaviour of isoxazoles under negative ion chemical ionization conditions

[M ? H⁺]^? ions of isoxazole (la), 3-methylisoxazole (1b), 5-methylisoxazole (1c), 5-phenylisoxazole (1d) and benzoylacetonitrile (2a) are generated using NICI/OH^? or NICI/NH₂^? techniques. Their fragmentation pathways are rationalized on the basis of collision-induced dissociation and mass-analysed ion kinetic energy spectra and by deuterium labelling studies. 5-Substituted isoxazoles 1c and 1d, after selective deprotonation at position 3, mainly undergo N ? O bond cleavage to the stable α-cyanoenolate NC ? CH ? CR ? O^? (R = Me, Ph) that fragments by loss of R? CN, or R? H, or H₂O. The same α-cyanoenolate anion (R = Ph) is obtained from 2a with OH^?, or NH₂^?, confirming the structure assigned to the [M ? H⁺]^? ion of 1d, On the contrary, 1b is deprotonated mainly at position 5 leading, via N? O and C(3)? C(4) bond cleavages, to H? C ≡ C? O ^? and CH₃CN. Isoxazole (1a) undergoes deprotonation at either position and subsequent fragmentations. Deuterium labelling revealed an extensive exchange between the hydrogen atoms in the ortho position of the phenyl group and the deuterium atom in the α-cyanenolate NC ? CD = CPh ? O^?.