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.     

Structure and fragmentation of [C3H7O]+ ions formed by chemical ionization

The unimolecular metastable and collision-induced fragmentation reactions of [C₃H₇O]⁺ ions produced by gas-phase protonation of acetone, propanal, propylene oxide, oxetan and allyl alcohol have been studied. The CID studies show that protonation of acetone and allyl alcohol yield different stable ions with distinct structures while protonation of propanal or propylene oxide yield [C₃H₇O]⁺ ions of the same structure. Protonated oxetan rearranges less readily to give the same structure(s) as protonated propanal and propylene oxide. The [C₃H₇O]⁺ ions fragmenting as metastable ions after formation by CI have a higher internal energy than the same ions fragmenting after formation by EI. Deuteronation of the C₃H₆O isomers using CD₄ reagent gas shows that loss of C₂H₃D proceeds by a different mechanism than loss of C₂H₄. The results are discussed in terms of potential energy profile for the [C₃H₇O]⁺˙ system proposed earlier.     

A relation between ion structures and experimental overall cross-sections for collisionally activated decomposition

Collisionally activated dissociation (CAD) mass spectra sometimes appear to be identical in spite of the fact that the precursor ion structures are known to differ. It is shown that determination of the experimental overall cross-section for collisionally activated decomposition yields valuable extra information. After applying it to examples of known structure, [C₄H₅N]^+·, [C₅H₅N]^+· and [C₅H₅O]⁺, it is used to study a more complex problem, that of [C₆H₆]^+· ions from four isomeric precursors.     

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.     

Chemical ionization mass spectra of selected C3H6O compounds

The chemical ionization mass spectra of five isomers of C₃H₆O (acetone, propionaldehyde, oxetane, propylene oxide and allyl alcohol) have been determined using a variety of reagent gases (H₂, D₂, N₂/H₂, CO₂/H₂ and CO/H₂). The [C₃H₇O]⁺ ions produced by protonation of these isomers undergo very similar reactions to those reported for analogous [C₃H₇O]⁺ metastable ions; however, decomposing ions generated by chemical ionization appear to have somewhat higher internal energies. The results of ²H labelling studies (D₂ reagent gas or labelled analogues of C₃H₆O) indicate that protonation occurs mainly on oxygen and are consistent with previous investigations of metastable oxonium ions. The protonated acetone ion is particularly stable, in agreement with the higher activation energies for fragmentation of this isomer than for other [C₃H₇O]⁺ structures. As the calculated heat of protonation of C₃H₆O is reduced by changing the reagent gas, so the extent to which fragmentation occurs decreases. This is discussed in the context of competition between fragmentation and collisional stabilization of the excited [C₃H₇O]⁺* ion. It is concluded that on average a large fraction (approaching 1) of the exothermicity of the protonation reaction resides in the [C₃H₇O]⁺* ions produced initially.     

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.     

Characterization of the C3H6O+· ion from 2-methoxyethanol. Mixture analysis by dissociation and neutralization—reionization

The C₃H₆O^+· ion formed upon the dissociative ionization of 2-methoxyethanol is identified by a combination of several tandem mass spectrometry methods, including metastable ion (MI) characteristics, collisionally activated dissociation (CAD), and neutralization—reionization mass spectrometry (NRMS). The experimental data conclusively show that 2-methoxyethanol molecular ion, namely, HOCH₂CH₂OCH ₃ ^+· , loses H₂O to yield mainly the distonic radical ion ·CH₂CH₂OCH ₂ ⁺ along with a smaller amount of ionized methyl vinyl ether, namely, CH₂=CHOCH ₃ ^+· . Ring-closed products, such as the oxetane or the propylene oxide ion are not observed. The proportion of ·CH₂CH₂OCH ₂ ⁺ increases with decreasing internal energy of the 2-methoxyethanol ion, which indicates a lower critical energy for the pathway leading to this product than for the competitive generation of CH₂=CHOCH ₃ ^+· . The present study also uses MI, CAD, and NRMS data to assess the structure of the distonic ion⁺ (CH₃)CHOCH₂· (ring-opened ionized propylene oxide) and evaluate its isomerization proclivity toward the methyl vinyl ether ion.     

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.     

Collisional activation spectra. Gas phase [C12H10O]+˙ and [C12H9O]+ ions from dephenyl ether,diphenyl carbonate,and o-, m- and p-phenylphenol

[C₁₂H₁₀O]⁺˙ ions in the mass spectra of diphenyl ether and diphenyl carbonate have the same structure, but one which differs from that of these ions from phenylphenol. [C₁₂H₉O]⁺ ions from all three compounds have isomerized to a common structure.     

Formation of aniline-like ions by electron-impact-induced elimination of CO from formanilide

The metastable peak intensity ratios for elimination of HNC vs DNC from the [M ? CO]⁺· ion of deuterium labelled analogues of formanilide show that the formyl hydrogen atom migrates to nitrogen prior to or during CO expulsion to form a [C₆H₇N]⁺· ion of aniline-like structure. An examination of metastable peaks does not allow similar conclusions to be reached for methyl substituted formanilides. Low abundance [C₆H₆O]_+· ions are formed by HNC elimination from the formanilide molecular ion in a reaction where three covalent bonds to the formyl carbon are broken.     

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.     

Mass spectra of organometallic compounds: XIV—Some monometallic olefing manganese tricarbonyl derivatives

The metastable ion supported fragmentation process in the mass spectra of the cyclohexadienyl derivative C₆H₇Mn(CO)₃, the cycloheptadienyl derivative C₇H₉Mn(CO)₃, the 1,2,3,4,5-and 1,2,3,5,6-pentahaptocyclootadienyl derivatives C₈H₁₁Mn(CO)₃, the cyclooctatrienyl derivative C₈H₉Mn(CO)₃ and the substituted cyclopentadienyl derivative (CH₃)₂NCH₂C₅H₄Mn(CO)₃, are described. Losses of carbonyl groups, generally stepwise, from the molecular ions to give the corresponding [M – 3CO]⁺· ions are first observed. Further fragmentation of the carbonyl-free [M – 3CO]⁺· ions can involve a variety of processes such as the following: (a) elimination of a neutral manganese atom to give a hydrocarbon fragment; (b) elimination of a neutral hydrocarbon fragment to give an [MnH]⁺· ion; (c) dehydrogenation; (d) elimination of a 2-carbon C₂H₂ or C₂H₄ fragment; (e) elimination of a C₃H₄ or C₃H₆ fragment as a neutral species when it is bridging two carbon atoms bonded to manganese, as in C₈H₉Mn(CO)₃ and 1,2,3,4,5,h⁵-C₈H₁₁Mn(CO)₃, respectively. Fragmentation of the [M – 3CO]⁺· ion in (CH₃)₂NCH₂C₅H₄Mn(CO)₃ presents the following additional features: (a) elimination of C₆H₆ with a nitrogen shift from carbon to manganese; (b) elimination of a neutral dimethylamino fragment to give [C₆H₆Mn]⁺·, which then loses neutral C₆H₆, C₆H₅ or Mn fragments and thus is formulated tentatively as [(fulvene)Mn]⁺· or [C₆H₅MnH]⁺· rather than [(benzene)Mn]⁺·.     

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.     

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 ions [CH3S]+, [C2H5S]+ and [CH3O]+ formed by electron-impact

The mass spectra of deuterated species shows that both the isomeric ions [CH₂?SH]⁺ and [CH₃? S]⁺ are formed in the ratio 2:1 from CH₃SH; the ions [CH₃CH?SH]⁺ and [CH₃CH₂S]⁺ in the ratio 0·8:1 from CH₃CH₂SH; and [CH₂?OH]⁺ and [CH₃? O]⁺ in the ratio 6·7:1 from methanol. The heats of formation of [CH₃S]⁺ and [C₂H₅S]⁺ are of the order of 222 and 203 Kcal.mole^?1 respectively. The isomeric ions cannot be distinguished on thermodynamic grounds.     

Chemical ionization of phenyl n-propyl ether and methyl substituted analogs: Propene loss initiated by competing proton transfer to the oxygen atom and the aromatic ring

The mechanism of propene loss from protonated phenyl n-propyl ether and a series of mono-, di-, and trimethylphenyl n-propyl ethers has been examined by chemical ionization (CI) mass spectrometry in combination with tandem mass spectrometry experiments. The role of initial proton transfer to the oxygen atom and the aromatic ring, respectively, has been probed with the use of deuterated CI reagents, D₂O, CD₃OD, and CD₃CN (given in order of increasing proton affinity), in combination with deuterium labeling of the β position of the n-propyl group or the phenyl ring. The metastable [M + D]⁺ ions of phenyl n-propyl ether—formed with D₂O as the CI reagent—eliminate C₃H₅D and C₃H₆ in a ratio of 10:90, which indicates that the added deuteron is incorporated to a minor extent in the expelled neutral species. In the experiments with CD₃OD as the CI reagent, the ratio between the losses of C₃H₅D and C₃H₆ from the metastable [M + D]⁺ ions of phenyl n-propyl ether is 18:82, whereas the ratio becomes 27:73 with CD₃CN as the reagent. A similar trend in the tendency to expel a propene molecule that contains the added deuteron is observed for the metastable [M + D]⁺ ions of phenyl n-propyl ether labeled at the β position of the alkyl group. Incorporation of a hydrogen atom that originates from the aromatic ring in the expelled propene molecule is of negligible importance as revealed by the minor loss of C₃H₅D from the metastable [M + H]⁺ ions of C₆D₅OCH₂CH₂CH₃ irrespective of whether H₂O, CH₃OH, or CH₃CN is the CI reagent. The combined results for the [M + D]⁺ ions of phenyl n-propyl ether and deuterium-labeled analogs are suggested to be in line with a model that assumes that propene loss occurs not only from species formed by deuteron transfer to the oxygen atom, but also from ions generated by deuteron transfer to the ring. This is substantiated by the results for the methyl-substituted ethers, which reveal that the position as well as the number of methyl groups bonded to the ring exert a marked effect on the relative importances of the losses of C₃H₅D and C₃H₆ from the metastable [M + D]⁺ ions of the unlabeled methyl-substituted species.     

The structure of decomposing [C7H7O]+ ions: Benzyl versus tropylium ion structures

The unimolecular dissociation reactions for [C₇H₇O]⁺ ions generated by fragmentation of a series of precursor molecules have been investigated. The metastable kinetic energy values and branching ratios associated with decarbonylation and expulsion of a molecule of formaldehyde (CH₂O) from the [C₇H₇O]⁺ ions are interpreted as the hydroxybenzyl and hydroxytropylium [C₇H₇O]⁺ not interconverting to a common structure on the microsecond time-scale. In addition, similar measurements on protonated benzaldehyde, methylaryloxy and phenyl methylene ether [C₇H₇O]⁺ ions are interpreted as the dominant fraction of these decomposing ions having unique structures on the microsecond time-scale. These results are supported by experimental heats of formation calculated from ionization/appearance energy measurements. The experimental heats of formation are determined as: hydroxybenzyl ions, 735 kJ mol^?1; hydroxytropylium ions, 656 kJ mol^?1; phenyl methylene ether ions, 640 kJ mol^?1; methylaryloxy ions 803 kJ mol^?1. The combination of the results reported in this paper with previously reported experimental data for stable [C₇H₇O]⁺ ions (see Ref. 1, C. J. Cassady, B. S. Freiser and D. H. Russell, Org. Mass Spectrom.) is interpreted as evidence that the relative population of benzyl versus tropylium [C₇H₇O]⁺ ion structures from a given precursor molecule is determined by isomerization of the parent ion and not by structural equilibration of the [C₇H₇O]⁺ ion.     

The structure of [YC6H5O]+· ions in the spectra of aryl alkyl ethers

The [YC₆H₅O]^+· ions from YC₆H₄OC₂H₅ appear to correspond in structure to the molecular ions of the analogous substituted phenols, [YC₆H₄OH]^+·.     

Dissociation of positively charged aliphatic epoxides. II. [C5H10O]+˙ epoxides and α,β unsaturated ethers

The unimolecular dissociations of C₅ epoxides ions mono- or disubstituted at C₁ give exclusive loss of CH₃ and exclusive formation of methoxy vinyl carbenium ion, both in the source and in the 2nd field-free region. In the case of the 1,2-disubstituted ion in the 2nd field-free region the loss of ethene is the only pathway, while a competition occurs for the trisubstituted ion leading to [C₃H₆O]⁺˙ and [C₄H₇O]⁺˙ ions, the structure of which are demonstrated. The first step of the different mechanisms is the cleavage of the heterocyclic C? C bond.     

N—H...O and O—H...O hydrogen‐bonded supramolecular networks in 4‐chloroanilinium, 2‐hydroxyanilinium and 3‐hydroxyanilinium hydrogen phthalates

The title salts, 4‐chloroanilinium hydrogen phthalate (PCAHP), C₆H₇ClN⁺·C₈H₅O₄⁻, 2‐hydroxyanilinium hydrogen phthalate (2HAHP), C₆H₈NO⁺·C₈H₅O₄⁻, and 3‐hydroxyanilinium hydrogen phthalate (3HAHP), C₆H₈NO⁺·C₈H₅O₄⁻, all crystallize in the space group P2₁/c. The asymmetric unit of 2HAHP contains two independent ion pairs. The hydrogen phthalate ions of 2HAHP and 3HAHP show a short intramolecular O—H...O hydrogen bond, with O...O distances ranging from 2.3832 (15) to 2.3860 (14) Å. N—H...O and O—H...O hydrogen bonds, together with short C—H...O contacts in PCAHP and 3HAHP, generate extended hydrogen‐bond networks. PCAHP forms a two‐dimensional supramolecular sheet extending in the (100) plane, whereas 2HAHP has a supramolecular chain running parallel to the [100] direction and 3HAHP has a two‐dimensional network extending parallel to the (001) plane.