Electron impact mass spectrometry of [C7H8N]+ and [C6H7]+ and [C6H7]+ fragment ions produced from o-toluidine and N-methylaniline

An energetic study of the production of [C₇H₈N]⁺ and [C₆H₇]⁺ fragment ions from o-toluidine and N-methylaniline is reported. The mechanisms for the formation of the ions are suggested. Metastable peaks associated with the formation and fragmentation of reactive [C₇H₈N]⁺ and [C₆H₇]⁺ ions were detected and kinetic energy released were determined. The results indicate that the [C₇H₈N]⁺ ion is formed at threshold from o-toluidine with an aminotropylium structure whereas for N-methylaniline the ion is formed with anN-phenylmethaniminium structure. [C₆H₇]⁺ ions are believed to be formed at threshold from the two precursors with a protonated benzene structure.     

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.     

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 photo-ionization study of organosulfur ring compounds: Thiirane,thietane and tetrahydrothiophene

The gas phase heats of formation of several organosulfur cations were determined from thiirane, thietane and tetrahydrothiophene precursor molecules by photoionization mass spectrometry. Heats of formation at 0 K and 298 K are reported for the following ions: [H₂CS]^+˙, [H₃CS]⁺, [C₂H₃S]⁺, [C₂H₄S]^+˙, [C₃H₅S]⁺, [C₃H₆S]^+˙, [C₄H₇S]⁺ and [C₄H₈S]^+˙. The [C₄H₇S]⁺ (m/z 87), [C₂H₄S]^+˙ (m/z 60), [C₂H₃S]⁺ (m/z 59), [C₄H₇]⁺ (m/z 55), [C₄H₆]^+˙ (m/z 54) and [CH₂S]^+˙ (m/z 46) ions are produced from metastable tetrahydrothiophene ions at photon energies between 10.2 and 10.7 eV. Decay rates of internal energy selected parent ions to the m/z 60, 59, 55 and 54 fragments were measured by threshold photoelectron-photoion coincidence, the results of which are compared to statistical theory (RRKM/QET) calculations. The [C₂H₄S]^+˙ ion from tetrahydrothiophene is found to have the thioacetaldehyde structure. From the measured [C₂H₄S]^+˙ onset a ΔH = 50±8 kJ mol^?1 was calculated for the thioacetaldehyde molecule.     

Ionisation et Fragmentation en Spectrométrie de Masse. VII—Structure de l'Ion [C6H6O]+ Issu de la Décomposition du Phényléthyléther Sous Impact Electronique

The activation energy is calculated for the fragmentation [C₆H₅OC₂H₅]⁺ → [C₆H₆O]⁺ +C₂H₄. Estimation of the enthalpy difference between the final state and the molecular ion supports the formation of a phenol-like structure for the [C₆H₆O]⁺ ion. The activation energy for backward reaction is compared with the mean kinetic energy release. Whether this is a concerted or non-concerted fragmentation is discussed.     

On the formation of anisole from phenoxyacetamides

A major fragmentation route of N-t-butyl-N-isopropyl-and N-ethylphenoxyacetamide is the formation of [C₇H₈O]⁺. Deuterium labeling implicates hydrogen transfer from nitrogen in all three species and from the t-butyl groups of the t-butylamide. The anisole structure is assigned to the [C₇H₈O]⁺· ion on the basis of its secondary fragmentation.     

Electron impact and ammonia chemical ionization mass spectra of n-azabicyclo[m.2.0]alkan-(n + 1)-ones (m = 3–6, n = 6–9)

Electron impact induced fragmentation of the title compounds obeys a route where the lactam moiety, OCNH, is cleaved first, with the accompanying formation of a cycloalkene ion. This can be verified by low-resolution, high-resolution, B/E and B²/E spectra as well as by collisional activation spectra of, for example, the ions m/z 82 and 67 from 7-azabicyclo[4.2.0]octan-8-one and from cyclohexene. The only, and fairly weak, fragment ions including O and N are [C₃H₃O]+, [C_kH_2k-2N]⁺ (k = 5–8) and [C₃H₆N]⁺. The ammonia chemical ionization spectra are also characteristic for all four lactams and show the same dominant ions in all cases, namely [M + 1]⁺, [M + 1 + NH₃]⁺˙ and [2 M + 1]⁺˙.     

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.     

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.     

The structures of [C5H5O]+ ions formed from isomers of nitrophenol in the gaseous phase

Translational energy release measurments on metastable ions are used in the comparison of the structures of isomeric ions. Metastable ions, m₂⁺, formed from m₁⁺ ions as the result of a high energy process in the ion source are compared with isomeric metastable ions formed as daughters from fragmentation of metastable m₁⁺ ions in a field. In the case of o-, m- and p-nitrophenol the structure of the [C₅H₅O]⁺ ions formed from [C₆H₅O]⁺ ions by these two independent methods is different as verified by comparison of the behaviour of [C₅H₅O]⁺ ions formed from several other compounds.     

Study of ion structures produced by the reaction of 2-propyl cations with water and methanol; covalently bound v. Hydrogen-bridged adducts

In contrast to an earlier report,¹ the collisonally induced dissociation of protonated 2-propanol and t-butyl alcohol yields spectra that are indistinguishable from those of the corresponding [C₃H₇/H₂O]⁺ and [C₄H₉/H₂O]⁺ ions generated by the (formal) gas phase addition reactions in a high pressure ion source of [s-C₃H₇]⁺ and [t-C₄H₉]⁺ ions with the n-donor H₂O. Similarly, [s-C₃H₇/CH₃OH]⁺ ions generated by both gas phase protonation of n- and s-propyl methyl ethers and addition reactions of [C₃H₇]⁺ to CH₃OH display mode-of-generation-independent collisionally induced dissociation characteristics. However, analysis of the unimolecular dissociation (loss of propene) of the [C₃H₇/CH₃OH]⁺ system, including a number of its deuterium, ¹³C- and ¹⁸O-labelled isotopomers, supports the idea that prior to unimolecular dissociation, covalently bound [C₃H₇- O(H)CH₃]⁺ ions intercovert with hydrogen-bridged adduct ions, analogous to the behaviour of the distonic ethene-, propene- and ketene-H₂O radical cations.     

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.     

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.     

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.     

Electron impact studies. CXII—The properties of positive ions produced by charge stripping from negative ions. [C6H5O]+, [C6H5S]+ and [C7H7S]+

The dissociative spectrum of the [C₆H₅S]⁺ ion derived by charge inversion from [C₆H₅S]^?, shows a variety of fragmentations including the competitive losses of H?, C₃H₄ and the formation of [CHS]⁺. The spectrum of a deuteriated derivative shows that these three processes are preceded or accompanied by H/D scrambling. The corresponding [C₆H₅O]⁺ species also undergoes hydrogen scrambling prior to fragmentation. In marked contrast, the ion [p-MeC₆H₄S]⁺ does not undergo hydrogen randomization between the methyl and aryl groups, and positional integrity is retained during fragmentation. These results are compared with the properties of the same ions produced by conventional ionization.     

Competing ring cleavages in substituted 4-arylcyclohexanols and their methyl ethers under electron impact

Ring cleavage α to the oxygen function leads to [C₃H₅O]⁺ and [C₄H₇O]⁺ ions in the mass spectra of 4-arylcyclohexanols and their methyl ethers, respectively. Cleavage α to the aryl group gives rise to [C₃H₇O]⁺ (from the alcohols), [C₄H₉O]⁺ (from the ethers) and [ArC₃H₄]⁺ (from both) ions. The competition between the two ring cleavages explains the effect of the substituents of the aryl groups on the relative abundances of the resulting ions.     

An investigation of the decomposition of the common intermediate ions produced by electron impact. IV—[C6H5CO]+ ions from several alkyl benzoates

The decomposition of the [C₆H₅CO]⁺ ions produced from eight alkyl benzoates by electron impact has been studied. By calculating the heat of formation of [C₆H₅CO]⁺ ions from the appearance potential value, it is shown that the ions from C₆H₅COOR when R?H, CH₃, C₂H₅ have some excess energy, and those where R = n-C₃H₇, iso-C₃H₇, n-C₄H₉, iso-C₄H₉, iso-C₅H₁₁ are produced with more excess energy. It is also shown that by taking this excess energy into account, there is a linear relationship between the heat of formation of the activated complex produced in the reaction [C₆H₅CO]⁺→[C₆H₅]⁺ + CO and the vibrational degree of freedom of the neutral fragment ? OR.     

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.     

An energy-resolved study of the fragmentation of ionized 1-Penten-3-ol

A detailed energy-resolved study of the fragmentation of CH₂?CHCH(OH)CD₂CD₃ (1-d₅) has been carried out using metastable ion studies and charge exchange techniques, combined with collision-induced dissociation studies to establish the structures of fragment ions. At low internal energies (metastable ions) the molecular ion of 1-d₅ rearranges to the 3-pentanone structure and fragments by loss of C₂H₅ or C₂D₅ leading to the acyl structure, [CH₃CH₂C?O]⁺ or [CD₃CD₂C?O]⁺, for the fragment ion. However, with increasing internal energy of the molecular ion this rearrangement process decreases rapidly in importance and loss of C₂D₅ by direct cleavage, leading to [CH₂?CHCH?OH]⁺, becomes the dominant fragmentation reaction. As a result the [C₃H₅O]⁺ ion seen in the electron impact mass spectrum of 1-penten-3-ol has predominantly the protonated acrolein structure.     

Structure and formation of gaseous [C4H6O]+˙ ions: 1—The enolic ions [CH2C(OH)?CHCH2]+˙ and [CH2CH?CHCH(OH)]+˙ and their relationship with their keto counterparts

The [C₄H₆O]^+˙ ion of structure [CH₂?CHCH?CHOH]^+˙ (a) is generated by loss of C₄H₈ from ionized 6,6-dimethyl-2-cyclohexen-1-ol. The heat of formation ΔH_f of [CH₂?CHCH?CHOH]^+˙ was estimated to be 736 kJ mol^?1. The isomeric ion [CH₂?C(OH)CH?CH₂]^+˙ (b) was shown to have ΔH_f, ? 761 kJ mol^?1, 54 kJ mol^?1 less than that of its keto analogue [CH₃COCH?CH₂]^+˙. Ion [CH₂?C(OH)CH?CH₂]^+˙ may be generated by loss of C₂H₄ from ionized hex-1-en-3-one or by loss of C₄H₈ from ionized 4,4-dimethyl-2-cyclohexen-1-ol. The [C₄H₆O]^+˙ ion generated by loss of C₂H₄ from ionized 2-cyclohexen-1-ol was shown to consist of a mixture of the above enol ions by comparing the metastable ion and collisional activation mass spectra of [CH₂?CHCH?CHOH]^+˙ and [CH₂?C(OH)CH?CH₂]^+˙ ions with that of the above daughter ion. It is further concluded that prior to their major fragmentations by loss of CH₃˙ and CO, [CH₂?CHCH?CHOH]⁺˙ and [CH₂?C(OH)CH?CH₂]^+˙ do not rearrange to their keto counterparts. The metastable ion and collisional activation characteristics of the isomeric allenic [C₄H₆O]^+˙ ion [CH₂?C?CHCH₂OH]^+˙ are also reported.