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.     

Cyclopropane intermediates in the rearrangement and fragmentation of olefinic molecular ions

Methyl loss from deuterium-labelled molecular ions of 4-methyl-2-pentene, 2-methyl-2-pentene and 1,1,2-trimethylcyclopropane has been investigated for metastable molecular ions and for molecular ions formed by charge exchange with COS⁺˙, XE⁺˙ and CO⁺˙. For metastable ion fragmentation reactions all three compounds exhibit very similar behavior and show specific and essentially equal loss of each of the original methyl groups as well as specific loss of a methyl where the hydrogens derive exclusively from the non-methyl hydrogens of the original molecules. The former results are interpreted in terms of interconversion of the three molecular ions through a ring-opened form of the trimethylcyclopropane molecular ion. The loss of the non-methyl hydrogens as CH₃ is interpreted in terms of isomerization to the 2,3-dimethyl-2-butene structure. With increasing internal energy direct allylic cleavage of the unrearranged methylpentene molecular ions increases in importance while the trimethylcyclopropane molecular ion shows an increased preference for loss of the C(2) methyl group. With increasing internal energy loss of the original non-methyl hydrogens as CH₃ decreases markedly in importance.     

On the mechanism of isomerization reactions in the esters of some simple α,β-unsaturated carboxylic acids

It is shown from a detailed examination of first field free region metastable peak shapes that the molecular ion of the methyl ester of acrylic acid rearranges into 2- and 3-butenoic acid ions prior to metastable fragmentations involving the losses of H₂O, CH₃˙ and CO. The key intermediate ion in this ester-acid isomerization is shown to be the enol form of ionized γ-butyrolactone. The C-5 homologues methylmethacrylate and ethylacrylate display a similar mechanism for H₂O loss, but the loss of CH₃˙ shows additional mechanistic complexities. It was shown from the metastable peak shapes of ¹³C and deuterium labelled compounds that the larger part of the methyl loss does not occur from acid type ions, but directly from methyl substituted enol ions of γ-butyrolactone. The mechanistic proposals also account for the presence of a pronounced loss of CH₃˙ from the isomeric ester methylcrotonate and the absence of H₂O loss in both methylcrotonate and the methyl ester of 3-butenoic acid.     

Internal energy effects on metastable characteristics. The structure of [C3H7O]+ ions

The effect of changes in the internal energy distribution of the fragmenting ion on the ratio of metastable ion intensities for two competing fragmentation reactions has been investigated both theoretically and experimentally. Model calculations have shown that if the competing reactions have significantly different activation energies the metastable intensity ratio does depend on the internal energy distribution although large changes are necessary before the ratio changes by more than a factor of two. Experimentally the metastable characteristics of [C₃H₇O]⁺ ions of nominal structures [CH₃CH₂O⁺?CH₂] (I), [(CH₃)₂C?O⁺H] (II), [CH₃CH₂CH?O⁺H] (III) and [CH₃O⁺?CHCH₃] (IV) have been examined. For each structure the metastable characteristics are found to be distinctive and independent of changes in the internal energy distribution of the fragmenting ion where these changes result from altering the precursor of the [C₃H₇O]⁺ ions. It is suggested that these internal energy changes can be estimated from the fraction of [C₃H₇O]⁺ ions which fragment in the ion-source. It is concluded that structures I to IV represent stable and distinct ionic structures.     

Mass spectra of aliphatic dicarboxylic acids and their dimethyl esters: Cyclic structures for the [M − H2O]+˙ ions from the diacids and [M − MeOH]+˙ ions from the dimethyl esters

Methyl 2-oxocycIoalkane carboxylate structures are proposed lor the [M ? MeOH]^+˙ ions from dimethyl adipate, pimelate, suberate and azelate. This proposal is based on a comparison of the metastable ion mass spectra and the kinetic energy releases for the major fragmentation reaction of these species with the same data for the molecular ions of authentic cyclic β-keto esters. The mass spectra of α,α,α′,α′-d₄-pimelic acid and its dimethyl ester indicate that the α-hydrogens are involved only to a minor extent in the formation of [M ? ROH]^+˙ and [M ? 2ROH]^+˙ ions, while these α-hydrogens are involved almost exclusively in the loss of ROH from the [M ? RO˙]⁺ ions (R = H or CH₃). The molecules XCO(CH₂)₇COOMe (X = OH, Cl) form abundant ions in their mass spectra with the same structure as the [M ? 2MeOH]^+˙ ions from dimethyl azelate.     

13C labelling study of the interconversion of ethylbenzene, 7-methylcycloheptatriene and p-xylene ions

The isomerization of the molecular ions of ethylbenzene, 7-methylcycloheptatriene and p-xylene by skeletal rearrangement prior to the formation of [C₇H₇]⁺ ions has been investigated by using ¹³C labelled compounds. The results obtained for ions generated by 70 eV and 12 eV electron impact, and fragmenting in the ion source, the 1st field free region and the 2nd field free region, respectively, are compared with those obtained from D labelled derivatives. It is shown that at long reaction times metastable p-xylene ions lose a methyl radical after scrambling of all C atoms and H atoms, while the unstable molecular ions in the ion source react by specific loss of one of the methyl substituents. Both unstable and metastable ethylbenzene ions fragment by two competing mechanisms, one corresponding to specific loss of the terminal methyl group, and the other involving scrambling of all C and H atoms. These results are discussed by use of a dynamic model developed for the mutual interconversion and fragmentation of the molecular ions of ethylbenzene, methylcyclo-heptatriene and p-xylene. The experimental results can be explained by an equilibrium between metastable methylcycloheptatriene ions and p-xylene ions with sufficient energy for skeletal rearrangement, while about 40% of the metastable ethylbenzene ions fragment after rearrangement to methylcycloheptatriene ions and about 60% of the ethylbenzene ions rearrange further to xylene ions before fragmentation. Metastable methylcycloheptatriene ions, mainly lose a methyl group without a skeletal rearrangement, however, because the rearranged ions are kinetically trapped as ‘stable’ xylene ions or ethylbenzene ions.     

Losses of isobaric neutral species from molecular ions of isomeric nitroanisoles

Alternative losses of the isobaric neutral species CH₂O and NO˙ have been assessed for molecular ions of isomeric nitroanisoles fragmenting in the ion source and the first field free region of a double focusing mass spectrometer. Mass analyses of the primary fragment ions indicate that specific loss of CH₂O occurs from molecular ions of 2-nitroanisole, while specific loss of NO˙ occurs from molecular ions of 3-nitroanisole. Although the peak due to [M? NO]⁺ ions is negligible in the mass spectrum of 2-nitroanisole, evidence is presented to show that they are transient intermediates in the consecutive fragmentation for loss of the elements of CNO₂ from the molecular ions.     

The behaviour of arylsulfonylmethyl sulfonates under conditions of electron-impact. The loss of CH2O

The mass spectra of ten arylsulfonylmethyl arenesulfonates and two arylsolfonylmethyl trifluoromethanesulfonates are discussed. A notable fragmentation of these compounds is the loss of CH₂O from the molecular ions. A probable structure for the [M ? CH₂O]^?. ions, as based on the measurement of metastable rations, is given and a fragmentation mechanism leading to this structure is proposed.     

Fragmentation of 1- and 3-methoxypropene ions another part of the C4H8O+• potential energy surface

The fragmentation mechanisms of metastable ionized 1? and 3?methoxypropene have been examined in detail by using ionization and appearance energy measurements, metastable ion and collisional activation mass spectra, and a variety of isotopically labeled molecules. These metastable C₄H₈O^+? ions fragment by loss of H; CH₃, and H₂CO, and the experimental observations allowed the construction of the potential energy diagram which describes their interconversion and the participation of four other distonic and carbene C₄H₈O^+? ions. It was found that these two methyl alkenyl ether ions had no common reaction channel with either the 2?methoxy isomer or with any of the alcohol, keto, or enol C₄H₈O^n+? isomers which previously have been extensively studied.     

Metastable ion studies. 4—mass spectral fragmentation and isomerization in the esters of chlorinated propenoic acids

Metastable peaks have been used to study the fragmentation pathways of the methyl and trideuteriomethyl chloropropenoates and chloromethyl propenoate. The molecular ion peaks of the unsaturated esters are more intense than those of the saturated esters, α-Cleavage, [M? OCH₃]⁺, produces the base peak in almost all compounds, the relative abundances of the additional peaks being low for chloromethyl propenoate. The losses of H₂O, CH₃^. and COOH^. indicate the isomerization of some ionized chloro esters to the chlorinated 2-butenoic acid molecular ions. An intense loss of H₂O observed for methyl 2-chloropropenoate indicates its most facile isomerization, [ester]^+˙ → [acid]^+˙, whereas the isomerization in methyl trichloropropenoate could not be observed. The molecular ion of chloromethyl propenoate, however, also seems to partly rearrange to the chlorinated 3-butenoic acid ion, since the first field free region metastable peak shows a weak loss of CO. The new reaction pathways, i.e. the losses of CHO^˙, CH₂O and CH₂CO from ionized chloromethyl propenoate, were detected.     

Collisionally induced dissociative ionization of neutral products from unimolecular ion fragmentations. 1—Neutral product structures

A method is described for the investigation of the structure of neutral products from the unimolecular (metastable) dissociative ionizations of mass selected ions, by means of the collisionally induced dissociative ionization of the neutral species themselves. The neutral species, with kilovolt translational energies, enter a positively charged collision cell situated in the second field free region of a standard ZAB-2F mass spectrometer. Dissociative ionization of the neutrals results therein from their collisions with He target gas. The resulting ions are analysed by means of the electric sector and the relative ion abundances are shown to be structure characteristic. For such experiments the neutral flux should be c. ≥ 0.5% of the selected precursor ion flux; the collision gas pressure must be insufficient to cause significant precursor ion fragmentation in the field free region preceding the collision cell. It was shown that HNC is generated in the fragmentation of aniline molecular ions, whereas HCN is the neutral product in the dissociative ionizations of pyridine, benzonitrile and benzyl cyanide. The neutral radical [C, H₃, O˙] formed together with [CH₃CO]⁺ from ionized methyl acetate has the structure ˙CH₂OH, but that from the analogous fragmentation of the methyl propanoate molecular ion has the structure CH₃O˙. Allyl radicals were shown to be generated from [(CH₃)₂CHCH₂OH]⁺˙ together with [CH₃OH₂]⁺ ions.     

Electron-impact induced fragmentation of some natural products containing bis-2,2-dimethylchromene and bis-2,2-dimethylchroman ring systems

Derivatives of eriostoic and eriostemoic acids (II and III) and their tetrahydro analogs (VI and IX) which contain the bis-2,2-dimethylchromene and the bis-2,2-dimethylchroman ring systems respectively, fragment subsequent to electron-impact in characteristic fashion. The former yield mass spectra dominated by the loss of a methyl radical from their molecular ions while the latter exhibit preferential fragmentation of the heterocyclic ring system. In particular the hydrogen transfer process accompanying elemination of C₄H₇ from the parent ion of bis-2,2-dimethylchromans was investigated by deuterium labeling studies. Fragment ions containing the bis-2,2-dimethylchroman ring undergo further fragmentation by the loss of C₄H₈ rather than C₄H₇. An unusual elimination of CH₅ (accompanied by the appropriate metastable ion) from the molecular ions of the 3,5-dinitrobenzoate esters (XIV and XV) was observed in their mass spectra.     

Isomerization and fragmentation of methylfuran ions and pyran ions in the gas phase

The mutual interconversion of the molecular ions [C₅H₆O]⁺ of 2-methylfuran (1), 3-methylfuran (2) and 4H-pyran (3) before fragmentation to [C₅H₅O]⁺ ions has been studied by collisional activation spectrometry, by deuterium labelling, by the kinetic energy release during the fragmentation, by appearance energles and by a MNDO calculation of the minimum energy reaction path. The electron impact and collisional activation mass spectra show clearly that the molecular ions of 1–3 do not equilibrate prior to fragmentation, but that mostly pyrylium ions [C₅H₅O]⁺ arise by the loss of a H atom. This implies an irreversible isomerization of methylfuran ions 1 and 2 into pyran ions before fragmentation, in contrast to the isomerization of the related systems toluene ions/cycloheptatriene ions. Complete H/D scrambling is observed in deuterated methylfuran ions prior to the H/D loss that is associated with an iostope effect k_H/k_D = 1.67–2.16 for metastable ions. In contrast, no H/D scrambling has been observed in deuterated 4H-pyran ions. However, the loss of a H atom from all metastable [C₅H₅O]⁺ ions gives rise to a flat-topped peak in the mass-analysed ion kinetic energy spectrum and a kinetic energy release (T₅₀) of 26 ± 1.5 kJ mol^?1. The MNDO calculation of the minimum energy reaction path reveals that methylfuran ions 1 and 2 favour a rearrangement into pyran ions before fragmentation into furfuryl ions, but that the energy barrier of the first rearrangement step is at least of the same height as the barrier for the dissociation of pyran ions into pyrylium ions. This agrees with the experimental results.     

Mass spectra of chlorinated veratroles (1,2-dimethoxybenzenes)

The behaviour of all nine chlorinated veratroles (1,2-dimethoxybenzenes) under electron impact has been investigated. The most common fragmentation processes are interpreted using metastable ion analysis and deuterium labelled compounds. For all compounds studied, the most common fragmentation route seems to be the primary loss of a methyl radical followed by loss of carbon monoxide. The ion formed has a well-known quinonoid structure and fragments by several routes elucidated by metastable ion analysis. In general, the spectra of the positional isomers are shown to be practically similar and it is apparent that e.g. the 3- and 4-chloro isomers can be differentiated only from the abundance ratio of the [M? CH₃? CO? CH₃]⁺ and [M? CH₃? CO? H₂O]⁺ ions.     

Mechanism of CO loss for protonated 1-indanone and isomeric [C9H9O]+ ions

In order to establish the mechanism of CO loss occurring during metastable decomposition of protonated 1-indanone, fragmentations of monocyclic [C₉H₉O]⁺ isomers have been studied. These ions of known structure were prepared by CI protonation and fragmentation of the corresponding acids chlorides. It is demonstrated that the wide component of the [MH? CO]⁺ metastable peak induced by protonated 1-indanone fragmentation is the result of fragmentation of the [C₆H₅CH₂CH₂CO]⁺ isomer ion.     

Stereochemical applications of mass spectrometry. II—Chemical ionization mass spectra of isomeric dicarboxylic acids and derivatives

The H₂ and CH₄, chemical ionization mass spectra of the cis dicarboxylic acids, maleic and citraconic acid, show much more extensive loss of H₂O from [MH]⁺ than the trans isomers, fumaric acid and mesaconic acid. Similarly, esters of maleic acid show a much more facile loss of ROH (R=alkyl or phenyl) from [MH]⁺ than do esters of fumaric acid. Similar differences are observed in the chemical ionization mass spectra of the isomeric phthalic and isophthalic acids and derivatives, where the ortho isomers show more extensive fragmentation of [MH]⁺ than the meta isomers. The facile fragmentation of [MH]⁺ for the cis and ortho isomers is attributed to ROH elimination involving interaction between the two carboxylate functions and forming the stable cyclic anhydride structure for the fragment ion. By contrast ROH elimination from [MH]⁺ for the trans and metu isomers requires a symmetry-forbidden [1,3]-H migration in the carboxyl protonated species and cannot lead to the cyclic anhydride structure. The chemical ionization mass spectra of cis and trans cyclohexane-1,2-dicarboxylic acids are essentially identical and show extensive fragmentation of the [IMH]⁺ ion. Experiments using deuterium labelling show extensive carboxyl group interactions for both isomers. The chemical ionization mass spectra of maleanilic and phthalanilic acids and of the related anhydrides and imides also are reported, as are the electron impact mass spectra of diphenyl maleate, diphenyl fumarate, diphenyl phthalate, maleanilic acid and phthalanilic acid.     

The fragmentation and isomerization of internal energy selected acetaldehyde molecular cations

The fixed wavelength photoelectron—photoion coincidence technique has been employed to study the fragmentation behaviour of excited acetaldehyde molecular cations with internal energies up to 7 eV. The recorded breakdown curves of the parent ion as well as the C₂H₃O⁺, CHO⁺ and CH₃⁺ fragment ions enable to reject state specific fragmentation behaviour of the title compound into the CHO⁺ and CH₃⁺ fragment ion channels. The present data give evidence of a fast isomerization of the CH₃CHO⁺ cation from its first electronically excited state ā(²A″) to the oxirane cation in its electronical ground state X?(²B₂).     

Electron ionization induced metastable decompositions of isomeric trimethylsilylmethanol, (CH3)3SiCH2OH, and methoxytrimethylsilane, (CH3)3SiOCH3

The metastable decompositions of trimethylsilylmethanol, (CH₃)₃SiCH₂OH (MW: 104, 1) and methoxytrimethylsilane, (CH₃)₃SiOCH₃ (MW: 104, 2) upon electron ionization have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectroscopy and D labeling. The metastable ions of 1 ^·+ decompose to give the fragment ions m/z 89 (CH ₃ ^· loss) and 73 (^·CH₂OH loss), whereas those of 2 ^·+ only yield the fragment ion m/z 89 (CH ₃ ^· loss). The latter fragment ion is generated by loss of a methyl radical from the trimethylsilyl group via a simple cleavage reaction as shown by D labeling. However, the fragment ions m/z 89 and 73 from 1 ^·+ are generated following an almost statistical exchange of the original methyl and methylene hydrogen atoms in the molecular ion as shown also by D labeling. This exchange indicates a complex rearrangement of the molecular ion of 1 ^·+ prior to metastable decomposition for which as key step a 1,2-trimethylsilyl group migration from carbon to oxygen is suggested. A different behavior is also found between the source-generated m/z 89 ions from 1 ^·+ which decompose in the metastable time region to give ions m/z 61 by loss of ethylene and those from 2 ^·+ which decompose in the metastable region to yield ions m/z 59 by elimination of formaldehyde.     

Two competing fragmentation processes in dimethoxybenzenes depending on their positional isomers: Elimination of CH3 and CHnO (n = 1–3) and formation of methoxycyclopentadienyl and protonated phenol ions

All the metastable transitions observed above m/z 39 in the first field-free region were compared for the three positional isomers of dimethoxybenzene. The observed isomer-dependent fragmentation processes, in particular the formation and decomposition of the m/z 95 (C₆H₇O)⁺ ion, are discussed in terms of two competing fragmentations [elimination of CH₃ and CH_nO (n = 1–3) and formation of methoxycyclopentadienyl and protonated phenol ions] and the relative energies of several isomers of the C₆H₇O⁺ ion calculated with molecular orbital theory.     

Fumarate,maleate and maleic anhydride complexes of platinum(O)

Dimethyl fumarate (dmf), diethyl fumarate (def), dimethyl maleate (dmm), and maleic anhydride (ma) react with [Pt(cod)₂] (cod = cyclo-octa-1,5-diene) and with [Pt(C₂H₄)₃] to give ‘mixed’ olefin platinum(O) complexes, e.g., [Pt(cod)(def)], [Pt(cod)(ma)], [Pt(C₂H₄)(dmf)₂] or [Pt(C₂H₄)(dmm)₂]. Tris-(olefin)platinum complexes [Pt(def)₃] and [Pt(dmf)₃] have also been obtained.