Mass spectrometry of aralkyl compounds with a functional group—VII. On the structure of the ions C8H9+ from C6H5C2H4X- and C9H11+ from C6H5C3H6X-compounds

The C₈H₉⁺-ion, formed from the molecular ions of 2-phenyl-1-bromoethane, 1-phenyl-1-bromoethane and of 1-phenyl-1-nitroethane by loss of the bromine atom and of the nitro group, splits off a molecule of acetylene after an almost complete randomization of hydrogens, as proved by deuteration. An eight-membered ring structure for the C₈H₉⁺-ion is proposed to explain these results. By loss of the nitro group from the molecular ions of 1-phenyl-1-nitropropane and of 1-phenyl-2-nitropropane the well-known phenylated cyclopropane ion3 (C₉H₁₁)⁺ is generated. Mass spectra of analogues, specifically deuterated in the side-chain, show that in this ion a randomization of hydrogen atoms in the cyclopropane ring as well as a hydride transfer from the cyclopropane ring to the phenyl cation occur.     

Formation and structure of [C8H8O]+˙ ions,generated from gas phase ions of phenyl-cyclopropylcarbinol and 1-phenyl-1-(hydroxymethyl)cyclopropane

The structure and formation of [C₈H₈O]^+. ions generated from phenylcyclopropylcarbinol and 1-phenyl-1-hydroxymethylcyclopropane upon electron impact, have been studied using kinetic energy release measurements, by determination of ionization and appearance energies and by collisional activation. It is shown that the non-decomposing [C₈H₈O]^+˙ ions have exclusively the structure of the enol ion of phenylacetaldehyde, although it is less stable than the enol ion of acetophenone by about 45 kJ mol^?1. This has been interpreted as an indication that the [C₈H₈O]^+˙ ions from phenylcyclopropylcarbinol are formed by an attack of either the phenyl ring or the hydroxyl group upon the C-1? C-2 (or C-1? C-3) bond of the cyclopropane ring under a simultaneous expulsion of ethene and migration of the attacking group to the C-1 position. The [C₈H₈O]^+˙ ion from 1-phenyl-1-(hydroxymethyl)cyclopropane is formed by opening of the cyclopropane ring via a benzylic cleavage. A kinetically controlled hydrogen shift in the resulting ring opened ion prior to or during ethene loss then leads to the formation of [C₈H₈O]^+˙ ions which have the structure of the enol ion of phenylacetaldehyde.     

The mass spectrum of 1-heptyl iodide

The mass spectra of twelve ¹³C labeled compounds have been measured in order to investigate the fragmentation of the 1-heptyl iodide molecular ion. A mechanism is proposed for each reaction leading to the ions containing the iodine atom. The loss of olefin from the heptyl ion derived from the 1-heptyl iodide proceeds via extensive rearrangement of the linear ion. A non-linear intermediate structure obtained via protonated substituted cyclopropane is suggested to explain the major fragmentation reactions of the heptyl ion.     

Alkyl–oxygen bond formation in the mass spectra of α-chloromethyl aryl sulphones

When subjected to electron-impact, chloromethyl aryl sulphones (II, X = Cl) fragment predominantly by the loss of CH₂Cl from the molecular ion followed by the loss of SO₂ and in most cases the appropriate metastable peaks are present to confirm the transitions. In addition, alkyl–oxygen bond formation in the molecular ion was revealed by the presence of prominent peaks corresponding to the [R? ?-SO]⁺ ions. In most spectra no evidence for aryl–oxygen bond formation could be found and thus the presence of the chlorine atom was able to effectively reverse the direction of skeletal rearrangement reported for aryl methyl sulphones.     

Studies in mass spectrometry. IV—Fragmentation of aryltriflates. Substituent effects on [ArO]+ ion abundances: Looking for a best suited electron energy in the steady state approach. Experimental evidence for the ‘through bond’ stabilization of aryl cations by meta electron donor Substituents

The fragmentation patterns of 24 substituted phenyltrifluoromethanesulfonates has been determined by exact mass and metastable transition measurements. The influence of the ring substituent(s) on the abundance of the [ArO]+ ions has been investigated at low energies and a new standard parameter for plotting against Hammett's ó constants is proposed. The direct loss of CF₃SO₃ from the molecular ion of 3,5-(diisopropyl)-phenyl-, 3,5-(dicyclopropyl)phenyl- and 3,5-di(α-methylcyclopropyl)-phenyl-methanesulfonate, and further observations on the fragmentation processes of these compounds are consistent with M.O. calculations on the ‘through bond’ stabilization of the [Ar]+ σ state by meta electron donor substituents.     

Mass spectra of some 1-substituted-4-acetoacetyl-3-methylpyrazol-5-ols

The fragmentation of 1-phenyl-, l-(2′-pyridyl)- and 1-(4′-methyl-2′-quinolyl)-4-acetoacetyI-3-methyIpyrazol-5-ols (compounds 1, 2 and 3, respectively) on electron impact has been studied and the major processes interpreted. The common feature in the mass spectra of these compounds is the loss of ketene, acetonyl radical, acetone and two molecules of ketene from the molecular ion. Whereas the ion generated after the last process, which corresponds to 1-substituted-3-methyIpyrazol-5-ols, loses methyl cyanide in the case of 1, similar ions in the case of 2 and 3 lose ?₂HO moiety, necessitating an intramolecular hydrogen transfer followed by ring fission and subsequent loss of methyl cyanide. All these and other related processes have been substantiated with the help of accurate mass measurements of the fragment ions and B/E linked-scan spectra.     

Deuterium labelling and thermochemical studies of dissociative ionization of methyl substituted monosilacyclobutanes. Ring expansion in the molecular ion of 1,1,3-trimethyl-1-silacyclobutane

Dissociative ionization of 1,1-dimethyl-1-silacyclobutane, 1,1-bis(trideuteromethyl)-1-silacyclobutane, 1,1,3-trimethyl-1-silacyclobutane, 1,1-bis(trideuteromethyl)-3-methyl-1-silacyclobutane and 1,1,2-trimethyl-1-sila-cyclobutane have been studied. Low energy electron impact fragmentation of 1,1-bis(trideuteromethyl)-3-methyl-1-silacyclobutane results mainly in the loss of ethene with the involvement of the C-methyl group from the rearranged molecular ion. No fragment ions indicating rearrangement of the molecular ion have been detected in mass spectra of 1,1-bis(trideuteromethyl)-1-silacyclobutane. The ionization energies for 1,1,3-trimethyl-1-silacyclobutane, 1,1,2-trimethyl-1-silacyclobutane and 1,1-dimethyl-1-silacyclopentane, and also the appearance energies for the [M ? 28]^+˙ and [M ? 42]^+˙ ions, have been measured by photoioniza-tion mass spectrometry. The heats of formation of these ions and of 1,1,3-trimethyl-1-silacyclobutane and 1,1-dimethyl-1-silacyclopentene molecules have been calculated as have the enthalpies of the transformation processes.     

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.     

The structures and formation of [C4H6N]+ ions 2—ion structures derived from 2-methylpropanenitrile

The loss of a hydrogen atom from ionized 2-methylpropanenitrile is preceded by a drastic rearrangement of the molecular ion. The result of this fragmentation is the generation of two stable structurally different [C₄H₆N]⁺ ions, formed via different pathways. Their structures can be established by a careful comparison of the metastable ion spectra, collision activation spectra, and charge stripping spectra from the compound and its three deuterium labeled analogues and from [C₄H₆N]⁺ ions generated from reference compounds via electron impact ionization or in selected ion/molecule reactions.     

Mass spectra of heterocyclic compounds. II-Hexahydro-1,3,5,-triazines

The twelve tri-N-sulphonylhexahydro-1,3,5-triazines (R = alkyl or aryl) exhibit a common fragmentation pattern; most of the fragments are directly or indirectly generated from the [M ? RSO₂]⁺ ion. Tri-N-Benzylsulphonylhexahydro-1,3,5-triazine (R = PhCH₂) departs from the others mainly because the key ion [M ? RSO₂]⁺ suffers a skeletal rearrangement with loss of SO₂. The rationalizations presented are supported by metastable evidence, accurate mass measurements and deuterium labelling.     

Multiple-stage mass spectrometry in structural characterization of organophosphorus compounds

Multiple-stage mass spectrometry involving consecutive collision-activated dissociation reactions was used to examine the structures of fragment ions commonly formed on electron ionization of organophosphorus esters. The compounds studied include several aryl thiophosphates, some of which are analogs of common pesticides. Energy-resolved collisionactivated dissociation experiments allow the dissociation of the molecular ions of these compounds in such a manner that only a few fragment ions dominate the spectrum. An abundant fragment ion of m/z 109, formed from all of the compounds studied, can have at least four different stable structures: (CH₃O)₂PO⁺, CH₃CH₂OP(O)OH⁺, CH₂ =CHOP(H)(OH)₂ ⁺, and (CH₂O)₂P(H)OH⁺. The structure of the fragment ion of m/z 109 was found to reflect the phosphorus-containing part of the compounds studied. Another abundant fragment ion obtained for all the aryl esters studied is structurally characteristic of the aromatic moiety of the molecule. This fragment ion is the result of a complex rearrangement involving transfer of an alkylene group to the aromatic ring from the phosphoruscontaining part of the molecular ion. The utility of these fragment ions in the structural characterization of unknown organophosphorus esters is discussed.     

Electrospray ionization mass spectrometric study of 1,3,4-oxadiazole–copper complexes

The complexes of 2,5-disubstituted-1,3,4-oxadiazoles, namely 2,5-diphenyl-1,3,4-oxadiazole (1), 2,5-bis(2-pyridyl)-1,3,4-oxadiazole (2) and 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (3), with copper cation were studied by electrospray ionization mass spectrometry (ESI-MS). The ability of the compounds studied to form complexes with copper (under the ESI conditions) can be ordered as 2 > 1 > 3. The compounds studied tend to form both 1 : 1 and 2 : 1 chelate complexes with both copper(II) and copper(I). The complexes with copper(I) are formed in the ESI process. The influence of solvent polarity, solution flow-rate, counter ions (Cl⁻, NO₃⁻, CH₃COO⁻, SO₄²⁻, acetylacetonates) on the type of the ions observed was studied. Copyright © 2004 John Wiley & Sons, Ltd.     

The loss of SO2H from an imido-sulfite under electron impact: Structural elucidation by collision activation of the resulting fragment

Electron impact mass spectrometry of an imido-sulfite 5,6-benzo-2-imino-N-(2,4,6-trichlorophenyl)-1,3-dioxa-2-thiacycloheptane shows an intense fragment ion corresponding to the expulsion of SO₂H from the molecular ion. A mechanism that requires a rearrangement of the molecular ion is proposed. Structural elucidation of the [M? SO₂H]⁺ ion was obtained by recording its metastable ion and collisional activation spectra. Comparison of these spectra with similar spectra recorded from a precursor ion of known structure demonstrated that they were identical. Thus, the results support the proposed structure which derives from the expulsion of SO₂H from the molecular ion of the compound 5,6-benzo-2-imino-N-(2,4,6-trichlorophenyl)-1,3-dioxa-2-thiacycloheptane.     

A study of gaseous 3-membered ring oxonium and sulphonium ions

The 1-methyl-1-thionia cyclopropane 3 and 1-phenyl-1-thionia cyclopropane 4 ions are stable, with lifetimes greater than 10^?5 sec, and can be identified from their collisional activation spectra. Their metastable counterparts (lifetime window 10^?6–10^?5 sec) have undergone ring opening to the isomeric structures CH₃S⁺CHCH₃9 and C₆H₅S⁺CHCH₃11 prior to decomposition. The 1-methyl-1-oxonia cyclopropane 1 and 1-phenyl-1-oxonia cyclopropane 2 ions could not be generated: instead acyclic structures CH₃O⁺CHCH₃5 and C₆H₅O⁺CHCH₃7 were found for both metastable and long living species. Loss of a phenoxy radical from C₆H₅OCH₂CH₂OCH₃ is shown to be preceded by a reciprocal hydrogen transfer and is not due to a SN_i-type reaction.     

Mass spectrometry of heterocyclic compounds VIII–electron-impact-induced fragmentation of 1,2-diphenyl-pyrazolidine-3,5-dione and some 4-substituted derivatives

The mass spectra of 1,2-diphenyl-pyrazolidine-3,5-dione and twenty-one 4-substituted derivatives are reported. Their fragmentation patterns have been studied by deuterium labelling, exact mass measurements, metastable studies by the defocusing technique and low energy spectra. Hydrogen rearrangements from the 4-position of the heterocycle and/or from the ß-position of the 4-substituent groups, lead to the main primary fragment ions [C₁₂H₁₁N₂]⁺ (m/e 183) as shown by the metastables. The 4,4-d₂ derivative shows an appreciable isotope effect even for molecular ions decomposing in the ion source. By comparison with the metastable abundances of competitive reactions, the molecular ions (m/e 252) of the 4-unsubstituted compound appear to be structurally different from the corresponding m/e 252 fragment ions formed from 4-derivatives by the loss of 4-substituent with H rearrangement. If only vinylic or aromatic hydrogen atoms are present, primary cleavage of the heterocyclic ring occurs with loss of OH·, C₃O₂ and C₃HO₂. Important rearrangements leading to elimination of C₆H₆N and C₆H₇N are typical for unsaturated substituents on position four having allylic hydrogen atoms. Fragment ions, identical to molecular ions of some compounds discussed here, are obtained by electron-impact and/or thermal decompostion of some complex compounds containing more than one 1,2-diphenyl-pyrazolidine-3,5-dione system. The [C₆H₅N₂]⁺ (m/e 105) and [C₆H₅]⁺ (m/e 77) ions are common fragments of all the title compounds. Any hydrogen scrambling reactions between phenyl and heterocycle or 4-substituent groups can be excluded.     

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.     

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.     

The solution photochemistry of αβ-unsaturated sulphones

1-Phenyl-2-(benzenesulphonyl)-ethylene and 1-phenyl-2-(benzenesulphonyl)-prop-1-ene have been shown to undergo $\text{Z}$,$\text{E}$-photoisomerisation, whereas 2-benzenesulphonylindene readily forms [_π2 + _π2] photoadducts with 2,3-dimethylbut-2-ene, cyclopentene, and cyclohexene.     

Mass spectra of organoarsenic compounds—II: Electron-impact-induced fragmentation of 2-substituted-1,2,2-dioxarsenanes

The electron-impact-induced fragmentation of 2-substituted 1,3,2-dioxarsenanes has been studied. The main fragmentation modes have been determined with the use of high resolution mass measurements and by application of the metastable defocusing technique. The predominant fragmentation for the 2-alkyl-1,3,2-dioxarsenanes proceeds via the loss of the 2-substituent from the molecular ion. In the case of 2-phenyl-1,3,2-dioxarsenanes elimination of the phenyl group competes with the formation of a C₆H₅ As ion as well as loss of aldehyde from the molecular ions.     

Mass spectrometry of pyridine compounds–IV: The formation and decomposition of the [M – methyl]+ ion [C8H10N]+ from 4-t-butylpyridine in comparison with that of the [M – methyl]+ ion [C9H11]+ from t-butylbenzene,as studied by D- and 13C-labelling

A strong secondary isotope effect is observed in the preferred loss of methyl vs. trideutero-methyl from the molecular ions of appropriately labelled 4-t-butylpyridine and t-butylbenzene decomposing in the first and second field free regions of a double focusing mass spectrometer. This has been rationalised by invoking the theory of radiationless transitions², which can account for the higher population of activated states responsible for loss of methyl vs. that for trideuteromethyl. ¹³C-Labelling at the central carbon atom of the t-butyl group indicates that the [M – methyl]⁺ ions, decomposing further by elimination of ethylene, cannot be represented exclusively by a pyridylated (or phenylated) cyclopropane ion if present at all. It is concluded that ions with structures generated by 1,2-hydrogen-, 1,2-pyridyl- (or 1,2-phenyl-) and 1,2-methyl shifts must also play a role. D-labelling further shows an extensive randomisation of side-chain hydrogen atoms in the [M-methyl]⁺ ions of 4-t-butylbenzene; in this case, however, the expelled ethylene also contains ring hydrogen atoms (≤2). Presumably this is caused by exchange between the side-chain and ortho-hydrogen atoms in the initially generated phenyldimethylcarbinyl carbenium ion.