Isomerization and fragmentation of aliphatic ether radical cations: Interconversion of distonic ions and cyclopropane intermediates

The reactions of propyl ether radical cations close to threshold are initiated by (reversible) formation of γ-disitonic isomers, R$ \mathop {\rm O}\limits^ + $ (H)CH₂CH₂CH₂^·. The three methylene groups in these ions lose their positional identity by ring closure/ring opening via [cyclopropane + alcohol]^+· intermediates. Extensive hydrogen exchange occurs within the C₃-chain. When R is not methyl the γ-distonic isomer undergoes further intramolecular hydrogen atom transfer reactions that lead to formation of α- and β-distonic ions. The α-distonic isomers expel ethyl and propyl radicals by C? O bond cleavage.     

Formation of β-distonic oxonium ions: Study of ROCH2CH2OR′ radical cations

Radical cations derived from the ethers ROCH₂CH₂OR′ (R, R′ = H, CH₃, C₂₅) were studied, since β-distonic oxonium ions are often prepared from ionized ethers of glycol. The first step in the fragmentation is a 1,5-transfer of an α-hydrogen to oxygen of a terminal alkoxy group leading to a δ-distonic oxonium ion. This step is thermo-neutral and reversible in the ROCH₂CH₂OH radical cations and exothermic and irreversible in the dialkyl ether radical cations. Depending on R and R,′ these δ-distonic oxonium ions fragment by three reactions: the loss of an alcohol or a water molecule, the formation of a β-distonic oxonium ion ˙CH₂CH₂O(H)⁺R and a 1,4-H migration between carbon atoms. Competition between these processes is discussed.     

Destabilized carbenium ions: α-imidoyl carbenium ions and the electron impact mass spectra of α-haloaldimines and α-haloketimines

A series of α-chloro- and α-bromoketimines compounds (1-9) with different substituents at the α-position and at the imino group has been investigated by electron impact mass spectrometry as possible precursors of the correspondingly substituted α-imidoyl carbenium ion, an important class of destabilized carbenium ions. The main fragmentation of the molecular ions of compounds, 1-9 in the ion source corresponds to an α-cleavage at the imino group; however, fragment ions are also formed by loss of the α-halo substituent. These fragment ions correspond at least formally to α-imidoyl carbenium ions. Their further reactions in dependence on the type of substituents at the imino group and at the α-C atom, were studied by mass-analysed ion kinetic energy and collisional activation mass spectrometry. The results agree with the initial formation of destabilized α-imidoyl carbenium ions but indicate an easy rearrangement of these ions in the presence of suitable alkyl substituents by 1,2- and 1,4-hydrogen shifts to more stable isomers.     

Mass spectra of stereoisomeric cis- and trans-2-alkyl-3-aryl(hetaryl)-4-(methoxycarbonyl)-3,4-dihydro-1h-isoquinol-1-ones and 1,2,3,4-tetrahydroisoquinolines

The molecular ions of the trans isomers of the investigated compounds are more stable than those of the cis isomers. Their principal fragmentation pathways involve retrodiene fragmentation of the molecular ions, which proceeds with greater probability in the case of the cis isomers, or loss of substituents from the 4 or 3 position. The latter process is characteristic only for tetrahydroisoquinoline derivatives, is not observed for 3,4-dihydro-1H-isoquinol-1-one derivatives, and proceeds more readily in the case of the trans isomers.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1395–1397, October, 1980.     

Atmospheric pressure chemical ionization negative mass spectra of the dinitrotoluene isomers

The atmospheric pressure chemical ionization of the dinitrotoluene isomers in ambient air was studied with a quadrupole mass spectrometer operating in the negative mode. The isomers can be grouped on the basis of the product ions: 2,5-, and 2,6- and 3,5-dinitrotoluene give the molecular anion with little fragmentation; 2,3- and the 3,4-dinitrotoluene behave similarly but with more extensive fragmentation; 2,4-dinitrotoluene gives the quasimolecular [DNT ? H]^? ion with little fragmentation. The results are discussed in terms of the molecular structure of the isomers.     

甘露糖磷酰氨基酸缀合物的ESI-MS研究

一系列全乙酰保护甘露糖-1-磷酰氨基酸酯缀合物的α构型和β构型异构体的ESI-MSn裂解规律研究表明, β构型异构体会出现一系列特征的m/z 433, 391, 371的碎片离子, 且［M-CH2CHCH3+Na］+碎片丰度较大. ［M-糖基+Na］+和［糖基+Na］+碎片相对丰度较小. α构型分子反之. 同时对全乙酰保护的［糖基+Na］+ m/z 353碎片离子进行三级碎裂, 其主要特征是消除CH2CO和AcOH中性分子的碎片离子, 而且其它位羟基构型的差异对质谱中碎裂方式影响不大.     

Differentiation of isomeric 2-aryldimethyltetrahydro-5-quinolinones by electron ionization and electrospray ionization mass spectrometry

A series of isomeric 2-aryl-6,6-dimethyltetrahydro-5-quinolinones (set I) and 2-aryl-7,7-dimethyltetrahydro-5-quinolinones (set II) were studied under positive ion electron ionization (EI) and electrospray ionization (ESI) techniques. Under EI conditions, the molecular ions were found to be less stable in set I isomers, and they resulted in abundant fragment ions, i.e., [M-CH(3)](+), [M-CO](+.), [M-HCO](+), [M-(CH(3),CO)](+), and [M-(CH(3),CH(2)O)](+), when compared with set II isomers. In addition, the set I isomers showed specific fragment ions corresponding to [M-OH](+) and [M-OCH(3)](+). The retro-Diels-Alder (RDA) product ion was always higher in set II isomers. The ESI mass spectra produced [M + H](+) ions, and their decomposition showed favorable loss of CH(3) radical, CH(4) and C(2)H(6) molecules in set I isomers. The set II isomers, however, showed predominant RDA product ions, and specific loss of H(2)O. The selectivity in EI and ESI was attributed to the instability of set I isomers by the presence of a gem-dimethyl group at the α-position, and it was supported by the data from model compounds without a gem-dimethyl group. Density functional theory (DFT) calculations successfully corroborated the fragmentation pathways for diagnostic ions. This study revealed the effect of a gem-dimethyl group located at the α-position to the carbonyl having aromatic/unsaturated carbon on the other side of the carbonyl group.     

Collisional activation mass spectra of pentene and hexene molecular ions

The collisional activation spectra of the molecular ions from fourteen pentene and hexene isomers show substantial differences, some of which can be rationalized using known ion fragmentation mechanisms. These differences, and their negligible dependence on ion internal energy, are advantageous for the structural characterization of such isomeric ions, including those produced by fragmentation of larger molecules. The decompositions of metastable ions of the pentenes were nearly identical, but those of the hexenes showed some differences useful for structural charactrization.     

High-energy collision-induced dissociation of small polycyclic aromatic hydrocarbons

High-energy collision-induced dissociation (CID) experiments on polycyclic aromatic hydrocarbons (PAHs) having 2-6 rings, naphthalene, anthracene, phenanthrene, fluoranthene, pyrene and coronene, were performed, and the relative abundances of their fragment ions were investigated as a function of collision energy. The results revealed that the PAHs except naphthalene showed a bimodal-type distribution of positive fragmentation ions, which is closely similar to the fragment-ion distribution reported for the CID of three-dimensional fullerene, C(60)(+) and C(70)(+). The three-ring isomers of anthracene and phenanthrene and the four-ring isomers of fluoranthene and pyrene can be distinguishable in their spectra under an electron ionization energy of 70 eV, but the high-energy CID spectra of the three- and four-ring isomers were almost identical. The fragmentation corresponding to fragment ions in the low-mass region of the bimodal CID spectra could be interpreted by the simple statistical model that fragment ions are formed by random evaporation from the molecular ions after a considerable structural rearrangement, 'phase transition', occurring at some high-energy state.     

Methylene radical cation transfer from ionized oxirane to CH3SCH3 and C6H5SCH3 in the gas phase: Formation of sulphur distonic ions and/or insertion in a carbon–sulphur bond

The α-distonic sulphur-containing ion $ {}^ \cdot {\rm CH}_2 \mathop {\rm S}\limits^ + \left({{\rm CH}_3 } \right)_2 $ has been generated by transfer of CH from ionized oxirane to dimethyl thioether and distinguished from the molecular ion of ethyl methyl thioether by collision induced dissociation (CID) experiments. In particular, the α-distonic ion expels CH₂ to a minor extent following collision, whereas the molecular ion of ethyl methyl thioether does not undergo this reaction. The metastable C₃H₈S^+˙ ions formed by CH transfer to dimethyl thioether and ionization of ethyl methyl thioether decompose by competing losses of CH₃R˙, CH₄ and C₂H₄. The elimination of ethene is taken as evidence for isomerization of the α-distonic ion to the molecular ion of ethyl methyl thioether prior to spontaneous dissociation. Evidence for the formation of stable α-distonic sulphur-containing ions by transfer of CH from ionized oxirane to methyl phenyl thioether has not been obtained. The collision-induced and spontaneous reactions of the ions formed by CH transfer to methyl phenyl thioether indicate that a mixture of the radical cations of CH₃C₆H₄SCH₃, C₆H₅SCH₂CH₃ and C₆H₅CH₂SCH₃ is generated implying that attack on the phenyl group occurs in addition to a formal insertion of a methylene entity in a C? S bond.     

The mass spectra of alkyl- and phenyl-4-imidazolin-2-ones

The mass spectra of a variety of alkyl- and aryl-4-imidazolin-2-ones have been determined and the fragmentation mechanisms have been analyzed by deuterium labelling, high resolution and metastable transitions allowing certain differentiations of positional isomers. In contrast to the benzoid systems the mass spectra of isomeric alkyl-4-imidazolin-2-ones are distinctive. The influence of the position of substituents is demonstrated by phenyl-4-imidazolin-2-ones establishing an exact prediction of fragmentation pathways. Fragment ions (e.g. [M-HNCO].⁺) which are the result of rearrangement processes were excluded for structure determinations. The ion structures involved were elucidated by collisional activation comparing model ions. Alkyl-phenyl-4-imidazolin-2-ones give almost identical mass spectra, but the positional isomers can easily be distinguished by different fragmentation patterns in both metastable and collisional activation spectra of the molecular ions.     

The mass spectrometry of DT-and monosubstituted biphenyls: Dependence of fragmentation patterns on position of substitution

This paper describes the mass spectroscopy of a series of biphenyl derivatives substituted in the 2,2′, 4,4′ and 2 positions. The substitutent functional groups are carbomethoxy, carbothoxy, carboxylic acid and hydroxymethyl. In addition, some results are reported on the spectroscopy of the d₅-carboethoxy derivatives, the 2- and 2,2′-αd₂- hydroxymethyl derivatives and fluorene-4-methanol. The molecular ions of the 4,4′-disubstituted biphenyl derivatives are far more stable than those of the 2,2′ isomers. It is also observed that the fragmentation patterns of these two sets of isomers are sharply different. Paradoxically, the 2-substituted biphenyl derivatives give relatively stable molecular ions and their fragmentation patterns are frequently different from those of the corresponding 2,2′-disubstituted biphenyls. The bulk of the evidence presented in this paper suggests that the usual sort of ‘ortho effect’ is not a significant factor in the fragmentation mechanisms proposed for the 2,2′-disubstituted biphenyl derivatives.     

Discrimination of isomers by liquid ionization mass spectrometry with techniques of collisional activation and adduct ion formation

Liquid ionization mass spectrometry is a soft ionization technique used with liquid samples under atmospheric pressure. It facilitates the handling of reagents and the observation of ion–molecule reactions in the ion source. The differentiation of isomers by characteristic fragment ions, for example those resulting from asymmetrical cleavage of a cyclobutane ring, and by molecular adduct ion formation was studied. The samples studied were cyclobutane derivatives, alkyl 4-(3-oxo-3-pIienyl-l-aIkenyl)benzoate dimers, and reagents having two functional groups were used to produce adduct ions to clarify the difference between isomers. The reagents act on a sample molecule at two functional groups to form hydrogen bonds. Some correlations were observed between the structure of the sample and the relative abundances of molecular adduct ions and also fragment ions produced by collisionally activated dissociation.     

Mass spectrometry in structural and stereochemical problems—CXCIX Contrasting fragmentations of singly- and doubly-charged o-, m- and p-hydroxyalkylphenones and their trimethylsilyl ethers

High resolution mass spectrometry, metastable defocusing and deuterium labeling of trimethylsilyl (TMS) ethers have been used to study the electron-impact induced fragmentations of o-, m- and p-hydroxyalkylphenones and their TMS ether derivatives. These derivatives have proven useful in contrasting the fragmentation patterns of singly- and doubly-charged ions because of the competing fragmentations: α-cleavage and a McLafferty rearrangement from the ketone moiety and methyl cleavage from the TMS group. A proximity effect was responsible for a markedly increased methyl radical loss from the o-TMS ether. This fragmentation was minor with the m- and p-isomers. Significantly intense doubly-charged ions were formed from ketonic cleavage and by the loss of a TMS methyl radical. The sequence of fragmentation depended on the size of the alkyl group attached to the ketone carbonyl. There was no evidence found for a McLafferty rearrangement occurring from the doubly-charged molecular ion of the TMS ethers of the hydroxyalkylphenones but the rearrangement occurred from the doubly-charge molecular ion of bis-3-(1-oxopentyl)-4-hydroxy-phenyl-methane and, of course, from the singly charged [M]^+. The bis-p-hydroxyphenylmethane derivatives were studied in an effort to increase the intensity of the doubly-charged ions as it was expected that the charges would be separated by a longer distance.     

(E)-4-alkoxycarbonylalkylthiochalcones: differentiation of isomeric derivatives by electron ionization mass spectrometry

The principal fragmentation pathways of the molecular ions of 18 new (E)-4-alkoxycarbonylalkylthiochalcones have been investigated. It has been shown that the data derived from electron ionization mass spectra (the relative abundance of the fragment ions and values of coefficients micro) can be used to differentiate the isomers. The fragmentation rules deduced here could help in the characterization of other chalcones of these types.     

Electron ionisation induced fragmentation of ethyl 5(1H)-oxo- and 7(1H)-oxo-1-aryl-2,3-dihydroimidazo[1,2-a]-pyrimidine-6-carboxylates: evidence for an unusually regioselective rearrangement of M(+*) ions.

The 70-eV electron ionisation (EI) mass spectra of the title compounds show clear differences between the 5-oxo and 7-oxo isomers due to regioselective fragmentations involving the ester function. Exceptionally abundant metastable peaks due to molecular ions fragmenting to [M -CO2](+.) were observed exclusively for the 7-oxo isomers, suggesting that the sufficiently long-lived molecular ions undergo a slow rearrangement preceding this fragmentation reaction. The results are contrasted to the available literature data on the ester group fragmentations involving the loss of CO2 and the EI mass spectrometry of pyrimidone beta-oxo esters. A reaction mechanism is proposed for the elimination of CO2 following ethyl group migration to the pyrimidone carbonyl oxygen.     

Electron ionization (EI) mass spectra of exo-endo double-bond isomers of polycyano "push-pull" pentadienes derived from cycloalkylidene malonic acid derivatives

The title compounds, which exist in solutions as mixtures of exo/endo double bond isomers due to the "push-pull" effect of the electron-donating and electron-withdrawing substituents, were studied by mass-spectrometric methods. Their fragmentation routes under electron impact were established and confirmed by metastable ion analysis and accurate mass measurements. The results demonstrated that the relative amounts of exo/endo isomeric molecular ions are in close agreement with the isomeric ratios observed in solutions by the NMR, although the mass spectra of the tetracyano derivatives indicated a small fraction of molecular ions existing in the endo form, which could not be detected in solution by the NMR methods.     

Gas phase substitution reactions by radical cations Part 5: Reaction of the CC ring-opened oxirane radical cation with 1,2-, 1,3- and 1,4-dichlorobenzene

Under conditions of chemical ionization in the high pressure source of a mass spectrometer, the α-distonic CC ring-opened oxirane radical cation transfers a methylene group to 1,2-, 1,3- and 1,4-dichlorobenzene. The structures of the M + 14]^·+ product ions have been established by collisionally induced dissociation of these ions compared with reference ions and application of principal component analysis. 1,2-Dichlorobenzene yields 80% 2-chlorobenzyl chloride, 5% 2,3-dichlorotoluene and 15% 3,4-dichlorotoluene. The [M + 14]^·+ ions from 1,3-dichlorobenzene are 64–67% 3-chlorobenzyl chloride, 27–28% 2,6-dichlorotoluene and 7% 2,4- or 3,5-dichlorotoluene. From 1,4-dichlorobenzene mainly 4-chlorobenzyl chloride is formed, together with some 2,5-dichlorotoluene. In this case there is also an unidentified contribution, probably by 1,4-dichlorocycloheptatriene ions. Possible formation of distonic product ions does not occur in the cases of 1,2- and 1,3-dichlorobenzene, and from 1,4-dichlorobenzene it is considered to be unlikely.     

Electron Impact Induced Fragmentation of Aromatic Alkoxyimines II [5]. Formation and Transformation of Heterocyclic Radical Cations in the Gas Phasea

 The molecular ion 1 of N-(n-propoxy)benzaldimine I rearranges by an 1,5-H-shift to the δ-distonic ion 2 which subsequently cyclizes to the α-distonic ion 3. Homolytic cleavage of the N–O bond in 3 results in the δ-distonic ion 4 which expels CH₂O leading to the β-distonic ion 5. Ion 5 is also formed from the molecular ions of tetrahydrooxazines II and III and from M^+• of phenylazetidine IVa. In a subsequent step, ion 5 cyclizes to the N-protonated 3,4-dihydroisoquinolinium ion 6. The syntheses of II–IV and their derivatives are described.     

Study of the fragmentation pathway of α-aminophosphonates by chemical ionization and fast atom bombardment mass spectrometry

The diastereoisomers of α-aminophosphonates are key intermediates in the synthesis of enantiomerically pure α-aminophosphonic acids, which are analogs of α-amino acids. Although several methods have been reported for the diastereoselective synthesis of α-aminophosphonates, their mass spectrometry (MS) fragmentation patterns have not yet been fully investigated. The work described here involved a detailed study of the fragmentation of enriched α-aminophosphonate diastereoisomers by chemical ionization (CI-MS) and fast atom bombardment (FAB)-MS. The complete characterization of the different conventional MS fragmentation pathways is represented and this intriguing exercise required the use of tandem mass spectrometry (MS/MS) experiments and high-resolution accurate mass measurements. All α-aminophosphonates gave prominent pseudomolecular ions, protonated molecules [MH](+) , and their fragmentations mainly showed a loss of dimethyl phosphite to give the corresponding iminium ions as base peaks for α-aminophosphonates bearing methylbenzyl and 2,2-dimethylbutyl fragments. The loss of the chiral fragment from the iminium ions bearing the (S)-1-(1-naphthyl)ethyl group gave rise to a base peak due to aryl cations. The nature of all fragment ions were confirmed by high-resolution mass spectrometry (HRMS).