Zum Mechanismus Massenspektrometrischer Fragmentierungsreaktionen—IV: Zur Bildung von Phenonium-Ionen bei der Elektronenstoß-Fragmentierung von ß-Phenyläthylbromiden

The apparently equal binding of all 9 H-atoms, demonstrated with the aid of deuterium labeling by N. M. M. Nibbering et al.,³ in [C₈H₉]⁺-ions generated from β-phenylethyl bromides has been confirmed. This result has been explained by a cyclooctatrienyl structure (a) of the ions. A comparison of the heats of formation of [C₈H₉]⁺-ions of different origin reveals, however, that this is not true for [C₈H₉]⁺-ions of β-phenylethyl bromide in the ground state. As a result of a study of the influence of substituents at the phenylring of β-phenylethyl bromide on the intensities of the [M ? Br]⁺-ions, R. H. Shapiro et al.⁴ proposed a phenonium structure (c) for the [C₈H₉]⁺-ions. The substituent effect on intensities and AP of [M ? CH₂Br]⁺- and [M ? Br]⁺-ions of β-phenylethyl bromides is quite different and can be explained much better by a ‘classical’ structure of the [C₈H₉]⁺- ions. This structure can also be deduced by a comparison of the IP and AP of [M ? Br]⁺-ions of benzyl-, β-phenylethyl- and γ-phenylpropyl-bromide.     

Mass spectrometry of trialkylsilyl and acyl derivatives of 2′-deoxynucleosides

The electron impact mass spectra of monosilyl and mixed acyl-silyl derivatives of 2′-deoxynucleosides are described in detail. (Silyl = tert-butyldimethylsilyl, cyclo-tetramethylene-isopropylsilyl, or cyclo-tetramethylene-tert-butylsilyl; acyl = acetyl or trifluoroacetyl.) The interpretation of the fragmentation pathways was aided by metastable ion decomposition studies, precise mass and deuterium labelling measurements. Mass spectrally, the acyl substituents are mostly ‘passive’ and have (with possibly one exception) little fragmentation directing capability. In contrast, the silyl groups have powerful fragmentation directing properties. Elimination of the bulky alkyl radical R^˙ (tert-butyl or isopropyl) from the molecular ion produces the siliconium ion, [M–R]⁺, which is the precursor for most of the other prominent ions in the spectra. These arise from ‘siliconium ion rearrangements’ resulting from the interaction of the positively charged siliconium ion centre with the electron dense regions (i.e. oxygens) in the molecule, to form cyclic silyloxonium ions which subsequently decompose. Since the interacting oxygen and silicon must be sterically accessible, the fragment ion types and their abundances are very dependent upon structure. Consequently, [M–R]⁺ ions formed from 3′- or 5′-O-silyl groups give rise to different sets of daughter ions which, for the most part, are not found, or have very low abundances, in the mass spectra of underivatized or trimethylsilylated nucleosides. Detailed information on sugar and base moieties and isomeric substitution is readily obtained.     

Mass spectrometry of stable free radicals—II: pyrrolidine nitroxides

3-Substituted-2,2,5,5-tetramethylpyrrolidine nitroxides are stable free radicals used extensively in the synthesis of ‘spin labels’. The high resolution mass spectra of these nitroxides substituted with ? CH₂OH, ? OH, ? NH₂ and ?o have been recorded on magnetic tape and the elemental compositions of the ions calculated by computer. Ionisation by electron bombardment(70eV), gives rise to an even-electron molecular ion species. [M+1]⁺. ions are observed in the spectra of all compounds examined, except in the case of the 3-carbonyl compound, 2,2,5,5-tetramethylpyrrolid-3-one-1-oxyl. Loss of a methyl radical from these ions leads to the appearance of ions at [M -14]⁺. The predominant fragmentation for those compounds in which the substituents can supply electrons to the ring, is the sequential elimination of isobutene, nitric oxide and a hydrogen radical. In the case of the 3-hydroxy compound, these ions account for 23 percent of the total ion current. 2,2,5,5-Tetramethylpyrrolid-3-one-1-oxyl, which bears an electron-withdrawing substituent gives rise to a fragmentation pattern somewhat different from those of the other compounds. The main features are the absence of a peak at [M + 1]⁺˙ and the general phenomenon of fewer peaks but with higher intensities.     

Ionization and appearance potentials in organic chemistry. II—Appearance potentials and enthalpy differences for the stereoisomers of methylbicyclo[4.4.0]decanes

The ionization potentials for the stereoisomers of trans-fused 2- and 3-methylbicyclo[4.4.0]decanes and the appearance potentials for the ions at m/e 137 [M–CH₃]⁺, 109, 96, 95 and 82 were measured by the electron impact method. The ionization potentials and appearance potentials of the [M–CH₃]⁺ ions appeared to be equal for each of the epimers. Appearance potentials of the other ions were always lower for the less stable epimer. No quantitative correlation was observed between the difference in the appearance potentials for any ions and the differences in enthalpies of the ground states.     

Diastereochemical differentiation of bicyclic diols using metal complexation and collision‐induced dissociation mass spectrometry

Metal complex formation was investigated for di‐exo‐, di‐endo‐ and trans‐2,3‐ and 2,5‐disubstituted trinorbornanediols, and di‐exo‐ and di‐endo‐ 2,3‐disubstituted camphanediols using different divalent transition metals (Co²⁺, Ni²⁺, Cu²⁺) and electrospray ionization quadrupole ion trap mass spectrometry. Many metal‐coordinated complex ions were formed for cobalt and nickel: [2M+Met]²⁺, [3M+Met]²⁺, [M–H+Met]⁺, [2M–H+Met]⁺, [M+MetX]⁺, [2M+MetX]⁺ and [3M–H+Co]⁺, where M is the diol, Met is the metal used and X is the counter ion (acetate, chloride, nitrate). Copper showed the weakest formation of metal complexes with di‐exo‐2,3‐disubstituted trinorbornanediol yielding only the minor singly charged ions [M–H+Cu]⁺, [2M–H+Cu]⁺ and [2M+CuX]⁺. No clear differences were noted for cobalt complex formation, especially for cis‐2,3‐disubstituted isomers. However, 2,5‐disubstituted trinorbornanediols showed moderate diastereomeric differentiation because of the unidentate nature of the sterically more hindered exo‐isomer. trans‐Isomers gave rise to abundant [3M–H+Co]⁺ ion products, which may be considered a characteristic ion for bicyclo[221]heptane trans‐2,3‐ and trans‐2,5‐diols. To differentiate cis‐2,3‐isomers, the collision‐induced dissociation (CID) products for [3M+Co]²⁺, [M+CoOAc]⁺, [2M–H+Co]⁺ and [2M+CoOAc]⁺ cobalt complexes were investigated. The results of the CID of the monomeric and dimeric metal adduct complexes [M+CoOAc]⁺ and [2M–H+Co]⁺ were stereochemically controlled and could be used for stereochemical differentiation of the compounds investigated. In addition, the structures and relative energies of some complex ions were studied using hybrid density functional theory calculations. Copyright © 2009 John Wiley & Sons, Ltd.     

Die massenspektrometrische Fragmentierung von Neopentylpolyolestern. 1—Pentaerythrittetrafettsäureester

The fragmentation of the homologous fatty acid tetraesters of pentaerythritol (C-2 to C-14) upon electron impact was investigated. The main fragment ions are [M? RCOO]⁺ and [M? RCOOH]⁺, for which cyclic acetal structures are postulated. Subsequent fragmentation was elucidated by ‘direct analysis of daughter ion’ (DADI) measurements and high resolution measurements. Esters of branched fatty acids can be distinguished from esters of n-fatty acids by characteristic ions. Isomeric esters of n-fatty acids cannot be separated by gas chromatography but identification is also possible by mass spectrometry.     

The electron impact fragmentation of 3- and 4-styrylpyridazine

The electron impact mass spectrometric fragmentation of trans-3- and trans-4-styrylpyridazine is reported in detail, including a comparison with other aza-stilbenes. With regard to a distinction between the two isomeric styrylpyridazines, the intensity ratio of the M⁺ and [M-1]⁺ ions, the general degree of fragmentation and the elimination pathways of nitrogen proved to be most characteristic.     

Electrospray ionization multiple-stage linear ion-trap mass spectrometry for structural elucidation of triacylglycerols: Assignment of fatty acyl groups on the glycerol backbone and location of double bonds

Linear ion-trap multiple-stage mass spectrometric approach (MS ⁿ ) towards nearly complete structural elucidation of triacylglycerol (TAG) including (1) assignment the fatty acid substituents on the glycerol backbone and (2) location of the double bond(s) on the unsaturated fatty acyl groups is reported. The characterization is established by the findings that MS² on the [M+Li]⁺ ions of TAG yields more abundant ions reflecting losses of the outer fatty acid substituents either as free acids (i.e., [M+Li-R₁CO₂H]⁺ and [M+Li-R₃CO₂H]⁺ ions) or as lithium salts (i.e., [M+Li-R₁CO₂Li]⁺ and [M+Li-R₃CO₂Li]⁺ ions) than the ions reflecting the similar losses of the inner fatty acid substituent (i.e., [M+Li-R₂CO₂Li]⁺ and [M+Li-R₂CO₂Li]⁺ ions). Further dissociation (MS³) of [M+Li-R _n CO₂H]⁺ (n=1, 2, or 3) gives rise to the ion series locating the double bonds along the fatty acid chain. These ions arise from charge-remote fragmentations involving β-cleavage with γ-H shift, analogous to those seen for the unsaturated long-chain fatty acids characterized as initiated ions. Significant differences in abundances in the ion pairs reflecting the additional losses of the fatty acid moieties, respectively, were also seen in the MS³ spectra of the [M+Li-R _n CO₂H]⁺ and [M+Li-R _n CO₂Li]⁺ ions, leading to confirmation of the fatty acid substituents on the glycerol backbone. MS ⁿ on the [M+Na]⁺ and [M+NH₄]⁺ adduct ions also affords location of fatty acid substituents on the glycerol backbone, but not the position of the double bond(s) along the fatty acid chain. Unique ions from internal losses of the glycerol residues were seen in the MS³ spectra of [M+Alk-R _n CO₂H]⁺ (n=1, 2, 3) and of [M+Alk-R _n CO₂Alk]⁺ (Alk=Li, Na, NH₄; n=1, 3). They are signature ions for glycerides and the pathways leading to their formation may involve rearrangements.     

In-beam electron impact mass spectrometry of aliphatic alcohols

The characteristics of the in-beam electron impact mass spectra of six isomers of undecanol as well as several 1-alkanols have been examined. In addition to the characteristic ions observed in the conventional electron impact spectra, the [2M+1]⁺, [2M+1-H₂O]⁺, [2M+1-2H₂O]⁺, [2M-R or R′]⁺, [2M-H₂O? R or R′]⁺, [2M? 2H₂O? R or R′]⁺ and [M+1? H₂O]⁺ peaks are common in the in-beam electron impact mass spectra of the undecanol isomers of structure RCH(OH)R′. Deuterium labelling experiments have shown that the extra proton in the protonated dimer ions, [2M+1]⁺, originates from the hydroxy group. The processes which produce the important peaks in the high m/e regions are discussed.     

Mass spectra of chlorinated aromatics formed in pulp bleaching: I—chlorinated catechols

A systematic study on the electron impact mass spectra of all nine chlorinated catechols in presented. Metastable ion analysis was used to elucidate the fragmentation pathways. The influence of the position of the chloro substituents can be used to distinguish the structural isomers. In this respect the most characteristic fragment ions are [M? CHl]⁺˙, [M? HCOOH]⁺˙, [M? COCl]⁺, [M? HCl? CO]⁺˙, [M? CHOCl]⁺˙ and [M? HCl? HCl]⁺˙.     

Mass spectrometric behaviour of 1-imino-substituted pyrazolo[1,2-a]-[1,2,4]triazoles under electron and chemical ionization

The spectra of five pharmacologically interesting substituted pyrazolo[1,2-a][1,2,4]triazole hydroiodides were measured under electron and chemical ionization. In the electron ionization spectra, in addition to the intense molecular ion peak of the free base (M⁺*), there was also a relatively intense molecular ion peak of the hydroiodide form, which is unusual since the hydroiodides are rarely so stable. The phenylimino and phenylamino substituents of the triazole ring affected the fragmentation behaviour of the compounds very much. The chemical ionization reagent gases used in this work were methane, isobutane, deuterated ammonia and acetone. In all the cases practically only [M+H]⁺ ions were observed, the only exception being acetone which also gave rise to intense [M+C₂H₃O]⁺ and [M⁺C₃H₇O]⁺ adduct ions. None of the reagent gases used was able to cause any fragmentation.     

Charge-remote and charge-proximate fragmentation processes in alkali-cationized fatty acid esters upon high-energy collisional activation. A new mechanistic proposal

The effect of the metal ion on the high-energy collision-induced dissociation (CID) of alkali metal-cationized n-butyl and methyl ester derivatives of palmitic and oleic acid was examined. The results show that the alkali metal ion has a pronounced effect and does not act as a mere ‘spectator’ ion with respect to the fragmentation process. While C–H cleavage is a dominant process for [M+Li]⁺ as well as [M+Na]⁺ precursor ions, C–C cleavage is also significant for the [M+Na]⁺ ions. Homolytic mechanisms involving the formation of a transient biradical cation are proposed which enable us to rationalize in a straightforward manner all product ions formed by both charge-remote and charge-proximate fragmentations. The mechanistic proposal is discussed in view of available knowledge on electron impact, CID and related processes. In order to predict how the alkali metal ion could affect the reactivity of the postulated biradical state formed following electronic excitation of the alkali metal-cationized molecules, quantum chemical calculations were performed on methyl and n-butyl acetate as model substances. The decreased spin density at the carbonyl oxygen atom in the biradical state may provide an explanation for the greater tendency towards C–C cleavage reactions of the sodium-cationized fatty acid esters relative to the corresponding lithium complexes. © 1988 John Wiley & Sons, Ltd.     

Synthesis and electron impact mass spectra of 3-substituted 2-acylaminoindazoles

3-Substituted-2-acylaminoindazoles 2 were prepared via oxidative cyclization of o-aminoaryl ketone acylhydrazones 1 with iodosobenzene diacetate. Their electron ionization mass spectra were recorded and in addition to the molecular ions show common fragmentation pathways corresponding to the [M-N₂]⁺, [M-NHCOX]⁺ and [M-COX]⁺ ions, with some influence on the skeletal fragmentation by different substituents.     

The behaviour of some α,α′-dibromoketones on electron impact. An analogy to the reductive removal of bromine in solution

The relative importance of the rearrangement ions [M ? Br ? CO]⁺, [M ? Br₂ ? CO]⁺ and [M ? HBr₂ ? CO]⁺ in the mass spectra of the title compounds is compared with the amounts of α-methoxyketone formed on reduction of these compounds with a Zn/Cu couple in methanol. It is suggested that the quantitative correlation found reflects the electron releasing powers of the substituents on the α carbons.     

Electron impact mass spectra of some dioximes of o-diacyl benzenes

The electron impact mass spectra of the new synthesized dioximes of o-diacyl benzenes (2) are reported. In addition to the molecular ion, characteristic peaks appear at values corresponding to the [M ? OH] ⁺, [M ? NOH]⁺ and [M ? NHOH]⁺ ions. No initial dehydration of the molecular ion has been observed.     

Mechanism of stereospecific alcohol elimination from cyclohexane trans-1,3-dicarboxylates under electron impact ionization

Diesters of cyclohexane trans-1,3-dicarboxylic acid give rise to major [M ? ROH]^+·. ions under electron impact ionization. A mass spectral study of the isomeric mixed methyl ethyl esters of the diacid, substituted by a methyl group at position 1 and deuterium labelled at position 3, indicates a stepwise mechanism for this alcohol elimination; the 3-hydrogen (or deuterium) is transferred to the carbonyl of the 1-ester group in the initial step. Subsequent migration of that hydrogen (or deuterium) to the alkoxyl of position 3 results in the highly site- and stereospecific alcohol elimination. CID spectra of the [M ? ROH]^+. ions obtained from the stereoisomeric diesters clearly show that they have different structures (or are different mixtures of structures).     

Participation of halogen atoms in hydrogen transfer between remote positions in gas-phase ionized isomeric haloresorcinols

Mono, di- and trihaloresorcinols substituted by a halogen atom at position 2 exhibit a highly specific elimination of H₂O on electron impact ionization and under conditions of collisionally induced dissociation (CID). The high isomer specificity suggests the intermediacy of a hydrogen transfer from one of the hydroxy groups to the adjacent halogen atom. A subsequent hydrogen migration to the other hydroxy group readily explains the facile elimination of H₂O from the M^+· ions of these particular isomers. An analogous three-step hydrogen transfer has not been observed in 2,3-dihalo-l,4-hydroquinones. 4-Bromo- and 4-icdoresorcinol undergo elimination of the halogen atom followed by a very fast loss of CO under CID conditions, affording [M ? Hal]⁺ ions of low abundance and highly abundant[M ? Hal ? CO]⁺ ions. The elimination of CO is suppressed in the isomeric 5-haloresorcinols, resulting in very highly abundant [M ? Hal]⁺ ions. This behavior suggests that a ‘hidden hydrogen transfer’ accompanies the elimination of the halogen atom from the molecular ions of 4-haloresorcinols.     

7H-benz [kl]acridin-7-ylium closed-shell ion produced from 1,8-bis(phenylamino)naphthalenes under electron impact: Tandem mass spectrometric structure elucidation

Not only strongly basic aromatic amines such as ‘proton sponges’ show characteristic [M – Me₂NH – H]⁺ peaks corresponding to cyclization to stable heterocyclic ions under electron impact. The fragmentation of the title compounds, which are weak bases, leads to [M – RNH₂ – C₆H₅]⁺ heteroaromatic ions. These are the 7-phenyl-7H-benz[kl]acridine radical cation and the delocalized stable 7H-benz[kl]acridin-7-ylium ion as shown by unimolecular metastable ion spectra, collision-induced decomposition (CID) tandem mass spectrometry (MS/MS) and accurate mass measurements. The high-collision-energy CID tandem mass spectra of these ions are perfectly superimposable on those of the ions produced by the reference substance, 7-phenyl-7H-benz[kl]acridine obtained as a by-product in the Ullmann phenylation of 1,8-diaminonaphthalene. This combination of MS/MS experiments also provides strong support for the gas-phase reaction mechanism leading to the benz[kl]acridine ions and for the structures of these ions.     

Ion-molecule reactions in the gas phase. part XXII. Reactivity of the cis- and trans-indanediols with dimethyl ether ions

Specific reactivity of cis- and trans-indanediols has been investigated under dimethyl ether (DME) chemical ionization conditions. Several unusual species, such as [M + 29]⁺ and [M + 27]⁺ ions, are produced in high yield. From DME pressure variations and tandem mass spectrometry experiments (low-energy collisions with Ar and NH₃) including some labeled compounds, it appears that [M + 29]⁺ ions are generated by nucleophilic substitution according to a S_Ni pathway from the proton bound[M + DMEH]⁺ adduct ion. On the other hand, [M + 27]⁺ ions are produced from the covalent [M + DME ? H]⁺ adduct ions via a stepwise process inducing a water loss. This latter dehydration occurs from the adducts prepared by [DME ? H]⁺ attachment to the homobenzylic hydroxy site, which allows internal proton transfer from the charged position to the benzylic hydroxy group, promotingthe loss of water. In addition, trans indanediol labeled with ¹⁸O has been used to obtain evidence for the regioselectivity of both water-loss mechanisms from the benzylic site.     

Mercury(II) and silver(I) receptors based on tetrathiacalix[4]arene hydrazones

The extraction ability of phenyl hydrazone derivatives of cone- and 1,3-alternate tetrathiacalix[4]arenes towards to some alkali, alkaline earth, transition and heavy metal ions has been investigated by picrate extraction method. The synthesized cone-tetrathiacalix[4]arene hydrazones show a high efficiency coupled with an excellent selectivity towards for Ag⁺ and Hg²⁺ ions. The stoichiometry of complexes and the extraction constants have been determined. The influence of calix[4]arene platform’s structure and the electron donor substituents in phenyl fragments on the extraction efficiency has been discussed.