bluuebunny/crossref_metadata_2025 huggingface dataset:
Collision‐induced fragmentation reactions of doubly charged ions of polycyclic aromatic hydrocarbons
Authors: Sim P. G. , Jamieson W. D. , Boyd R. K.
Published: May 2005

Unimolecular and collision‐induced reactions of molecular dications M2 + of polycyclic aromatic hydrocarbons (PAHs) have been investigated. For larger PAHs (three or more rings) charge‐separation reactions, and both dissociative and non‐dissociative electron‐capture collisions, are less important. In both low energy (0‐400 eV) and high energy (16 keV) collision regimes, the dominant processes are expulsion of hydrogen atoms and of neutral acetylene molecules. In the high‐energy collision regime, neutral free radicals are also expelled from these M2 + precursors. Some degree of isomer differentiation is possible via relative intensities of doubly charged fragment ions in the low‐energy collision regime, but not all isomers can be thus distinguished and it seems unlikely that such a scheme could be made predictive. The CID spectra are largely independent of precursor size or structure at lower masses (m/z ⩽ 70). These findings are reviewed in the light of literature data on expulsion of small neutral species from aromatic dications. It is proposed that the charge‐separation reactions characteristic of smaller aromatic systems are charge‐site‐initiated cleavages, while expulsion of small neutral species such as acetylene is a radical‐site‐initiated reaction from M2 + ions possessing considerable diradical character. In all cases an important driving force is the avoidance of anti‐aromatic character for a benzene ring from which two electrons have been removed. The similarity of the CID spectra at low masses is analogous to that discussed recently (S. J. Pachuta et al., J. Am. Chem. Soc. 110, 657 (1988)) for fragmentations of the corresponding singly charged molecular radical cations.
Mass spectra of substituted and annelated benzofurazan‐1‐oxides
Authors: Dyall Leonard K.
Published: May 2005

Electron impact positive ion spectra of ten substituted or annelated benzofurazan‐1‐oxides are reported. While most of the molecular ions lose either NO˙ + NO˙, or NO˙ + CO, some also lose CO as an initial fragment. One of the fragmentation pathways for 4‐methylbenzofurazan‐1‐oxides involves initial ˙CHO loss.

With the annelated benzofurazan‐1‐oxides (naphtho[1,2‐c]furazan oxide and quinolo[3,4‐c]furazan oxide), loss of N2O2 is followed by a retro‐Diels–Alder elimination of butadiyne or propynenitrile, respectively from the aryne radical cation. In the case of quinolo[5,6‐c]furazan oxide, loss of N2O2 from the molecular ion must be followed by substantial rearrangement to enable the observed loss of propynenitrile to take place.
Ion formation in fast atom bombardment mass spectrometry: a divided probe investigation of gas‐phase reactions
Authors: Miller Jack M. , Balasanmugam Kesgapillai , Fulcher Adrian
Published: May 2005

Some ion‐formation processes during fast atom bombardment (FAB) are discussed, especially the possibility of reactions in the gas phase. Divided (two halves) FAB probe tips were used for introducing two different samples into the source at the same time. Our results showed [M + A]+ ions (where M = crown ethers and A = alkali metal ions), can be produced, at least in part, in the gas phase when crown ethers and sources of alkali metal ion are placed on two halves of the FAB probe tip. The extent of this ion formation depends on the volatility of the crown ether and on steric factors. Cluster ions such as (M + LiCl)Li+, (2M + LiCl)Li+, [2M + K]+ and [2M + Na]+ are also observed to form in the gas phase. Unimolecular decompositions contribute to some ions detected in FAB. When the alkali ion salt and the crown ether are mixed together the probability of [M + A]+ ion formation increases significantly, regardless of the volatility of the crown ether.
Mass spectra of doubly charged ions
Authors: Barber Michael , Bell David J. , Morris Michael , Tetler Lee W. , Woods Michael D. , Monaghan John J. , Morden William E.
Published: May 2005

The mass spectra of biological molecules, whose molecular mass exceeds 10 kDa, invariably contain multiply charged ions. For example, a survey scan of a small protein will produce singly, doubly and triply protonated molecules, the intensity of the doubly charged species often being greater than that of the singly charged entity. Although the spectra resulting from doubly charged peptides have not previously been studied, collisional activation of such doubly charged species may result in significant additional information pertaining to molecular structure. The techniques employed to study ions originating from multiply charged species were linked scanning of constant B/E and tandem mass spectrometry, namely low collision energy spectra acquired on a BEQQ hybrid instrument. The methodology was applied to model compounds whose tandem mass spectrometry characteristics are well known, e.g. gramicidin S and angiotensin I. The results for the product ions of the [M + 2H]2+ species of the models were obtained which highlight the methodology required for high‐mass materials.
Trends in the energies of cation and anion states of alkan‐1‐ols
Authors: Badrinathan D. Mathur C. , Marathe V. R.
Published: May 2005

We report results of a systematic investigation of the energies of positive‐ion and negative‐ion states in alkan‐1‐ol molecules CnH2n+1 OH (n = 1–5) in low‐energy electron impact experiments using a beam of nearly mono‐energetic electrons. The measurements have been conducted in a crossed electron‐beam/molecular‐beam apparatus wherein positive ions are detected using a quadrupole mass spectrometer whereas unstable negative ions are observed as resonance structures in the electron transmission function. Theoretical calculations of anion and cation energies have been carried out using the semi‐empirical HAM/3 technique.
Quantitative variability in the 252Cf plasma desorption mass spectra of triglycerides and waxes
Authors: Showell John S. , Fales Henry M. , Sokoloski Edward A.
Published: May 2005

A series of mixtures of solid triglycerides and waxes are examined using 252Cf plasma desorption mass spectrometry. The relative abundances of diagnostic ions are found to vary with sample preparation, handling, and composition. It is proposed that the less efficiently packing, shorter chain, compounds are forced to the analysis site, i.e. the surface. This takes place at varying rates that reflect the bulk properties of the mixture as revealed by its phase diagram and thermal history. Mixtures that remain liquids during analysis show ion abundances correctly reflecting their composition. Mixtures of isotopomeric glycerides also show normal ion abundances.
The fragmentation of ionized alkyl phenyl ethers
Authors: Blanchette Marcia C. , Holmes John L. , Lossing F. P.
Published: May 2005

The energetics of olefin loss from ionized alkyl phenyl ethers have been determined by ionization and appearance energy measurements. It is concluded that the reaction is governed by one or more of three features: (i) the strength of the bond between the phenoxy radical and the alkyl ion; (ii) the ease of isomerization of the alkyl ions, chiefly by H‐shifts therein, and (iii) the strength of the CH bond (primary, secondary and tertiary) involved in the H transfer to oxygen which precedes the olefin loss. The possible participation in the reaction of distonic ions and proton bound radical‐molecule pairs is also discussed.
Fused‐ring tropylium ion formation and fragmentation
Authors: Standley Laurel J. , Hites Ronald A.
Published: May 2005

Fused‐ring tropylium ions, formed in the electron impact mass spectra of methyl phenanthrenes and methyl anthra‐cenes were studied by mass spectrometric and computational methods. These ions form through a cyclopropyl intermediate. As a result of an equilibrium between the benzylium and tropylium ion configurations, partial carlton atom scrambling occurs in the fused‐ring structures. Fragmentation of these ions occurs only in the ring incorporating the methyl carbon. These conclusions are supported by MNDO calculations and by 13C‐labeled methyl‐phenanthrene and methylanthracene fragmentation studies.
Studies of phthalic acid and related compounds by positive‐ and negative‐ion chemical ionization tandem mass spectrometry (± CI/MS/MS)
Authors: Gillespie Todd A. , Urogdi Laszlo , Katritzky Alan R. , Yost Richard A.
Published: May 2005

The positive‐ and negative‐ion chemical ionization (± CI) behavior of phthalic acid and four related compounds (phthalic anhydride, phthalide, biphthalyl, and anthraquinone‐2‐carboxyIic acid) are characterized by tandem mass spectrometry with low‐energy collisionally activated dissociation. Principal fragmentation pathways have been established, and molecular and fragment ions arising from these compounds have been differentiated from adduct and dimer ions, as well as from ions due to impurities. The ubiquitous nature of these compounds in analytical samples makes it important to identify the corresponding ions, and thus to be able to exclude them from consideration in the identification of other sample components.
Fragmentation of enamides derived from 6,7‐dimethoxy‐4‐methyl‐2H‐1,3‐benzothiazin and 3,4‐dihydro‐6,7‐dimethoxy‐1‐methylixopuinoline
Authors: Wang Jian , Smith Richard W. , Fodor Lajos , Macleant David B.
Published: May 2005

The electron impact mass spectra of several enamides have been examined. The enamides were prepared by reaction of aroyl halides with 6,7‐dimethoxy‐4‐methyl‐1,3‐benzothiazin, and with 3,4‐dihydro‐6,7‐dimethoxy‐l‐methylisoquinoline. The fragmentation pathways that have been proposed are supported by ion composition determinations and by mass‐analysed ion kinetic energy spectrometry experiments carried out on the molecular ions and major fragment ions. The spectra are characterized by a loss of carbon monoxide from the molecular ion of each compound, a process which is accompanied by migration of the aryl group.
Ab initio and RRKM evidence that the McLafferty rearrangement of ionized n‐butanal is stepwise
Authors: Hudson Charles E. , Griffin Lawrence L. , McAdoo David J.
Published: May 2005

Hartree‐Fock calculations with geometry optimization at a series of C(2)C(3) distances predict a negligible reverse critical energy for the second step of the McLafferty rearrangement of ionized n‐butanal,

in contrast to a previous conclusion. The most favorable geometry for departure of the C2H4 has a dihedral angle of about 90° between the parting fragments. RRKM calculations were used to estimate rates of reactions associated with the McLafferty rearrangement These calculations indicate that the McLafferty rearrangement of the n‐butanal ion is stepwise. The RRKM calculations predict competition of H exchange with decomposition up to much higher energies than actually occurs.
Mass spectrometry of some new pyrimidine derivatives: Electron impact‐induced decomposition of 5‐alkyl‐5‐acylamino‐6‐imino‐1,2,5,6‐tetrahydro‐2(1H)‐thioxopyrimidin‐4(3H)‐ones
Authors: Plaziak Adam S. , Golankiewicz Bozenna , Zeidler Joanna , Golankiewicz Krzysztof
Published: May 2005

The mass fragmentation of some new 5‐substituted derivatives of 6‐imino‐1,2,5,6‐tetrahydro‐2(1H)‐thioxopyrimidin‐4(3H)‐one are reported. Electron impact‐induced decompositions occurred primarily on the substituents localized in the 5‐position of the series.
Energy‐dependent reversal of secondary isotope effects on simple cleavage reactions: Tertiary amine radical cations with deuterium at remote positions
Authors: Ingemann Steen , Hammerum Steen , Derrick Peter J. , Fokkens Roel H. , Nibbering Nico M. M.
Published: May 2005

Deuterium substitution at remote sites gives rise to inverse secondary kinetic isotope effects on the α‐cleavage of a number of tertiary amines in the ion source, but to normal isotope effects on reactions occurring in the field‐free regions. The change from normal to inverse secondary isotope effects when the internal energy of the reacting ions increases is consistent with transition‐state models that involve slight lowering of vibrational frequencies also for bonds well removed from the site of cleavage. The isotope effect on the reactions of ions of high internal energy is caused by the influence that even small changes of isotope‐dependent frequencies have on the state sums (a statistical weight effect favouring loss of the labelled radical), whereas the behaviour of low‐energy ions is determined by the zero‐point energy changes which favour loss of the unlabelled fragment.
Upon the existence of [Al3Cl10]− in room temperature chloroaluminate ionic liquids
Authors: Abdul‐Sada Ala'a K. , Greenway Anthony M. , Seddon Kenneth R. , Welton Thomas
Published: May 2005

A negative‐ion fast‐atom‐bombardment mass spectrometric study of the room temperature chloroaluminate ionic liquids AlCl3‐[MeEtim]Cl ([MeEtim]+ = 1‐methyl‐3‐ethylimidazolium cation) has produced the first unambiguous experimental evidence for the existence of [Al3C110]−.
Determination of hydroxyaromatic compounds in complex mixtures by tandem mass spectrometry
Authors: Stach Joachim , Zimmer Dieter , And Monika MÖDer , Herzschuh Rainer
Published: May 2005

Derivatization of alkylated hydroxyaromatics with N‐methyl‐bis(trifluoracetamide) is used for a rapid screening for alkylated hydroxyaromatic compounds in complex mixtures by tandem mass spectrometry. Applications are based on a detailed investigation of the fragmentation reactions of derivatized alkylated phenols, 2,3‐dihydroindenols, indenols, 1,2,3,4‐terrahydronaphtbols and naphthols. As shown by daughter‐ion mass spectra obtained in different field‐free regions of a BEQQ‐instrument, the loss of CF3COOH, CF3CO˙ or CF3COO˙, respectively, is common for the compounds studied and can be used for their detection by means of neutral mass loss scans.
Structures of [M–44]+ ions in the electron impact and fast atom bombardment mass spectra of the β‐blocker nipradilol with a nitrate ester group
Authors: Yoneda Michiaki , Tsujimoto Kazuo , Ohashi Mamoru , Shiratsuchi Masami , Ohkawa Yasushi
Published: May 2005

Both electron impact (EI) and fast atom bombardment (liquid secondary ion mass spectrometry) (FAB(LSIMS)) spectra of tbe antihypertensive drug nipradilol (3,4‐dihydro‐8‐(2‐hydroxy‐3‐isopropylamino)propoxy‐3‐nitroxy‐2 H‐l‐benzopyran) and it analogues exhibit intense [M ‐ 44]+ peaks. However, high‐resolution measurements and tandem mass spectrometric techniques clarified that the composition of the ion produced by EI is [M‐C2H4O]+, whereas that of the ion produced by FAB(LSIMS) is [M ‐ NO2 + 2H]+. FAB collision‐activated dissociation‐tandem mass spectrometry revealed that the structure of the latter ion corresponds to the MH+ of the hydrolysis product of the nitrate ester group.
Formation of [M − H]+ ions in electron impact mass spectra of nitrosocarbonyl compounds α‐ and γ‐nitrosocarbonyl compounds
Authors: Konopski Leszek
Published: May 2005

The presence of the [M + H]+ ions and the absence of the monomer molecular ions M+· in the mass spectra of some tertiary α‐ and γ‐nitrosocarbonyl compounds is reported. This effect is caused by the rearrangement of the mobile hydrogen in the α‐carbonyl position in the fragmentation pattern of the dimer molecular ions 2M+·.
Measurement of onset potentials for proton transfer reactions with a triple quadrupole instrument
Authors: Bursey Maurice M. , Blackburn Thomas R. , Meyerhoffer William J. , Mattson William A.
Published: May 2005

Problems with the treatment of data from the study of both exothermic and endothermic ion‐molecule reactions are discussed. Recommendations are (i) to extrapolate data from an obviously linear region in the plot of ratios of secondary to primary ion intensity which appears ∼1–3 V above the value of the onset potential; (ii) to use ionization methods that give low internal energies to primary ions; (iii) to set for moderately high resolution; (iv) to check the stability of the primary ion current frequently; (v) to pay special attention heed to source and collision region pressure stability; and (vi) to keep collision region pressures below 0.6 mPa.
Mass spectrometry of bisquinolizidine alkaloids: Multiflorine and its derivatives
Authors: Wyrzykiewicz E. , Wysocka W.
Published: May 2005

The mass spectral fragmentations of multiflorine, 13α‐hydroxymultiflorine, 5‐dehydromultiflorine, 5‐dehydro‐13α‐hydroxymultiflorine, 5‐dehydro‐11,12‐secomultiflorine and 11,12‐seco‐12‐didehydromultiflorine were investigated. Fragmentation pathways, the elucidation of which was assisted by accurate mass measurements and metastable transitions, are proposed. These results create a reliable basis for distinguishing the above‐mentioned from isomeric (previously reported) quinolizidine alkaloids.
Unimolecular depurination of protonated deoxyribonucleosides in the gas phase: A fast atom bombardment tandem mass spectrometric investigation
Authors: Liguori Angelo , Greco Francesco , Sindona Giovanni , Uccella Nicola
Published: May 2005

The spontaneous or collision‐induced unimolecular dissociations of base‐protonated 2′‐deoxyribonucleosides afford, in the gas phase and in the absence of solvent, high yields of protonated nucleobases. The release of neutral bases corresponds to a high‐energy reaction path and does not compete with the formation of the protonated bases. A determinant solvent effect has to be invoked to explain the opposite behaviour, which is exhibited by the same precursors, dissociating unimolecularly, in aqueous acidic solution.
Mass spectrometric characteristics of a novel series of ring‐c monoaromatic tricyclic terpanes found in Tasmanian tasmanite
Authors: de Almeida Azevedo Débora , de Aquino Neto Francisco Radler , Simoneit Bernd Rolf Tatsuo
Published: May 2005

The mass spectra of a series of ring‐C monoaromatic tricyclic terpanes from a Tasmanian tasmanite sediment were measured. Their fragments are interpreted to include the aromatic ring‐C and the side‐chain, shifting by 14 u from one homologue to the next. Suggestions about the fragmentation pattern are made and compared with those for tricyclic terpanes with similar structures.
Interaction of remote functional groups (amide and amine) in steroidal compounds after electron ionization
Authors: Longevialle Pierre , Bouchoux Guy , Hoppilliard Yannik
Published: May 2005

Electron impact ionization of steroidal compounds with two diferently substituted functional groups (amide and amine) at remote positions leads to the interaction of these groups in ion‐neutral complexes after the detachment of one of them. Two situations may be encountered, depending on which group is more easily detached from low‐energy parent ions: (i) when the formation of the immonium ion is preferred, a very efficient proton transfer to the amide is observed leading to abundant [M− imine]+· and subsequent daughter ions; (ii) when the detachment of the amide is preferred, hydrogen exchanges occur with the amine group, and fragment ions may be observed resulting from the addition of both groups in a proton‐bound structure.
Fragmentation reactions of some deprotonated acetophenones
Authors: Chowdhury Swapan K. , Harrison Alex. G.
Published: May 2005

The collision‐induced dissociation reactions of the enolate ions, [XC6H4COCD2]−, derived from labelled acetophe‐none and the fluoro‐. chloro‐ and nitro‐substituted acetophenones, were studied as a function of collision energy over the range 5‐100 eV on the laboratory scale, enabling breakdown graphs to be established. For the acetophenone and fluoroacetophenone enolates the major primary fragmentation reaction forms [XC6H4]− + D2CCO, although the formation of XC6H4D + [ DCCO]− is favoured thermocbemically. This result implies that there is a significant energy barrier for D+ abstraction in the [XC6H4]−−D2CCO ion‐dipole complex formed initially on collisional activation of the enolate. The ortho‐ and meta‐fluorophenyl anions fragment further by elimination of HF. By contrast, chlorophenyl anions show no loss of HCl but may fragment to form Cl−. In addition, Cl− is formed from the chloroacetophenone enolate ions at low collision energies, possibly by a displacement reaction of [DCCO]− on the chlorobenzene before the ClC6H4D−[DCCO]− ion‐dipole complex separates. The lowest energy fragmentation channel for the nitro‐substituted enolate ions is elimination of NO. At higher collision energies the nitrophenyl anion is formed and itself fragments to form NO2−; this reaction appears to be particularly facile for the ortho isomer. In a number of cases exchange of hydrogen between the ring and the enolic position is observed, particularly at low collision energies. A pronounced effect of collision gas identity on the breakdown graph for acetophenone enolate is reported and possible origins are discussed.
Mass spectra of liquid crystals. IV—diphenylethyne derivatives (nematic tolans)
Authors: Leclercq P. A. , van den Bogaert H. M.
Published: May 2005

The 70 eV electron impact mass spectra of 19 dipheaylethyne derivatives are presented and discussed in terms of observed substituent effects.
Photoelectron spectroscopy of the oxygen dimer and clusters
Authors: Carnovale Frank , Peel J. Barrie , Rothwell Richard G.
Published: May 2005

The He I photoelectron spectra of gas‐phase oxygen dimer and oxygen clusters have been measured in a pulsed beam. The spectrum of the dimer (O2)2 is characterized by broad bands which show 1:1 correspondence with those of the monomer O2. The vertical ionization energies are 0.2 ± 0.1 eV lower than those of the monomer. The photoelectron bands observed for mixtures of small clusters are identical to the dimer bands except for further shifts of up to ∼0.3 eV to lower ionization energies. The clusters spectra show bandwidths and bandshapes which are effectively the same as those measured for thin films of condensed O2, indicating that the oxygen dimer (O2)2 is the ionization chromophore in each case. This supports the dimer‐ion hypothesis proposed for the ionization of van der Waals clusters of closed‐shell molecules. Ab initio calculations on (O2)2+ show good agreement with the experimental data, and suggest that the ground dimer‐ion electronic state is of rectangular rather than trans‐planar geometry.
Theoretical study of the structure and unimolecular decomposition pathways of ethyloxonium, [CH3CH2OH2]+
Authors: Swanton David J. , Marsden David C. J. , Radom Leo
Published: May 2005

Ab initio molecular orbital calculations with split‐valence plus polarization basis sets and incorporating valence‐electron correlation have been performed to determine the equilibrium structure of ethyloxonium ([CH3CH2OH2]+) and examine its modes of unimolecular dissociation. An asymmetric structure (1) is predicted to be the most stable form of ethyloxonium, but a second conformational isomer of Cs symmetry lies only 1.4 kJ mol−1 higher in energy than 1. Four unimolecular decomposition pathways for 1 have been examined involving loss of H2, CH4, H2O or C2H4. The most stable fragmentation products, lying 65 kJ mol−1 above 1, are associated with the H2 elimination reaction. However, large barriers of 257 and 223 kJ mol−1 have to be surmounted for H2 and CH4 loss, respectively. On the other hand, elimination of either C2H4 or H2O from ethyloxonium can proceed without a barrier to the reverse associations and, with total endothermicities of 130 and 160 kJ mol−1, respectively, these reactions are expected to dominate at lower energies. A second important equilibrium structure on the surface is a hydrogen‐bridged complex, lying 53 kJ mol−1 above 1. This complex is involved in the C2H4 elimination reaction, acts as an intermediate in the proton‐transfer reaction connecting [C2H5]+ +H2O and C2H4 + [H3O]+ and plays an important role in the isotopic scrambling that has been observed experimentally in the elimination of either H2O or C2H4 from ethyloxonium. The proton affinity of ethanol was calculated as 799 kJ mol−1, in close agreement with the experimental value of 794 kJ mol−1.
Elimination of carbon monoxide by electron impact on quinoline N‐oxide, carbostyril and 8‐hydroxyquinoline
Authors: Blumenthal Thomas , Gillis Richard G. , Porter Quentin N. , Yeoh Li Li
Published: May 2005

Under electron impact, the molecular ions of quinoline N‐oxide, carbostyril and 8‐hydroxyquinoline lose carbon monoxide giving a fragment ion C8H7N (m/z 117), which was shown by collision‐activated dissociation in each case to have the structure of the molecular ion of indole. Its formation from 8‐hydroxyquinoline requires an unusual rearrangement. Isoquinoline N‐oxide loses HCN rather than CO and gives a fragment which has the structure of the molecular ion of benzofuran. When the first three compounds were subjected to flash vacuum pyrolysis, quinoline N‐oxide at 500–700°C gave carbostyril and indole was detected by gas chromatography/mass Spectrometry. At 900°C carbostyril and 8‐hydroxyquinoline both gave indole in small amounts, detected by gas chromatography/mass Spectrometry.
Collision‐Induced dissociations of deprotonated peptides. Dipeptides containing aspartic or glutamic acids
Authors: Waugh Russell J. , Bowie John H. , Hayes Roger N.
Published: May 2005

Deprotonated dipeptides, on collisional activation, fragment by the characteristic process NH2CH(R1) CONHCH(R2)CO2− → NH2−C(R1)CONHCH(R2)CO2H → −NHCH(R2)CO2H + NH2C(R1)CO, when R1 and R2 = H or alkyl. However, when one of the constituent amino acids is either aspartic acid or glutamic acid, the standard cleavage becomes minor in comparison with fragmentation through the α‐side‐chain of Asp or Glu. For example, [Asp‐Leu ‐ H]− and [Leu‐Asp ‐ H]− both fragment principally by loss of water, a fragmentation not normally noted for peptides. In addition, [Leu‐Asp ‐ H]− loses CO2 and also forms HO2CCHCHCO2−˙. These fragmentations establish that Asp is the C‐terminal amino acid. In contrast, isomeric Glu dipeptides, e.g. [Glu‐Ala ‐ H]− and [Ala‐Glu ‐ H]− undergo similar fragmentation, both competitively losing H2O and CO2. Both spectra also contain a product ion at m/z 128, identified as the pyroglutamate anion. Product ion and deuterium‐labelling studies have been used in an attempt to elucidate the complex fragmentation mechanisms in these systems.
Studies in organic mass spectrometry. Part 9—mass spectra of 9,10‐disubstituted 2,3,6,7‐tetraalkoxy‐9,10‐dihydroanthracenes: A remarkable loss of radicals from even‐electron ions
Authors: Ceraulo Leopoldo , Agozzino Pasquale , Ferrugia Mirella , Lamartina Liliana , Natoli Maria Concetta
Published: May 2005

The electron impact‐induced fragmentations of 23 9,10‐disubstituted 2,3,6,7‐tetraalkoxy‐9,10‐dihydroanthracenes, including six stereoisomeric pairs, were studied. The loss of the 9‐(or 10‐)substituent constitutes the main fragmentation route. The resulting ions give rise to a remarkable violation of the so‐called ‘even‐electron rule’ as they eject the second substituent at the 10‐(or 9‐)position as a radical. The latter aspect was particularly investigated with the aid of low‐energy (nominal value 15 eV) and mass‐analysed ion kinetic energy spectra.
Determination of enzyme kinetic parameters of cyclic CMP‐specific phosphodiesterase by quantitative fast atom bombardment tandem mass spectrometry
Authors: Newton Russell P. , Khan Jalal A. , Ghosh Dipankar , Langridge James I. , Brenton A. Gareth , Harris Frank M. , Walton Terence J.
Published: May 2005

The determination of cytidine 3′,5′‐cyclic monophosphate‐specific phosphodiesterase activity by means of fast‐atom bombardment (FAB) mass Spectrometry with mass‐analysed ion kinetic energy (MIKE) spectrum scanning is described. Initial efforts to determine the activity of the enzyme by this method were unsuccessful owing to the obfuscation of sample‐related peaks by peaks emanating from the incubation buffer and cation adducts; dilution of buffer and a desalting procedure overcame these difficulties. In the resulting positive‐ion FAB mass spectra, characteristic peaks of the enzyme substrate and product could be readily identified and the protonated molecular ions selected for MIKE scanning. By spiking enzyme incubates with known amounts of substrate and product, and measuring peak heights in the MIKE spectra of both spiked and unspiked samples, the substrate/product ratio at the end of a series of phosphodiesterase incubations was determined. From the data obtained, the Km and Vmax of the phosphodiesterase were calculated as 6.08 mM and 11 μmol min−1 mg−1, respectively, showing good agreement with the analogous values of 8.06 mM and 5.8 μmol−1 min−1 mg−1 obtained by radioactive assay.
Resonance electron‐capture mass spectra of azobenezene and its monosubstituted derivatives
Authors: Vasil'ev Yury V. , Mazunov Viktor A. , Nazirov Eduard R.
Published: May 2005

The negative‐ion mass spectra of azobenezene and its derivatives were obtained using the resonance electron‐capture technique. The molecular negative ion peaks (M−·) were found to be dominant for almost all of these compounds. Different mechanisms of formation of molecular ions in two energy regions are proposed and the pathways of fragmentation are discussed. It is shown that fragment ions are formed predominantly by direct bond breaking except for those which derive from long‐lived resonance states of M−·.
Electron impact mass spectrometry of phenyl thiophosphoroorganic amides
Authors: Moldovan Zaharie , Culea Monica , Palibroda Nicolae , Fenesan Ion , Popescu Rodica
Published: May 2005

The interpretation of the electron impact mass spectra of methyl phenyl phosphinomorpholinylamidothioate, O‐methyl phenyl phosphonomorpholinylamidothioate, phenyl phosphonomorpholinylamidochloridothioate and O‐ethyl phenyl phosphonomorpholinyhimidothioate is presented. Elucidation of the fragmentation pathways was aided by exact mass measurements and observation of peaks due to metastable transitions detected for the first and last of the above compounds.
Hydrogen‐carbon, carbon‐carbon double rearrangement induced by proximity effects. 1–formation of methoxybenzyl ions in the electron impact mass spectra of substituted 1,1‐bis(dimethoxyphenyl)methanes
Authors: Ceraulo Leopoldo , Natoli Maria Concetta , Agozzino Pasquale , Ferrugia Mirella , Lamrtina Liliana
Published: May 2005

The 75 eV electron impact mass spectra of 1,1‐bis(dimethoxyphenyl)methanes bearing o‐methoxy groups are dominated by intense peaks corresponding, at least formally, to benzyl ions [(CH3O)2C6H3CH2]+ (b). They arise from ions [((CH3O)2C6H3)2CH]+ (a), which are in turn formed from molecular ions by loss of an alkyl radical through benzylic cleavage. The analysis of compounds labelled with 2H or 13C at methoxy groups led to the determination of the mechanism. Hydrogen migration, as hydride, followed by electrophilic substitution by the methylene carbon of the phenyl methylene ether cation through a six‐centred transition state is responsible for the formation of benzylic ions b.
Destabilized carbenium ions: α‐imidoyl carbenium ions and the electron impact mass spectra of α‐haloaldimines and α‐haloketimines
Authors: Sürig Thomas , Grützmacher Hans‐Friedrich , de Kimpe Norbert
Published: May 2005

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.
Partial methanolysis and fast atom bombardment mass spectrometry of peptides containing sulphurated ammo acids
Authors: Foti Salvatore , Saletti Rosaria , Marietta Donata
Published: May 2005

Sequence determination by partial methanolysis and fast atom bombardment (FAB) mass spectrometry of peptides containing cysteine and methionine was investigated. Cysteine‐containing peptides require methylation of the sulphydryl group by methyl iodide to give a stable S‐methylcysteinyl residue prior to partial methanolysis and mass spectrometry. Methionine‐containing peptides undergo partially a methylation on sulphur during methanolysis, with formation of an S‐methylsulphonium ion which under FAB conditions is extracted from the matrix and eliminates methyl sulphide in the gas phase. The presence of additional peaks due to chemical modifications or gas‐phase fragmentations, however, does not interfere with the sequence information of the spectra.
Chemical reactions in fast atom bombardment mass spectrometry
Authors: Vékey Károly , Zerilli Luigi F.
Published: May 2005

Examples of various chemical reactions occurring in the matrix or in the selvedge region in fast atom bombardment (FAB) mass spectrometry are discussed. These are categorized as oxidations and reductions; substitutions; clusterings and additions; and sample decomposition or transformation. Some reactions observed showed significant time behaviour and in one case it was possible to determine rate constants. These data suggest that chemical reactions can be accelerated significantly by fast atom bombardment.
Collision‐activated dissociation mass spectra of crown ethers, perfluorinated crown ethers and related macrocycles
Authors: Maleknia Simin , Liou Chien‐Chung , Brodbelt Jennifer
Published: May 2005

The collision‐activated dissociation (CAD) mass spectra for a series of crown ethers, perfluoro crown ethers, cryptands and several dicyclohexano substituted crown ethers are reported. The CAD spectra were acquired with a triple quadrupole mass spectrometer, and in some cases spectra were recorded as a function of collision energy. In general, the protonated crown ethers dissociate via a series of losses of C2H4O units. The perfluoro crown ethers dissociate predominantly via losses of C2F4O units. The dicyclohexano ethers fragment in analogous ways in conjunction with cleavage of the cyclohexano rings. CAD spectra are also reported for acyclic ether systems.
Energy disposal in gas‐phase nucleophilic displacement reactions
Authors: Vanorden Steven L. , Pope R. Marshall , Buckner Steven W.
Published: May 2005

The partitioning of reaction exothermicity into relative translational energy of the products of gas‐phase SN2 (F− + CH3Cl) and nucleophilic aromatic substitution (F− + C6H5Cl) reactions has been investigated using kinetic energy release Fourier transform ion cyclotron resonance spectroscopy. The chloride product ion is observed to be highly translationally excited for the SN2 reaction, indicating a cold internal energy distribution for the products. For the chlorobenzene reaction the products are not generated with large translational energies. The results are compared with a statistical model. Ion‐intensity profiles for the CH3Cl reaction deviate significantly from the statistical model whereas the chlorobenzene results are consistent with this model. The kinetic energy release for the CH3C1 reaction is compared with energy‐disposal results for the photodissociation and dissociative electron‐attachment processes of halomethanes. In all three cases a node in the molecular orbital between the carbon atom and the departing halogen results in a repulsive energy release. Ion‐retention curves for the nucleophilic aromatic substitution reaction are consistent with the existence of a long‐lived ion‐dipole complex on the exit channel for this reaction.
Loss of CO from the molecular ions of o‐, m‐ and p‐anisoyl fluorides, CH3OC6H4COF, with fluorine atom migration
Authors: Tajima Susumu , Tobita Seiji , Mitani Motohiro , Akuzawa Kazuko , Sawada Hideo , Nakayama Masaharu
Published: May 2005

Loss of CO from the molecular ions ([CH3OC6H4COF]+˙) of o‐, m‐ and p‐anisoyl fluorides has been investigated by mass‐analysed ion kinetic energy (MIKE) spectrometry. This reaction involves fluorine atom migration from the carbonyl group to the benzene ring.

In the cases of o‐ and p‐anisoyl fluorides, the fluorine atom migrates via a three‐membered transition state to form the molecular ions ([CH3OC6H4F]+˙) of o‐ and p‐fluoroanisoles, respectively. On the other hand, in the case of m‐anisoyl fluoride, the fluorine atom migrates from the carbonyl group to the benzene ring via a three‐ or four‐membered transition state.
Alkane loss from collisionally activated alkylmethyleneimmonium ions
Authors: Veifth Hans J. , Gross Jürgen H.
Published: May 2005

The collision‐induced dissociation (CID) spectra of five alkylmethyleneimmonium ions (H2C‐N+R1R2, (a) R1 = R2 = C2H5, (b) R1 = n‐C3H7, R2 = H, (c) R1 = n‐C3H7, R2 = CH3, (d) R1 = n‐C3H7, R2 = C2H5, (e) R1 = R2 = n‐C3H7) are reported and discussed in terms of the mechanism of alkane loss. The most abundant alkane losses result from 2‐azaallylic bond cleavages within R1 and R2 leading to daughter ions of m/z 84. Ion d (R1 = n‐C3H7, R2 = C2H5) was chosen for a deuterium‐labelling study because it exhibited methane loss nearly free from interferences with other fragmentations. The methane lost consists to a great extent (95%) of the methyl moiety of R2. Whereas the methyl moiety obviously stays intact during the fragmentation process, the hydrogen additionally needed originates from all positions of R1 and the double‐bonded methylene in an approximately random distribution, suggesting extensive hydrogen migrations preceding the transfer step.
Selective isotopic exchange of polyfimctional ions in tandem mass spectrometry: Methodology, applications and mechanism
Authors: Ranasinghe Asoka , Cooks R. Graham , Sethi Satinder K.
Published: May 2005

Isotopic exchange of mass‐selected odd‐ and even‐electron molecular ions of aromatic compounds upon collision with deuterated gases was investigated as a function of reagent gas, interaction time and collision energy. Use of ND3 as reagent allows exchange of all active hydrogens for the compound types studied, providing a count of the total number of active hydrogens present in the analyte. CH3OD exchanges specific types of active hydrogens, such as phenolic and carboxylic hydrogens, without exchanging amino hydrogens. This selectivity assists in the identification and enumeration of different types of active hydrogens present in polyfunctional compounds. The H–D exchange patterns serve to differentiate isomeric aromatic compounds containing methoxy, amino, hydroxy and carboxylic acid substituents. Trapping of mass‐selected ions in the collision region of a triple quadrupole mass spectrometer greatly enhances the degree of H–D exchange, thereby facilitating determination of the number of active hydrogens in the analyte. Triple stage mass spectrometric experiments, performed in a pentaquadrupole mass spectrometer, help elucidate the exchange process. Isotopic exchange in the collision region of a tandem mass spectrometer also provides insights into the site of protonation in molecules containing several functional groups. The proximity of the functional groups and the proton affinity difference between the analyte and the reagent gas are important factors in site‐specific H–D exchange in polyfunctional compounds. An investigation of the effects of collision energy reveals that cluster ion formation plays a major role in the exchange mechanism operating in the triple quadrupole and that H–D exchange, ion‐molecule adduct formation and endothermic fragmentation are competitive reaction channels.
Electron impact mass spectra of some acridones substituted with hydroxyl, methoxyl, alkyl and alkenyl groups
Authors: Lin Shaw‐Tao , Wu Tian‐Shung , Furukawa Hiroshi
Published: May 2005

Forty‐four acridones were examined by mass Spectrometry. The fragmentations were studied, and included cleavage of the carbonoxygen bond of methoxyl groups, cleavage of the carbon–carbon bonds of γ,γ‐dimethylpyranyl groups, cleavage of isoprenyl groups and the formation of doubly charged molecular ions and daughter ions. The tendency for fission of carbonoxygen bond in the methoxyl group for acridones is about 300 : 200 : 8 : 1 for the C(2), C(4), C(5) and C(6) positions, respectively. The isoprenyl group is very sensitive to its position in fragmentation, and mass Spectrometry can be a powerful aid in distinguishing between C(2)‐ and C(4)‐isoprenyl isomers.
Electron impact mass spectrometry of ethyl esters of halogenated acetic acids
Authors: Catinella Silvia , Favretto Donata , Traldi Pietro , Bravo Pierfrancesco , Viani Fiorenza
Published: May 2005

The behaviour of a series of differently halogenated acetic acid ethyl esters was studied using electron impact and metastable ion techniques. The presence of different halogens strongly influences the fragmentation pathways and also the kinetic energy release values associated with some decomposition routes. By mass‐analysed ion kinetic energy spectrometry, a composite metastable peak was detected for the transition CF2BrCO+ → CF2Br+, and rationalized on the basis of two different structures for the precursor ion.
Boundary effects and collisional activation in a quadrupole ion trap
Authors: Paradisi Cristina , Todd J. F. J. , Traldi Pietro , Vettori Umberto
Published: May 2005

The effects of storing ions at different values of the stability parameters az and qzwere studied in a quadrupole ion trap, using helium or argon as buffer. A region was localized near the boundaries of the stability diagram in which the ions experience an increase in their kinetic energy. This is reflected in the occurrence both of fragmentation due to collisional activation and of a certain extent of ion loss due to unstable trajectories. The results of this excitation, referred to as ‘boundary effects,’ depend on the specific qzat which the ion storage is performed and on the buffer gas used, and point to a simpler means of obtaining tandem mass spectra with the ion trap without the need to apply resonant tickle voltages between the end ‐cap electrodes.
Differentiation of anomeric C‐glycosides by mass spectrometry using fast atom bombardment, mass‐analysed ion kinetic energy and collision‐activated dissociation
Authors: Brakta Mohamed , Chaguir Brahim , Sinou Denis , Banoub Joseph , Becchi Michel
Published: May 2005

Positive‐ion fast atom bombardment mass spectrometry appears to be a useful method for the differentiation of anomeric C‐glycosides. The mass‐analysed ion kinetic energy (MIKE) and collision‐activated dissociation (CAD) MIKE spectra of selected positive ions can be used as fingerprints of the α‐ or β‐anomers. The main fragmentation routes and particularly the formation of the [M − H]+ ion and the [M + H − PhCH2OH]+ ion were traced for each anomer.
Bimodal internal energy distribution as a consequence of Cul‐de‐sac isomerization reactions
Authors: Hammerum Steen , Vulpius Tore , Audier Henri‐Edouard
Published: May 2005

A simple model is developed to examine the internal energy distribution for metastable ions and the probability of reaction in the field‐free regions of sector mass spectrometers in systems where the reactant ions in the ion source react by dissociation in competition with reversible isomerization to non‐dissociating species. The internal energy distribution for the reacting ions is strongly dependent on whether dissociation is preceded by isomerization and back‐isomerization, and it is shown that a range of reasonable combinations of rate constants for the forward and reverse isomerization result in a bimodal distribution of the internal energy for the ions that dissociate in the field‐free regions. The isomerization effectively provides a kinetic reservoir of higher energy reactant ions. The reaction probability of the metastable ions varies strongly with internal energy, and the back‐isomerized ions may in some instances be responsible for most of the reactions observed. A bimodal energy distribution will give rise to composite metastable peaks even though neither non‐statistical behavior nor multiple reaction channels are involved.
Competing oxygen transfers from nitro group to sulphur and allenic double bonds in allenyl o‐nitrophenyl sulphides on electron impact
Authors: Ramana D. V. , Ramakrishna N. V. S.
Published: May 2005

Interesting oxygen transfers from the o‐nitro group to allenic carbons were observed in allenyl o‐nitrophenyl sulphides under electron impact conditions giving rise to abundant fragment ions at m/z 151, 138 and 136. However, a similar oxygen transfer to the sulphur leads to another intense ion at m/z 145. The proposed fragmentation pathways and the ion structures were established through high‐resolution data, collision‐activated dissociation mass‐analysed ion kinetic energy and B/E linked‐scan spectra.
Effect of structure on the electron‐capture negative ionization mass spectrometric response of steroids
Authors: Mayer Helen K. , Reusch William , Watson J. Throck
Published: May 2005

The key to enhanced electron‐capture negative ion mass spectrometry (ECNI‐MS) response of ketosteroids is extensive α,β‐unsaturation. Combinations of double bonds, carbonyl groups, epoxides and halogens within a steroid nucleus were correlated with their ECNI responses. The greatest ECNI responses result from extensive conjugation, such as the linearly conjugated 4‐ene‐3,6‐dione system and the cross‐conjugated 1,4‐diene‐3,11‐dione system. In general, an epoxy ketosteroid has a relative response 40 times greater than that of its corresponding enone. In an α‐halo ketone, an axial halogen substitution will increase the relative response by an order of magnitude, but an equatorial halogen substituent will not affect the response. The fragmentation of ketosteroids as related to ECNI response is discussed.
Reactivity of electron impact ionized large‐ring cycloalkylamines. Loss of CnH2n + 1. Alkyl radicals from long‐chain aliphatic compounds with a terminal enamine or ester function (lipids)
Authors: Lefèvre O. , Mollova N. , Longevialle P.
Published: May 2005

Large‐ring Cycloalkylamines rearrange after electron impact ionization into long‐chain enamines. A general mechanism is proposed for their fragmentation, and a parallel is established with the fragmentation of long‐chain esters (lipids). It is shown that the parent ions rearrange into a series of interconverting branched onium ions from which alkyl radicals are lost. The mechanism of the interconversion is discussed.
Electron impact fragmentation of pyranocoumarin derivatives. Tandem mass spectrometric study of abundant singly and doubly charged fragment ions at high and low collision energy
Authors: Rubino F. M. , Mascaro P. , Palmisano G. , Appendino G. , Cravotto G. , Tagliapietra S. , Nano G. M.
Published: May 2005

A series of novel pyranocoumarin derivatives were analysed by electron impact mass spectrometry and tandem mass spectrometry at low aud high collision energy. The collision‐induced decomposition tandem mass spectra of the [M  CH3]+ ions yielded several structurally valuable fragments and those of the [M  CH3]2+ ions allowed some charge‐separation reactions to be recognized.
Chemical ionization and electron impact mass spectra of symmetrical S‐alkyl alkanesulfonothioates (thiosulfonates)
Authors: Freeman Fillmore , Keindl Monica C.
Published: May 2005

The chemical ionization (CI) and electron impact (EI) mass spectra of eight symmetrical S‐alkyl alkanesulfonothioates (thiosulfonates) are reported.