bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractHartree‐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

AbstractThe 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

AbstractDeuterium 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

AbstractA 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

AbstractDerivatization 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

AbstractBoth 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

AbstractThe 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

AbstractProblems 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractElectron 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

AbstractThe 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.
Photoelectron spectroscopy of the oxygen dimer and clusters
Authors: Carnovale Frank , Peel J. Barrie , Rothwell Richard G. 
Published: May 2005

AbstractThe 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

AbstractAb 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

AbstractUnder 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

AbstractDeprotonated 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractSequence 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

AbstractExamples 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.