bluuebunny/crossref_metadata_2025_split huggingface dataset:
Conversion of acetamido‐ and nitro‐substituted ortho‐azidonitro derivatives of 2,3,4,5‐tetrahydrobenzo [b][1,4]dioxocin to the corresponding furoxans and furazans in the gas phase
Authors: Takakis Ioannis M. , Hadjimihalakis Phaedon M. , Gross Michael L. , Hayes Roger N. , Haas George W. 
Published: May 2005

AbstractElectron impact ionization of the five isomeric 2,3,4,5‐tetrahydro‐9‐azido‐7,8‐dinitro‐, 2,3,4,5‐tetrahydro‐8‐azido‐7,9‐dinitro‐, 2,3,4,5‐tetrahydro‐8‐azido‐7,10‐dinitro‐, 2,3,4,5‐tetrahydro‐7‐azido‐8,9‐dinitro‐, 2,3,4,5‐tetrahydro‐7‐azido‐8,10‐dinitro‐ and the related 2,3,4,5‐tetrahydro‐7‐acetamido‐8‐azido‐9‐nitro‐, 2,3,4,5‐tetrahydro‐7‐acetamido‐9‐azido‐8‐nitro‐, 2,3,4,5‐tetrahydro‐9‐acetamido‐7‐azido‐8‐nitro‐, 2,3,4,5‐tetrahydro‐10‐acetamido‐7‐azido‐8‐nitrobenzo[b] [1,4]dioxocin derivatives furnished, after elimination of nitrogen, the corresponding nitro and acetamido dioxocino‐annelated benzofuroxans. Further loss of oxygen from the latter afforded the corresponding benzofurazans. It was shown in two cases that these processes occur primarily upon electron impact ionization, without excluding some small fraction undergoing a thermal degradation process. The proposed fragmentation patterns are supported by high‐resolution and mass‐analyzed ion kinetic energy spectroscopic data. Similar work on the unsubstituted 6,7‐dihydro[1,4]dioxino[2,3‐f]‐ and 7,8‐dihydro‐6H‐[1,4]dioxepino[2,3‐f]‐2,1,3‐benzoxa‐diazole 1‐oxide reveal that loss of oxygen from the molecular ion to furnish the corresponding benzofurazans is the result of electron impact ionization (at least in part).
Comparison between the mass spectrometric behaviour and condensed‐phase reactivity of products of addition of phthalimidesulphenyl chloride to aryl acetylenes
Authors: Capozzi G. , Menichetti S. , Nativi C. , Seraglia R. , Traldi P. 
Published: May 2005

AbstractThe electron impact‐induced mass spectrometric behaviour of six products of the addition of phthalimidesulphenyl chloride to aryl acetylenes was studied with the aid of daughter ion spectroscopy. The electron impact‐induced decomposition processes parallel those observed in the condensed phase. In particular for the diaryl adducts the formation of 3‐chlorobenzothiophene derivatives results in a highly favoured process under the two sets of operating conditions.
Identification of short‐chain α‐hydroxyaldehydes produced by lipid peroxidation in bovine liver
Authors: Loidl Angelika , Spiteller Gerhard 
Published: May 2005

AbstractA method to detect short‐chain α‐hydroxyaldehydes in mammalian tissues is described. Aldehydes or their masked precursor molecules are converted into dithiolane derivatives. These are separated by thin‐layer chromatography and registered by multiple ion monitoring after trimethylsilylation. Fragmentation of 2‐(1′‐trimethylsilyloxy)‐1,3‐dithiolanes was studied by using electron impact ionization.
Electron‐impact‐induced Ortho effects in (O‐carboxyphenyl)diphenylphosphine
Authors: Srinivas R. , Rao G. K. Viswanadha , Ravinder V. 
Published: May 2005

AbstractProximity effects of the carboxyl group with the phosphorus atom in the molecular ion of (o‐carboxyphenyl)diphenylphosphine (1) leads to the loss of ĊHO to form the base peak at m/z 277, which is totally absent in the (m‐ and p‐carboxyphenyl)diphenylphosphines. The mechanism of the loss of ĊHO and the fragmentation of 1 was investigated in detail using metastable ion, collision‐induced dissociation and high‐resolution studies.
Reactivity of vinylamine radical cation with neutral olefins: An example of ‘ionic’ free‐radical chemistry in the gas phase
Authors: Bouchoux Guy , Penaud‐Berruyer Florence 
Published: May 2005

AbstractHydrogen atom abstraction and cycloaddition–cycloreversion reactions between ionized vinylamine and neutral olefins were studied in a Fourier transform ion cyclotron resonance spectrometer. The efficiencies of both processes are appreciably less than unity. The trend in reaction efficiencies is consistent with the presence of a substantial intrinsic barrier but interpretation based on constraints inherent to the approach of the reactants cannot be discarded. The regioselectivity observed during the cycloaddition–cycloreversion reactions closely parallels the nucleophilic attack of an olefin by a free radical. The thermochemistry of these reactions and partial potential energy surfaces were examined with the help of experimental data and molecular orbital calculations (MP2/6–31G*//3–21G).
Photoelectron spectra of the isomers of methoxypyridine
Authors: Dunne Simon J. , Summers Lindsay A. , von Nagy‐Felsobuki Ellak I. 
Published: May 2005

AbstractThe He I photoelectron spectra of 2‐, 3‐ and 4‐methoxypyridine are reported and interpreted in terms of a composite‐molecule model and ΔSCF/MP ionization energies. For pyridine, this ab initio method correctly locates the nitrogen lone‐pair molecular orbital with respect to experiment and shows quantitatively the importance of incorporating both correlation and relaxation effects. Comparison of experimental and ab initio ionization energies of the methoxypyridines with the semi‐empirical HAM/3 method further substantiates the utility of HAM/3 for the interpretation of photoelectron spectra.
Multiphoton excitation of ferrocene and vanadocene at 351 nm in comparison with 248 and 193 nm. Wavelength dependent competition between ionization and dissociation
Authors: Opitz Joachim , Bruch Detlev , von Bünau Günther 
Published: May 2005

AbstractMultiphoton ionization of ferrocene and vanadocene upon nanosecond laser excitation at 351 nm is reported in comparison with results at 248 and 193 nm. A similar behaviour was found for both compounds at each wavelength. The excitation and fragmentation mechanism was elucidated by the determination of the number of absorbed photons for the ion formation and the analysis of the metastable ion decay. On biphotonic excitation at 248 nm extensive fragmentation of the neutral parent molecule to metal atoms is observed, followed by the ionization of these metal atoms after absorption of two further photons. In contrast, at 193 nm the molecular ion is formed after absorption of two photons and dominates the fragmentation pattern up to moderate laser intensities. At high laser intensities the metal ion, formed after resonant absorption of three photons, becomes prominent. At 351 nm the exclusive formation of molecular ions is observed only near the threshold of the laser intensity for ion detection. Metal ion formation prevails at higher laser intensities involving the absorption of five photons. Most likely this absorption mechanism is resonant.
Mass spectrometric study of some 2,3‐diaryl‐1,3‐thiazolidin‐4‐ones under electron impact
Authors: Woolston C. R. J. , Lee J. B. , Swinbourne F. J. 
Published: May 2005

AbstractThe mass spectral fragmentations of some 2,3‐diaryl‐1,3‐thiazolidin‐4‐ones were established by comparing spectra and by using exact mass measurements. The major fragment ions were identified and their relative importances related to substituent effects and bond strengths. Good correlations were observed between the relative abundances of some of the breakdown products and the Hammett σ constants of substituents in the aromatic rings.
Novel aryloxy migrations in substituted diphenyldithiocarbonates under electron impact conditions
Authors: Ramana D. V. , Mahalakshmi P. , George M. 
Published: May 2005

AbstractUnusual expulsion of ‘SO’ was observed from the molecular ions of substituted diphenyldithiocarbonates under electron impact conditions. An initial aryloxy migration to sulphur followed by further rearrangement is proposed for this process, based on the substituent effects. The diarylthioketone radical–cation structure assigned for the [M – SO] ion was confirmed through the collision‐activated dissociation B/E linked‐scan spectra.
Organic cluster ions from continuous‐flow fast atom bombardment
Authors: Jarvis V. M. , Villanueva M. A. , Bostwick D. E. , Moran T. F. 
Published: May 2005

AbstractCluster ions from fast atom bombardment of liquid alcohols and nitriles were examined using a continuous‐flow technique. Protonated molecular MnH+ species are the dominant cluster ions observed in molecules of formula M. The abundances of the MnH+ cluster ions decrease monotonically with increasing n, and within a homologous series the MnH+ abundance diminishes more rapidly for higher molecular mass compounds. Reaction products (ROH)n(H2O)H+ and (ROH)n(ROR)H+ are observed also in the case of alcohols, and the ion abundances decrease with increasing n. Radiation damage yields fragment ions and ionic alkyl reaction products which are captured in solvent clusters. Semi‐empirical molecular orbital methods were used to examine the energetics of cluster ion formation and decomposition pathways. Metastable decomposition processes exhibit only evaporative loss of monomers, with the probability of loss increasing sharply with n. The evaporative ensemble model of Klots was used to predict the cluster size‐dependent trends of metastable dissociation processes observed for alcohol and nitrile cluster ions.
Substitution reactions in the secondary ion mass spectrometry of N‐methylpyridinium halides
Authors: Yoneda Michiaki , Tanizaki Ken , Tsujimoto Kazuo , Ohashi Mamoru 
Published: May 2005

AbstractSecondary ion mass spectra of N‐methylpyridinium halides (C+X−, where C+ is a pyridinium cation and X− is a halogen anion) exhibit the C+ ions, a series of cluster ions ((C+)n(X−)n–1) and, furthermore, remarkable [CX – R]+ ions (R = H or Me). The mechanism of the formation of [CX – R]+ ions was investigated by the use of deuterated compounds and B/E and B2/E constant linked‐scan measurements. A possible explanation is proposed in which the ions are produced through substitution reactions between species constituting the C2X+ cluster ions in the gas phase.
Mechanism of production of ions in electrospray mass spectrometry
Authors: Ashton D. S. , Beddell C. R. , Cooper D. J. , Green B. N. , Oliver R. W. A. 
Published: May 2005

AbstractA mechanism for the production of multiply charged molecular ion species in electrospray mass spectrometry (ES‐MS) is still required. A concise discussion of a recently published ionic solution equilibrium model offering a partial mechanism is presented. That publication proposed that the ion abundance–charge profile could be fitted by one or a series of superimposed Gaussian functions, in accord with a solution equilibrium model. It is shown that indeed a simulated mass spectrum based on a solution odel can compare well with the observed spectrum. However, some new and recently published experimental evidence is presented which shows that the ES mass spectra of many proteins give rise to multiply charged molecular ions which carry higher charges than those calculated by the model. Further, the ion abundance–charge profile is very sensitive to some experimental parameters, e.g. cone voltage; it does not necessarily reflect the solution or gaseous ion populations in the mass spectrometer source. Therefore, the concept that gaseous multi‐charged ions originate from equivalently charged solvated ions in electrically neutral solution must be treated with caution.
Negative‐ion fragmentation pathways in 2,4,6‐trinitrotoluene
Authors: Langford M. L. , Todd J. F. J. 
Published: May 2005

AbstractThe mass spectrum of 2,4,6‐trinitrotoluene vapour in air was recorded and the negative‐ion fragmentation pathways of the molecular anion were investigated down to the fourth generation. The dissociation of the parent anion led to the production of three fragment anions with masses 46, 197 and 210 u where the last two daughter ions initiated two entirely separate fragmentation pathways. These pathways were studied and the fragmentation mechanisms identified. The background negative‐ion mass spectrum, generated by the glow discharge ion source in the airstream which carries the sample vapour into the source, was examined in detail and an assignment of this mass spectrum is included.
Comparative investigations of the secondary ion emission of metal complexes under MeV and keV ion bombardment
Authors: Feld Herbert , Rading Derk , Leute Angelika , Benninghoven Alfred 
Published: May 2005

AbstractA series of ionic and neutral Group VIII transition metal complexes with molecular masses up to 2500 u were analysed by time‐of‐flight secondary ion mass spectrometry (SIMS) and plasma desorption mass spectrometry (PDMS). The secondary ion emission, the secondary ion yields and the yield ratios Y(PDMS)/Y(SIMS) of 20 ionic and neutral metal complexes were determined. Both techniques generally provide both molecular and fragment ion information. Characteristic fragmentation patterns give useful data for structural characterization. Additionally, the stabilities of different secondary ion species were compared by their half‐lives. Both PDMS and SIMS are very sensitive, yielding optimum spectra from total sample sizes as low as 5 nmol, and the sample consumption is negligible.
Isomerization and isotope effects in sterically congested cluster ions
Authors: Szulejko J. E. , McMahon T. B. 
Published: May 2005

AbstractThe association reactions of methylated acetone with acetone and dimethyl ether were studied over an extended temperature range (30–250°C) by high‐pressure mass spectrometry. In addition, the association reactions of various deuterium‐substituted analogues of both the reactant ion and neutral molecules were investigated. Two distinct isomeric forms are apparent from the examination of the behaviour of the association equilibrium constant as a function of reaction temperature: (i) a low‐temperature, covalently bound isomer characterized by larger −ΔH° and −ΔS° values and significant equilibrium isotope effects on the entropy of association; and (ii) a high‐temperature, electrostatically bound isomer of lower −ΔH° and −ΔS° values and with insignificant equilibrium isotope effects. The equilibrium isotope effects and larger −ΔS° values are more pronounced for the methylated acetone–dimethyl ether adduct than for the less sterically congested methylated acetone‐acetone adduct. Molecular models of the low‐temperature isomers show severe restriction or complete loss of internal rotations and is the origin of the observed equilibrium isotope effects. No such phenomenon is expected or observed in the more loosely bound high‐temperature, electrostatic isomers. Potential energy surfaces for the transfer of methylcation from methylated acetone to a second acetone molecule and to dimethyl ether are proposed.
Dependence of benzyl alcohol loss onC‐terminal amino acid in tandem mass spectrometry ofN‐protected tripeptides
Authors: Schwartz Brenda L. , Erickson Bruce W. , Bursey Maurice M. , Marbury G. Dean 
Published: May 2005

AbstractThe effect of the basicity and chain length of theC‐terminal amino acid on the fragmentation ofN‐protected tripeptides was investigated with a hybrid tandem instrument. Positive‐ion unimolecular decomposition and colisionally activated decomposition studies on the [M + H]+ions of a series ofN‐benzyloxycarbonyl (Cbz or Z)‐protected tripeptides, Cbz–Gly–Leu–Xxx–OMe and Cbz–Gly–Pro–Xxx–OMe, where Xxx = Arg, Lys, Orn, Gln and Glu, indicate that substitution of leucine with proline at the second position in the tripeptides facilitates the loss of benzyl alcohol (108 u) from the precursor. Additionally, higher gas‐phase basicity and longer chain length of theC‐terminal side‐chain group promote the formation of the [M + H – 108]+ion.
Sequence‐ion studies in peptides: The generation of C″ ions—part 2
Authors: van Dongen W. D. , de Koster C. G. , Heerma W. , Haverkamp J. 
Published: May 2005

AbstractThe mechanism for the formation of Cx″‐type ions in which x ≤ n – 2, from protonated peptides containing n amino acids, produced under conditions of fast atom bombardment and collisional activation, was investigated. Comparison of the tandem mass spectra of the [M + H]+ ions of two model peptides (VGSE and AAA) with those of their corresponding [Md + D]+ ions, in which all exchangeable hydrogens are replaced with deuterium, revealed that in the case of VGSE the hydrogen atom from the hydroxyl group in the serine residue and/or from the amide group of the adjacent amino acid must be involved in the migration process which leads to the formation of C2″‐type ions. The second peptide (AAA) shows the exclusive transfer of the amide hydrogen atom in the formation of the C1″ ion. The mechanism differs from an earlier reported reaction channel which appeared to be applicable to the formation of C‐type ions. The most plausible neutral species which is eliminated in the formation process of the Cx″ ion from [M + H]+ is proposed on the basis of relative stability calculations.
Leucine and isoleucine in chemical ionization and plasma desorption mass spectrometry: A comparative study
Authors: Bouchoux Guy , Bourcier Sophie , Hoppilliard Yannik , Mauriac Christine 
Published: May 2005

AbstractThe NH3 and CH4 chemical ionization mass spectra of leucine and isoleucine are compared with their plasma desorption mass spectra. Mass‐analysed ion kinetic energy (MIKE) and collision‐induced dissociation (CID)/MIKE experiments are also presented and the mechanisms of the formation and dissociation of immonium ions [M – CO2H2]+ are discussed. A comparison between the various ionization techniques reveals that plasma desorption produces pseudo‐molecular ions whose internal energy lies between zero and 300 kJ mol−1.
Reaction of protonated acetaldehyde with methanol in the gas phase
Authors: König S. , Kofel P. , Reinhard H. , Sägesser M. , Schlunegger U. P. 
Published: May 2005

AbstractThe ion–molecule reactions between protonated acetaldehyde and methanol were explored on a quadrupole, quistor, quadrupole tandem mass spectrometer. In addition to proton transfer to methanol and subsequent methanol ionmolecule chemistry, the protonated acetaldehyde–methanol adduct and methylated acetaldehyde could be detected. Experiments with labelled counterparts showed that the formation of methylated acetaldehyde corresponds to substitution of the oxygen‐bound hydrogen of protonated acetaldehyde by the methyl group of methanol. The reaction rates involved and H–D exchange rates were determined.
Mass spectrometric studies of selected coumarins and psoralens. I—negative ions
Authors: Tkaczyk Martin , Zobel Alicja M. , Plomley Jeffry B. , March Raymond E. 
Published: May 2005

AbstractAn investigation of the negative‐ion chemistry of 13 coumarin and three psoralen compounds is reported. Negative‐ion chemical ionization was effected using O−˙ and OH− as the reagent ions; it was found that O−˙ leads preferably to [M – H + O]− whereas OH− mainly furnishes [M – H]−. The results of metastable anion fragmentation and collision‐induced dissociation of mass‐selected anions are presented.
Gas‐phase unimolecular chemistry of ethyl butyl ketone cations
Authors: Bouchoux Guy , Flammang Robert , Jaudon Pascale , Lefevre Odile 
Published: May 2005

AbstractMass spectra of the four isomeric ethylbutyl ketones, C2H5CO‐n‐C4H9 (1), C2H5CO‐iso‐C4H9 (2), C2H5CO‐sec‐C4H9 (3) and C2H5 CO‐t‐C4H9 (4), are reported. The structure of the fragment ions produced from precursors of both high and low internal energy were identified by collision experiments in a six‐sector mass spectrometer. Metastable molecular ions 1+˙ and 2+˙ behave identically in expelling CH3˙, C3H6 and C2H5˙ to produce the same fragment ions: [C2H5C(OH)CHCHCH3]+, [C2H5C(OH)CH2]+˙ and a mixture of [CHOHCHCHC2H5]+ and [CHOHCHC(CH3)2]+, respectively. The key isomerization step between 1+˙ and 2+˙ is a 1,2‐enol–olefin shift. Metastable molecular ions 3+˙ and 4+˙ give essentially ethene and ethyl losses, respectively; the fragment ions are [C2H5C(OH)CHCH3]+˙ and [t‐C4H9CO]+. Keto–enol isomerization also occurs to a significant extent for ions 1+˙ and 2+˙ and 3+˙. In contrast, 1,2‐ or 1,3‐ethylhydroxycarbene migrations are only marginal processes.
Structures and stabilities of C3H4O+˙ isomers: A G2 theoretical study
Authors: McKee Michael L. , Radom Leo 
Published: May 2005

AbstractSix stable isomers andtwo transition structures were characterized on the C3H4O+˙ potential energy surface using the G2 procedure. Heat capacity corrections were made to allow the direct calculation of heats of formation at 298 K. The most stable isomer is the methylketene radical cation (1, ΔHf 298 = 797.0 kJ mol−1), followed by a distonic ion \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^ \cdot {\rm H}_2 {\rm CH}_2 \mathop {\rm C}\limits^ +=\!={\rm O} $\end{document} (2), only 46.5 kJ Mol−1 higher in energy (ΔHf 298 = 843.5 kJ mol−1). Ionized cyclopropanone (7) represents a transition structure for exchange of terminal methylene groups in the distonic ion with a corresponding barrier of 57.6 kJ mol−1. There is amuch larger barrier (106.7 kJ mol−1) for the 1,2‐hydrogen migration in the distonic ion (2) to form the mor estabe ionized methylketene (I), while the fragmentation of 2 to C2H4+˙ + CO is highlyendothermic (by 113.1 kJ mol−1). The predicted ease of formation of the distonic ion on ionization of neutral cyclopropanone (7n) and the higher barrier for further reaction would suggest that ionized cyclopropanone should show reactivity characteristic of a radical as observed for ionized cyclobutanone. Three additional C3H4O+˙ isomers, hydroxyallene radical cation (3), acrolein radical cation (4) and oxetene radical cation (5), lie within 15 kJ mol−1 of one another in energy, while a cyclic form (6) of the distonic ion is much less stable. G2 heats of formation and adiabatic ionization energies were also calculated for four of the neutral C3H4O analogs. WHile the ionization energies are in good agreement with available experimental values, as are the heats of formation for acrolein (4n) and cyclopropanone (7n), the G2 heat of formation of methylketene (−65.1 ± 10 kJ mol−1) is almost 40 kJ mol−1 higher than the ‘standard’ value, which it is considered shoul dbe revised. Calculations iwth the G2(MP2) method, which is less computationally intensive than G2, yields of formation for the neutral and ionic species within 24 kJ mol−1 of the G2 values.
New [C2, H3, N, O]+˙ isomers: A metastable and collision‐induced fragmentation study
Authors: Hop Cornelis E. C. A. , Snyder David F. 
Published: May 2005

AbstractMetastable and collision‐induced fragmentation in a masspsectrometer were used to characterize several [C2, H3, N, O]+˙ isomers. CO2 loss from ionized oxazolidine‐2,5‐dione yields the distonic ions H2CNHCO+˙ and HNCH2CO+˙ and not the cyclic isomer, . Kinetic energy relese measurement indicate that CO2 loss from the molecular ions is accompanied by a significant barrier for the reverse reaction. In this respect the behavior of oxazolidine‐2,5‐dione parallels that of the isoelectronic compound sucinic anhydride. O(H)CC(H)NH+˙ kions are generated via CO loss from 2(3H)‐oxazolone molecular ions. There is no evidence to support generation of the cyclic isomer, . For comparison, data wer obtained for the known isomers H3CNCO+˙, H3COCN+˙, H3CCNO+˙, HOCH2CH+˙ H2NCCOH+˙ H2N(H)CCO+˙ and H2CC(H)NO+˙. The H3COCN+˙ and H3CCNO+˙ structures were assigned to the m/z 57 fragment ions from methyl cyanoformate and acetohydroxamic acid, respectively.