bluuebunny/crossref_metadata_2025_split huggingface dataset:
Stereoelectronic control of the retro diels–alder reaction in 8‐(N‐pyrrolidyl)bicyclo[4.3.0]nona‐3,7‐diene isomers
Authors: Tureček František , Pancíř Jiří , Stahl Daniel , Gäumann Tino 
Published: May 2005

AbstractThe cis‐ and trans‐annulated isomers of 8‐(N‐pyrrolidyl)bicyclo[4.3.0]nona‐3,7‐diene show different propensities for the retro Diels–Alder fragmentation following electron impact ionization. Molecular ions of the cis‐annulated isomer decompose predominantly via the retro Diels–Alder reaction to give [C9H13N] +· fragments of the appearance energy (AE)=8.45±0.05eV and critical energy Ec=133±8kJ mol−1. The trans‐annulated isomer gives abundant [M–H]+ (AE=9.34±0.08eV) and [M–C6H6]+· fragments, in addition to [C9H13N]+· ions of AE=8.98±0.05eV and Ec=181±8kJ mol−1. The ionization energies (IE) were determined as IEcis=7.07±0.05 eV and IEtrans=7.10±0.06eV. The stereochemical information is much less pronounced in unimolecular decompositions of long‐lived (metastable) molecular ions which show very similar fragmentation patterns for both geometrical isomers. Nevertheless, the isomers exhibit different kinetic energy release values in the retro Diels–Alder fragmentation; T0.5=3.8±0.3 and 4.8±0.2 kJ mol−1 for the cis and trans isomer respectively. Topological molecular orbital calculations indicate that the retro Diels–Alder reaction prefers a two‐step path, with a subsequent cleavage of the C(5)C(6) and C(1)C(2) bonds. The open‐ring distonic intermediate represents the absolute minimum on the reaction energy hypersurface. The cleavage of the C(1)C(2) bond is the rate‐determining step in the decomposition of the cis isomer, with the critical energy calculated as 137 kJ mol−1. The cleavage of the C(5)C(6) bond becomes the rate‐determining step in the trans‐annulated isomer because of stereoelectronic control. The difference in the energy barriers to this cleavage in the isomers (ΔE=95k Jmol−1) provides a quantitative estimate of the magnitude of the stereoelectronic effect in cation radicals.
Mass spectra of bromohydrins of uracil derivatives
Authors: Dezor‐Mazur Maria , Golankiewicz Krzysztof 
Published: May 2005

AbstractThe mass fragmentation of bromohydrins of uracil derivatives is discussed. Fragmentation pathways have been proposed on the basis of high‐resolution data and metastable transitions. The distinct influence of substituents in position 5 of uracil on the fragmentation pathway has been observed.
OH˙ loss and molecular rearrangements of quinoxalineN‐oxides studied by interpretation of mass spectra and application of linear discriminant analysis
Authors: Bartoszek M. , Salzwedel D. , Stumm G. , Niclas H.‐J. 
Published: May 2005

AbstractQuinoxalineN‐oxides substituted at theorthoposition to the NO group give characteristic [M – OH]+fragments. With the di‐N‐oxides the peak intensities depend on the electron‐withdrawing strength of the 2‐ and 3‐substituents. Linear discriminant analysis was used to study the fragmentation of quinoxalineN‐oxides as determined by the number of NO groups. Results of peak selection and discriminant analysis (Fisher quotients and discriminant vector coefficients) were interpreted with regard to the mass spectrometric decomposition of quinoxaline and quinoxalineN‐oxide molecules. For the substituted quinoxaline TV‐oxides, fragmentations involving molecular rearrangements like those observed for unsubstituted quinoxalineN‐oxidles were also found. For these compounds, partial rearrangement to quinoxalinones is confirmed.
Distinguishing between N‐methylated imidazolinones using mass spectrometry
Authors: Cardaciotto Stanley J. , Mowery Patrick C. , Thomson Michael L. , Wayne Richard S. 
Published: May 2005

AbstractImidazolin‐4‐ones are a new class of herbicides which contain both an amide and amino nitrogen in a five‐membered ring. During the manufacturing process, it is possible to methylate either nitrogen. This presented an interesting analytical problem in distinguishing between the two N‐methylated derivatives of ASSERT® (a mixture of 6‐(4‐isopropyl‐4‐methyI‐5‐oxo‐2‐imidazolin‐2‐yl)‐ m‐ and p‐toluic acid methyl esters) at low levels in a complex mixture. After developing a high‐performance liquid chromatography method for separating and isolating components in the mixture, mass spectrometric techniques, which included electron impact (EI) and collisional activation decomposition (CAD), were used to distinguish between two isomeric N‐methylated derivatives. The data from the El and CAD experiments enabled amidomethyl and aminomethyl structures to be assigned. Accurate mass measurements and proton nuclear magnetic resonance spectroscopy were used to support the assignments.
Mass spectrometric fragmentation of the tautomers of 1,3‐diketones. A gas chromatographic/mass spectrometric study
Authors: Masur Monika , Grützmacher Hans‐Fr. , Münster Helmut , Budzikiewicz Herbert 
Published: May 2005

AbstractThe diketo and ketoenol tautomers of aliphatic 1,3‐diketones can be easily separated by gas chromatography. The mass spectra of tautomers of pentane‐2,4‐diones substituted at C(l) and C(3), separated in this way, have been obtained. The fragmentation mechanisms are discussed. The mass spectra of the tautomers are quite different, and the main fragmentation pathways can be easily linked to the structures of the (non‐interconverting!) tautomeric molecular ions. Furthermore, isomers differing by the position of the substituent can also be identified by their mass spectra.
Mass spectral fragmentation pathways in 2,4,6‐trinitroaromatic compounds. A tandem mass spectrometric collision induced dissociation study
Authors: Yinon Jehuda 
Published: May 2005

AbstractA collision induced dissociation study of a series of 2,4,6‐trinitroaromatic compounds was carried out using a tandem BB mass spectrometer. Fragmentation pathways of the compounds were determined in the electron impact mode. It was found that dominant fragmentation pathways include loss of OH˙ due to an ortho effect and loss of NO˙ due to the formation of resonance‐stabilized products.
Stereocontrolled regiospecificity of the water loss from trans‐sobrerol radical cation upon electron ionization
Authors: Selva Antonio , Ferrario Francesco , Ventura Paolo , Pellegata Renato 
Published: May 2005

AbstractWater loss from frans‐sobrerol upon electron impact ionization selectively involves the tertiary OH group, predominantly occurring by a stereocontrolled H‐transfer from C(5) position in a rate‐determining step process, as proved by 18O and deuterium labelling. Monomethyl ethers behave accordingly. Ionic structures of the water‐loss product or products are investigated by metastable ion and collision activation mass‐analysed ion kinetic energy spectroscopy, using model ions generated from some substrates, which are chemically related to fralts‐sobrerol in condensed phase, i.e. α‐pinane epoxide, cis‐sobrerol and pinol. A substantial conversion of cis‐sobrerol molecular ions to ionized pinol by loss of water has been demonstrated.
A comparison of three experimental techniques for ion structure studies via collision‐induced reactions: The [C5H8]+˙ example
Authors: Curtis J. M. , Brenton A. G. , Bcynon J. H. , Boyd R. K. 
Published: May 2005

AbstractStructure differentiation between [C5H8]+˙ ions, formed by electron ionization of various precursors, has been used as a test case for comparison of three experimental techniques involving collision‐induced dissociation (CID). Low‐energy CID in an rf‐only quadrupole collision cell has been studied in the range 1–150 eV laboratory collision energy. These data have been compared with those obtained using mass‐analyzed ion kinetic energy spectroscopy at 8 keV energy, and with results from dissociative charge‐stripping (DCS) coupled with a second electron capture collision (EC) in order to remove intense interferences (DCS/EC). The greatest degree of structure differentiation was possible using the DCS/EC technique. The other two methods were comparable in this regard, although effects of pre‐collision internal energy was apparent for collision energies much below 30 eV. Day‐to‐day reproducibility of spectra was most difficult to obtain for the low‐energy CID technique. Of the [C5H8]+˙ ions thus tested, the isoprene molecular ion was clearly the best match to the fragment ion formed from limonene.
Organic reactions at surfaces: A study of carnitine by secondary ion mass spectrometry
Authors: Hand Owen W. , Hsu Bih‐Hsiung , Cooks R. Graham 
Published: May 2005

AbstractCarnitine inner salt, (CH3)3N+ CH2CH(OH)CH2COO−, and carnitine hydrochloride, (CH3)3N+CH2CH (OH)CH2COOH Cl−, in the solid state undergo ion‐beam‐induced intermolecular methyl transfer reactions as shown by (CH3)3N+ CH2CH(OH)CH2COOCH3 ions at m/z 176 in their positive ion spectra. In the case of carnitine HCl, the product ion is three times as abundant as the intact cation. For the inner salt however, the product is less than one‐tenth as abundant as [M + H] +. In both cases, the reaction can be precluded by dissolution of the sample, supporting an intermolecular mechanism. The negative ion spectra for these compounds contain no [M − CH3]− ions, suggesting that simple transmethylation does not occur. Rather it is proposed that the inner salt abstracts a methyl group from the intact carnitine cation to yield [M + CH3]+ and a neutral species, the driving force being a minimization of the total number of charges desorbed into the gas phase. Thermodynamic data favor this mechanism as do data for other carnitine salts. The reaction appears to be inhibited when one reactant is present in excess. This is the case for carnitine HNO3 and CH3SO3H, which tend to liberate the intact cation since the anions are large and polarizable. It is also the case for small, hard anions like fluoride, which appear to favor release of the inner salt, hence the cation at m/z 162 is of low abundance and the transmethylation product (m/z 176) is absent. The extent of the reaction is also dependent on the methods of preparation of the sample, and deposition of the salts from solution greatly reduces the extent of methyl transfer. [M − CH3]− is observed when glycerol is used as a matrix, possibly due to a matrix‐analyte methyl transfer reaction.
Destabilized carbenium ions: Secondary and tertiary α‐carbomethoxybenzyl cations
Authors: Wolf Rainer , Dommröse Anne‐Marie , Grützmacher Hans‐Fr. 
Published: May 2005

AbstractThe secondary α‐carbomethoxybenzyl cations a and the tertiary α‐carbomethoxybenzyl cations d have been generated by electron impact‐induced fragmentation from appropriately α‐substituted methyl phenylacetate and 2‐phenylpropionates 1–4. The ions a and d are further examples of destabilized carbenium ions with a push–pull substitution at the carbenium ion centre. The characteristic reaction of these ions is a rearrangement by a 1,2‐shift of the methoxy group concomitant to the elimination of CO. This rearrangement reaction is associated with a very large and non‐statistical kinetic energy release (a : T 50 = 570 meV; d : T 50 = 760 meV), which is attributed to tight transition states along the reaction coordinates corresponding to the three‐membered cyclic oxonium ions b and h, respectively. The tertiary ion d can be distinguished from its more stable isomers f and g by the mass‐analysed ion kinetic energy and collisional activation spectra. The investigation of specifically deuterated analogues of ions d and g reveals an isomerization of d to g via a species protonated at the phenyl group but no equilibration between d and g. This isomerization exhibits a large isotope effect for the hydrogen transfer, indicating similar energy barriers for the isomerization and for the CO elimination of d.
Studies in cluster ion formation. Part II. Concentration and matrix effects on the cluster ions of tetraalkylammonium halide salts
Authors: Tolun Emirgül , Todd John F. J. 
Published: May 2005

AbstractThe roles of the matrix and salt concentration on the formation of cluster ions during fast atom bombardment of tetraalkylammonium halide salts have been studied. The occurrence of anomalous ion intensity regions at certain cluster numbers (n) is found to depend strongly on the salt concentration in the matrix. In addition the time‐dependent nature of the spectra is examined and cluster ion formation is explained by postulating that equilibrium processes occur during bombardment of the salt solution.
Sites of protonation in phenothiazines: Methane and ammonia chemical ionization mass spectra
Authors: Flurer Rick A. , Busch Kenneth L. 
Published: May 2005

AbstractPositive ion methane and ammonia chemical ionization mass spectra for ten phenothiazine derivatives are reported. The fragmentations observed in the chemical ionization mass spectra are rationalized in terms of the location of the added proton. High‐resolution measurements are used to confirm empirical formulae of the ions in the mass spectra. Changes in the mass spectra with a change in the chemical ionization reagent gas from methane to ammonia are described. A comparison with positive ion secondary ion mass spectra of the same compounds show that the amount of fragmentation is higher in the secondary ion mass spectra, but the same types of ions are observed in spectra produced by both ionization methods.
Characterization of C8H10 alkylbenzenes by negative ion mass spectrometry
Authors: Harrison Alex G. , Tong H. Y. 
Published: May 2005

AbstractThe O−˙ chemical ionization mass spectrri of the C8H10 alkylbenzenes, o‐, m‐. andp ‐xylene and ethylbenzene, show formation of [M − H + O]−, [M − H]−, [M − H2]−˙ and, for the xylenes, [M − CH3 + O]− as primary reaction products; the relative importance of these products depends on the isomer. However, [OH]− is a primary product from reaction of O−˙ with both the C8H10 isomers and hydrogen‐containing impurities; [OH]− reacts further with the alkylbenzenes to produce [M − H]− with the result that the chemical ionization mass spectra depend on experimental conditions such as sample size and the presence of impurities. The collision‐induced charge inversion mass spectra of the [M − H + O]− and [M − H]− products allow only distinction of ethylbenzene from the xylenes. However, the collision‐induced charge inversion mass spectra of the [M − H2]−˙ ions show differences which allow identification of each isomer.
The elusive methene ylides CH2CIH, CH2FH and CH2OH2
Authors: Hop Cornelis E. C. A. , Bordas‐Nagy Joszef , Holmes John L. , Terlouw Johan K. 
Published: May 2005

AbstractThe possible existence of the ylides CH2CIH, CH25FH and CH2OH2 as stable neutral species in the gas phase has been investigated by the neutralization–reionization (NR) mass spectrometry of their radical cations using a double‐focusing mass spectrometer of reversed geometry. The experiments were, for the most part, performed under single‐collision conditions with Xe as the neutralization target gas and He and O2 as reionization agents. For each ylidion a peak was observed in their NR mass spectrum which indicated that the neutral ylide had apparently been produced. However for CH2FH+˙ and CH2OH2+˙ the m/z 34 and m/z 32 peaks, respectively, were attribut‐able to interferences from the natural isotopic abundance of ions of lower mass. For CH2CIH+˙, the NR recovery signal was found to arise from the presence of CH3CI+˙ as an impurity in the ylidion flux. This was proved by examination of the collisional activation mass spectra of the [C, H3, CI]+˙ ions produced in the NR mass spectra of the conventional ions and ylidions, an experiment performed using a triple‐sector mass spectrometer.
Further evidence for the stability of methylenechloronium ylide, H2CCIH. A neutralization‐reionization ion‐dissociation experiment
Authors: Wesdemiotis C. , Feng R. , Baldwin M. A. , McLafferty F. W. 
Published: May 2005

AbstractRecent theoretical and experimental studies are in conflict with our previous neutralization–reionization evidence for the surprising stability of simple ylides such as H2CCIH. Neutralization of [H2CCIH]+˙ ions from CICH2COOH with Hg followed by reionization of the resulting neutral molecules with O2 produces intense [CCIH3]+˙ parent ions. Collisionally activated dissociation of these mass‐selected ions shows that 47 ± 5% are the [H2CCIH]+˙ isomer. This demonstrates that stable H2CCIH molecules can be formed by neutralization, surviving intact for the lifetime of the experiment (10−6 s). Field ionization conditions effective in ionizing excited argon atoms do not produce appreciable ionization of the neutrals from [H2CCIH]+˙, suggesting that the stable H2CCIH neutrals are not Rydberg state molecules.
Comparison of thermospray and ion spray mass spectrometry in an atmospheric pressure ion source
Authors: Covey T. R. , Bruins A. P. , Henion J. D. 
Published: May 2005

AbstractIon spray is an approach to liquid chromatography, mass spectrometry which includes features common to the electrospray and ion evaporation interfaces. Thermospray is a liquid chromatographic/mass spectrometric technique which utilizes heat and electrolytes in the mobile phase to generate sample ions. In this paper the operation of these two techniques at atmospheric pressure are compared with respect to the effects of solvent composition and electrolyte ion concentration for the production of ions from compounds that are ionized in solution (safranin orange, acid black 1 and testosterone sulfate) and un‐ionized in solution (methyl red, adenosine and diethylstilbestrol). The results indicate that at atmospheric pressure ion spray produces ions by the ion evaporation mechanism while thermospray produces ions by both gas‐phase chemical ionization and ion evaporation processes.
Elemental composition from moment analyses of the low‐resolution isotope pattern
Authors: Blom Karl F. 
Published: May 2005

AbstractThe basic principles of an approach to mass spectral elemental analysis based on the moments of the low‐resolution isotopic pattern are presented. The moment analyses are faster and more general than conventional methods of isotopic pattern analysis. The procedure is applied to the analysis of compounds which exhibit ‘pure’ molecular ions and the analysis of compounds which produce significant fragment ions at masses overlapping the molecular ion. It appears as though reasonable selectivity can be achieved for low relative molecular mass compounds (relative molecular mass < 400 a) using standard instrumentation and data acquisition softy are. Preliminary studies of instrumental factors affecting the measurement of moments are discussed.
Collisionally activated mass spectra of [MH] ions of polyhydroxy and hydroxy ether compounds
Authors: Brumley William C. , Andrzejewski Denis , Sphon James A. 
Published: May 2005

AbstractCollision‐induced decomposition/mass‐analyzed ion kinetic energy or collisionally activated mass spectra of [M − H]− ions of polyhydroxy compounds and other alcohols and ethers are reported. The [M − H]− ion of each compound is produced under OH− negative ion chemical ionization mass spectrometric conditions. Characteristic fragmentations are observed that include production of [M − H − 2]−, [M − H − 18]− and [M − H − 32]− ions. Certain other fragment ions in the collisionally activated mass spectra make it possible to distinguish among structural isomers. In polyhydroxy compounds, fragmentation increases as the number of hydroxyl groups increase, and carbon‐carbon bond cleavage becomes favored.
The reactive states of ions: Antiaromatic seven‐membered N‐substituted‐1H‐azepines and isomeric aromatic substrates
Authors: Kulkarni P. S. , Mirajkar S. P. , Bambal R. B. , Ayyangar N. R. 
Published: May 2005

AbstractComparison of the electron impact and chemical ionization mass spectra of N‐substituted‐1H‐azepines and isomeric aromatic substrates reveals significant differences. The data indicate that the substituted azepincs examined are stable to electron impact at low temperatures and retain their antiaromatic 8π electron seven‐membered cyclic system prior to their fragmentation. The electron impact data are supported by low electron‐volt and metastable scan techniques. Electron impact or chemical ionization induced transformation of azepines to aromatic substrates similar to the thermal transformation of neutral analogues is not observed at low temperatures. Chemical ionization mass spectra of N‐ethoxycarbonyl‐1H‐azepines and N‐phenylcarbamates were complicated owing to thermal decomposition reactions at high temperatures.
Keto‐to‐enol isomerization in the molecular ion of dimethylmethylphosphonate
Authors: Holtzclaw James R. , Wyatt Jeffrey R. 
Published: May 2005

AbstractA mechanism for keto‐to‐enol isomerization in dimethylmethylphosphonate (DMMP) has been proposed based on deuterium‐labeling studies, a model compound and thermodynamic data. An electron ionization study of H/D exchange occurring in CD3‐labeled DMMP suggests that rapid keto‐to‐enol isomerization occurs in the ion source and a reaction mechanism based on sequential 1,4‐H migrations rather than by the direct 1,3‐H transfer or by sequential 1,2‐H migrations is proposed. The examination of the mass‐analyzed ion kinetic energy/collision‐induced dissociation spectrum of the methylphosphonic acid molecular ion suggests that keto‐to‐enol isomerization does not occur for this species and that 1,2‐ and 1,3‐H migrations are not favored. Available thermodynamic data were employed to construct a potential energy surface for keto‐to‐enol isomerization of the DMMP molecular ion. The thermodynamic data show that the energy barrier to isomerization is below the internal energy required for decomposition of the DMMP molecular ion. Additionally, the ΔHf° for the intermediate and enolic isbmers are shown to be significantly less than the ΔHf° for the keto form of the DMMP molecular ion.
Use of fast atom bombardment mass spectrometry for the characterization of nitroaromatic isomers
Authors: Balasanmugam Kesagapillai , Miller Jack M. 
Published: May 2005

AbstractPositive and negative ion fast atom bombardment (FAB) mass spectra of some monosubstituted nitroaromatic isomers are reported. Generally ions carresponding to [M + H]+ and M+ are observed in the positive ion FAB spectra; ions such as [M − H] − and M−˙ are observed in the negative ion FAB spectra. The use of FAB mass spectra to distinguish the isomers is discussed. Comparisons of FAB, chemical ionization and electron impact mass spectra of the same isomers (wherever possible) are reported. The structural information obtained in the negative ion FAB spectra complement those obtained in the positive ion FAB spectra.
Rearrangement and dissociative processes in the [C3H6O]+˙ potential energy surface. Methyl vinyl ether: An example of non‐ergodic behaviour?
Authors: Lifshitz C. 
Published: May 2005

AbstractThe unimolecular dissociation reaction of the methyl vinyl ether radical cation to acetyl cation plus methyl radical was studied by means of 13C labelling and tandem mass spectrometric techniques. The results are in good agreement, in all major aspects, with previous data for 13C‐labeled propene oxide. The data are discussed in terms of non‐ergodic behaviour. Isomerization of the methoxyethylidene radical cation intermediate to the acetone radical cation is ruled out on the basis of the labelling experiments and comparison with the hydroxymethene radical cation system.
The chemical ionization (CH4 and i‐C4H10) mass spectra of tetracyanoquinodimethane and tetracyanoethylene—the result of a complex system of competing reactions
Authors: Budzikiewicz H. , Poppe A. 
Published: May 2005

AbstractIt is shown that depending on the experimental parameters upon electron impact and chemical ionization (CI) radical addition with subsequent CI, and formation of dihydro products which can undergo subsequently the reaction mentioned above, contribute to varying extents to the CI mass spectra of tetracyanoquinodimethane and tetracyanoethylene. These results are of importance for systems where radical reactions under CI conditions are to be expected.