bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cis/trans stereochemical effects in the negative chemical ionization/OH− mass spectra of strained‐ring azabicycloalkanes using MIKE and CA/MIKE spectrometry
Authors: Tabet Jean‐Claude , Tondeur Yves , Troin Yves , Vallee‐Goyet Danielle , Gramain Jean‐Claude , Fraisse Daniel 
Published: May 2005

AbstractStereospecific decomposition reactions of isomeric (cis and trans) deprotonated molecules from azabicycloalkane derivatives as azetidinols generated under negative chemical ionization (NCI)/OH− have been examined using mass‐analysed ion kinetic energy (MIKE) and collisional activation (CA)/MIKE spectra. These measurements together with the ones obtained on specifically labelled compounds enabled us to determine the origin of the stereochemical effects. The results indicate that the hydroxylic proton constitutes the preferential (≃90%) site for the deprotonation process. Subsequent fragmentations of the deprotonated species observed in the second field‐free region of a reversed geometry instrument are affected by the stereochemistry of the hydroxylic group. The isomer with the hydroxyl group in the cis position relative to the hydrogen at the ring junction mainly loses H2O, while the trans isomer eliminates CH3˙, both processes occurring with high specificity. Labelling studies indicate that two major pathways exist for the elimination of H2O from the cis isomer and the loss of CH3˙ from the trans isomer. The course of the reaction is determined by the ability of the stereoisomers to transfer a proton during the first decomposition step. When the size of the lactam ring is increased from a five‐membered ring to a six‐ or seven‐membered ring, these stereochemical effects tend to become less pronounced.
Electron impact induced fragmentations of 2‐chlorovinyl methyl sulphides
Authors: Paradisi C. , Scorrano G. , Visentini A. , Traldi P. 
Published: May 2005

AbstractA series of 2‐chlorovinyl methyl sulphides, CH3SCR1CR2Cl (R1, R2 = H, Me, Et, t‐Bu, Ph; R1 = H, R2 = CH3) has been analysed under electron impact conditions. The most significant common fragmentations involve primary loss of Cl˙ and methyl radicals. Analysis of the phenyl‐substituted derivative suggests that loss of CH3˙ is a lower activation energy process than loss of Cl˙. The characteristic loss of CH3SCl from the same derivative has been examined with respect to the structure of the resulting fragment by means of suitable model compounds.
Decompositions of metastable [C6H12O]+ ions with the oxygen on the third carbon: Ring‐size preferences in [CnH2nO]+˙ ions
Authors: Griffin Lawrence L. , Holden Katherine , Hudson Charles E. , McAdoo David J. 
Published: May 2005

AbstractThe isomerizations preceding the metastable decompositions in the mass spectrometer of a number of [C6H12O]+˙ ions with the oxygen on the third carbon are characterized utilizing deuterium labeling. Hydrogens are transferred in these ions by three‐, five‐ and six‐membered ring rearrangements, with propensities determined by features of the individual reactions. Three‐membered ring hydrogen transfers between α and β‐carbons are preferred to all five‐membered ring hydrogen transfers. However, six‐membered ring hydrogen transfers take place to the apparent exclusion of three‐membered ring hydrogen transfers to enol carbons when the products are of comparable stability. The low‐energy [C6H12O]+˙ isomerizations characterized are predictable from the behavior of their lower homologs. It is concluded that the determinants of these reactions are the same as those of other highly reactive organic intermediates.
Reminiscences of the early days of mass spectrometry in the petroleum industry
Authors: Meyerson Seymour 
Published: May 2005

AbstractThis retrospect traces the development of commercial mass spectrometers for use in the petroleum industry, starting with the formation of Consolidated Engineering Corporation in 1937 and installation of their first instruments in 1942 and 1943. During the middle and late forties, just keeping the machines running placed major demands on the time, energy and ingenuity of mass spectrometrists. The fifties saw progressive exploration of the chemical basis of mass spectra, culminating in the Eighth Annual Meeting of ASTM Committee E‐14 on Mass Spectrometry in 1960. This meeting marked a turning point in the development of the field—its transformation from what was originally almost a cult to a widely accepted and recognized branch of physical organic chemistry.
Unsaturated ketenes: A study of their formation and rearrangement by tandem mass spectrometry and low‐temperature infrared spectroscopy
Authors: Maquestiau André , Pauwels Pascal , Flammang Robert , Lorenčak Primož , Wentrup Curt 
Published: May 2005

AbstractA combination of collisional activation mass spectrometry and low‐temperature infrared spectroscopy has been used to monitor the production and isomerization of vinyl‐ and methylene‐ketenes. Vinyl‐ and 2‐propenyl‐ketenes were produced by flash vacuum pyrolysis of β,γ‐unsaturated acid chlorides. Vinyl(carboxyl)ketenes, vinylketenes and methyleneketenes were obtained from Meldrum's acid derivatives (5‐alkylidene‐1,3‐dioxane‐4,6‐diones). The formation of methyleneketenes by pyrolysis of α, β‐unsaturated acid chlorides is only indicated in the mass spectrometry experiments. Carbonylcyclopropane was obtained by pyrolysis of cyclopropylcarbonyl chloride or the corresponding Meldrum's acid derivative. The methyleneketenes isomerize to vinylketenes in the gas phase, particularly under conditions involving long contact times. Carbonylcyclopropane thermally decarbonylates to allene, not methylacetylene. Molecular ions of vinylketenes are obtained via pyrolysis of either acid chlorides or Meldrum's acid derivatives. Molecular ions of alkylmethyleneketenes are obtained pure only by electron impact induced fragmentation of Meldrum's acid derivatives.
Negative ion atmospheric pressure ionization mass spectrometry: Ionization of aromatic hydrocarbons
Authors: Wickramanayake Palitha P. , Siu K. W. Michael , Berman Shier S. 
Published: May 2005

AbstractA corona discharge atmospheric pressure ionization source generates the reagent ions, OH− and O− ions in addition to better known O2− ions, when ambient air is used as the carrier. All three ions are gas‐phase bases that could form negative ions from organics via proton abstraction. Ionization of simple aromatic hydrocarbons by O2− is thermodynamically not feasible. Simple aromatic hydrocarbons are ionized only by O− and/or OH− to form [M  H]− ions. However, [M  H]− ions do not appear in the mass spectrum as they undergo stabilization via clustering with predominantly oxygen atoms.
Energy‐dependent fragmentation of the p‐ethyltoluene molecular ion
Authors: Nacson Sabatino , Harrison Alex G. 
Published: May 2005

AbstractThe p‐ethyltoluene molecular ion fragments by loss of either the methyl group attached to the ring or the β‐methyl of the ethyl group. Using specific isotopic labeling and charge exchange techniques the relative importance of the losses of the two methyl groups has been studied as a function of internal energy from metastable ions to ions of 7 eV internal energy. The ratio of loss of the β‐methyl to loss of the ring‐methyl increases from 3.4 for metastable ions to 6.7 for ions of 7 eV internal energy. This variation is interpreted in terms of an energy‐dependent competition between fragmentation and hydrogen migration in a dimethylcy‐cloheptatriene intermediate. In variable energy collision‐induced dissociation studies the ratio of loss of the β‐methyl to loss of the ring‐methyl decreases with increasing collision energy. It is speculated that this different behavior is related to the differences in preparation of the molecular ions. Those subjected to collisional activation are prepared initially as ground‐state ethyltoluene ions and are vibrationally excited upon collision and may fragment largely from the original structure. By contrast, in the charge exchange and electron impact experiments the molecular ions with sufficient energy to fragment are probably formed initially in electronically excited states and the rearrangement to the cycloheptatriene structure may be more facile in these excited states or during the internal conversion processes leading to vibrationally excited ground‐state ions.
Hydrogen radical/molecule reactions in the negative ion mass spectrometry of dicyano‐methane derivatives of 9‐fluorenone and benzophenone
Authors: Her Guor‐Rong , Dolnikowski Gregory G. , Watson J. Throck 
Published: May 2005

AbstractHydrogen radical/molecule reactions are postulated to rationalize the formation of major fragment ions in the methane negative and positive chemical ionization mass spectrometry of the dicyanomethane derivatives of 9‐fluorenone and benzophenone. Evidence for the formation of [M + H]− anions that are not represented by significant peaks in the mass spectra is provided by mass spectra of deuterium‐labeled analogs and by tandem mass spectrometry studies of the [M + 1]− anions of the analyte.
Evidence for the Amino–Claisen rearrangement occurring in the molecular ions of N‐allylaniline
Authors: Vandezonneville Alexandra , Flammang Robert , Maquestiau Andre , Kingston Eric , Beynon John H. 
Published: May 2005

AbstractOne of the most intense peaks in the mass spectrum of N‐allylaniline is at m/z 106 (97%). High resolution analysis and collision‐induced dissociation studies confirm that this peak contains mostly [C7H8N]+ ions having the anilinomethene structure, but also a small contribution is seen from [C8H10]+ ions which result from the loss of the elements of HCN from molecular ions, following an Amino–Claisen rearrangement. The occurrence of a thermal rearrangement in the sample molecules cannot, however, be completely ruled out. Studies on metastable molecular ions of N‐allylaniline and collision‐induced dissociation of the m/z 106 ions formed from these show that, in the case of molecular ions with energies closer to threshold, the rearrangement reaction competes much more effectively with the direct cleavage reaction.
Rationalization of scrambling processes among [C2H3O]+ ions
Authors: Nobes Ross H. , Radom Leo 
Published: May 2005

AbstractScrambling data for the three observed [C2H3O]+ isomers, namely [CH3CO]+ (a), [CH2COH]+ (b) and (c), are rationalized by using ab initio molecular orbital calculations. For ions a and c, processes leading to scrambling of the carbon atoms require substantially more energy than the threshold for decomposition to [CH3]+ + CO. Accordingly, little or no carbon scrambling is predicted nor is any observed in the metastable dissociation of a and c. The observed carbon scrambling in b prior to metastable dissociation to [CH3]+ + CO has previously been explained in terms of a mechanism involving the oxiranyl cation (c). However, this mechanism is shown to be unlikely because of the high energies involved. An alternative lower‐energy pathway involving the intermediacy of protonated oxirene (h) is proposed. Such a mechanism is fully compatible with the experimental data.
Inter‐ and intra‐annular proton exchange in gaseous benzylbenzenium ions (protonated diphenylmethane)
Authors: Kuck Dietmar , Bather Wolfgang 
Published: May 2005

AbstractTwo distinct proton exchange reactions occur in metastable gaseous benzylbenzenium ions, generated by isobutane chemical ionization of diphenylmethane and four deuterium‐labelled analogues. Whereas the proton ring‐walk at the benzenium moiety is fast giving rise to a completely random intraannular proton exchange, the interannular proton exchange is surprisingly slow and competes with the elimination of benzene. A kinetic isotope effect of kH/kD= 5 has been determined for the interannular proton transfer, and a particularly high energy barrier of 50–75 kJ mol−1 has been estimated. These observations are attributed to steric restrictions of the ring‐to‐ring proton transfer in benzylbenzenium ions and contrasted to the fast interannular proton exchange in the higher homologues.
Electron impact induced fragmentations of some 2,2‐disubstituted 1,3‐oxathiolanes
Authors: Vainiotalo Pirjo , Nevalainen Vesa 
Published: May 2005

AbstractThe electron impact induced fragmentations of nine 2,2‐disubstituted 1,3‐oxathiolanes have been studied by means of exact mass measurement and metastable ion analysis. The ring cleavage almost always takes place so that the C(2)S and C(5)O bonds are broken, leading to the most stable products. The nature of the substituents determines the primary fragmentations of molecular ions. Ring cleavage is important only if both substituents are alkyl groups or if the carbon attaching to the ring has an alkyl character. The loss of the substituent becomes the most favourable process if it is attached to the ring through the electron‐deficient carbon atom.
Doubly‐charged ethane ions: Solution to the dilemma of stability predicted by theory and instability observed in experiment
Authors: Shields G. C. , Moran T. F. 
Published: May 2005

AbstractPotential energy curves have been computed for [C2H6]2+ ions and the results used to interpret the conspicuous absence of these ions in 2E mass spectra and in charge‐stripping experiments. The energies and structures of geometry‐optimized ground‐state singlet and excited‐state triplet [C2H6]2+ ions have been determined along with energies for different decomposition barriers and dissociation asymptotes. Although singlet and triplet [C2H6]2+ ions can exist as stable entities, they possess low energy barriers to decomposition. Vertical Franck‐Condon transitions, involving electron impact ionization of ethane as well as charge‐stripping collisions of [C2H6]+ ions, produce [C2H6]2+ ions which promptly dissociate since they are formed with energies in excess of various decomposition barriers. Appearance energies computed for doubly‐charged ethane fragment ions are in accordance with experimental values.
Mass spectrometry of imidazole‐4(5)‐carboxaldehyde and some 1‐methyl and nitro derivatives
Authors: Cert Arturo , Delgado‐Cobos Pedro , Trujillo Pérez‐Lanzac Mariana 
Published: May 2005

AbstractThe mass spectra of imidazole‐4(5)‐carboxaldehyde, its two 1‐methyl derivatives, 4(5)‐nitroimidazole, 5(4)‐nitroimidazole‐4(5)‐carboxaldehyde and 1‐methyl‐5‐nitroimidazole‐4‐carboxaldehyde are presented and discussed in comparison with those of other imidazole‐carboxaldehydes and nitroimidazoles earlier reported. The imidazole‐carboxaldehydes and their 1‐methyl derivatives exhibit the characteristic fragmentation of aromatic aldehydes, and differences between the isomers can be observed. The nitroimidazoles show the fragmentation typical of aromatic nitrocompounds. In the o‐nitroimidazole‐carboxaldehydes, the typical losses of aldehydes do not occur, but primary ortho effects between the formyl and nitro groups give rise to important fragmentation routes. In their 1‐methyl derivatives, the presence of the methyl group adjacent to the nitro group originates additional double and secondary ortho effects. For some of these transformations, fragmentation mechanisms are proposed.
Gas phase ion chemistry of methyl acetate, methyl propanoate and their enolic tautomers. An experimental approach
Authors: Burgers Peter C. , Holmes John I. , Hop Cornelis E. C. A. , Terlouw Johan K. 
Published: May 2005

AbstractFrom a combination of isotopic substitution, time‐resolved measurements and sequential collision experiments, it was proposed that whereas ionized methyl acetate prior to fragmentation rearranges largely into \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 \mathop {\rm C}\limits^ + ({\rm OH}){\rm O}\mathop {\rm C}\limits^{\rm .} {\rm H}_2 $\end{document}, in contrast, methyl propanoate molecular ions isomerize into \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^. {\rm H}_2 {\rm CH}_2 \mathop {\rm C}\limits^ + ({\rm OH}){\rm OCH}_3 $\end{document}. Metastably fragmenting methyl acetate molecular ions are known predominantly to form H2ĊOH together with \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 - \mathop {\rm C}\limits^ + = {\rm O} $\end{document}, whereas ionized methyl propanoate largely yields H3CO˙ together with \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 {\rm CH}_2 - \mathop {\rm C}\limits^ + = {\rm O} $\end{document}. The observations were explained in terms of the participation of different distonic molecular ions. The enol form of ionized methyl acetate generates substantially more H3CO˙ in admixture with H2ĊOH than the keto tautomer. This is ascribed to the rearrangement of the enol ion to the keto form being partially rate determining, which results in a wider range of internal energies among metastably fragmenting enol ions. Extensive ab initio calculations at a high level of theory would be required to establish detailed reaction mechanisms.
Double collision experiments
Authors: Villeneuve Sylvie , Burgers Peter C. 
Published: May 2005

AbstractA variety of double collision experiments, whereby fast species undergo collisional interactions in two distinct regions of a mass spectrometer, are described. These include two‐stage charge reversal of negative ions, two‐stage double electron transfer from targets to cations, neutralization‐reionization experiments as well as delayed analysis of organic cations formed in a one‐step charge reversal of anions. Experiments have been performed on a number of systems of current interest in gas‐phase ion chemistry. It is concluded that autoelectron detachment of benzyl anions leads to benzyl radicals, whereas the collisionally induced electron detachment produces a mixture of benzyl and tropyl radicals. By contrast, electron detachment from [H3CNH]− is not a metastable process and occurs only after excitation to produce H3CNH˙ radicals, which do not rearrange into the thermodynamically more stable H2CNH2˙. It is shown that in the double electron transfer reactions H+ + Xe→H˙ + Xe+˙ and H˙ + Xe→H− + Xe+˙, excited states are produced. From double collision experiments on methyl formate ions, it is concluded that the non‐decomposing ions have undergone rearrangement on the time‐scale of 10 μs into the distonic isomer, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm H} - \mathop {\rm C}\limits^ + ({\rm OH}){\rm O}\mathop {\rm C}\limits^. {\rm H}_2 $\end{document}. Finally, it is shown that short‐lived (<0.2 μs) [H2, C, N]+ ions generated by charge‐reversal of [H2CN]− have the [H2CN]+ structure, whereas most of the long‐lived (10 μs) ions have the [HCNH]+ structure.
Studies in cluster ion formation. Part I. Fast atom bombardment mass spectrometry of tetraalkylammonium halides: Factors influencing cluster ion size and stability
Authors: Tolun Emirgul , Todd John F. J. 
Published: May 2005

AbstractPositive and negative ion fast atom bombardment mass spectra of tetraalkylammonium halide salts (NR4X, where X = Cl, Br, I and NR4 = NMe4, NEt4) have been studied and intense cluster ion formation has been observed. The cluster ion intensity distributions were found to show enhancements at certain cluster numbers (n). The negative cluster ions of NMe4X salts showed anomalous ion intensity regions, which differed from both the positive cluster ions of all NR4X salts and also the corresponding negative clusters of NEt4X salts. The influence of anion and cation size on cluster ion formation and abundances has been studied and it has been established that smaller anion and cation size favours the formation of larger cluster ions. The possible structures of the cluster ions exhibiting relative increased stabilities are discussed.
On the shift of H2O between the carbons of the radical cation [CH2CH2OH2]+˙
Authors: McAdoo David J. , Hudson Charles E. 
Published: May 2005

AbstractComputations predict that H2O will shift rapidly between the carbons of \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm .} {\rm H}_2 {\rm CH}_2 \mathop {\rm O}\limits^ + {\rm H}_2 $\end{document} over most of the energy range between the threshold for formation of the ion and the onset of its decomposition. This prediction is important to understanding the relationships and contrasts between the chemistries of free radicals and carbonium ions. We present experimental evidence that the theoretical predictions are correct.
Configurational assignment of fragment ions by CID. Stereochemistry and mechanism of retro‐Diels–Alder fragmentation accompanied by hydrogen transfer in gas‐phase cations
Authors: Weisz A. , Iberkleid E. , Mandelbaum A. , Blum W. , Richter W. J. 
Published: May 2005

AbstractLow energy CID mass spectra of m/z 173, C8H13O4+, obtained from the diethyl ester of cis,syn,cis‐l,2,3,3a,4,5,5a,6,7,8‐decahydroindacene‐4,5‐dicarboxylic acid and cis,syn,cis‐l,2,3,4,5,6,7,8,8a,9,10,10a‐dodecahydrophenanthrene‐9,10‐dicarboxylic acid indicate they have the structure of protonated diethyl maleate. This finding together with previous deuterium labelling results suggest that the formation of this ion from both precursors takes place by migration of a hydrogen atom from an allylic δ‐position followed by the concerted cleavage of two allylic CC bonds in analogy to the ground state retro‐Diels–Alder fragmentation.
Mass spectra of chlorinated veratroles (1,2‐dimethoxybenzenes)
Authors: Knuutinen J. , Korhonen I. O. O. 
Published: May 2005

AbstractThe behaviour of all nine chlorinated veratroles (1,2‐dimethoxybenzenes) under electron impact has been investigated. The most common fragmentation processes are interpreted using metastable ion analysis and deuterium labelled compounds. For all compounds studied, the most common fragmentation route seems to be the primary loss of a methyl radical followed by loss of carbon monoxide. The ion formed has a well‐known quinonoid structure and fragments by several routes elucidated by metastable ion analysis. In general, the spectra of the positional isomers are shown to be practically similar and it is apparent that e.g. the 3‐ and 4‐chloro isomers can be differentiated only from the abundance ratio of the [MCH3COCH3]+ and [MCH3COH2O]+ ions.
Dissociations of positively charged aliphatic epoxides. I. Structure elucidation of the γ‐cleavage products
Authors: Bouchoux Guy , Hoppiiliard Yannik , Jaudon Pascale 
Published: May 2005

AbstractDissociative ionization of 1,2‐epoxy n‐alkanes gives rise to abundant [C4H7O]+ ions of structure [CH3OCHCHCH2]+. This conclusion is drawn from metastable ion analysis and from collisional activation spectra. This fragmentation involves the CC ring opening and a 1,4‐H migration leading to the corresponding enol ether [CH3OCHCHCH2R]+. precursor of [CH3OCHCHCH2]+ fragment. The same isomerization scheme applies to 1,2‐epoxy methyl substituted alkanes and 2,3‐epoxy n‐alkanes.
Ortho effects in organic molecules on electron impact. 14—Concerted and stepwise ejections of SO2 and N2 from N‐arylidene 2‐nitrobenzenesulphenamides
Authors: Ramana D. V. , Sundaram N. , George M. 
Published: May 2005

AbstractUnexpected ortho interaction of the nitro group has been noticed during the mass spectral fragmentations of N‐arylidene 2‐nitrobenzenesulphenamides, where the molecular ions expel SO2 and N2 both in concerted and stepwise processes. Loss of a hydrogen or the substituent from this fragment leads to a very abundant ion in all the compounds studied. Based on chemical evidence and linked‐scan studies, a 1,2‐phenylenetropylium cation structure has been postulated for the [M–SO2–N2–H/substituent]+ ion.