bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical ionization mass spectrometry of derivatives of L‐dopa and L‐tryptophan and their detection in tumour samples
Authors: Duarte M. F. N. , Hutchinson D. W. , Jennings K. R.
Published: May 2005

Conditions have been established for the detection by chemical ionization mass spectrometry (CIMS) of compounds related to L‐dopa (3‐hydroxy‐L‐tyrosine) and L‐tryptophan without derivatization. The least fragmentation of the parent ion occurs in positive ion CIMS when methylamine is used as reagent gas. The compounds can also be detected by negative ion CIMS using carbon tetrachloride as reagent gas. While the total ion current in the latter technique is lower than that obtained with positive ion CIMS, the background noise in the mass spectra of samples obtained from natural sources is greatly reduced. CIMS has been used to show the presence of aromatic amino acids in acid extracts of samples of two different classes of tumour without sample derivatization.
Gas phase reactions of protonated 1,3‐diphenylpropyne and some isomeric [C15H13]+ ions
Authors: Bäther Wolfgang , Kuck Dietmar , Grützmacher Hans‐Fr.
Published: May 2005

Metastable (3‐phenyl‐2‐propynyl)benzenium ions, generated by electron impact induced fragmentation from the appropriately substituted 1,4‐dihydrobenzoic acid, react by loss of ˙CH3 and C6H6. The study of deuterated derivatives reveals that hydrogen/deuterium exchanges involving all hydrogen and deuterium atoms precede the fragmentations. The results suggest a skeletal rearrangement by electrophilic ring‐closure reactions giving rise to protonated phenylindene and protonated 9,10‐methano‐9,10‐dihydroanthracene prior to the elimination of C6H6 and ˙CH3, respectively. A study of isomeric [C15H13]+ ions by collision‐induced decomposition and by deuterium labelling shows that these ions interconvert by hydrogen migrations and skeletal rearrangements.
Structural investigation of the antibiotic sporaviridin: 11—Molecular secondary ion mass spectral studies on the constituent pentasaccharides viridopentaoses
Authors: Harada Ken‐ichi , Kimura Ikumi , Masuda Katsuyoshi , Suzuki Makota
Published: May 2005

Molecular secondary ion mass spectra of three pentasaccharides, viridopentaoses A, B and C, using various matrices, are discussed. The appearance of the molecular ion species is dependent upon the relative proton affinities between the sample and the organic matrix material. However, the presence of sodium ion, rather than proton, also greatly influences the appearance of the molecular ion species. The glycosidic linkages are mainly cleaved between the glycosidic oxygen atom and the anomeric carbon atom to give informative sugar sequence ions. These fragmentations have been confirmed by the linked scanning technique (B/E).
Amidine mass spectral fragmentation patterns
Authors: Barker James , Jones Michael , Kilner Melvyn
Published: May 2005

Electron impact mass spectrometry of a range of amidines (R′NC(R)NHR′) including formamidines, acetamidines, benzamidines and tert‐butylamidine, has been undertaken, and comparisons made of the fragmentation pathways followed by the different families of compounds. Fragmentation of all the molecular ions is characterized by skeletal carbon‐nitrogen bond cleavage to form [R′NCR]+ and [R′NH]+ fragments, both of which are observed. For formamidines (RH), the positive charge remains with the [R′NH]+ fragment which leads to the base peak at m/z93 corresponding to [R′NH2]+˙. In contrast, for acetamidines and benzamidines the charge prefers to remain with the [R′NCR]+ fragment which gives the base peak for these compounds. The spectra of unsubstituted amidines (HNC(R)NH2) are characterized by cleavage of the carbon substituent from the NCN skeleton, [CN2H3]+ (m/z 43) being produced in all cases.
Bifunctional even‐electron ions: 1—Fragmentation behaviour of ω‐methoxy‐, ω‐hydroxy‐ and ω‐chloro‐oxonium ions
Authors: Záhorszky U. I.
Published: May 2005

Bifunctional oxonium ions—generated from tertiary aliphatic alcohols containing an additional hydroxy, methoxy or chloro group at the end of an alkyl side‐chain—do not markedly exhibit fragmentations typical of ordinary oxonium ions, but show as the main reactions those caused by functional group interaction, through‐space interaction being the dominant factor. The main primary fragmentation is loss of the additional functional group X as HX, followed by loss of the side‐chain originally separating the two functional groups, leading to carbonyl cations. This typical reaction sequence is initiated by proton migration from the oxonium moiety to the additional functional group. The reaction behaviour of the bifunctional ions is discussed. The lowest homologues show specific deviations from the general fragmentation behaviour.
The [C5H8]+˙ radical cation: Structural studies by energy‐resolved mass spectrometry
Authors: Mason R. S. , Jennings K. R. , Verma S. , Cooks R. G.
Published: May 2005

Control of the degree of collisional excitation of an ion can be achieved by varying the collision energy or the number of collisions. The former experiment yields a series of daughter spectra which is analogous to a breakdown curve and which serves to characterize the ion undergoing collisionally activated dissociation (CAD). Recognition of differences in the spectra between isomeric ions is facilitated if the curves obtained from the energy‐resolved data are considered in pairs and their differences are plotted. This procedure serves to demonstrate that ionized 1,3‐ and 1,4‐pentadiene are structurally distinct, even though they are not distinguishable in conventional CAD. 2‐Pentyne and 3‐octyne give [C5H8]+˙ fragment ions which are closely related to the 1,3‐ and 1,4‐pentadiene structures, respectively. Any particular ion can be characterized by another form of difference spectrum: one comparing data at two collision pressures each taken over a range of collision energies. This procedure yields conclusions regarding ion structure which match those reached from the energy‐resolved experiments.
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

Stereospecific 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

A 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

The 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

This 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

A 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

A 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

The 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

Hydrogen 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

One 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

Scrambling 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

Two 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

The 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

Potential 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

The 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

From 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

A 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

Positive 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

Computations 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

Low 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

The 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

Dissociative 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

Unexpected 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.
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

The 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

The 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

QuinoxalineN‐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

Imidazolin‐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

The 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

A 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

Water 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

Structure 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

Carnitine 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

The 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

The 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

Positive 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

The 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

The 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

Recent 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

Ion 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

The 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

Collision‐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.