bluuebunny/crossref_metadata_2025 huggingface dataset:
Isomerization and fragmentation of methylfuran ions and pyran ions in the gas phase
Authors: Spilker Rudiger , Grutzmacher H.‐F.
Published: May 2005

The mutual interconversion of the molecular ions [C5H6O]+ of 2‐methylfuran (1), 3‐methylfuran (2) and 4H‐pyran (3) before fragmentation to [C5H5O]+ ions has been studied by collisional activation spectrometry, by deuterium labelling, by the kinetic energy release during the fragmentation, by appearance energles and by a MNDO calculation of the minimum energy reaction path. The electron impact and collisional activation mass spectra show clearly that the molecular ions of 1–3 do not equilibrate prior to fragmentation, but that mostly pyrylium ions [C5H5O]+ arise by the loss of a H atom. This implies an irreversible isomerization of methylfuran ions 1 and 2 into pyran ions before fragmentation, in contrast to the isomerization of the related systems toluene ions/cycloheptatriene ions. Complete H/D scrambling is observed in deuterated methylfuran ions prior to the H/D loss that is associated with an iostope effect kH/kD = 1.67–2.16 for metastable ions. In contrast, no H/D scrambling has been observed in deuterated 4H‐pyran ions. However, the loss of a H atom from all metastable [C5H5O]+ ions gives rise to a flat‐topped peak in the mass‐analysed ion kinetic energy spectrum and a kinetic energy release (T50) of 26 ± 1.5 kJ mol−1. The MNDO calculation of the minimum energy reaction path reveals that methylfuran ions 1 and 2 favour a rearrangement into pyran ions before fragmentation into furfuryl ions, but that the energy barrier of the first rearrangement step is at least of the same height as the barrier for the dissociation of pyran ions into pyrylium ions. This agrees with the experimental results.
Mass spectrometric studies of cyclopropanes with electronegative substituents. Indication of methoxy group randomization in (±) dimethyl 2‐methoxy‐3,3‐dimethylcyclopropane‐1,1‐dicarboxylate
Authors: Kolsaker Per , Kvarsnes Arnfinn , Storesund Hans‐Johan
Published: May 2005

(±) 2‐Methoxy‐3,3‐dimethylcyclopropane‐1,1‐carboxylate was studied by electron impact and chemical ionization mass spectrometry. By using selective deuteration of the methyl groups, it was established that extensive randomization of the methoxy groups takes place prior to the decomposition of the molecular ion.
Low‐energy mass spectra of some aliphatic ketones
Authors: Abbatt Jonathan A. , Harrison Alex G.
Published: May 2005

The charge exchange mass spectra of a selection of C5‐C7 ketones have been measured using [CS2]+˙, [COS]+˙ and [N2O]+. as reagent ions. The low energy charge exchange with [CS2]+˙ or [COS]+˙ provides simple primary ion mass spectra, which readily permit structure elucidation in contrast to metastable ion spectra. In several cases, isomer distinction is easier from the charge exchange mass spectra than from the electron impact mass spectra. The energy transfer from [N2O]+˙ is sufficiently high for complex spectra resembling electron impact mass spectra to be obtained.
The behaviour of stereoisomeric ions in the gas phase: 3—The case of homosubstituted bicyclooctenes
Authors: Grassi Maria , Audisio Guido , Traldi Pietro
Published: May 2005

In the electron impact mass spectroscopy of four 2,3,5,6‐bicyclo(2.2.2)–7‐octenetetramethoxycarbonyl stereoisomers differences in relative abaundances of product ions and also different fragmentation pathways are observed. The stereospecificity is retained also under positive ion chemical ionization (CI(methane), CI(isobutane)) and negative ion chemical ionization (NICI) (OH−) conditions. Interesting correlations between fragmentation and molecular symmetry are suggested.
Discrimination of isomers by liquid ionization mass spectrometry with techniques of collisional activation and adduct ion formation
Authors: Niki Yoshiko , Tsuchiya Masahiko
Published: May 2005

Liquid ionization mass spectrometry is a soft ionization technique used with liquid samples under atmospheric pressure. It facilitates the handling of reagents and the observation of ion–molecule reactions in the ion source. The differentiation of isomers by characteristic fragment ions, for example those resulting from asymmetrical cleavage of a cyclobutane ring, and by molecular adduct ion formation was studied. The samples studied were cyclobutane derivatives, alkyl 4‐(3‐oxo‐3‐pIienyl‐l‐aIkenyl)benzoate dimers, and reagents having two functional groups were used to produce adduct ions to clarify the difference between isomers. The reagents act on a sample molecule at two functional groups to form hydrogen bonds. Some correlations were observed between the structure of the sample and the relative abundances of molecular adduct ions and also fragment ions produced by collisionally activated dissociation.
Mass spectra of some 1‐(6′‐substituted‐4′‐methyl‐2′‐quinolyl)‐3‐methylpyrazol‐5‐ols and their 4‐substituted analogues
Authors: Singh Shiv P. , Vaid Radhe K. , Prakash Om
Published: May 2005

Electron impact induced fragmentation of some 1‐(6′‐substituted‐4′‐metbyI‐2′‐quinolyI)‐3‐methylpyrazoI‐5‐ols follows a route where the pyrazole moiety is preferentially cleaved with successive losses of two moieties of 41 u. High‐resolution measurements have established that the first loss is due to the Ċ2HO moiety, which necessitates an intramolecular hydrogen transfer followed by ring fission. The resultant ion loses CH3CN in a subsequent step. The origin of many fragment ions was traced with the use of B/E linked‐scan spectra.
The effect of charge on hydroxyl loss from ortho‐substituted nitrobenzene ions
Authors: McLuckey Scott A. , Glish Gary L.
Published: May 2005

An abundant loss of hydroxyl in decompositions of ortho‐substituted nitroarene cations is commonly observed when the substituent contains one or more labile hydrogen atoms. The major loss of hydroxyl also takes place from many but not all of the corresponding molecular anions. Data are reported for the collisionally activated decompositions of the cations and anions of o‐nitrotoiuene, o‐nitrophenol and o‐nitroaniline. Data for some dinitro ions are also reported. The results can be rationalized on the basis of a greater degree of charge developed at the substituent in the transition state of the anions that leads to a rearranged ion. It is from this structure tint hydroxyl is lost via simple bond cleavage. This can be viewed most simply as a proton transfer from the substituent to the nitro group in the anion as opposed to hydrogen transfer in the analogous step for the cation. The degree to which hydroxyl loss occurs is therefore largely determined by the tendency for hydrogen (cations), or proton (anions), transfer from the substituent to the nitro group.
Comparison of the stable products formed by fast atom bombardment and γ‐irradiation of glycerol
Authors: Keough T. , Ezra F. S. , Russell A. F. , Pryne J. D.
Published: May 2005

The major end‐products formed by fast atom bombardment (FAB) and γ‐irradiation of glycerol have been identified using capillary gas chromatography/mass spectrometry. The product distributions differ dramatically. Products resulting from the recombination of either carbon‐centered or oxygen‐centered radicals are evident for both sources of irradiation. However, the major FAB‐generated products are formed by the recombination of carbon‐centered [G – H]· radicals with other carbon‐centered radicals while the major γ‐radiolysis products result from the recombination of oxygen‐centered [G–H]· radicals with lower‐weight carbon‐centered species. γ‐Radiolysis of an N2O‐saturated aqueous solution of glycerol, experimental conditions that strongly favor the formation of carbon‐centered [G – H]· radicals at the expense of the oxygen‐centered species, yielded some products that were identical to those produced by FAB of neat glycerol. Finally, the free radicals produced by γ‐radiolysis of glycerol were investigated by electron spin resonance spectroscopy and spin trapping with 5,5‐dimethyl‐1‐pyrrolne‐TV‐oxide. The presence of only the carbon‐centered free radical was established by this technique. The implications of these results are discussed.
Mass spectroscopy of natural products. XXIII, Comparative positive and negative ion mass spectral studies of scapanin‐type diterpenoidst
Authors: Schmidt Jiirgen , Huneck Siegfried , Franke Peter , Connolly Joseph D.
Published: May 2005

The mass spectral behaviour of 15 scapanin‐type diterpenoids (8α,12‐epoxylabdanes) was elucidated using accurate mass measurements and deuterium‐labelled derivatives. The results obtained allow a localization of the functional groups in the diterpenoid system. Furthermore, the negative ion mass spectra of the scapanin‐type diterpenoids are discussed and compared with the positive ion spectra.
Doubly charged ion mass spectra of alkyl‐substituted furans and pyrroles
Authors: Nelson P. R. , Kee Fung C. A. Fung , Sedgwick J. B. , Shields G. C. , Abbey L. E. , Moran T. F.
Published: May 2005

Doubly charged ion mass spectra of alkyl‐substituted furans and pyrroles were obtained using a double‐focusing magnetic mass spectrometer operated at 3.2 kV accelerating voltage. Molecular ions were the dominant species found in doubly charged spectra of lower molecular weight heterocydic compounds, whereas the spectra of the higher weight homologues were typified by abundant fragment ions from extensive decomposition. Measured doubly charged ionization and appearance energies ranged from 22.8 to 47.9 eV. Ionization energies were correlated with values calculated using self‐consistent field–molecular orbital techniques. A multichannel diabatic curve‐crossing model was developed to investigate the fundamental organic ion reactions responsible for development of doubly charged ion mass spectra. Probabilities for Landau–Zener type transitions between reactant and product curves were determined and used in the collision model to predict charge‐transfer cross‐sections, which compared favorably with experimental cross‐sections obtained using time‐of‐flight techniques.
Mass spectrometric investigation of benzyliderieaniline
Authors: Srzić Dunja , Čevizović Nada , Meić Zlatko
Published: May 2005

The mass spectral fragmentation of benzylideneaniline has been studied by deuterium labelling. Under electron impact, molecular ions undergo simple fission at the phenyl–N, phenyl–C and CN bonds, hydrogen migration reactions, and skeletal rearrangement fragmentations. Except in the skeletal rearrangement reactions, hydrogen scrambling does not feature in benzylideneaniline upon electron impact.
Mass spectra of phenoxyacetyl derivatives. Mechanism of loss of ‘CO’ from phenyl phenoxyacetates
Authors: Vairamani M. , Mirza Urooj Ali
Published: May 2005

The mechanism of loss of ‘CO’ in phenyl phenoxyacetates has been studied by collision‐induced dissociation spectra. Interesting ortho effects have been reported in the o‐methoxy and o‐nitro isomers.
Surface‐induced dissociation of isomeric [C5H6]+˙ ions
Authors: Mabud Md. A. , Ast T. , Cooks R. G.
Published: May 2005

Collisions of [C5H6]+˙ ions of 25 eV tianslational energy with a surface result in dissociation, the extent of which depends upon the nature of the neutral precursor. A correlation between the degree of fragmentation and the reported degree of charge stripping upon gaseous collisions (R. Flammang et al, Org. Mass Spectrom. 20, 253 (1985)) supports the suggestion that mixtures of cyclic and linear structures in differing proportions are present in the [C5H6]+˙ ion beam.
A mass spectrometric study of dimethyl ester trimethylsilyl ether derivatives of some 3‐hydroxydicarboxylic acids
Authors: Svendsen John S. , Sydnes Leiv K. , Whist Jon E.
Published: May 2005

The fragmentation pathways for the dimethyl ester trimethylsilyl ether derivatives of some 3‐hydroxydicarboxylic acids have been found by using B/E 2nd B2/E linked scans and isotope substitution techniques. Most of the fragments are due to ionization at silicon, which induces a fragmentation pattern that intimately reflects the structure of the compounds.
Quantum mechanical investigation of effects in formation of carbonyl and thiocarbonyl cations. Part I. Fragmentation reactions of 1,2‐hemithiodione radical cations
Authors: Nevalainen Vesa , Vainiotalo Pirjo
Published: May 2005

In order to study decomposition reactions of ionic oxygen and sulphur‐containing compounds, such as hemithiodione radical cations, a quantum chemical investigation of the formation of formyl, thioformyl, acyl and thioacyl cations and radicals was performed. Calculations were carried out mainly at the 6–31G* level involving complete geometry optimizations. In the ionization of aldehydes and thioaldehydes, no important energy differences were found between the oxygen and sulphur analogues studied. A stepwise generation of formyl and thioformyl cations from formaldehyde and thioformaldehyde, by hydrogen atom abstraction followed by expulsion of unpaired electrons from the resulting radicals, showed the radicalization of formaldehyde to be only 12.6 kJ mol−1 more favoured than that of thioformaldehyde. The electron expulsion from formyl radical was 23.8 kJ mol−1 more favoured than that from thioformyl radical. Substitution of hydrogens of formyl and thioformyl groups by methyls lowered the total formation energies of carbonyl and thiocarbonyl cations 119.2 and 96.2 kJ mol−1. The formation energy difference between acyl and thioacyl cations was also very small.
High‐pressure collisional activation mass spectrometry of aromatic halides
Authors: Blom Karl , Munson Burnaby
Published: May 2005

The high‐pressure collisional activation mass spectra with methane as the reagent/collision gas are reported for five aromatic halides. The major decomposition of the protonated aromatic halides is hydrogen halide elimination. The energy‐resolved mass spectra and the chemical reactivities of fragment ions with the methane collision gas are used to establish dissociation pathways and structures of fragment ions. The high‐pressure collisional activation mass spectra are compared with conventional collisionally induced dissociation and chemical ionization mass spectra.
Loss of methyl from ionized 2‐hydroxy‐1,3‐butadiene
Authors: Turefček František , Maquin Francis , Hill Nicholas , Stahl Daniel , Gäumann Tino
Published: May 2005

The loss of methyl from unstable, metastable and collisionally activated [CH2CHC(OH)CH2]+˙ ions (1+˙) was examined by means of deuterium and 13C labelling, appearance energy measurements and product identification. High‐energy, short‐lived 1+˙ lose methyl groups incorporating the original enolic methene (C(1)) and the hydroxyl hydrogen atom (H(0)). The eliminations of C(1)H(1)H(1)H(4) and C(4)H(4)H(4)H(0) are less frequent in high‐energy ions. Metastable 1+˙ eliminate mainly C(1)H(1)H(1)H(4), the elimination being accompanied by incomplete randomization of the five carbon‐bound hydrogen atoms. The resulting [C3H3O]+ ions have been identified as the most stable CH2CHCO+ species. The appearance energy for the loss of methyl from 1 was measured as AE[C3H3O]+ = 10.47 ± 0.05 eV. The critical energy for 1+˙ → [C3H3O]+ + CH3˙ is assessed as Ec ⩽ 173 kJ mol−1. Reaction mechanisms are proposed and discussed.
Computer‐assisted composition analysis of unknown compounds by simultaneous analysis of the intensity ratios of isotope patterns of the molecular ion and daughter ions in low‐resolution mass spectra
Authors: Tenhosaari Aarni
Published: May 2005

A microcomputer program has been developed which calculates compositions of unknown compounds by using the intensity ratios of isotope patterns in low‐resolution electron impact mass spectra. The patterns originating from the molecular ion, recognizable daughter ions, and ions from common neutral fragment losses are simultaneously used for the composition analysis, thus improving the reliability compared to previously published methods. The method is most suitable for analysing spectra of compounds giving intense, well‐shaped and separate peak patterns of daughter ions. The program is also able to analyse the compositions of the analytically useful compounds of boron.
Desorption due to charge exchange in low‐energy collisions of organofluorine ions at solid surfaces
Authors: Vincenti M. , Cooks R. G.
Published: May 2005

Collisions of organofluorine ions at a metal surface result in efficient emission of adsorbate species as gas‐phase ions. The experiments are done at 120° scattering angle in a hybrid (BQ) mass spectrometer; the primary ions, mass‐selected by a magnetic sector (B), are allowed to collide with a target at a selected kinetic energy in the tens of eV range and the emitted ions are mass‐analyzed using a quadrupole mass filter (Q). It is proposed that the impinging ions undergo neutralization accompanied by desorption of hydrocarbon ions and that the amount of internal energy deposited in the desorbed ions is strongly dependent on the collision energy and affects their degree of fragmentation. Competing processes include reflection and fragmentation of the colliding particle, along with such ion/adsorbate reactions as hydrogen atom abstraction by the fluorinated ion. Small even‐electron ions, such as [CHF2]+ and [C2H2F]+ are more effective in promoting chemical sputtering of the surface adsorbate as compared to larger ions (e.g. [C3F5]+) and odd‐electron ions (e.g. [C2F4]+˙ and [C2HF2]+˙). At low energies some odd‐electron fluorinated ions undergo collision without any secondary ions being emitted from the surface. In these cases the parent ions are apparently neutralized, but without sufficient energy transfer to cause hydrocarbon ion desorption. Non‐fluorinated organic ions yield fragment ions and ion/surface reaction products under the condition of these experiments, but do not cause significant desorption of hydrocarbon ions.
Mass spectrometric investigation of some [1]benzopyrano[3,4‐c]pyrazole and [1]benzopyrano[4,3‐d]isoxazole derivatives
Authors: Kostova Ivanka N. , Mollova Nevena N.
Published: May 2005

The fragmentation behaviour of some [1]benzopyrano[3,4‐c]pyrazole and [1]benzopyrano[4,3‐d]isoxazole derivatives under electron impact is described.
Internal energy requirements for remote site fragmentation
Authors: Wysocki Vicki H. , Bier Mark E. , Cooks R. Graham
Published: May 2005

Attempts to induce remote site fragmentation by using low‐energy collisional activation with a neutral target gas or with a surface (surface‐induced dissociation) are reported. These data, in conjunction with knowledge of the internal energy deposition associated with these activation methods, provide information on the amount of internal energy needed to promote remote site fragmentation in selected compounds. The internal energy required for remote site fragmentation of lithiated oleyl alcohol was recently estimated to be 1.3–1.9 eV. Our experiments suggest that internal energies much greater than 2 eV are required to produce detectable remote site fragmentation in several different ions, including lithiated oleyl alcohol, n‐dodecylbenzenesulfonate anion, stearylsulfate anion, stearate anion and protonated octadecylamine. The results also suggest that the internal energy required to record spectra which show remote site fragmentation is compound‐dependent, i.e. strongly dependent on the nature of the ionic group.
Mass spectral fragmentation pathways in phthalate esters. A tandem mass spectrometric collision‐induced dissociation study
Authors: Yinon Jehuda
Published: May 2005

A collision‐induced dissociation study of a series of phthalate esters was carried out using a tandem BB mass spectrometer. Fragmentation pathways of the phthalates were determined in the electron impact mode. Two major daughter ions are formed, one by a McLafferty rearrangement and hydrogen transfer and the other by loss of an alkoxy radical Another major daughter ion, at m/z 149—which is the base peak in the electron impact mass spectra of most phthalate esters—is being formed through four alternate pathways.
[C7H7] radicals studied by neutralization reionization mass spectrometry; the ortho‐tolyl to benzyl radical rearrangement
Authors: Buschek J. M. , Holmes John L.
Published: May 2005

The collision‐induced dissociation characteristics of the benzyl, tropyl and tolyl cations which serve to identify these [C7H7]+ isomers (the m/z 74–77 relative abundances) were not preserved in their neutralization‐reionization (NR) mass spectra. However, analyses of mixtures of the [C7H7]+ isomers made by comparison of NR mass spectra gave similar results to those obtained from collisional activation (CA) mass spectra. The radicals produced by electron transfer from Xe to tolyl cations were not observed to rearrange to benzyl radicals on the time‐scale of the NR experiment, a result in conflict with other gas‐phase studies.
Fragmentation of ligands and stability of inorganic skeletons in the mass spectra of polynuclear beryllium and zinc complexes
Authors: Sipachev Viktor A. , Nekrasov Yurii S.
Published: May 2005

Electron impact induced fragmentations of 12 beryllium and zinc μ4‐oxohexa‐μ‐acido and μ4‐oxopenta‐μ‐acido‐μ‐alkoxo complexes containing carboxylato, fluoro‐ and chloroacetato, alkoxo and nitrato groups as ligands are discussed. The spectra are interpreted in terms of a model of weakly interacting fragments. It is demonstrated that in the systems under consideration, structural rather than electronic factors play the determining role in the stabilization of fragment ions. All the spectra are dominated by terra‐ and trinuclear complex cations containing OM4 and OM3 (M = Be, Zn) units, respectively. Their relative abundances correlate with the number of ligand vibrational degrees of freedom, being independent of ligand electronic characteristics.
Stereochemical effects in mass spectrometry. 7. Determination of absolute configuration of some organic molecules by reaction mass spectrometry
Authors: Chen Yao‐Zu , Li Hung , Yang Hou‐Jun , Hua Su‐Ming , Li Hai‐Quan , Zhao Fan‐Zhi , Chen Nen‐Yu
Published: May 2005

It was found that in the chemical ionization (isobutane) mass spectra of some asymmetric secondary alcohols and α‐amino acids, when a pair of enantiomers (such as R‐ and S‐2‐phenyIbutyric anhydride, R‐ and S‐mandelic acid, R‐ and S‐2‐methylbutanoic acid or R‐ and S‐α‐phenyl ethyl amine) were used as reaction reagents, the relative abundances of characteristic ions formed by the stereoselective reaction between sample and reagent of the same configuration were much higher than those ions formed by the sample and a reagent of a different configuration. The absolute configuration of the sample molecule may be predicted by examination of mass spectra of the sample measured with R‐ and S‐reagent respectively. This approach proved to be a convenient way for determination of the absolute configurations of organic molecules on a micromole level by mass Spectrometry.
Protonation of some cyclopropylarenes in the gas phase
Authors: Galli Carlo , Speranza Maurizio
Published: May 2005

The capability of the cyclopropyl group to conjugate with Unsaturated systems or to delocalize an adjacent positive charge is compared with that of a double bond, and is tested by the determination of the gas‐phase basicity (GB) of various methyl‐substituted cyclopropylbenzenes, α‐methyl styrenes, and biphenyls by Fourier‐transform ion cyclotron resonance. The site of the gas‐phase protonation in the cyclopropylbenzene is established at the three‐membered ring, based on analogy with the trends of GBs in the styrene system upon methyl substitution at the aromatic meta and para positions. The diverse nature of the conjugative interaction of a cyclopropyl group and of a double bond toward an aromatic ring is manifested, instead, in the different behaviour toward a steric perturbation. The cyclopropyl group adopts the bisected conformation in cyclopropylbenzene, because it tolerates the steric perturbation of a methyl group, intentionally placed in the ortho position, more easily than the double bond, which adopts a coplanar conformation in the isomeric α‐methyl styrene.
Cationated cyclopropanes as reaction intermediates in mass spectra: an earlier incarnation of ion‐neutral complexes
Authors: Meyerson Seymour
Published: May 2005

The appearance a decade ago of the concept of ion‐neutral complexes as intermediates in unimolecular decompositions constituted a major advance in mechanistic interpretation of mass spectra. The concept, however, was not new; it goes back to the introduction of the cationated cyclopropane in the mid‐1950s. Its development did not occur via a smooth progression; rather, its history bridges a sharp discontinuity. The early workers, in a curious historical quirk, failed to generalize on the idea beyond complexes in which the neutral component is cyclopropane or a closely related species. Hence it took an independent rediscovery of the concept to usher in the rapid growth and wide application that characterize its present stage of development.
Rearrangement reactions of the molecular ions of some substituted aliphatic oxiranes
Authors: Grützmacher Hans‐Friedrich , Pankoke Doris
Published: May 2005

The fragmentation reactions of glycidic methyl ester (1) and of its derivatives (2–6) substituted by one, two and three methyl groups, respectively, at the oxirane ring, of the corresponding glycidols (7–12), and of the glycidyl ethers (13–16) in the 70 eV mass spectra have been studied using isotopic labelling and mass‐analysed ion kinetic energy spectrometry. It is shown that the typical reaction of these aliphatic oxirane radical cations carrying a nucleophilic methoxy group and hydroxy group, respectively, at the side chain corresponds under high‐energy conditions to a rearrangement by a methoxy group or a hydroxy group migration to the β‐carbon atom of the oxirane moiety. This rearrangement is very likely mediated by the isomerization of the molecular ions into distonic ions via CC bond cleavage within the oxirane ring.
Doubly charged porphyrin ion tandem mass spectrometry: Implications for structure elucidation
Authors: Beato Brian D. , Yost Richard A. , Quirke J. Martin E.
Published: May 2005

Under electron ionization (EI) conditions, porphyrins yield unusually high intensities of doubly charged molecular and fragment ions. These doubly charged ions offer unique opportunities for the structure elucidation of porphyrins by tandem mass spectrometry (MS/MS). First, they fragment to a greater extent than the corresponding singly charged ions under both EI/MS and EI/MS/MS conditions. Second, doubly and singly charged porphyrin ions often fragment via different pathways, and can therefore yield different structural information. This paper describes several ways in which analyses of doubly charged porphyrin ions with a triple quadrupole tandem mass spectrometer can be useful in structure elucidation of porphyrins. The effect of the metal atom on the fragmentation of metalloporphyrins in an EI source is demonstrated by correlating the extent of doubly charged fragment ion formation to a stability index. Doubly charged porphyrin ions are shown to yield predominantly doubly charged daughter ions upon collisionally activated dissociation (CAD), and are also shown to fragment to a greater extent than corresponding singly charged porphyrin ions. Advantages and disadvantages of doubly charged porphyrin ion MS/MS for structure elucidation are discussed.
Collisionally activated dissociation of some pyridinium cations: Novel fragmentation pathways
Authors: Katritzky Alan R. , Watson Clifford H. , Dega‐Szafran Zofia , Eyler John R.
Published: May 2005

Three novel collisionally induced dissociation pathways, additional to the usual formation of pyridine or pyridinium cation, are described for laser‐desorbed N‐substituted pyridinium cations. Particularly prevalent is the formation of an ion of m/z 94, corresponding to [PyCH3]+. Doubly charged pyridiniums tend to lose H+, and one example of the apparent formation of the neutral radical C5H6N˙ is reported.
Unimolecular and collision‐induced dissociation reactions of peracetylated methyl glucopyranoside
Authors: Guevremont Roger , Pathak Ved P. , Wright Jeffrey L. C.
Published: May 2005

The mechanism of the collision‐induced fragmentation of peracetylated methyl‐α‐D‐glucopyranoside was investigated using deuterium‐labelled acetates and sequential mass spectrometry. Loss of the substituent at C(1), the anomeric carbon, yields an ion of m/z 331, [C14H19O9]+. This ion further dissociates via two pathways, the first including m/z 271, [C12H15O7]+, 169, [C8H9O4]+ and 109, [C6H5O2]+, and the second including m/z 211, [C10H11O5]+, 169, [C8H9O4]+ and 127 [C6H7O3]+. The first path proceeds via loss of acetate at C(3), followed by a single‐step concerted loss of acetates from C(2) and C(4), and ending with loss of acetate from C(6). The second path proceeds predominantly via loss of acetates from C(3) and C(4), elimination of ketene from the C(2)‐acetate and finally loss of ketene from the acetate at C(6). This path is also characterized by an ill‐defined series of parallel decomposition reactions involving acetates from other sites on the molecule. At low collision energy, and in the absence of collision gas (unimolecular reaction conditions), the former pathway predominates; m/z 331 dissociates via loss of acetate at C(3), followed by a single‐step concerted loss of acetates from C(2) and C(4).
Collision‐Induced consecutive dissociation reactions of the monocarboxylate anions generated from the dimethyl and diethyl esters of glutaric acid and its 3,3‐dimethyl analogue
Authors: Maas Wilfried P. M. , Nibbering Nico M. M.
Published: May 2005

The collision‐induced dissociation reactions of the monocarboxylate anions, generated from the dimethyl and diethyl esters of glutaric acid and its 3,3‐dimetbyl analogue in a chemical ionization source, were studied as a function of the potential applied to the collision cell in combination with 2H labelling experiments. It was shown that many of the product anions are formed in consecutive steps. The mechanisms associated with these steps appear to be initiated by a functional group interaction between the carboxylate anion and the ester group, 1,5‐hydrogen migrations both to the carboxylate anion and the uncharged ester group and charge remote fragmentation. In one of the collision‐induced dissociation channels a methyl anion is generated as a granddaughter product anion, which contains the hydrogen atoms exclusively originating from positions 2 and 4 as shown by the applied 2H labelling.
Field desorption mass spectrometric analysis of mixtures of surfactants and inorganic salts
Authors: Zhu Peifen , Su Keman
Published: May 2005

A field desorption mass spectrometric method is proposed for the analysis of mixtures of surfactant and inorganic salts. Different compounds in the mixture are determined with different heating currents and distinguished on the basis of the exterior character of the mass spectrum. The effectiveness of the method is demonstrated with known mixtures of surfactants and inorganic salts. Two unknown additives for electric tin plating were tested with the method.
Unimolecular reactions of the isolated immonium ions CH3CH = NH+C4H9, CH3CH2Ch = NH+C4H9 and (CH3)2C = NH+C4H9
Authors: Bowen Richard D. , Colburn Alex. W. , Derrick Peter J.
Published: May 2005

The reactions of ten metastable immonium ions of general structure R1R2CNH+C4H9 (R1 = H, R2 = CH3, C2H5; R1 = R2 = CH3) are reported and discussed. Elimination of C4H8 is usually the dominant fragmentation pathway. This process gives rise to a Gaussian metastable peak; it is interpreted in terms of a mechanism involving ion‐neutral complexes containing incipient butyl) cations. Metastable immonium ions ontaining an isobutyl group are unique in undergoing a minor amount of imine (R1R2CNH) loss. This decomposition route, which also produces a Gaussian metastable peak, decreases in importance as the basicity of the imine increases. The correlation between imine loss and the presence of an isobutyl group is rationalized by the rearrangement of the appropriate ion‐neutral complexes in which there are isobutyl cations to the isomeric complexes containing the thermodynamically more stable tert‐butyl cations. A sizeable amount of a third reaction, expulsion of C3H6, is observed for metastable n‐C4H9 +NHCR1R2 ions; in contrast to C4H8 and R1R2CNH loss, C3H6 elimination occurs with a large kinetic energy release (40–48 kJ mol−1) and is evidenced by a dish‐topped metastable peak. This process is explained using a two‐step mechanism involving a 1,5‐hydride shift, followed by cleavage of the resultant secondary open‐chain cations, CH3CH+ CH2CH2NHCHR1R2.
Electron impact excitation of ions from larger organic molecules
Authors: Wang Bing‐Huang , McLafferty Fred W.
Published: May 2005

[M + Na]+ ions (m/z 1164) from gramicidin S can be dissociated in a Fourier‐transform mass spectrometer with ∼ 85% efficiency using a 50 μA beam of 70 eV electrons for 500 μs. Although the fragment ions are smaller, on average, than those from collisionally activated dissociation or 193 nm photodissociation, these also provide extensive sequence information.
Role of molecular oxygen in fragmentation processes of abscisic acid methyl ester in electron capture negative ionization
Authors: Heath Timothy G. , Gage Douglas A. , Zeevaart Jan A. D. , Watson J. Throck
Published: May 2005

Ion‐molecule reactions with oxygen promote fragmentation in the electron capture negative ionization mass Spectrometry of methylated abscisic acid. It is shown that low‐energy collisions of the molecular anion (m/z 278) with O2 in the center quadrupole of a triple quadrupole mass spectrometer result in the formation of an adduct ion which subsequently fragments to yield two prominent ions, one at m/z 141 which contains the side‐chain and a second at m/z 152 which includes the ring. Two minor products are represented by peaks at m/z 179 and 136. These ion‐molecule reactions can also occur in the ion source when oxygen is present, which may cause fluctuations in relative intensities of the fragment ion peaks. The principal fragments of mass 141 and 152, and also those of mass 179 and 136, do not form in the absence of O2. Product ion mass spectra following collisions of M−· with 18O2 in the center quadrupole show that the side‐chain fragment ion shifts from m/z 141 to m/z 143, indicating retention of an oxygen atom from the gas‐phase reaction with molecular oxygen. It is proposed that on reaction with oxygen, a peroxy radical anion intermediate at m/z 310 is formed which then fragments to yield an enolate anion (m/z 141) or a quinone‐like radical anion (m/z152). The induced fragmentations of the 2‐cis and 2‐trans isomers of methylated abscisic acid appear to be identical. Analysis of isotopically labeled compounds of both abscisic acid and structurally similar metabolites reveal that oxygen‐activated fragmentation in these selected cases provides diagnostic ions which allow one to determine the position(s) of isotopic enrichment in the molecule.
The acid effect in fast atom bombardment
Authors: Shiea Jentaie , Sunner Jan
Published: May 2005

The effects on fast atom bombardment (FAB) mass spectra of analyte bases in glycerol on adding acids were investigated. The MH+ ion intensity usually increased but sometimes decreases were also found. For the practical use of FAB, it is importaat to clarify the reasons for this acid effect. By critical evaluation, a variety of different explanations were found, e.g. changes in solubility, surface activity, volatility, radiation‐induced chemistry and desorption energetics. No unambiguous examples of efficient preformed ion formation were found. However, it is still possible that preformed ion formation may be important in the uppermost surface layer. The mechanistic implications of these findings are briefly discussed.
Mass spectra of liquid crystals. V.—cyclohexyl and phenyl esters of cyclohexanecarboxylic and benzole acids
Authors: Leclercq P. A. , van den Bogaert H. M.
Published: May 2005

The 70 eV electron impact mass spectra of 38 cyclohexanecarboxylate and 15 benzoate derivatives are presented. This part includes data on six substituted cyclohexyl and 47 derivated phenyl esters, including eleven alkoxyphenyl and 17 dicyanoalkoxypnenyl esters. Based on the observed peak shifts by various substituents, the elemental formulae of the main fragment ions are rationalized.
Structural study of Mn(III)‐tetraarylporphyrin complexes by fast atom bombardment mass spectrometry
Authors: Rubino F. M. , Mascaro P. , Banfi S. , Quici S.
Published: May 2005

A series of 27 Mn(III)‐tetraarylporphyrins bearing heterogeneous substituents on the phenyl rings at the meso positions were subjected to positive ion fast atom bombardment mass spectrometry The source spectra yielded the molecular ion and a few peaks confirmative of the chemical structure. Major processes were hydrogenolytic loss of the aryl rings or of their substituents. Molecules carrying a side‐chain underwent loss of the chain either as a solution process (amide linkage) or as a gas‐phase process (phenolic ether linkage). Collisionally activated dissociation mass‐analysed ion kinetic energy spectroscopy allowed discrimination between the two types of fragmentation processes and some other previously unreported gas‐phase reaction channels to be recognized.
Thermal denaturation of some proteins and its effect on their electrospray mass spectrat
Authors: Blanc J. C. Y. Le , Beuchemin D. , Siu K. W. M. , Guevremont R. , Berman S. S.
Published: May 2005

The effects of beat on the electrospray mass spectra of eight globular proteins in solution were studied. These ranged from hardly noticeable to a dramatic shift in the mass spectrometric profile and a concomitant increase in ion abundance. This change is believed to be the result of thermal denaturation of the protein species in solution resulting in a transition from a more compact to a less compact conformation. We accounted for this transition by means of a recently proposed model based on aqueous solution acid/base equilibria. For cytochrome c, profiles calculated by means of this model agree well with experimental data. The ΔH of the denaturation reaction of cytochrome c in aqueous solution containing 0.2% acetic acid was calculated from experimental data to be 103.8 ± 9.2 kJ mol−1, in good agreement with previous measurements.
Electron impact, chemical ionization and negative chemical ionization mass spectra, and mass‐analysed ion kinetic energy spectrometry–collision‐induced dissociation fragmentation pathways of some deuterated 2,4,6‐trinitrotoluene (TNT) derivatives
Authors: Yinon Jehuda , Fraisse Daniel , Dagley Ian J.
Published: May 2005

Mass‐analysed ion kinetic energy spectrometry (MIKES) with collision‐induced dissociation (CID) has been used to study the fragmentation processes of a series of deuterated 2,4,6‐trinitrotoluene (TNT) and deuterated 2,4,6‐trinitrobenzylchloride (TNTCI) derivatives. Typical fragment ions observed in both groups were due to loss of OR′ (R′ = H or D) and NO. In TNT, additional fragment ibns are due to the loss of R2′O and 3NO2, whilst in TNTCI fragment ions are formed by the loss of OCI and R2′OCI. The TNTCI derivatives did not produce molecular ions.

In chemical ionization (Cl) of both groups. MH+ions were observed, with [M – OR′]+fragments in TNT and [M – OCI]+fragments in TNTCI. In negative chemical ionization (NCI) TNT derivatives produced M−′, [M–R′]−, [M–OR′]−and [M–NO]−ions, while TNTCI derivatives produced [M–R]−, [M–Cl]−and [M – NO2]−fragment ions without a molecular ion.
Alkene loss from metastable methyleneimmonium ions: Unusual inverse secondary isotope effect in ion‐neutral complex intermediate fragmentations
Authors: Veith Hans J. , Gross Jürgen H.
Published: May 2005

The mechanism of propene elimination from metastable methyleneimmonium ions is discussed. The first field‐free region fragmentations of complete sets of isotopically labelled methyleneimmonium ions (H2C = \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^{\rm +} $\end{document}+R1R2: R1 = R2 = n‐C3H7; R1 = R2 = i‐C3H7; R1 = n ‐C3H7; R2 = C2H5; R1 = n‐C3H7; R2 = CH3; R1 = n‐C3H7; R2 = H) were used to support the mechanism presented. The relative amounts of H/D transferred are quantitatively correlated to two distinct mathematical concepts which allow information to be deduced about influences on reaction pathways that cannot be measured directly. Propene loss from the ions examined proceeds via ion‐neutral complex intermediates. For the di‐n‐propyl species rate‐determining and H/D distribution‐determining steps are clearly distinct Whereas the former corresponds to a 1,2‐hydride shift in a 1‐propyl cation coordinated to an imine moiety, the latter is equivalent to a proton transfer to the imine occurring from the 2‐propyl cation generated by the previous step. For the diisopropyl‐substituted ions which directly form the 2‐propyl cation‐containing complex, the rate‐determining hydride shift vanishes. The 2‐propyl cation‐containing complex can decompose directly or via an intermediate proton‐bridged complex. Competition of these routes is not excluded by the experimental results. Assuming a 2:1:3 distribution, a preference for the α‐ and β‐methylene of the initial n‐propyl chain as the source of the hydrogen transferred is detected for n‐propylimmonium ions containing a second alkyl chain R2. This preference shows a clear dependence on the steric influence of R2. During the transfer step isotopic substitution is found to affect the H/D distribution strongly. For the alternative route of McLafferty rearrangement leading to C2H4 loss, specific γ‐H transfer is observed.
Is the McLafferty rearrangement of ketones concerted or stepwise? The application of kinetic isotope effects
Authors: Stringer Michael B. , Underwood Dennis J. , Bowie John H. , Allison Colin E. , Donchi Kevin F. , Derrick Peter J.
Published: May 2005

Intramolecular 13C and 2H isotope effects have been measured for unimolecular losses of ethene (the McLafferty rearrangement) from metastable molecular ions of 2‐ethyl‐1‐phenylbutanl‐1‐one, 3‐ethylpentan‐2‐one and heptan‐4‐one. Primary and secondary deuterium isotope effects are observed at the γ‐(terminal) and β‐positions, respectively. Large primary 13C isotope effects occur at β‐positions and for the y positions of γ‐ethylpentan‐2‐one and heptan‐4‐one. The carbon isotope effects in the cases of the doubly isotopically labelled CH3COCH(C2H5)(13CH2CH3) and CD3COCD(C2D5)(13CD2CD3) are 1.17 (±0.01) and 1.04 (±0.01), respectively. All of these isotope effects are consistent with a stepwise mechanism in which more than one step is rate determining.
Structural characterization of sulfonated azo dyes using liquid secondary ion mass spectrometry/tandem mass spectrometry
Authors: Richardson Susan D. , McGuire John M. , Thruston Alfred D. , Baughman George L.
Published: May 2005

Eight monosulfonated and disulfonated azo dyes were analyzed using liquid secondary ion mass spectrometry/tandem mass spectrometry, in the negative ion mode, under low‐energy collision conditions (110–150 eV). Many structurally characteristic fragment ions were obtained, several of which have not been reported previously using other mass spectrometric techniques. Among the structurally important ions observed were those due to loss of SO2, SO3 and various aromatic substituents, such as NO2 and NHCOCH3. Losses of N2 were also proposed. In addition, product ions due to cleavage at the azo linkage were observed, and also SO3− and HSO3− ions. Several of the azo cleavage product ions detected did not contain sulfonate groups. Possible pathways for the formation of product ions are proposed. The structural information obtained should help to better characterize and identify sulfonated azo dyes in the future.
Experimental aspects of the collision‐induced decomposition of ions in a four‐sector tandem mass spectrometer
Authors: Bordas‐Nagy Jozsef , Despeyroux Dominique , Jennings Keith R. , Gaskell Simon J.
Published: May 2005

The collision‐induced decomposition (CID) of ions in the collision cell of a four‐sector tandem mass spectrometer (EBEB) was studied as a function of cell potential and collision gas over the mass range 246–1847. When the cell is at a potential of 2 kV or greater above earth, the loss of light ions by scattering is negligible and the detection sensitivity for light ions increases owing to their being reaccelerated on leaving the cell. The CID spectra given by helium and argon are very similar for ions derived from samples of m/z ⩽ 1000 but at higher masses argon is the more efficient collision gas.
Gas‐phase reactions of C3Hn+ ions
Authors: Prodnuk Stephen D. , Grocert Scott , Bierbaum Veronica M. , DePuy Charles H.
Published: May 2005

The kinetics and reactivity of linear C3H+ and cyclic and linear isomers of C3H2+ and C3H3+ were studied in a tandem flowing afterglow‐selected ion flow tube (FA‐SIFT). The C3Hn+ ions represent possible ionic precursors to neutral cyclopropenylidene, c‐C3H2, a molecule that has been found to be one of the most common species in interstellar space. Accordingly, rate coefficients and product branching ratios were measured for reactions of each ion with a variety of organic and inorganic compounds, with particular (but not exclusive) attention to neutral species likely to be present in the interstellar medium. The chemistry of C3H+ is found to be dominated by oxygen abstraction reactions to produce the relatively stable HC3O+ ion, and by reactions in which C2H2 or CO are eliminated. Cyclic and linear isomers of C3H2+ readily abstract H atoms to produce the stable cyclic or linear C3H3+ ions, presumably the ionic precursors to interstellar C3H2. C3H3+ ions react predominantly by clustering with the neutral reagent in three‐body association reactions. C3H3+ also proton transfers to several species to produce directly neutral C3H2.
Ion—neutral complex intermediate for loss of water from fatty acid carboxylates
Authors: Jensen Nancy J. , Haas George W. , Gross Michael L.
Published: May 2005

Fatty acid carboxylates desorbed into the gas phase by fast atom bombardment (FAB) undergo both metastable‐ion and collisionally activated dissociation (CAD) to eliminate water. Upon CAD, the water elimination competes with charge‐remote fragmentation. Deuterium labeling results are in accord with the elimination occurring via an ion‐neutral complex involving hydroxide ion and neutral ketene. Evidence that the precursor of the ion‐neutral complex is the enol form of the carboxylate was obtained by forming the enolate directly by decarboxylation of appropriate dicarboxylic acids. The [M  H  CO2]− ions of these species undergo a more facile elimination of water than do carboxylates desorbed directly upon FAB.
The barrier to methane elimination from ionized butan‐2‐one: Theory and experiment
Authors: Hudson Charles E. , Ahmed Mohammed S. , Olivella Santiago , Giam C. S. , McAdoo David J.
Published: May 2005

Experimental and theoretical approaches are used to characterize the losses of methyl and methane from ionized butan‐2‐one (a). Formation of a with an energy content near its dissociation threshold by charge exchange demonstrates that even very close to the threshold, methane elimination competes poorly with methyl loss. The fragmentations of a generated by reaction of CH3CH3 with CH2CO+˙ support this conclusion. Ab initio calculations were used to locate a transition state for methane elimination from a though the ion‐neutral complex [CH3CH2CO+ ˙CH3]. These calculations place the top of the barrier to methane elimination 6–7 kJ mol−1 above the threshold for methyl loss and the products of methane elimination about 35 kJ mol−1 below that threshold. This barrier prevents methane elimination from competing strongly with methyl loss. A binding energy of 18 kJ mol−1 was obtained for the complex [CH3CO+ ˙CH2CH3]. The difference between the theoretical energies for the products of methane and methyl loss obtained by including electron correlation are in good agreement with experimental results. However, the calculated difference [ΔHf(CH3CH2CO+) + ΔHf(˙CH3)] minus ΔHf(a) was 23.1 kJ mol−1 smaller than the experimental difference. Although there is clearly a barrier to the H transfer in the elimination of methane from a, whether that transfer takes place by surmounting the barrier or by tunneling through it cannot clearly be determined owing to uncertainties in the barrier height.