bluuebunny/crossref_metadata_2025_split huggingface dataset:
Stereoelectronic effects on the retro‐Diels‐Alder fragmentation of ionized bicyclo[4.3.0]nona‐3,7‐dienes
Authors: Tureček František , Pancíř Jiří , Stahl Daniel , Gäumann Tino 
Published: May 2005

AbstractThe electron impact and collision‐induced dissociation mass spectra of cis‐ and trans‐annulated bicyclo[4.3.0]nona‐3,7‐dienes differ in their relative abundances of [C5H6]+˙ fragments formed by the retro‐Diels‐Alder decomposition. The formation of [C5H6]+˙ is not preceded by hydrogen migration in the short‐lived and long‐lived molecular ions. The appearance energy of [C5H6]+˙ from both annulation isomers is identical within experimental error: AEcis([C5H6]+˙)=10.56±0.10 eV and AEtrans([C5H6]+˙)=10.54±0.15 eV. The barrier to the retro‐Diels‐Alder fragmentation lies 68–76 kJ mol−1 above the thermo‐chemical threshold corresponding to [C5H6]+˙ + C4H6. Investigation of the two‐dimensional reaction coordinate by the Topological Molecular Orbital treatment shows that the lowest energy path for the retro‐Diels‐Alder reaction involves a two‐step dissociation of the C(5)C(6) and C(1)C(2) bonds in the molecular ion, the latter step overcoming a barrier, calculated as 80 kJ mol−1 above the thermochemical threshold. The stereochemical difference between the geometric isomers is due to stereoelectronic assistance of the π orbitals of the cis‐annulated isomer in the cleavage of the C(5)C(6) bond. Other mechanisms of the retro‐Diels–Alder reaction are discussed.
Hydrogen chemical ionization mass spectrometry of metalloporphyrins
Authors: Evershed Richard P. , Wolff George A. , Shaw Graham J. , Eglinton Geoffrey 
Published: May 2005

AbstractThe hydrogen chemical ionization (H2 CI) mass spectra of a range of metal(II) (Ni, Cu, Co, Pt), metal (III) (Al, Mn, Ga, Fe (bearing a single axial ligand)) and metal(IV) (Si, Ge, Sn (bearing two axial ligands) and V (as VO2+)) porphyrins have been determined, The spectra are highly dependent on the coordinated metal, rather than the axial ligand(s) (where present). Ni(II), Cu(II), Mn(II or III), Ga(III), Ge(IV), Fe(III) and Sn(IV) porphyrins fragment via hydrogenation and demetallation, followed by cleavage of the resulting porphyrinogens at the meso(bridge) positions to give mono‐ and di‐pyrrolic fragments. Tripyrrolic fragments are also observed in the case of Ni(II), Cu(II) and Sn(IV). Fragmentations of this type are similar to those observed for free‐base porphyrins. In the case of Pt(II), Co(II), Al(III), Si(IV) and V(IV) (as vanadyl), the dipyrrolic fragment ions are either very weak or completely absent; hence their H2CI spectra contain limited structural information. This variable CI behaviour may be related to the relative stabilities of the metalloporphyrins together with the multiple stable valency states exhibited by several metals.
Chemical ionization induced competing and consecutive heterolytic ring cleavages in the mass spectra of substituted tetrahydro‐1,3,2‐oxazaphosphorin‐2‐oxides
Authors: Kulkarni P. S. , Gogte V. N. , Modak A. S. , Sahasrabudhe S. D. , Tilak B. D. 
Published: May 2005

AbstractThe chemical ionization mass spectra of some substituted heterocyclic systems, viz. tetrahydro‐1,3,2‐oxazaphosphorin‐2‐oxides, reveal polar cycloreversion reactions. The zwitterion intermediate formed by the heterolysis of the CO bond undergoes stepwise clean heterolysis further resulting in the expulsion of the potential nucleophilic and electrophilic groups. The competitive and consecutive heterolysis of the ring bonds leads to the formation of the protonated phenylimine ions. Competing with heterolysis, elimination reactions involving hydrogen transfer leading to the formation of α‐phenylethyl ions are also observed. The chemical ionization mass spectrometry of the heterocyclic system shows many features of the Grob type of fragmentation mechanism.
The mass spectrometry of 2‐methylthio(glyco)oxazoline derivatives of pentoses and hexoses
Authors: Byrd Gary D. , Davidson Robert M. , Edward White V , Coxon Bruce , Margolis Sam A. 
Published: May 2005

AbstractThe fragmentation of 2‐methylthio(glyco)oxazolines under electron impact has been investigated by low‐ and high‐resolution mass spectrometry. Field desorption was used in those cases where the molecular ion was weak or missing. Fragmentation pathways were determined by monitoring metastable transitions and through the use of labeled compounds. The results support the anticipated structure for these compounds and show the sensitivity of the mass spectra toward ring size.
Ammonium adduct ion in ammonia chemical ionization mass spectrometry: 2—Mechanism of elimination of neutrals from the ammonium adduct ion
Authors: Nakata Hisao , Hoshino Yoshihiko , Takeda Naohito , Tatematsu Akira 
Published: May 2005

AbstractThe mechanism of elimination of ROH (R = H or CH3) from the ammonium adduct ion, [M+NH4]+, of 1‐adamantanol and its methyl ether is examined by using linked‐scan metastable ion spectra and by measuring the dependence of the peak intensity ratio [M+NH4]+/[M+NH4ROH]+ on ammonia pressure. For 1‐adamantanol both SNi and SN1 reactions are suggested in metastable ion decomposition, while only the SN1 mechanism is operative in the ion source. For 1‐adamantanol methyl ether the SN1 reaction predominates both in metastable ion decomposition and in the ion source reaction.
Electron impact fragmentation of some mixed cyclotetraphosphazenes
Authors: Bamgboye T. Tunde , Bamgboye Omolara A. 
Published: May 2005

AbstractThe electron impact fragmentations of some cyclotetraphosphazenes are reported and discussed. The major fragmentation path involves loss of two amine radicals and one chlorine radical in the series P4N4Cl8‐n(NMe2)n when n=2, and subsequent stages involve a ring contraction process with elimination of a P = N fragment, when n = 5 loss of amine radicals predominates on statistical grounds with little evidence of ring contraction. In the series P4N4F8‐n(NMe2)n fragmentation is dominated by loss of amino radicals when n = 4 and loss of fluorine radicals predominates on statistical grounds when n = 2. In the series P4N4F8‐nXn (n = 2 or 4, X = Cl or Br), when n = 2 and X = Br the major fragmentation path is the loss of two bromine radicals, whereas when X = Cl the more favoured path is the loss of two chlorine radicals. In both, subsequent stages involve ring contraction reactions with elimination of a PN fragment. When n = 4 and X = Br or Cl on bond energy grounds the more favoured fragmentation pattern is the loss of bromine or chlorine radicals, respectively.
Characterization of the high molecular mass chlorinated matter in spent bleach liquors (SBL): 3—Mass spectrometric interpretation of aromatic degradation products in SBL
Authors: Österberg F. , Lindström K. 
Published: May 2005

AbstractThe aromatic content of the high molecular matter in pulp mill bleachery effluents has been characterized after chemical degradation in the laboratory followed by derivatization and gas chromatographic/mass spectrometric analysis. Aromatic molecules with carboxy, hydroxy, methoxy and chloro substituents were separated and identified by recording their computer‐reconstructed ion current chromatograms. About 70 aromatic carboxylic acids were tentatively identified as degradation products. Aromatic compounds with hydroxy groups were derivatized to trideuteromethoxy groups in order to distinguish them by mass spectrometry from compounds with naturally occurring methoxy groups. Single and double hydrogen atom transfer between methyl ester and methoxy groups were observed in some molecules and given special attention when they occurred ortho to each other. A methoxy group substituted ortho to a methyl carboxylate resulted in an unusual fragmentation.
Rearrangements in the molecular ion of o‐(methyl‐d3‐thio)benzoic Acid: Comparison with o‐methoxy‐d3‐benzoic acid
Authors: Gillis Richard G. , Porter Quentin N. 
Published: May 2005

AbstractHydrogen/deuterium exchange and rearrangements in the molecular ion of o‐(methyl‐d3‐thio)benzoic acid lead to fragment ions [MOD]+ as well as [MOH]+ and m/z 106 and 107, just as in the molecular ion of o‐methoxybenzoic acid. However, the fragment ion m/z 108 has the composition C6H4S rather than C6H2D3CO as it does in the case of o‐methoxy‐d3‐benzoic acid. By varing the repeller potential at 10 eV (and thus the residence time in the ion source), the corresponding fragments are seen to be formed more slowly from the methylthio acid than from the methoxy acid, which leads to the conclusion that H/D exchange between carboxyl and labelled methylthio is slower than it is between carboxyl and labelled methoxyl.
Fragmentation of organic anions induced by exothermic addition reactions
Authors: Depuy Charles H. 
Published: May 2005

AbstractThe addition reactions of a series of carbanions with CO2, COS and CS2 have been studied in a flowing afterglow apparatus. Carbon dioxide simply forms the adduct, but the more highly exothermic additions to COS and CS2 lead both to addition and addition followed by fragmentation. A number of novel fragmentation pathways have been observed for additions to diazo anions and to anions derived by proton abstraction from allenes. In addition to these reactions, highly basic anions are observed to undergo sulfur atom transfer reactions with CS2.
Chemical ionization (NO) spectra of n‐alkenoic acids and their esters
Authors: Brauner A. , Budzikiewicz H. , Francke W. 
Published: May 2005

AbstractIt is shown that the position of one double bond in long‐chain unsaturated fatty acids and their esters can be determined by chemical ionization (NO) mass spectra. Homoconjugated dienoic acids (as linoleic acid) do not give a characteristic framentation pattern, in contrast to those which contain three or more homoconjugated double bonds.
Further studies on a site‐specific hydrogen transfer observed in electron capture negative ion chemical ionization mass spectrometry of hydroxyamine pentafluoropropionate derivatives
Authors: Low G. K.‐C. , Duffield A. M. 
Published: May 2005

AbstractFurther studies have demonstrated that the site‐specific hydrogen transfer process involved in the formation of the m/z 145 anion of β‐hydroxyamine pentafluoropropionate (PFP) derivatives observed under electron capture negative ion chemical ionization conditions occurs when the two functional groups are separated by up to five carbon atoms. Deuterium labelling has established that the site specificity, transfer of a hydrogen atom from the carbon adjacent to nitrogen to the OPFP group, is maintained in 4‐amino‐butan‐1‐ol‐N, O‐(PFP)2. The corresponding PFP derivatives of the N‐methylaminoalkanol‐(PFP)2 derivatives lack the m/z 145 species with m/z 163, [OPFP]− being the base anion. Substitution of alkyl groups on the carbon adjacent to oxygen results in a diminution of the ion intensity at m/z 145. with a marked increase in the intensity of m/z 144. The formation of the m/z 145 and 144 anions to proposed to proceed through the intervention of a fluoride ion‐molecule complex as outlined in Scheme 1 with the product ion distribution dependent on which of the two pathways is preferred.
Cyclopropane intermediates in the rearrangement and fragmentation of olefinic molecular ions
Authors: Laderoute Keith R. , Harrison Alex. G. 
Published: May 2005

AbstractMethyl loss from deuterium‐labelled molecular ions of 4‐methyl‐2‐pentene, 2‐methyl‐2‐pentene and 1,1,2‐trimethylcyclopropane has been investigated for metastable molecular ions and for molecular ions formed by charge exchange with COS+˙, XE+˙ and CO+˙. For metastable ion fragmentation reactions all three compounds exhibit very similar behavior and show specific and essentially equal loss of each of the original methyl groups as well as specific loss of a methyl where the hydrogens derive exclusively from the non‐methyl hydrogens of the original molecules. The former results are interpreted in terms of interconversion of the three molecular ions through a ring‐opened form of the trimethylcyclopropane molecular ion. The loss of the non‐methyl hydrogens as CH3 is interpreted in terms of isomerization to the 2,3‐dimethyl‐2‐butene structure. With increasing internal energy direct allylic cleavage of the unrearranged methylpentene molecular ions increases in importance while the trimethylcyclopropane molecular ion shows an increased preference for loss of the C(2) methyl group. With increasing internal energy loss of the original non‐methyl hydrogens as CH3 decreases markedly in importance.
Effects of stepwise excitation on fragmentation of gas phase ions: An especially striking case
Authors: Kenttämaa H. I. 
Published: May 2005

AbstractDramatically different fragmentation patterns are obtained for 4‐ethyl‐2,6,7‐trioxa‐1‐phosphabicyclo‐[2.2.2]octane‐1‐oxide upon electron ionization (EI) and for the corresponding molecular ion on collision‐induced dissociation (CID). Two reasons for this behaviour have been discovered. (i) Irreversible multistep isomerization of the molecular ions occurs prior to collisional activation in mass spectrometry/mass spectrometry (MS/MS). Isomerization reactions have been characterized by isotopic labelling and by examining structures of relevant unlabelled and labelled fragment ions by MS/MS. The extent of isomerization can be controlled by varying the amount of internal energy of the molecular ions. This has been done by changing the number of thermalizing collisions which the ions undergo with neutral molecules in the ion source. (ii) When multiple collisions are used to dissociate the molecular ions, the initially stable fragmentation products undergo extensive further decomposition. As a result, abundant phosphorus‐containing fragment ions are obtained for the bicyclic phosphate in high‐pressure CID, whereas electron ionization leads to predominant hydrocarbon ions. A minor change in the structure of this molecule has major effects on the fragmentation behaviour: high‐ and low‐energy collisional activation spectra of the molecular ion of the corresponding phosphite are identical with the 12 e V EI mass spectrum of the neutral.
On the origin of fast forward moving ions generated from a repelled ion beam
Authors: Burgers Peter C. 
Published: May 2005

AbstractMass‐selected positive ions having kilovolt translational energies can be reversed in direction by applying a large voltage to a collision chamber situated in the second field‐free region of a reversed geometry instrument. It is shown that such repelled positive ions impinge on the earthed plate, situated in front of the chamber, leading to a variety of negative ions. The mass‐to‐charge ratios of these anions have been measured.
A comparison of electron impact mass spectrometry of isomeric pyranocoumarins (pyrano[3,2‐c][1]benzopyran‐5‐ones) and pyranochromones (pyrano[2,3‐b][1]benzopyran‐5‐ones)
Authors: Bravo Pierfrancesco , Ticozzi Calimero , Daolio Sergio , Traldi Pietro 
Published: May 2005

AbstractThe mass spectrometric behaviour of two pairs of isomeric 2,2‐dimethylpyrano[3,2‐c][1]benzopyran‐5‐ones (pyranocoumarins) and 2,2‐dimethylpyrano[2,3‐b][1]benzopyran‐5‐ones (pyranochromones) and four pairs of isomeric 2‐hydroxymethyl‐2‐methylpyrano[3,2‐c][1]benzopyran‐5‐ones (pyranocoumarins) and 2‐hydroxymethyl‐2‐methylpyrano[2,3‐b][1] benzopyran‐5‐ones (pyranochromones) has been studied in detail with the aid of exact mass measurements, linked scans, collisionally activated decompositions and deuterium labelling experiments. The presence in both series of compounds of the same ions derived by structural interconversion of both molecular ions is emphasized, and structural information on the ions [C7H5O2]+ (m/z 121), highly characteristic for these classes of compounds, as for 4‐hydroxycoumarins, is reported.
Analysis of free fatty acids by a micro‐liquid chromatograph directly coupled with a mass spectrometer through a vacuum nebulizing interface
Authors: Matsumoto K. , Yoshida H. , Ohta K. , Tsuge S. 
Published: May 2005

AbstractA liquid chromatograph directly coupled with a quadrupole mass spectrometer through a vacuum nebulizing interface was applied to the analysis of various free fatty acids. Chemical ionization mass spectra of the C7C22 free fatty acids were first examined using either methanol or benzene as the reagents. Then the practical compositional analysis of the fatty acids were performed with various biological samples such as bean oil, rape oil, palm oil and milk fat where most of the fatty acids are included as their triglycerides.
Fast atom bombardment mass spectra of O‐isopropyl oligodeoxyribonucleotide triesters
Authors: Phillips Lawrence R. , Gallo Kathleen A. , Zon Gerald , Stec Wojciech J. , Uznanski Bogdan 
Published: May 2005

AbstractThe positive and negative fast atom bombardment (FAB) mass spectra of representative dimeric O‐isopropyl oligodeoxyribonucleotide triesters were recorded and examined. Features found to be shared among the FAB mass spectra of these compounds yielded important information regarding molecular weight, nucleobase composition and nucleotide sequence.
Charge stripping and the site of cationization of substituted aromatic compounds
Authors: Pachuta Steven J. , Isern‐Flecha Ileana , Cooks R. Graham 
Published: May 2005

AbstractThe site of protonation, methylation and ethylation of anilines, phenols and thiophenols can be determined by tandem mass spectrometry (MS/MS). The extent to which the cation attachment products formed from these compounds undergo charge stripping is related to the site of cation attachment. The formation of doubly charged ions for phenols and anilines is favored when cation attachment occurs on the ring. For thiophenols alkylation occurs almost exclusively on the substituent, and yet an abundant stripping peak is recorded. This behavior is opposite to that seen for anilines and phenols and is accounted for in terms of orbital energies in the three systems. The site of cation attachment, as deduced by charge stripping, agrees with conclusions drawn from the fragmentation behavior of these ions upon collision‐induced dissociation.
The effect of ion source temperature on the fragmentation of 2‐pentanone
Authors: Wood K. V. , McLuckey S. A. , Cooks R. Graham 
Published: May 2005

AbstractA dramatic source temperature effect is observed in the low collision energy collision‐induced dissociation spectra of molecular ions derived via 70 eV electron bombardment of 2‐pentanone and via charge exchange. The observation cannot simply be attibuted to the increased thermal energy of the 2‐pentanone molecular ions. A number of factors appear to contribute to the observed results, (i) the increase, with temperature, in the ratio of the concentration of enol to keto neutral molecules, (ii) temperature effects on the Franck‐Condon factors which control the yields and internal energies of the keto and enol molecular ions, (iii) differences in the relative stabilities of the keto and enol molecular ions, (iv) temperature effects on the equilibrium between the isomeric molecular ions. This last item is probably most important, with the concentration of magnified image being most significant. These data highlight the role that ion source temperature can play as a variable in elucidating ion structure problems.
An unusual pathway for the elimination of hydrogen cyanide from benzonitrile upon electron impact as revealed by 13C and 15N labelling
Authors: Molenaar‐Langeveld Tineke A. , Fokkens Roel H. , Nibbering Nico M. M. 
Published: May 2005

AbstractIt is shown by 15N and specific 13C labelling that ∼50% of the molecules of hydrogen cyanide, eliminated within ∼10−6 s upon electron impact of benzonitrile, contains the original cyano carbon atom, whereas the remaining percentage contains one of the phenyl ring carbon atoms at random. This is even more dramatic for the molecular ions of benzonitrile which decompose in the first and second field‐free regions of the VG Micromass ZAB‐2F high‐field mass spectrometer used. Then only 5–7% of the eliminated molecules of hydrogen cyanide contains the original cyano carbon atom. A cycloaddition‐cycloreversion process in the molecular ions, leading to ionized 1‐cyano‐1,3‐hexadien‐5‐yne as an intermediate in the hydrogen cyanide loss, is proposed to explain this.
Evidence for long‐lived excited states of [CnH2]2+ carbodications
Authors: Appling J. R. , Shields G. C. , Moran T. F. 
Published: May 2005

AbstractThe energetics, structures, stabilities and reactivities of[CnH2]2+ ions have been investigated using computational methods and experimental mass spectrometric techniques. Spontaneous decompositions of [CnH2]2+ into [CnH]+ + H+ products, observed for ions with odd‐n values, have been explained by invoking the formation of excited triplet states. Even‐n [CnH]+ ions possess triplet ground states with low‐lying excited states, whereas odd‐n ions have triplet states with energies several eV above ground singlet states. Radiationless transitions of vibrationally excited long‐lived triplet state ions into singlet state continua are suggested as possible mechanisms for spontaneous deprotonation processes of odd‐n [CnH2]2+ ions. Evidence for these long‐lived excited states has been obtained in bimolecular single electron transfer reactions.
Stereospecific alcohol elimination from dialkyl mesaconates under electron impact: Another example of a hidden hydrogen transfer
Authors: Bornstein Daniel , Weisz Adrian , Mandelbaum Asher 
Published: May 2005

AbstractUnder electron impact dimethyl and diethyl mesaconates give rise to abundant [M  MeOH]+ and [M  EtOH]+ ions, respectively. The geometrically isomeric citraconates yield [M  MeO]+ and [M  EtO]+ ions. Mixed methyl ethyl mesaconates eliminate both methanol and ethanol. These findings, together with the results of a deuterium labelling study, indicate that the elimination of alcohol from the molecular ions of the mesaconates is partially preceded by a hidden hydrogen transfer step.
Mass spectroscopic fragmentation reactions. XXXI—the fragmentation behaviour of o‐halobenzoic acids substituted in the 5‐position: A novel ortho effect
Authors: Budzikiewicz H. , Breuer M. 
Published: May 2005

AbstractA fragmentation sequence typical for o‐halobenzoic acids carrying a heteroatom substituent in the 5‐position is described.
Reactions induced by the γ‐hydrogen shift in the molecular ion of 1‐nitropropane
Authors: Hindawi S. K. , Fokkens R. H. , Pinkse F. A. , Nibbering N. M. M. 
Published: May 2005

AbstractIt is shown that the γ‐ or 1,5‐hydrogen shift in the molecular ion of 1‐nitropropane leads to three primary fragmentation reactions. They are the loss of a hydroxyl radical, a molecule of ethylene and a molecule of nitric oxide. The structures and chemistry of the resulting ions have been investigated by a series of experiments including deuterium labelling, spontaneous and collisionally induced dissociations and accurate mass measurements.
Mass spectrometric studies of heterocyclic organophosphorus compounds: 1,3,2‐benzodioxaphospholes
Authors: Hangzhou Guo , Tsao Kinghunt , Yu‐Gui Li , Wangjian  
Published: May 2005

AbstractThe mass spectral behaviour of a new type of organophosphorus compound, 1,3,2‐benzodioxaphospholes, under 70 eV electron impact has been studied by means of high and low resolution mass spectrometry as well as by B/E and B2/E linked scans. The influence of different substituents in the molecule (O, S or N) on the mass spectra is investigated. The reason for isomerization between oxygen and sulphur within a molecule studied by mass spectrometry is discussed.