bluuebunny/crossref_metadata_2025_split huggingface dataset:
The reaction intermediate spectrum. A new type of experiment in tandem mass spectrometry
Authors: O'Lear John Raymond , Wright Larry Gene , Louris John Nathan , Cooks R. Graham 
Published: May 2005

AbstractA new scan is described which responds to ions that are intermediates in the dissociation of a mass‐selected parent ion (mp) to give a mass‐selected daughter ion (md). The scan gives a simple mass v. abundance output for ions which satisfy this condition. It is implemented here on a BEQQ hybrid mass spectrometer using, in sequence, collision‐induced dissociation occurring at high energy in the first reaction region, and low‐energy collisional activation in the collision quadrupole. The experiment provides information on reaction sequences not available from single scans of other types. In the several cases examined, it is demonstrated that, among many conceivable fragmentation routes connecting a parent ion with a particular fragment ion, only a few are significant. Examination of reaction intermediate spectra also appears to be a fruitful new approach to mechanistic questions, as illustrated by consideration of the behavior of several isomeric octanones. These new spectra also have analytical value: they show good signal‐to‐noise ratios and allow ready distinction between isobaric and isomeric ions. A comparison of the reaction intermediate spectrum with a daughter spectrum obtained by the B/E linked‐scanning technique reveals the contributions of artifact peaks which result from poor parent ion mass resolution in the latter. Reaction intermediate spectra combine information from the daughter spectra of mp and the parent spectra of md and, as a specified portion of this data domain, have unique characteristics.
Nucleophilic substitution in certain bicyclic sesquiterpene alcohols under chemical lonization conditions
Authors: Madhusudanan K. P. , Srivastava Rashmi , Kulshreshtha D. K. , Fraisse D. 
Published: May 2005

AbstractSubstitution of amino for hydroxyl groups in certain sesquiterpene alcohols has been studied by chemical ionization mass spectrometry using ammonia and ammouia‐d3 as the reagent gases, and by mass‐analysed ion kinetic energy spectrometry and collision‐induced decomposition mass‐analysed ion kinetic energy measurements. Depending upon the source conditions and the nature of the substrates, both SNi and SN1 mechanisms have been found to operate. No evidence is obtained for an SN2 mechanism in these compounds. In centdarol and isocentdarol, addition of NH3 to the double bond, followed by elimination of H2O, also contributes to the substitution process. Attack of [NH4]+ on the epoxide function, followed by loss of H2O, appears to lead to the substitution ions in epoxycentdarol, epoxyisocentdarol and epoxyhimachalol.
Unimolecular dissociations of the [2‐hexanone]+˙ metastable ion
Authors: Bouchoux Guy , Tortajada Jeanine , Dagaut Jacques , Fillaux Joëlle 
Published: May 2005

AbstractThe metastable molecular ion of 2‐hexanone loses a methyl radical mainly (∼80%) from positions C(4) and C(6), in equal proportions, as indicated by 13C labelling. The necessary skeletal rearrangement of the butyl chain is interpreted in terms of a 1,2‐[enol‐olefin] +˙ shift. The results and the mechanisms concerning the minor eliminations of C2H4, C2H5˙, C3H5˙ and C3H6 neutrals are also discussed.
Mass spectral fragmentation of benzofurazan‐1‐oxide
Authors: Dyall Leonard K. 
Published: May 2005

AbstractFragmentations in the mass spectrum of benzofurazan‐1‐oxide have been studied using linked scan, accelerating voltage scan and mass‐analysed ion kinetic energy spectrometric techniques. Major pathways involve NO·+ NO· and NO·+CO loss, these double losses occurring in such rapid succession as to appear ‘concerted’ in some experiments. Minor pathways are loss of CO2, C2N2O2, or C2HN2O2 from the molecular ion. The major fragment ion, m/z 76, in the conventional mass spectrum is not detected in a mass‐analysed ion kinetic energy spectrometric experiment with the molecular ion until collision activation is provided. The conventional electron impact spectrum invariably includes ions from benzofurazan which is produced by thermal deoxygenation in the source.
Average mass approach to stable isotope dilution mass spectrometry
Authors: Blom Karl F. 
Published: May 2005

AbstractThe application of the fundamental equation for isotopic dilution mass spectrometry to real samples often requires using approximations which can lead to significant errors. A new approach relating the composition of isotopic mixtures to the average mass of the mass spectral pattern is presented. The fundamental equations are completely general; they should be applicable to a wide range of problems and may provide improved accuracy and precision in some cases.
Energy dependence of the fragmentation of ester enolate ions
Authors: Young Alex B. , Harrison Alex G. 
Published: May 2005

AbstractThe energy dependence of the fragmentation of a selection of ester enolate ions has been studied by variable, low‐energy collision‐induced dissociation experiments in the quadrupole collision cell of a hybrid BEQQ mass spectrometer. The dominant fragmentation reactions observed are
equation image
where ΔH1 − ΔH2=PA([RCCO]−) − PA([ŔO]−) (PA=proton affinity). The anion of lowest proton affinity is formed preferentially at low internal energies with the yield of the anion of higher proton affinity increasing with increasing internal energy. The [CH3OCOCOCH2]− anion derived from methyl pyruvate forms [CH3OCO]− by reaction (2); this anion readily fragments to [CH3G]−+ CO consistent with a structure represented by a dipole‐stabilized cluster of [CH3O]− and CO.Comparison of the 8‐keV with the 50‐eV collision‐induced dissociation mass spectra indicated that the average internal energy of the fragmenting ions is considerably lower in the high‐energy collisional experiments than it is in the low‐energy collisional experiments.
Wavelength‐dependent fragmentation in resonance‐enhanced multiphoton ionization mass spectrometry
Authors: Kinsel Gary R. , Segar Kimberly R. , Johnston Murray V. 
Published: May 2005

AbstractWavelength‐dependent effects in the resonance‐enhanced multiphoton ionization/fragmentation mass spectra of p‐chloroaniline and diphenyl ether are presented. For both molecules, the formation of low‐energy fragments can be discriminated against in favor of higher‐energy fragments by using ‘low’‐energy radiation (290 nm region) for ionization/fragmentation. The same low‐energy fragments become dominant when higher‐energy radiation (266 nm) is used. This unique behavior is explained in terms of the narrow distribution of parent ion internal energies created through resonance‐enhanced multiphoton ionization/fragmentation and the competing kinetic processes accessed by the parent ion as it absorbs each successive photon.
Hydrogen interchange prior to the fragmentation of protonated molecules
Authors: Harrison Alex G. 
Published: May 2005

AbstractProtonated ethanol shows extensive interchange of hydrogen between the carbons and oxygen prior to fragmentation to either [C2H5]++H2O or to [H3O]++C2H4. Protonated isopropanol shows only a small amount of interchange of hydrogens between carbon and oxygen prior to fragmentation, while protonated alkyl amines show practically no interchange of hydrogens between carbon and nitrogen prior to fragmentation. These results, and earlier results (Terlouw et al., Org. Mass Spectrom. 21, 665 (1986)) showing hydrogen interchange between carbon and oxygen in protonated methyl isopropyl ether, arc discussed in relation to the energetics for fragmentation of the protonated species [RHXH]+. It is concluded that as long as the proton affinity of XH is greater than the proton affinity of the olefin R by no more than ∼50 kJ mol−1 hydrogen interchange between the alkyl group and the heteroatom will occur.
Reduction of a quaternary phenanthridinium salt in a chemical ionization source when introduced by direct injection in acetonitrile
Authors: Arpino Patrick J. , Schmitter Jean‐Marie 
Published: May 2005

AbstractReduction of phenanthridinium methiodide to its dihydroderivative was observed when a solution of the sample in acetonitrile was injected in the chemical ionization source of a combined liquid chromatograph/mass spectrometer equipped with a direct liquid introduction interface and a polarized desolvation chamber.
Energy‐resolved collisional activation of dimethyl phosphonate and dimethyl phosphite ions in a quadrupole ion trap and a triple quadrupole mass spectrometer
Authors: Brodbelt Jennifer S. , Kenttämaa Hilkka I. , Cooks R. Graham 
Published: May 2005

AbstractCollision‐activated dissociation spectra of dimethyl phosphonate and dimethyl phosphite ions were measured as a function of the amplitude of a supplementary AC voltage applied across the end‐caps of an ion‐trap mass spectrometer. These spectra yield breakdown graphs which bear a close resemblance to those obtained by varying collision energy in a triple‐quadrupole mass spectrometer operating under multiple‐collision conditions. Variation in the time of excitation at the resonance frequency provides an alternative route to breakdown graphs. The results demonstrate that energy deposition occurs via multiple activating collisions in the ion trap. Maximum energy deposition observed is somewhat smaller under normal operating conditions in the ion trap than in the triple‐quadrupole mass spectrometer.
Analysis of natural products by tandem mass spectrometry employing reactive collisions with ethyl vinyl ether
Authors: Pachuta Robin R. , Kenttämaa Hilkka I. , Cooks R. Graham , Zennie Thomas M. , Ping Cai , Chang Ching‐Jer , Cassady John M. 
Published: May 2005

AbstractRepresentative natural products from the diterpene dilactone, psorospermin and quabalactone classes were protonated, mass‐selected and reacted with ethyl vinyl ether in a triple‐quadrupole mass spectrometer. Minor differences in the structures of the compounds led to different reactivities toward the reagent, as indicated by the relative abundances of products such as the ethylated and vinylated compounds. Additional information is obtained from the dissociation products formed upon non‐reactive, inelastic collisions with the neutral reagent. The daughter spectra have excellent signal: noise ratios and good reproducibility. The results demonstrate that the use of reactive collisions may supplement collision‐activated dissociation in chemical analysis of large organic molecules by tandem mass spectrometry.
Low‐resolution accurate mass measurement in self‐chemical ionization mass spectrometry
Authors: Caldwell Gary W. , Masucci John A. , Inners Ruth R. 
Published: May 2005

AbstractLow‐resolution (2000, 10% valley definition) accurate mass measurements using self‐chemical ionization (self‐CI) have been evaluated as an alternative to the conventional chemical ionization (CI) method. In conventional CI experiments a high pressure of reagent gas is required to induce ionization while in self‐CI no reagent gas is used and the self‐CI is produced presumably by molecular/fragment ion–molecule reactions. Nine compounds ranging in mass from 50–500 daltons were examined. Results obtained by the self‐CI method indicate that the elemental composition assignment can be obtained simultaneously for the protonated molecule and/or molecular ion. It is also shown that perfluorokerosene can be used routinely in self‐CI as an internal reference standard over a broad mass range (50–500 daltons). It is sometimes difficult to use a single reference standard in conventional low‐resolution CI accurate mass spectrometry over a similar mass range.
The structures of [C4H4]+˙ ions by photodissociation in an ion cyclotron resonance spectrometer
Authors: van der Hart W. J. 
Published: May 2005

AbstractThe photodissociation of [C4H4]+˙ fragment ions at the ion cyclotron resonance time‐scale competes with relaxation of the internal energy by infrared emission. As a result the fraction of photodissociating ions increases with light intensity. The experiments indicate that [C4H4]+˙ from benzene and 1,5‐hexadiyne consists of a mixture of 60% vinyl acetylene ions, 10% butatriene ions and 30% cyclic ions. This confirms previous conclusions from studies of the ion‐molecule reactions of [C4H4]+˙ with benzene.
Reduction of methylene blue during the ionization process
Authors: Burinsky David J. , Dilliplane Robert L. , DiDonato Gerald C. , Busch Kenneth L. 
Published: May 2005

AbstractMethylene blue is readily reduced during the process of ionization in methods as diverse as electron ionization, direct exposure probe electron ionization, direct exposure probe chemical ionization, solid sample secondary ion mass spectrometry, liquid sample secondary ion mass spectrometry and fast atom bombardment mass spectrometry. The reduction of methylene blue need not initially involve the abstraction of hydrogen from the solvent present in the latter two methods.
Charge neutralization of ions from benzene
Authors: Sedgwick J. B. , Nelson P. R. , Steiner P. A. , Moran T. F. 
Published: May 2005

AbstractCross‐sections have been measured for the charge neutrilization if ions from benzene in kiloelectron‐volt collisions with benezene target molecules. Measured values range from 65 Å2 for the symmetric [C6H6]+C6H6 resonant reactions to 8 Å2 for [C3H3]+C6H6 reactions. Cross‐sections computed using a simple resonance charge transfer model compare favourably with experimental data for the symmetric reactions. The cross‐sections for asymmetric reactions are smaller that those for they symmetric system and magnitudes of the asymmetric cross‐sections are correlated with recombination energies of the respective ions.
On the mechanism of the ethyl elimination from the molecular ion of 6‐methoxy‐1‐hexene
Authors: Molenaar‐Langeveld Tineke A. , Fokkens Roel H. , Nibbering Nico M. M. 
Published: May 2005

AbstractIt is shown by 13C and D labelling that the ethyl radical elimination from the molecular ion of 6‐methoxy‐1‐hexene is a very complex process involving at least two different channels. The major channel (80%) is induced by an initial 1,5‐hydrogen shift in the molecular ion from C(5) to C(l) leading via a series of steps to methoxy‐cyclohexnne, which then undergoes a ring contraction to 2‐methyl‐1‐methoxycyclopentane, being the key intermediate for the ethyl loss. The same key intermediate is formed in the other, minor channel (20%) by ring closure directly following an initial 1,6‐hydrogen shift in the molecular ion of 6‐methoxy‐1‐hexene from C(6) to C(l). Collision‐induced dissociation experiments on the [M − ethyl]+ ion from 6‐methoxy‐1‐hexene have further established that it has the unique structure of oxygen methyl cationized 2‐methyIpropen‐2‐al. This ion is also generated by ethyl loss from the molecular ion of 2‐methyl‐1‐methoxycyclopentane itself, as shown by collision‐induced dissociation experiments, thus confirming the key role of the intermediate mentioned.
Molecular states formed by ion beam neutralization of [C2H2]+ and [C2H3]+ on metal targets
Authors: Hudgins Douglas M. , Raksit Asit B. , Porter Richard F. 
Published: May 2005

AbstractA combination of charge‐stripping and beam‐scattering techniques has been used to study the molecular states formed when a fast beam of [C2H2]+ and [C2H3]+ in several isotopic forms are neutralized by electron transfer from metal target atoms (K, Na, Mg and Zn). For [C2H3]+ the isotopic compositions and relative abundances of product states were found to be insensitive to the method of ion preparation (electron impact and chemical ionization). Ground state neutrals are formed in partial abundance when Mg or Zn is used as a target atom. With low ionization potential targets (K and Na) excitel dissociative states of C2H2 and C2H3 are formed as major beam constituents. For these states decomposition products have been identified and fragmentation energies measured. The excited states of C2H2 and C2H3 lie alout 6.8 eV and 2.9 eV, respectively, above their stable ground states. The discussion focuses on the possible identity of the excited states and their structural relations to the precursor ions.
Peptides investigated by laser desorption—multiphoton ionization mass spectrometry
Authors: Grotemeyer J. , Schlag E. W. 
Published: May 2005

AbstractLaser evaporation of intact neutral molecules into a supersonic beam combined with multiphoton ionization (MUPI) is used to study the fragmentation behaviour of peptides. Owing to the separation of desorption and ionization, an optimization of these processes can be applied to the sample. The investigation of mixtures containing hydrophobic and hydrophilic peptides shows equal probabilities for detection of the two molecules, demonstrating that the neutral yield of both classes of compound is equal in the desorption process. A loss of sensitivity is not observed. By employing the feature of tunable fragmentation, it is possible to sequence peptides in the gas phase. At low laser intensities only the molecular ions are formed. By increasing photon intensities, fragmentation reactions are induced. Owing to the nature of the multiphoton ionization, these mass spectra (at moderate laser powers) contain few and only structurally dependent signals. The molecular ion of the sample investigated is detected in every case.
Neutralization–reionization efficiencies for gaseous [C3Hn]+ Ions
Authors: Wesdemiotis Chrysostomos , Feng Rong 
Published: May 2005

AbstractA neutralization‐reionization study of [C3Hn]+ ions, n = 6−0, shows that neutralization efficiencies are larger for hydrocarbon radical cations than they are for even‐electron carbocations with one more H atom. Reionization efficiencies to positive ions do not depend appreciably on the electronic properties of the neutral species undergoing reionization. However, reionization efficiencies to negative ions increase dramatically with the degree of unsaturation, by nearly two orders of magnitude, from C3H6 to C3. All C3Hn neutrals studied survive intact neutralization to produce a considerable fraction of stable, non‐dissociating [C3Hn]+ upon cationization. All C3Hn radicals (n = 5, 3, 1) can be transferred collisionally to stable even‐electron anions. For the C3Hn hydrocarbon molecules studied (n = 6, 4, 2, 0) the stability of the molecular radical anion increases substantially with the degree of unsaturation.
The functional relationship scan in tandem mass spectrometry
Authors: Vincenti M. , Schwartz J. C. , Cooks R. G. , Wade A. P. , Enke C. G. 
Published: May 2005

AbstractA type of tandem mass spectrometric scan is described in which m1 +, m2 + and m3, defined in terms of the process m1 + → m2 + + m3, are all varied while maintaining some prescribed relationship between them. The scan can be viewed as a variant upon the neutral loss scan in which the constant mass difference between parent and daughter ion masses is replaced by some other functional relationship. Specific examples of this scan are provided in two simple applications: (i) symmetrical proton‐bound dimers are recognized using the functional scan m2 = (m1 + 1)/2; and (ii) symmetrical Diels–Alder adducts comprised of diene and dienophile of equal masses are recognized in mixtures using the scan function m2 = (1/2)m1.
Mass spectrometry of arenediazonium salts by fast atom bombardment
Authors: Ambroz Hanna B. , Jennings Keith R. , Kemp Terence J. 
Published: May 2005

AbstractContrary to expectation, thermally labile arenediazonium salts (M) afford sizeable peaks due to ArN2 + and (ArN2 +)M ions under fast atom bombardment; the main fragment ion is usually the highly reactive aryl cation Ar+, the production of which is independent of whether its ground state is singlet or triplet.
A mass spectral rearrangement of 2‐(alkylamino)benzoic acid derivatives and related heterocyclic systems
Authors: Barbuch Robert J. , Peet Norton P. 
Published: May 2005

AbstractThe behavior of a series of 2‐(alkylamino)benzoic acid derivatives and related heterocylic systems has been investigated using electron impact mass spectrometry and fragmentation pathways have been formulated. Deuterium‐labeled and 13C‐labeled compounds were used in this study. An interesting rearrangement of the alkylamino compounds, in which the alkyl substituent is incorporated into a fulvene ion, was observed. This rearrangement had previously been observed for 1‐alkylisatins and l‐alkyl‐3‐arylimino‐2‐indolinones.
Negative ion chemical ionization mass spectrometry—binding of molecules to bromide and iodide anions
Authors: Caldwell G. W. , Masucci J. A. , Ikonomou M. G. 
Published: May 2005

AbstractThe analytical potential of negative ion chemical ionization (NICI) mass spectrometry utilizing dibromodifluoro‐methane (CF2Br2) and iodomethane (CH3I)/methane (CH4) as reagent gases is examined. The NICI mass spectrum of CF2Br2 contains Br−, [HBr2]− and [CF2Br3]− anions. Weak acids (i.e. those acids with approximately ΔH°(acid) values between 1674 and 1464 kJ mol−1) react with Br− to produce minor yields of the hydrogen−bonded bromide attachment [MH + Br]− anion or are unreactive. Strong acids (i.e. those acids with approximately ΔH°(acid) > 1464 kJ mol−1) produce primarily [MH + Br]− anions with a minor yield of proton transfer [M  H]− anion. The NICI spectrum of CH3I/CH4 is dominated by I−. Weak acids react with I− to yield minor amounts of [MH + 1]− or are unreactive. Strong acids produce only [MH + l]− anions. From a consideration of the gas‐phase basicity of the halide anion and the binding energy of the hydrogen‐bonded halide attachment adduct, thermochemical data are used as a potential guide to rationalize or predict the ions observed in NICI mass spectra.
One and two hydrogen atom migrations in the retro‐diels‐alder fragmentation of norbornene and bicyclo [2.2.2] octene derivatives under electron impact
Authors: Weisz Adrian , Mandelbaum Asher 
Published: May 2005

AbstractNorbornene derivatives with one or two carbonyl‐containing substituents at positions 5 and 6 (anhydrides, esters, amides, cyclic ketones) undergo an electron impact induced rerro‐Diels‐Alder fragmentation accompanied by the migration of one hydrogen atom (RDA + H) giving rise to the [dienophile + H]+ ions. Bicyclo[2.2.2]octene derivatives substituted at C(5) and C(6) by a ring with two carbonyl groups (anhydrides, imides, diketones) undergo an RDA fragmentation accompanied by the transfer of two hydrogen atoms (RDA + 2H). Bicyclo[2.2.2]octene 5,6‐diesters undergo both the RDA + H and RDA + 2H fragmentations, and the relative abundance of the resulting [dienophile + H]+ and [dienophile + 2H]+˙ ions is strongly affected by configuration.
Substituent effects in the mass spectra of diethyl phenyl phosphates
Authors: Kascheres Concetta , Sintoni Mirtes 
Published: May 2005

AbstractThe mass spectra of diethyl phenyl phosphates show substituent effects with electron‐donating groups favouring the molecular ion M+˙, and the [MC2H4]+˙, [M – 2C2H4]+˙ and [XPhOH]+˙ ions. The [PO3C2H6]+ (m/z 109) and [PO3H2]+ (m/z 81) ions are favoured by electron‐withdrawing groups. Results suggest that the formation of the [XPhC2H3]+˙ ion involves rearrangement of C2H3 to the position ortho to the phosphate group. Ortho effects are also observed.