bluuebunny/crossref_metadata_2025_split huggingface dataset:
Differences in the keto–enol equilibrium of [M + H]+ ions of bupropion formed by electron impact and fast atom bombardment ionization
Authors: Johnson Robert L. , Taylor Lester C. E. 
Published: May 2005

AbstractThe fragmentation of the protonated molecular ion of bupropion produced by collisionally induced decomposition is shown to depend on the ionization method used to form the [M + H]+ ion. The daughter ion products do not depend on the energy of decomposition, i.e. high‐ or low‐energy collisions, but on the ratio of the keto‐enol equilibrium as influenced by the ionization process.
Secondary isotope effects in the mass spectra of C2D5OC2H5 and CD3CH2OC2H5
Authors: Russell Morley E. 
Published: May 2005

AbstractSecondary isotope effects were examined in the loss of methyl from the molecular ions of C2D5OC2H5 and CD3CH2OC2H5 for ion source processes and collisional activation (CA) processes. In the ion source loss of CH3 was slightly favored over loss of CD3, but in CA the CH3 loss was favored to a much larger extent than expected. A possible explanation is given which invokes an energy distribution function peaked near the appearance energy for the methyl loss process.
Usefulness of field desorption mass spectrometry in determining molecular masses of carotenoids, natural carotenoid derivatives and their chemical derivatives
Authors: Takaichi Shinichi 
Published: May 2005

AbstractField desorption (FD) mass spectrometry was applied to the determination of the molecular masses of carotenoids, natural carotenoid derivatives and their chemical derivatives. All the carotenoids examined gave the molecular ion as the base peak with negligible fragment ions. Carotenoid glucoside and its fatty acid monoester were successfully determined without acetylation, whereas carotenoic acids (carboxylate and sulphate) needed to be converted into methyl esters prior to analysis. The applicable ranges of molecular masses and polarity were very wide. In addition, carotenoid glycoside gave only [M]+˙ without [M + H]+˙ and [M + cation]+˙. The numbers of carbonyl groups, primary and/or secondary hydroxyl groups and total hydroxyl groups could be directly determined according to the increase in mass units of the carotenoids after chemical reduction, acetylation and trimethylsilylation, respectively. Owing to the negligible fragment ions, FD analysis was also suitable for carotenoids containing small amounts of impurities or other carotenoids. Hence this technique is useful for determining the molecular masses of carotenoids and the number of modifiable groups in carotenoids.
Simplified electron ejection in Fourier transform ion cyclotron resonance mass spectrometry by suspended trapping
Authors: Cooper Brian T. , Buckner Steven W. 
Published: May 2005

AbstractSuspended trapping is used to eject electrons in negative‐ion Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometric experiments. In contrast to electron ejection by resonant excitation of the trapping motion, suspended trapping involves allowing the electrons to escape along the z‐axis (perpendicular to the trap plates) while the trapping potential is briefly removed. The duration of this event is sufficiently short (∼10 μs) so that ion losses are negligible; the overall effect is that of a ‘high‐pass mass filter’. Suspended trapping is simpler to implement and more generally applicable to various cell geometries than resonant electron ejection. The effectiveness of the suspended trapping technique is not compromised by the anharmonicity of the potential well in ‘elongated’ ICR traps, but depends simply on the time it takes the electrons to escape the cell. Finally, a small, positive offset potential (∼+0.25 V) applied to the trap plates during the suspended trapping event increases the efficiency of the ejection.
Collisions of fluorocarbon ions at solid surfaces: Electronic excitation, surface‐induced dissociation and chemical sputtering
Authors: Ast T. , Riederer D. E. , Miller S. A. , Morris M. , Cooks R. G. 
Published: May 2005

AbstractCollisions of C3F at self‐assembled hydrocarbon, deuterated hydrocarbon and fluorocarbon surfaces yield fragment ions which are characteristics of both electronic excitation and vibrational excitation. Direct electronic excitation is indicated by loss of F˙, which has been shown previously to be diagnostic of this type of excitation process. Electronic excitation is favored by low‐energy collisions at the hydrocarbon surface. Even the change to the corresponding deuterated surface produces a large effect in favor of the normal vibrational excitation process. This change in mechanism with the nature of the target shows up as a dramatic isotope effect in the surface‐induced dissociation (SID) mass spectra. The control over the excitation process exhibited by the effective mass of the target is probably exerted through its effect on the relative velocity of the collision partners. The fluorinated surfce is more effective than the others in conversion of translational into internal energy and in minimizing ion loss through neutralization and other processes which compete with SID. The fluorinated surfaces yield spectra that are largely free from chemical sputtering, a process which occurs even at ultra‐high vacuum for stainless‐steel surfaces which are not rigorously cleaned. The internal energy deposition associated with chemical sputtering increases with increaisng collision energy. Several of the fluorocarbon fragment ions generated from perfluoropropylene have also been examined at self‐assembled monolayer surfaces and they are well behaved in their SID and chemical sputtering reactions. This in contrast to the low efficiency of SID and high sputtering efficiency observed in previous studies at uncharacterized multi‐layer hydrocarbon‐covered surfaces.
Role of different 5‐substituents on the mass spectrometric behaviour of uracil
Authors: Favretto Donata , Traldi Pietro , Celon Eloisa , Resnati Giuseppe 
Published: May 2005

AbstractThe influence of different 5‐substituents on the mass spectrometric behaviour of uracil was systematically studied by means of mass‐analysed ion kinetic energy spectrometry. By applying this technique to the molecular ions and the major primary product ions formed by loss of HNCO, the electron impact‐induced fragmentation patterns of a series of substituted uracils could be elucidated. In addition to the common decomposition processes, distinctive fragmentation routes were identified for some compounds, and a rationale for the observed behaviour is given based on the electronic properties of the substituents.
Elimination of acetic acid from protonated 4,5‐diacetoxyphenanthrene and 2,2′‐diacetoxybiphenyl: An example of an ion chemistry proximity effect
Authors: Orlando M. , George M. , Gross M. L. 
Published: May 2005

AbstractProtonated 4,5‐diacetoxyphenanthrene and 2,2′‐diacetoxybiphenyl dissociate, owing to the interaction of the two acetoxy groups, by eliminating a molecule of acetic acid. This novel proximity effect, which occurs for a fast atom bombardment‐produced closed‐shell ion, was examined in detail using tandem mass spectrometric methods. This process, which was first observed in the decompositions of acetylated complex natural products such as stentorin and hypericin, may serve as a general process for determining the proximity of hydroxyl groups in polycyclic aromatic compounds.
Formation of CH3CH2OCH3+˙ and ˙CH2+OHCH2CH3 from ionized 2‐ethoxyethanol and theoretical and experimental studies of the reactions of ˙CH2+OHCH2CH3
Authors: McAdoo David J. , Hudson Charles E. , Ramanujam V. M. Sadagopa , George M. 
Published: May 2005

AbstractIt is concluded that C3H8O+˙ formed by dissociation of ionized 2‐ethoxyethanol (8) is a mixture of CH3CH2OCH3+˙ (7) and ˙CH2+OHCH2CH3 (2). Formation of 7 and CH3CH2+OHCH3 (12) is attributed to dissociations of species formed by the hydrogen transfers [CH3CH2OCH2+ ˙CH2OH] → [CH3CH2OCH2OCH3+˙ CH2O] → [CH3CH2+OHCH3HCO˙]. Production of 7 competes weakly with dissociation to CH3CH2 +OCH2 (13) and to 12. The low abundance of 7 is attributed to the simple dissociation 8 → 13 being both energetically and entropically favored, and a second H‐transfer to give 12 being energetically favored. The threshold for forming 7 is 45 kJ mol−1 above that for dissociation directly to 13, so formation of 7 is the first ion‐neutral complex‐mediated elimination found to have a threshold above that for the competing simple dissociation. The low abundance of 7 also demonstrates that ion–neutral complexes can be intermediates without obviously revealing their presence by direct dissociation. Experimental results suggest that 2 isomerizes to CH3CH2CH2OH+˙ (5) and then dissociates by eliminating water. Ab initio results support the feasibility of 2 → CH3+OHCH2CH2˙ (1) and 2 → 5. However, experimental observations suggest that 2 → 1 does not occur. This is attributed to strong competition from dissociation and isomerization to 5. The transition state for 2 → 5 resembles [CH3CH2CH2OH]+˙, and a cyclic transition state for 2 → 5 is ruled out. When the ethyl‐oxygen bond in 2 is simply lengthened, the charge is initially concentrated on ethyl, but it switches to CH2OH in a curve crossing at an apparent transition state for CO bond breaking.
Investigation of the ionization processes occurring in liquid‐assisted secondary ion mass spectrometry of derivatized monosaccharides
Authors: Paul G. J. C. , Théberge R. , Bertrand M. J. , Feng R. , Bailey M. D. 
Published: May 2005

AbstractSeveral derivatized monosaccharides, the 2‐deoxy‐D‐ribofuranoses, have been studied by liquid‐assisted secondary ion mass spectrometry (LSIMS) in order to gain insight into the factors affecting ionization in FAB/LSIMS. Examination of the mass spectra for these compounds obtained in eight liquid matrices (diethanolamine, ethylene glycol, glycerol, 2‐hydroxyethyl disulfide, 2‐hydroxyphenethyl alcohol, 3‐nitrobenzyl alcohol, sulfolane and thioglycerol) reveals that in all cases the anomalous [M – H]+ ion is the predominant species in the molecular ion region and that [M + Na]+ species are observed in the presence of Na+. The analysis of these compounds by chemical ionization with ammonia shows [M + H]+ as the major species while [M – H]+ is essentially absent. This indicates that the ionization processes occurring in the two techniques are not analogous. Thermodynamic considerations based on the gas‐phase hydride ion affinities of the protonated matrices do not support a predominant gas‐phase mechanism for the formation of [M – H]+ in LSIMS. However, it is possible using solvation energies to rationalize the formation of [M – H]+ in terms of condensed‐phase ionization processes which take place either in the liquid matrix or in the dense selvedge region immediately above the surface where extensive solvation is present. Electrospray data obtained for one of the derivatized monosaccharides indicates that the [M – H]+ is not performed in the condensed phase in LSIMS and that it is the product of fast ion beam‐induced processes. While the nature of the matrix is seen to have little effect on the intensities of [M – H]+ and [M + H]+ it is observed to be an important factor for the intensity of M+˙ for one of the monosaccharides. This effect can be related to the electron‐scavenging properties of the matrices and reinforces the hypothesis that condensed phase processes are significant in ionization.
Matrix‐assisted laser desorption mass spectrometry of homopolymer oligodeoxyribonucleotides. Influence of base composition on the mass spectrometric response
Authors: Schneider Klaus , Chait Brian T. 
Published: May 2005

AbstractMatrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS) has the potential for providing a rapid alternative to gel electrophoresis for DNA sequence analysis provided that an intense mass spectrometric response can be obtained from mixtures of DNA fragments containing up to 300 nucleotides. MALDI‐MS has not yet proved viable for such analyses because the MS response falls off rapidly for mixed‐base DNA fragments containing more than 20–30 nucleotides. Previous studies have demonstrated that base composition is a critical factor in the MALDI‐MS response of oligodeoxyribonucleotides. This paper describes an investigation of the physical roots of the observed influence of base composition on the mass spectrometric response, focusing on homopolymer oligodeoxyribonucleotides (dT7, dT10, dT18, dT36, dG7, dG10, dG18, dI18 and dU18) and dT5G5. Forty‐eight different matrix compounds were tested for their ability to produce laser desorption masses spectra from such homopolymer oligodeoxyribonucleotides. Considerably stronger mass spectrometric responses were obtained from polydeoxythymidines than from polydeoxyguanosines, polydeoxycytidines and polydeoxyadenosines. Although mass spectral peaks corresponding to dT18 were observed from 20 of the matrices studied, no discernible response was observed for dG18 from any of these matrices. To elucidate the physical basis for origins of the observed differences in response, a number of factors were investigated including the ionization efficiency, the tendency towards fragmentation and the extent to which the oligodeoxyribonucleotides were incorporated into the matrix crystals. The results of these experiments indicate that low ionization efficiency is not a likely main contributor to the low response to polydeoxyguanosines, fragmentation is a likely main contributor to the low response to polydeoxyguanosines, the overall incorporation of polydeoxyguanosines into matrix crystals is comparable to that for polydeoxythymidines and the exocyclic amino group of guanosine, adenosine and cytidine has a strong inhibitory effect on the mass spectrometric response.
Chemical ionization mass spectrometry of benzoyl peroxide: A radical aromatic substitution resulting in biphenylcarboxylic acid in the gas phase
Authors: Tu Ya‐Ping , Zou Da‐Peng , Tang Ming‐Sheng , Xin Bao‐Min , Jia Wei‐Ping , Liu Shu‐Ying 
Published: May 2005

AbstractA radical aromatic substitution resulting in biphenylcarboxylic acid is inferred for the decomposition of benzoyl peroxide from the chemical ionization and collision‐induced dissociation mass spectra. The thermolysis of benzoyl peroxide gives rise to a benzoyloxy radical, which undergoes rapid decarboxylation and hydrogen abstraction leading to phenyl radical and benzoic acid, respectively. Attack of the resulting phenyl radical on the benzoic acid results in biphenylcarboxylic acid. On the other hand, the phenyl radical abstracts a hydrogen atom to yield benzene, which is then subjected to the attack of a benzoyloxy radical, affording phenyl benzoate. This substitution reaction rather than the recombination of benzoyloxy and phenyl radicals is found to be responsible for the formation of phenyl benzoate under the present conditions.
Fourier transform ion cyclotron resonance study of the dehydrogenative polymerization of allene and propyne induced by W+ in the gas phase
Authors: Ferhati Azedine , Sozzi Georges , Ohanessian Gilles 
Published: May 2005

AbstractA Fourier transform ion cyclotron resonance study of the gas‐phase reaction of W+ with allene is reported. W+ successively dehydrogenates at least nine allene molecules, leading to the formation of a series of WC3nH ions, with WC27H as the largest product observed after a reaction delay of 40 s. Starting from the third reaction, there is a competition between elimination of H2 and C2H2 at each step. The activation of propyne was also investigated and found to lead to essentially the same sequence as with allene. The reaction of W+ with 1,1‐d2‐allene and collision‐induced dissociation spectra of product ions are used to discuss a plausible mechanism for the formation and structures of some of the species observed.
Matrix‐assisted laser desorption/ionization mass spectrometry with additives to 2,5‐dihydroxybenzoic acid
Authors: Karas Michael , Ehring Hanno , Nordhoff Eckhard , Stahl Bernd , Strupat Kerstin , Hillenkamp Franz , Grehl Matthias , Krebs Bernt 
Published: May 2005

AbstractSelected benzoic acid derivatives and related substances were used as additives to 2,5‐dihydroxybenzoic acid (2,5DHB) and the performance of the mixtures in matrix‐assisted laser desorption/ionization mass spectrometry was investigated. Using benzoic acid derivatives substituted at position 2 and/or 5 or related substances as a co‐matrix in the 1–10% range with 2,5DHB results in improved ion yields and signal‐to‐noise ratio of analyte molecules, especially for the high‐mass range. The enhanced performance is prominent for 2‐hydroxy‐5‐methoxybenzoic acid and exists for both proteins and oligosaccharides. It is suggested that the improvement is caused by a disorder in the 2,5DHB crystal lattice allowing ‘softer’ desorption. Charge transfer from matrix ions to additive molecules at the expense of analyte ionization gives a simple explanation for the deteriorating effects of some tested additives.
Fast atom bombardment mass spectral study of tetracycline antibiotics
Authors: Harada Ken‐Ichi , Masuda Katsuyoshi , Suzuki Makoto , Oka Hisao , Ikai Yoshitomo , Hayakawa Junko 
Published: May 2005

AbstractIn the fast atom bombardment mass spectra of tetracyclines, ammonia elimination fragment ions, [M + H – NH3]+ and [M + H – NH3 – H2O]+, appeared with considerable abundances. A plausible mechanism for the loss of ammonia from [M + H]+ was discussed based on results by measurements on various types of tetracyclines and benzamides as the model compounds and B/E linked scanning. The results indicated that the loss of ammonia occurred in the carboxyamide moiety in the A ring and was accelerated by an ortho‐hydroxy group. The same fragmentations were observed in the electrospray mass spectra of tetracyclines with a skimmer‐capillary voltage differential of 150 V.
Structures and thermochemistry of ions formed by methane chemical ionization of Fe(CO)5. Site of protonation and determination of FeH and FeCO bond dissociation energies of HFe(CO)n+ ions
Authors: Ekeberg Dag , Hagen Solveig Isaksen , Hvistendahl Georg , Schulze Christian , Uggerud Einar , Vedde John 
Published: May 2005

AbstractHigh‐energy collisional activation mass spectrometry of HFe(CO)5+ ions shows that Fe(CO)5 is protonated on the iron atom rather than on one of the ligands. This finding is supported by ab initio quantum chemical calculations. The value of the proton affinity of Fe(CO)5 was measured by high‐pressure mass spectrometry to be 857 kJ mol−1. The FeCO bond dissociation energies for HFe(CO)n+ (n = 1–5) were measured by energy‐variable low‐energy collisional activation mass spectrometry. The FeH bond dissociation energies in HFe(CO)n+ ions were also determined. A synergistic effect on the strengths of the FeH and FeCO bonds in HFe(CO)+ is noticed. It is demonstrated that the electronically unsaturated species HFe(CO)n+ (n = 3, 4) formed in exothermic proton‐transfer reactions with Fe(CO)5 form adducts with CH4. Adducts between C2H5+ or C3H5+ and Fe(CO)n are observed. These adducts are probably formed in direct reactions between the respective carbocations and Fe(CO)5.
Kinetic and thermodynamic considerations of the bracketing method: Entropy‐driven proton‐transfer reactions in a Fourier transform mass spectrometer
Authors: Gorman Gregory S. , Amster I. Jonathan 
Published: May 2005

AbstractSeveral reports of experimentally derived proton affinity values and gas‐phase basicity values for amino acids and peptides have recently appeared in the literature. Here, we show that the thermodynamic quantity that is measured by the Fourier transform mass spectrometry proton transfer bracketing of amino acids and peptides is gas‐phase basicity and not proton affinity. Both experimental and theoretical evidence supports this conclusion. The difference between the values determined by proton transfer bracketing measurements for lysine versus leucine is consistent with a difference in gas‐phase basicity rather than proton affinity. The rate of proton transfer from protonated lysine to a series of reference compounds have been measured. Entropy‐driven, endothermic proton transfer is found to occur at the collision rate. Recent ab initio and semi‐empirical calculations of the proton affinity of lysine are found to agree with the value that is derived from bracketing studies when one assumes that gas‐phase basicity is measured. While entropy‐driven reactions have been observed previously in high‐pressure mass spectrometers, this is the first evidence for such reactions at low pressure in a Fourier transform mass spectrometer.
Activation of aromatic rings by early transition metal ions in the gas phase. Implications for the metal‐catalyzed [2 + 2 + 2] cycloaddition of alkynes and nitriles
Authors: Pope R. Marshall , Buckner Steven W. 
Published: May 2005

AbstractThe reactions of a series of monocyclic and bicyclic arenes with early transition metal ions (Sc+, Y+, Nb+ and Ta+) and their oxides and dioxides were studied in a Fourier transform ion cyclotron resonance mass spectrometer. Ring cleavage of the nitrogen‐containing heterocycles results in loss of HCN as the dominant pathway. Thermochemical considerations, secondary reactions and correlations with solution cyclotrimerization reactions indicate that the MC4H4+ product is a metallacyclopentadiene. Based on correspondence between the reactivities of a series of early metals with their valence electron counts, the reactivities of quinoline and isoquinoline and the decomposition behavior of the products, a metallacycloheptatriene intermediate is proposed for the heteroaromatic ring cleavage reaction. These results are compared to metal complexes in solution which catalyze the [2 + 2 + 2] cyclotrimerization of alkynes and nitriles.
Collison‐induced dissociation and photodissociation of nitroaromatic molecular ions: A unique isomerization for p‐nitrotoluene and p‐ethylnitrobenzene ions
Authors: Cassady Carolyn J. , McElvany Stephen W. 
Published: May 2005

AbstractThe photodissociation and low‐energy collision‐induced dissociation of p‐nitrotoluene and p‐ethylnitrobenzene molecular ions were studied using Fourier transform ion cyclotron resonance mass spectrometry. The dissociation of these ions is highly dependent on the time‐scale of the experiment and the pressure of the nitroaromatic compound. Collisions of the ions with nitroaromatic neutral species increase the abundance of fragment ions due to NO elimination, while collisions with inert gases such as sulfur hexafluoride and argon have no effect. Evidence is presented for the occurrence of an ion–molecule reaction between p‐alkylnitrobenzene ions and nitroaromatic neutral species that induces isomerization of the ion. This isomerization is proposed to involve a nitro‐to‐nitrite rearrangement. Although the mechanism of this process is unknown, isotopic labeling experiments have shown that it does not involve nitrogen atom transfer between the two reactants. The dissociations of o‐nitrotoluene, m‐nitrotoluene and nitrobenzene ions are also discussed. For these ions, no pressure‐ or time‐scale‐dependent behavior was observed, indicating that an isomerization did not occur.
Investigation of the effects of cooling after laser desorption before post‐ionization
Authors: Steenvoorden R. J. J. M. , Van Der Hage E. R. E. , Boon J. J. , Kistemaker P. G. , Weeding T. L. 
Published: May 2005

AbstractTo evaluate the necessity for cooling laser‐desorbed molecules to obtain molecular ions and minimal fragmentation, mass spectra of a series of methoxybenzenes were measured in two different experimental configurations. In one geometry, laser‐desorbed molecules were entrained in a pulsed supersonic jet before ionization. In the other, the molecules were ionized directly after laser desorption. The samples were ionized with laser‐generated vacuum ultraviolet radiation in a single‐photon process or ultraviolet radiation in a multi‐photon process.