bluuebunny/crossref_metadata_2025 huggingface dataset:
The reactive states of ions: Antiaromatic seven‐membered N‐substituted‐1H‐azepines and isomeric aromatic substrates
Authors: Kulkarni P. S. , Mirajkar S. P. , Bambal R. B. , Ayyangar N. R.
Published: May 2005

Comparison of the electron impact and chemical ionization mass spectra of N‐substituted‐1H‐azepines and isomeric aromatic substrates reveals significant differences. The data indicate that the substituted azepincs examined are stable to electron impact at low temperatures and retain their antiaromatic 8π electron seven‐membered cyclic system prior to their fragmentation. The electron impact data are supported by low electron‐volt and metastable scan techniques. Electron impact or chemical ionization induced transformation of azepines to aromatic substrates similar to the thermal transformation of neutral analogues is not observed at low temperatures. Chemical ionization mass spectra of N‐ethoxycarbonyl‐1H‐azepines and N‐phenylcarbamates were complicated owing to thermal decomposition reactions at high temperatures.
Keto‐to‐enol isomerization in the molecular ion of dimethylmethylphosphonate
Authors: Holtzclaw James R. , Wyatt Jeffrey R.
Published: May 2005

A mechanism for keto‐to‐enol isomerization in dimethylmethylphosphonate (DMMP) has been proposed based on deuterium‐labeling studies, a model compound and thermodynamic data. An electron ionization study of H/D exchange occurring in CD3‐labeled DMMP suggests that rapid keto‐to‐enol isomerization occurs in the ion source and a reaction mechanism based on sequential 1,4‐H migrations rather than by the direct 1,3‐H transfer or by sequential 1,2‐H migrations is proposed. The examination of the mass‐analyzed ion kinetic energy/collision‐induced dissociation spectrum of the methylphosphonic acid molecular ion suggests that keto‐to‐enol isomerization does not occur for this species and that 1,2‐ and 1,3‐H migrations are not favored. Available thermodynamic data were employed to construct a potential energy surface for keto‐to‐enol isomerization of the DMMP molecular ion. The thermodynamic data show that the energy barrier to isomerization is below the internal energy required for decomposition of the DMMP molecular ion. Additionally, the ΔHf° for the intermediate and enolic isbmers are shown to be significantly less than the ΔHf° for the keto form of the DMMP molecular ion.
Use of fast atom bombardment mass spectrometry for the characterization of nitroaromatic isomers
Authors: Balasanmugam Kesagapillai , Miller Jack M.
Published: May 2005

Positive and negative ion fast atom bombardment (FAB) mass spectra of some monosubstituted nitroaromatic isomers are reported. Generally ions carresponding to [M + H]+ and M+ are observed in the positive ion FAB spectra; ions such as [M − H] − and M−˙ are observed in the negative ion FAB spectra. The use of FAB mass spectra to distinguish the isomers is discussed. Comparisons of FAB, chemical ionization and electron impact mass spectra of the same isomers (wherever possible) are reported. The structural information obtained in the negative ion FAB spectra complement those obtained in the positive ion FAB spectra.
Rearrangement and dissociative processes in the [C3H6O]+˙ potential energy surface. Methyl vinyl ether: An example of non‐ergodic behaviour?
Authors: Lifshitz C.
Published: May 2005

The unimolecular dissociation reaction of the methyl vinyl ether radical cation to acetyl cation plus methyl radical was studied by means of 13C labelling and tandem mass spectrometric techniques. The results are in good agreement, in all major aspects, with previous data for 13C‐labeled propene oxide. The data are discussed in terms of non‐ergodic behaviour. Isomerization of the methoxyethylidene radical cation intermediate to the acetone radical cation is ruled out on the basis of the labelling experiments and comparison with the hydroxymethene radical cation system.
The chemical ionization (CH4 and i‐C4H10) mass spectra of tetracyanoquinodimethane and tetracyanoethylene—the result of a complex system of competing reactions
Authors: Budzikiewicz H. , Poppe A.
Published: May 2005

It is shown that depending on the experimental parameters upon electron impact and chemical ionization (CI) radical addition with subsequent CI, and formation of dihydro products which can undergo subsequently the reaction mentioned above, contribute to varying extents to the CI mass spectra of tetracyanoquinodimethane and tetracyanoethylene. These results are of importance for systems where radical reactions under CI conditions are to be expected.
Rearrangements of ionized carbonyl compounds
Authors: McAdoo David J.
Published: May 2005

Developments in our knowledge of the isomerizations of low‐energy [CnH2nO]+˙ and [CnH2nO2]+˙ ions are traced. The reactions of these ions take place by a series of simple steps common to them all. This article represents an extension of the lifelong efforts of Professor McLafferty to provide a unified understanding of the reactions of ions in the gas phase.
Gas‐phase alkylation of 2‐methylfuran under chemical ionization conditions
Authors: Robin D. , Hoppilliard Y. , Audier H.
Published: May 2005

In the gas phase, under chemical ionization conditions, the sites of attachment of [CH3]+ and [C2H5]+ to 2‐methylfuran have been studied by tandem mass spectrometry. Spontaneous metastable and collision‐induced fragmentations have been compared to those observed for the protonated alkylfuran isomers. The results obtained show that alkylation occurs preferentially at the β‐position.
Charge localization by molecular orbital calculations. II. Formamide, thioformamide and N‐methylated analogues
Authors: Baldwin Michael A. , Welham Kevin J.
Published: May 2005

Ab initio molecular orbital calculations have been used to determine the degree of charge localization associated with the formation of radical cations on electron impact of formamide, thioformamide and their N‐methyl and N, N‐dimethyl analogues. The charge favours the nitrogen of formamides and the sulphur of thioformamides, but the degree of localization is calculated to be lower than in the radical cations of urea and thiourea.
Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane with ethers
Authors: Orlando Ron , Ridge D. P. , Munson Burnaby
Published: May 2005

Chemical ionization mass spectra of several ethers obtained with He/(CH3)4Si mixtures as the reagent gases contain abundant [M + 73]+ adduct ions which identify the relative molecular mass. For the di‐n‐alkyl ethers, these [M + 73]+ ions are formed by sample ion/sample molecule reactions of the fragment ions, [M + 73 − CnH2n]+ and [M + 73 − 2CnH2n]+. Small amounts of [M + H]+ ions are also formed, predominantly by proton transfer reactions of the [M + 73 − 2CnH2n]+ or [(CH3)3SiOH2]+ ions with the ethers. The di‐s‐alkyl ethers give no [M + 73] + ions, but do give [M + H]+ ions, which allow the determination of the relative molecular mass. These [M + H]+ ions result primarily from proton transfer reactions from the dominant fragment ion, [(CH3)3SiOH2]+ with the ether. Methyl phenyl ether gives only [M + 73]+ adduct ions, by a bimolecular addition of the trimethylsilyl ion to the ether, not by the two‐step process found for the di‐n‐alkyl ethers. Ethyl phenyl ether gives [M + 73]+ by both the two‐step process and the bimolecular addition. Although the mass spectra of the alkyl etherr are temperature‐dependent, the sensitivities of the di‐alkyl ethers and ethyl phenyl ether are independent of temperature. However, the sensitivity for methyl phenyl ether decreases significantly with increasing temperature.
Reactions between substrate molecules and chemical ionization reagent gases prior to ionization
Authors: Budzikiewicz H.
Published: May 2005

A survey of side reactions such as oxidation, hydrogenation, reductive removal of substituents and radical processes is presented, and the implications for mass spectroscopic analysis are pointed out.
Enhanced positive secondary ion emission from substituted polynuclear aromatic hydrocarbon/sulfuric acid solutions
Authors: Leibman Christopher P. , Todd Peter J. , Mamantov Gleb
Published: May 2005

Secondary ion mass spectra of singly substituted aromatic hydrocarbon/H2SO4 solutions showed intense aromatic molecular ion and protonated aromatic molecule peaks characteristic of dissolved aromatic compounds from a number of aromatic compound classes, including acids, aldehydes, ketones, nitriles and nitrogen heterocycles. The presence of simultaneously abundant peaks for molecular ions and protonated molecules in secondary ion mass spectra of each aromatic compound/sulfuric acid solution is consistent with known or expected gas‐phase proton transfer chemistry. The ratio of intensities, M+˙:[M + H]+, appears to be determined by sulfuric acid solution chemistry of the compound. Spectra obtained from 1–2 μl samples were relatively free from chemical noise and persisted for up to 20 min. Detection limits for some substituted aromatic compounds are estimated to be 10−12.
Unidirectional triple and double hydrogen rearrangement reactions in the radical cations of γ‐arylalkanols
Authors: Kuck Dietmar , Filges Ulrich
Published: May 2005

A novel fragmentation reaction accompanied by the unidirectional migration of three hydrogen atoms has been found in the radical cations of γ‐arylpropanols with electron‐donating substituents in the para position. This triple hydrogen (3H) rearrangement reaction is the dominant fragmentation channel of the long‐lived molecular ions of trans‐2‐(4′‐dimethylaminobenzyl)‐l‐indanol, 2, but it occurs also in simpler γ‐arylpropanol ions. Deuterium labelling of 2 reveals that the three hydrogen atoms originate with extraordinarily high specificity from the C(l), C(2) and O positions of the alcohol moiety. Cis‐ and 3′‐substituted isomers do not undergo this reaction. Along with the 3H rearrangement reaction a unidirectional double hydrogen (2H) rearrangement reaction takes place independently and with less specificity in the trans‐2‐(4′‐X‐benzyl)‐l‐indanol ions 1+˙ and 2+˙. No hydrogen exchange occurs during the 3H and 2H rearrangement reactions. Mechanistic alternatives of these unusual fragmentation reactions are discussed; the experimental evidence strongly favours pathways via several intermediate ion–neutral complexes.
Stabilization of even‐electron ions by cyclization of substituents on 3N‐ and 4N‐nitrogens in 4N‐substituted cytosines
Authors: Plaziak Adam S. , Celewicz Lech , Spychala Jaroslaw , Golankiewicz Krzysztof
Published: May 2005

Electron impact mass spectra of 4N‐substituted derivatives of cytosine are reported. The strong influence of 4N‐substituents on the mode of mass fragmentation occurring upon electron impact ionization was proved. The presence of α‐carbon in the 4N‐substituent extends the possibilities of fragmentation via rearrangement leading to formation of even‐electron bicyclic ions containing quaternary 3N‐nitrogen. When β‐carbon was present in the 4N‐substituent, the decomposition based on elimination of alkenes was detected as well as the rearrangement leading to formation of bicyclic even‐electron ions containing quaternary 3N‐ or 4N‐nitrogen.
A gas‐phase anionic analog of the wittig reaction. An ion cyclotron resonance study of the gas‐phase ion chemistry of silyl carbanions
Authors: Campanaro A. , Marvin C. H. , Morehouse S. P. , McMahon T. B.
Published: May 2005

Gas‐phase ion–molecule reactions of a variety of fluorosilyl carbanions with compounds containing double bonds to oxygen, XO, have been examined using pulsed ion cyclotron resonance spectroscopy. The predominant reaction channel observed for species not containing acidic hydrogens is a Wittig‐like process involving SiO bond formation and elimination of XCH2 species. The gas‐phase acidity of F3Si(CH3) has been determined and those of F2Si(CH3)2 and FSi(CH3)3 have been estimated. From the fluoride transfer reactions of F3SiCH2− the fluoride affinity of F2SiCH2 has been estimated and limits on the π bond strength in this silaethene obtained. Potential analytical applications of the Wittig reactivity have been discussed.
Mass spectra of halogenated esters: 8. Methyl esters of 2,3‐dichloro‐, bromochloro‐ and dibromopropenoic acids
Authors: Korhonen Ilpo O. O.
Published: May 2005

The mass spectral fragmentation of methyl esters of E and Z isomers of 2,3‐dichloro‐, 2‐bromo‐3‐chloro‐, 3‐bromo‐2‐chIoro‐ and 2,3‐dibromopropenoic acids have been investigated. The M+˙ peak is shown with all isomers, the [M − OCH3]+, [M − X]+, [M − OCH3 − CO]+, [M − OCH3 − CO − X]+˙ and [M − OCH3 − CO − X − X]+ ions constituting abundant peaks in all spectra. The results, particularly from the bromochloro isomers, show that a halogen atom is eliminated from the 3‐ rather than the 2‐ position and from the Z rather than the E isomer. Bromine as a bulky atom is preferentially lost.
Electron impact fragmentation of isomeric 2‐diazo‐2‐cyanoacetamides and 4‐cyano‐5‐hydroxy‐l,2,3‐triazoles
Authors: Lebedev Albert T. , Alekseeva Tatiana N. , Bakulev Vasiliy A. , Kolobov Mikhail Yu. , Petrosyan Valeriy S.
Published: May 2005

On the basis of electron impact mass spectra, high‐resolution mass spectra and metastable ion spectra, the isomerization of the molecular ions of N‐aryl‐2‐diazo‐2‐cyanoacetamides and 1‐aryl‐4‐cyano‐5‐hydroxy‐l,2,3‐triazoles into each other or into the common structure has been demonstrated. Corresponding N‐cyclohexyl derivatives are unable to undergo such a transformation.
Identification of platelet activating factor by tandem mass spectrometry
Authors: Plattner Ronald D. , Stack Robert J. , Al‐Hassan Jassim M. , Summers Brent , Griddle Richard S.
Published: May 2005

Platelet activating factor, a phospholipid (1‐O‐alkyl‐2‐O‐acetyl‐sn‐glycerophosphocholine), is a potent mediator of inflammation and acute hypersensitivity reactions. Chemical ionization mass spectra provide useful information for confirmation of structure. However at low levels the confirmation of structure in biological samples is difficult because the chemical noise level is too great. Collisionally activated decomposition daughter spectra of the protonated molecule and major fragment ions of authentic standards were recorded and are discussed. Selected tandem mass spectral scans provided the improvement in signal‐to‐noise to allow positive confirmation of structure at nanogram levels in partially purified extracts from a biological source, the epidermal cell secretions of the saltwater catfish Arius thalassinus.
Mass spectrometric investigation of substituted 1,3‐emthiolaneS‐oxides
Authors: Bortolini O. , Traldi P. , Licini G.
Published: May 2005

The electron impact mass spectrometry of four model 2‐phenyl‐2‐alkyl‐l,3‐dithiolaneS‐oxides is described and discussed with the aid ofB/Emass‐analysed ion kinetic energy collisional spectroscopy and labelling experiments, as well as of fast atom bombardment spectra. The mechanisms of the common fragmentation patterns are given.
Characterization of cyclotrimethylenetrinitramine (RDX) by N,H isotope analyses with pyrolysis–atmospheric pressure ionization tandem mass spectrometry
Authors: Snyder A. Peter , Kremer Johannes H. , Liebman Shirley A. , Schroeder Michael A. , Fifer Robert A.
Published: May 2005

Pyrolysis‐atmospheric pressure chemical ionization was used to study the thermal decomposition of the energetic material cyclotrimethylenetrinitramine (RDX) and characterization of the individual molecular ion products was accomplished by tandem mass spectrometry. The analysis was aided with pyrolysis mass spectra of the (15N)‐ and perdeuterated RDX isotopes, and molecular formulae were derived for the m/z 46, 60, 74, 75, 85 and 98 molecular ions in the RDX pyrolysis mass spectrum. Equivalent fragments between the daughter ion mass spectra of the unlabeled and labeled RDX were determined in order to define a structure for each pyrolysis feature. Daughter ion mass spectra of pure reference compounds confirmed the identity of five of the six molecular ions. Perdeuterated RDX analyses provided evidence that m/z 74 and 75 are N,N‐dimethylformamide and N‐nitrosodimethylamine, respectively; m/z 46, 60 and 85 were identified as the protonated forms of formamide, N‐methylformamide and dimethylaminoacetonitrile, respectively.
Ion chemistry of α‐diazoketones. 2. Synthesis of Ionized or protonated N‐tosylpyrrolidin‐3‐ones from 1‐diazo‐4‐tosylaminobutan‐2‐ones under electron impact or chemical ionization
Authors: Selva Antonio , Saba Antonio
Published: May 2005

Mass spectral syntheses of gaseous ionized or protonated N‐tosylpyrrolidin‐3‐ones from 1‐diazo‐4‐tosylaminobutan‐2‐one substrates under electron impact or protonating chemical ionization conditions, respectively, are supported by collisional activation experiments.
Photodissociation and structures of [C6H4]+ ˙ ions generated from cyanobenzene and other precursors
Authors: van Der Hart W. J. , Oosterveld E. , Molenaar‐Langeveld T. A. , Nibbering N. M. M.
Published: May 2005

Previous work on the electron impact induced loss of hydrogen cyanide from the radical cations of cyanobenzene has revealed that ring opening is important in the formation of the corresponding [C6H4]+ ˙ ions. Photodissociation experiments now show that these [C6H4]+ ˙ ions and those generated from 2‐ethynylpyridine, 1,3‐hexadiyn‐6‐nitrile and 1,2‐diiodobenzene all photodissociate in the visible region to [C4H2]+ ˙. The corresponding photodissociation spectra are all the same and have a maximum at about 370 nm, in agreement with spectra of ions with three conjugated double or triple bonds. Owing to the high reactivity, the low photodissociation rate and, possibly, the internal energy of the ions, the photodissociation kinetics are too complicated to be solved. The experiments nevertheless show that at least a major fraction of the [C6H4]+ ˙ ions has a ring‐opened structure. This conclusion is supported by MNDO calculations, which indicate that the heats of formation of the possible acyclic structures are about 150 kJ mol−1 lower than those of the o‐, m‐ and p‐benzyne structures.
Sequence‐specific fragmentation of benzyloxycarbonyl dipeptides by resonance‐enhanced multiphoton ionization
Authors: Segar Kimberly R. , Johnston Murray V.
Published: May 2005

The mechanism of fragment ion formation in resonance‐enhanced multiphoton ionization (REMPI) of benzyloxycarbonyl (CBZ)‐derivitized dipeptides is presented. At 266 nm, the entire multiphoton process can be thought of as a two‐part scheme where ionization occurs by resonant two‐photon ionization followed by photodissociation of the created ions. When the energy of two photons exceeds the molecular ionization energy by a significant amount, REMPI has the advantage of producing both parent ions and low appearance energy fragments in large amounts. For CBZ dipeptides, resonant two‐photon ionization at 266 nm produces parent ions as well as A type sequence ions with high abundance. On the other hand, a three‐photon process (resonant two‐photon ionization followed by parent ion photodissociation) forms sequence‐related ions which also involve complex fragmentations of the CBZ chromophore. These results are compared to mass spectra obtained by other ionization/dissociation methods and to REMPI mass spectra of related compounds. Factors related to molecular structure elucidation based upon REMPI mass spectra are discussed. Enhanced isomer distinction is demonstrated for CBZ‐leu‐ala‐OCH3 and CBZ‐ile‐ala‐OCH3 based upon REMPI fragmentation.
The distonic ion ˙CH2CH2CO+ and its formation from ionized cyclopentanone
Authors: Traeger John C. , Hudson Charles E. , McAdoo David J.
Published: May 2005

Collisional activation decomposition (CAD) spectra are interpreted as indicating that formation of ˙CH2CH2CO+ (1) from ionized cyclopentanone, succinic anhydride arid butyrolactone is important at 70 eV electron energy. However, photoionization appearance energy measurements and CAD spectra demonstrate that CH3CHCO+· (3) is formed from ionized cyclopentanone near threshold. Ab initio molecular orbital calculations place ΔH f (1) about 36 kJ mol−1 above ΔH f (3).
Comparative chemical ionization mass spectra of tricyclic antidepressant amines and related compounds
Authors: Schildcrout Steven M. , Geidner Robert M.
Published: May 2005

Positive‐ion methane chemical ionization (CI) mass spectra were obtained for seven underivatized tricyclic amines: amitriptyline, nortriptyline, protriptyline, imipramine, desipramine, cyclobenzaprine and cyproheptadine. Some discrepancies in previously reported spectra were noted. Spectra of protriptyline, cyclobenzaprine and cyproheptadine are reported for the first time. Comparisons are made among the CI spectra and with electron ionization spectra, and relative abundances of major CI ions among the seven compounds are rationalized in terms of substituent and geometric effects.

In most cases low‐mass iminium ions from anion abstraction were more abundant than [MH]+. Hydride abstraction and adduct formation with reagent‐gas ions were important. The three heterocyclic amines gave abundant [M]+˙ by electron transfer. Protonation at the nitrogen atom on the side chain followed by amine elimination gave tricyclic aromatic fragment ions.
Reactions of the trimethylsilyl ion with 1,2‐cyclopentanediol isomers in the collision region of a triple quadrupole instrument
Authors: Meyerhoffer William J. , Bursey Maurice M.
Published: May 2005

Ion‐molecule reactions with the trimethylsilyl ion were used to distinguish between cis‐ and trans‐1,2‐cyclopentanediol isomers. The ion kinetic energy of [Si(CH3)3]+ was varied from 0 eV to 15 eV (center of mass frame of reference). At low ion kinetic energies (<4 eV), there are significant differences in the relative stabilities and decomposition behavior of the adduct ions [M + Si(CH3)3]+. The cis‐1,2‐cyclopentanediol isomer favors decomposition of [M + Si(CH3)3]+ to yield the hydrated trimethylsilyl ion [Si(CH3)3OH2]+ at m/z 91. For the trans isomer, the formation of the hydrated trimethylsilyl ion is an endothermic process with a definite threshold ion kinetic energy.
N‐phosphoryl amino acids and peptides. Part II: Fast atom bombardment mass spectrometry of N‐di‐isopropyloxyphosphoryl and N‐dibutyloxyphosphoryl amino acids
Authors: Xue Chu‐Biao , Yin Ying‐Wu , Yu Xiao , Zhao Yu‐Fen
Published: May 2005

The positive ion fast atom bombardment mass spectra of N‐di‐isopropyloxyphosphoryl (Dipp) and N‐dibutyloxyphosphoryl (Dbp) amino acids or amino acid methyl esters are presented; and according to the observation of the metastable ions and the high‐resolution accurate mass measurement their fragmentation patterns are postulated. Both types of compounds were found to undergo similar fragmentations to produce (HO)2P(O)NCHR + H+, in most cases as the most abundant fragment ion of structural significance. An intrinsical difference between the two types of compounds is that N‐Dippamino acids appear to favour the successive losses of two molecules of propylene, while the loss of HCOOH seems to be preferred by the Dbp amino acids. For those compounds containing an extra functional group on the side chain of amino acids such as serine or glutamic acid some other type of fragmentation was observed besides the normal phenomenon.
Positive desorption chemical ionization/tandem mass spectrometry of natural isomeric morphinans: Regioselectivity of protonation, positional and stereochemical effects upon decomposition
Authors: Charles Brigitte , Cole Richard B. , Bruneton Jean , Tabet Jean‐Claude
Published: May 2005

Fragmentations of naturally occurring morphinan isomers have been studied using a triple quadrupole mass spectrometer. Protonated molecules were generated via desorption chemical ionization using ammonia and methane as reagent gases. Mechanistic analysis of conformationally specific and stereospecific decomposition pathways has been performed and corroborated using deuterium labelling. Strong evidence of regioselective protonation has been offered, suggesting significant differences in the distribution of protonation sites as a function of reagent gas proton affinity.
Radical reactions with selected organic compounds in the gas phase
Authors: Budzikiewicz H. , Drabner G.
Published: May 2005

Several organic compounds for which radical reactions prior to ionization under chemical ionization (CH4) have been discussed were subjected to gas‐phase reactions with methyl and ethyl radicals. The results and their implications for proposed reaction mechanisms in a methane plasma will be discussed.
On the stereospecificity of the structurally diagnostic m/z 84 ion in the mass spectra of gibberellins and kaurenes with a 15‐hydroxy substituent
Authors: Macleod John K. , Monahan Lilian C.
Published: May 2005

The presence of a 15‐hydroxy substituent in naturally‐occurring kaurenes and gibberellins gives rise to a structurally diagnostic ion at m/z84 (C5H8O) in their electron‐impact mass spectra, which shifts to m/z156 for the TMS derivative, and is unaffected by differences in the A and B rings. Studies on a model tricyclic system have shown that this ion is more intense when the stereochemistry of the B/C ring junction is cis than when it is trans. In accordance with a previously proposed pathway for the formation of this ion, deuterium labelling confirms that there is a site‐specific transfer of the β‐hydrogen from the cis ring junction to the C5 fragment ion, although a minor modification of the earlier mechanism is proposed.
The [CH5NO]+ ˙ potential energy surface: Distonic ions, lon‐dipole complexes and hydrogen‐bridged radical cations
Authors: Burgers Peter C. , Lifshitz Chava , Ruttink Paul J. A. , Schaftenaar Gijs , Terlouw Johan K.
Published: May 2005

By combining results from a variety of mass spectrometric techniques (metastable ion, collisional activation, collision‐induced dissociative ionization, neutralization‐reionization spectrometry, 2H, 13C and 18O isotopic labelling and appearance energy measurements) and high‐level ab initio molecular orbital calculations, the potential energy surface of the [CH5NO]+ ˙ system has been explored. The calculations show that at least nine stable isomers exist. These include the conventional species [CH3ONH2]+ ˙ and [HOCH2NH2]+ ˙, the distonic ions [OCH2NH3]+ ˙, [ONH2CH3]+ ˙, [CH2O(H)NH2]+ ˙, [HONH2CH2]+ ˙, and the ion‐dipole complex CH2NH2+ …︁ OH˙. Surprisingly the distonic ion [CH2ONH3]+ ˙ was found not to be a stable species but to dissociate spontaneously to CH2O + NH3+ ˙. The most stable isomer is the hydrogen‐bridged radical cation [HCO …︁ H …︁ NH3]+ ˙ which is best viewed as an immonium cation interacting with the formyl dipole. The related species [CH2O …︁ H …︁ NH2]+ ˙, in which an ammonium radical cation interacts with the formaldehyde dipole is also a very stable ion. It is generated by loss of CO from ionized methyl carbamate, H2NC(O)OCH3 and the proposed mechanism involves a 1,4‐H shift followed by intramolecular ‘dictation’ and CO extrusion. The [CH2O …︁ H …︁ NH2]+ ˙ product ions fragment exothermically, but via a barrier, to NH4+ ˙ HCO…︁ and to H3NC(H)O+ ˙ H˙. Metastable ions [CH3ONH2]+…︁ dissociate, via a large barrier, to CH2O + NH3+ + and to [CH2NH2]+ + OH˙ but not to CH2O+ ˙ + NH3. The former reaction proceeds via a 1,3‐H shift after which dissociation takes place immediately. Loss of OH˙ proceeds formally via a 1,2‐CH3 shift to produce excited [ONH2CH3]+ ˙, which rearranges to excited [HONH2CH2]+ ˙ via a 1,3‐H shift after which dissociation follows.
Collision‐induced dissociations of deprotonated peptides, dipeptides and tripeptides with hydrogen and alkyl α groups. An aid to structure determination
Authors: Eckersley Mark , Bowie John H. , Hayes Roger N.
Published: May 2005

The characteristic collision‐induced dissociations of [M  H]− ions of dipeptides and tripeptides involve proton transfer to the carboxylate centre as a prelude to fragmentation. Dipeptides show the process NH2CH(R1)CONHCH(R2)CO2− → NH2C(R1)CONHCH(R2)CO2H → −NHCH(R2)CO2H + NH2C(R1)CO (R = H or alkyl) while tripeptides show the analogous processes NH2CH(R1)CONHCH(R2)CONHCH(R3)CO2− → NH2CH(R1)CONHC(R2)CONHCH(R3)CO2− → NHCH(R3)CO2H + NH2CH(R1)CONHC(R2)CO and NH2CH(R1)CONHCH(R2)CONHCH(R3)CO2− → NH2C(R1)CONHCH(R2)CONHCH(R3)CO2H → −NHCH(R2)CONHCH(R3)CO2H + NH2C(R1)CO. These fragmentations provide ready identification of the peptide.
Identification of butyryl derivatives of cyclic nucleotides by positive ion fast atom bombardment mass spectrometry and mass‐analysed ion kinetic energy spectrometry
Authors: Newton Russell P. , Walton Terence J. , Basaif Salem A. , Jenkins Andrea M. , Brenton A. Gareth , Ghosh Dipankar , Harris Frank M.
Published: May 2005

The syntheses of the dibutyryl derivatives of the 3′,5′‐cyclic monophosphates of adenosine, guanosine and cytidine are described. The fast atom bombardment mass spectra of these compounds are discussed, together with the mass‐analysed ion kinetic energy spectra of their protonated molecular ions and of diagnostic fragments. A protocol for the identification of the derivatives is reported which includes criteria for confirming retention of the cyclic phosphodiester moiety, substitution of both heterocyclic base and ribose ring, and butyrylation of the 2′‐O‐position. The origins of significant fragments in the spectra are discussed.
Natural isotopes as a tool in the elucidation of elemental composition and fragmentation pathways by tandem mass spectrometry
Authors: de Hoffman Edmond , Auriel Michel
Published: May 2005

Comparison of daughter spectra obtained by tandem quadrupole mass Spectrometry from ions selected from an isotopic cluster of the same elemental composition can be efficiently used to find out the elemental composition of a fragment. This even applies with ions of high mass and with low abundance isotopes such as 18O. By this method, loss of CO and HCO˙ from the phenol molecular ion is confirmed and the absence of CO loss from the p‐tert‐butyl‐pbenol molecular ion is made obvious. Limitations of the method are discussed in the case of a bicyclic imidazolidinone.
Bifunctional even‐electron ions. V. Fragmentation behaviour of aliphatic hydroxonium ions containing an additional cyano group
Authors: Záhorszky U. I. , Schulze P.
Published: May 2005

The fragmentation behaviour of a series of bifunctional hydroxonium ions C2H5C(\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^ + {\rm H} $\end{document})(CH2)n‐CN (n = 0‐6), a n, has been investigated by electron impact mass spectrometry. The type of interaction‐induced reactions depend strongly on the chain distance between the oxonium and the cyano moiety, but the initial step in all cases is proton transfer from the hydroxy to the cyano group. The following selective primary fragmentations have been found: loss of C  NH (HCN) from a 0, loss of CH2,  C  NH from a 1, loss of HNCO from a 2‐a 4, loss of CH3CHC  O from a 4‐a 6, and loss of CO from a3. In addition, loss of NH3 and H2O, respectively, are observed as general fragmentations, except for the lowest homologues (a 0 and a 1). Mechanistic pathways of the selective reactions have been deduced with the aid of metastable ion spectra, high resolution, deuterium labelling and comparison of metastable ion spectra with those of independently generated ions.
Structural characterization of organic molecules by negative ions in laser microprobe mass spectrometry. Part 2—Salts
Authors: Van Vaeck L. , Bennett J. , van Espen P. , Schweikert E. , Gijbels R. , Adams F. , Lauwers W.
Published: May 2005

Laser microprobe mass spectrometry (LMMS) has been applied systematically to a variety of organic polyfunctional molecules, covering a wide range of structures and polarities. The microprobe generally offers a combination of desorption under relatively mild conditions with abundant fragmentation. We attempted an empirical approach by tentative hypotheses about desorption and ionization in LMMS to consistently rationalize the detected fragments. The complementary nature of structural data, carried by positive and negative ions, is characteristic for LMMS results of non‐ionic compounds. The analysis of salts represents, traditionally, an ultimate test case for soft methods in organic mass spectrometry. Hence, by a selected series of compounds, we have tried to assess to which extent the presence of preformed ions becomes an asset for LMMS analysis and affects the amount, the accessibility or the distribution of organic information between positive and negative fragments.
Mass spectra of 2‐thioacylaminothiazoles, 2‐thioacylaminobenzothiazoles and their ‘fixed structure’ amino and imino tautomeric forms
Authors: Jagodziński Tadeusz S. , Terentiev Piotr B.
Published: May 2005

The fragmentation patterns of the thioacyl derivatives of 2‐aminothiazole and 2‐aminobenzothiazole and of their ‘fixed structure’ imino and amino tautomeric forms give evidence of the predominance of the imino tautomer in the molecular ions of the trifluorothioacetyl compounds. On the other hand, the M+˙ ions of the thioacetyl and thiobenzoyl derivatives are mainly the amino tautomers. A rearrangement with elimination of RCN and formation of the 2‐thiazolothione (2‐benzothiazolothione) ion is characteristic for ail the compounds investigated. Additional evidence for the interpretation of the main fragmentation paths is provided by the mass spectra of the N‐trideuteromethyl and N‐deuterated derivatives, high‐resolution mass spectrometry being used to determine the elemental composition of some selected peaks.
Extensive substituent scrambling in substituted diphenylacetylenes on electron impact
Authors: Ramana D. V. , Rama Krishna N. V. S.
Published: May 2005

Extensive randomization of the substituents is found in mono‐ and disubstituted diphenylacetylenes under electron impact conditions. The study of diphenylacetylenes with a variety of substituents indicates that the electron‐withdrawing groups favour the substituent scrambling, AH the substituted diphenylacetylenes give an ion at m/z 176, having a common ion‐structure, arising as a result of the expulsion of a hydrogen radical and the substituent in monosubstituted diphenylacetylenes and the expulsion of both the substituents in disubstituted diphenylacetylenes. A ring‐expanded structure is postulated for this common fragment.
Multiply charged ions in ionspray tandem mass spectrometry
Authors: Siu K. W. M. , Gardner G. J. , Berman S. S.
Published: May 2005

Multiply charged anions of some 30 species containing multiple carboxylic, sulphonic and/or phosphoric groups generated by ionspray were studied by using tandem mass spectrometry (MS/MS). Two trends emerged: (1) lower‐charged ions are preferentially evaporated, and (2) for more highly charged ions, evaporability is a function of ion size. Triply and quadruply charged ions were observed for azo dyes with molecular masses in the 700‐900 Dalton range. Daughter‐ion mass spectra of selected multiply charged ions are presented.
Unimolecular decomposition of ethoxytrimethylsilane
Authors: Herold David A. , Futrell Jean H.
Published: May 2005

A study of the metastable spectra from ethoxytrimethylsilane and the mass shifts of the deuterium‐labeled species permitted the rationalization of the fragmentation mechanism for forming all major ions in the mass spectrum. A new mechanistic pathway for the formation of [Si(CH3)3]+ (m/z 73) is demonstrated. A strong metastable ion for elimination of neutral acetaldehyde from the parent ion was observed despite the absence of a detectable daughter ion.
Positive electron impact and chemical ionization mass spectra of some nitramine nitrates
Authors: Rowley J. A.
Published: May 2005

The positive electron impact (EI) and isobutane chemical ionization (CI) mass spectra of six nitramine nitrates were studied with the aid of some accurate mass measurements. In the EI spectra, β fission relative to both the nitramine and nitrate ester is important. In the CI spectra a major ion occurs at [MH – 45]+ and was found to be mainly due to [M + 2H  NO2]+. All of the compounds except N‐(2 hydroxyethyl)‐N‐(2′,4′,6′‐trinitrophenyl)nitramine nitrate gave an [MH]+ ion. The [MH – 45]+ ion in the isobutane CI mass spectra of tetryl is also due to [M + 2H  NO2]+.
Stabilization of even‐electron ions in 4‐amino‐substituted cytosines. Appearance of the strong ortho effect between an aryl substituent and the pyrimidinyl ring
Authors: Plaziak Adam S. , Spychala Jaroslaw , Golankiewicz Krzysztof
Published: May 2005

The mass fragmentation of 4‐arylaminocytosine derivatives was investigated and it was found that the ortho effect is mainly responsible for the strong stabilization of even‐electron ions formed during fragmentation. The ortho effect in this class of compounds completely eliminates other possible fragmentation patterns. This effect disappears when the aryl substituent is separated from the 4‐amino group of the cytosine moiety by a methylene group.
Thermospray liquid chromatography/mass spectrometry of quaternary ammonium saltst
Authors: Vicchio Domenick , Yergey Alfred L.
Published: May 2005

Various esters of choline and carnitine have been shown to undergo fragmentation dependent on both the structures of the molecules and the vapor and block temperatures when subjected to thermospray ionization mass spectrometry. The fragments observed vary with the structure of the ester portion and the backbone of the molecules, and are consistent with the compounds undergoing three types of reactions: (1) carbon—nitrogen bond cleavage to tertiary amines and alkyl compounds, (2) hydrolysis of the ester linkage, forming alcohols and organic acids, and (3) deacylation of the molecules to form acids and the corresponding quaternary alkene. The general applicability of the carbon—nitrogen bond cleavage mechanism was studied with other classes of quaternary compounds, but the sole products consistently detected for pyridinium, benzyldimethyldodecylammonium and tetraalkylammonium salts were amines. The fragments observed for the esters of choline and carnitine and also the anticholinergic 2,2‐diphenyl‐4‐diisopropylaminobutyramide methiodide suggest that the mechanism of the thermospray fragmentation pathway is similar to that of the Hofmann elimination reaction.
Stereochemical effects in mass spectrometry: Part 8‐chemical ionization mass spectra of cyclic glycols and monosaccharides using formaldehyde dimethyl acetal as reagent gas
Authors: Tu Ya‐Ping , Chen Yao‐Zu , Chen Su‐Nian , Wang Mei‐Lan , Jing Zhi‐Zhong
Published: May 2005

The chemical ionization mass spectra of cyclic glycols and monosaccharides using formaldehyde dimethyl acetal as reagent gas have been studied. In the gas phase, the stereospecific reaction of methylene acetal formation was observed. From the relative abundances of the characteristic ions the stereoisomers of these compounds may be definitely distinguished.
Structures of decomposing and non‐decomposing gas‐phase [C4H6O]+· radical cations, and their [C2H2O]+· and [C3H6]+· product ions
Authors: Dass Chhabil , Gross Michael L.
Published: May 2005

The structures of gas‐phase [C4H6O]+· radical cations and their daughter ions of composition [C2H2O]+· and [C3H6]+· were investigated by using collisionally activated dissociation, metastable ion measurement, kinetic energy release and collisional ionization tandem mass spectrometric techniques. Electron ionization (70 eV) of ethoxyacetylene, methyl vinyl ketone, crotonaldehyde and 1‐methoxyallene yields stable [C4H6O]+· ions, whereas the cyclic C4H6O compounds undergo ring opening to stable distonic ions. The structures of [C2H3O]+· ions produced by 70‐eV ionization of several C4H6O compounds are identical with that of the ketene radical cation. The [C3H6]+· ions generated from crotonaldehyde, methacrylaldehyde, and cyclopropanecarboxaldehyde have structures similar to that of the propene radical cations, whereas those ions generated from the remainder of the [C4H6O]+· ions studied here produced a mixed population of cyclopropane and propene radical cations.
α‐Distonic ions as transient species in the reactions of metastable alkyl benzoate radical cations
Authors: Audier Henri Edouard , Milliet Arielle , Sozzi Georges , Hammerum Steen
Published: May 2005

The key intermediates to the fragmentation of metastable methyl and ethyl benzoate radical cations are α‐ and β‐distonic isomers of the molecular ions. The α‐distocic isomers are also formed by fragmentation of longer chain alkyl benzoates, but may not be long‐lived, stable species. Rearrangement of the α‐distonic ions prior to fragmentation can take place, but (re)formation of the benzoate molecular ions does not occur.
Characterization of cyclotrimethylenetrinitramine (RDX) by pyrolysis H2O/D2O atmospheric‐pressure chemical ionization tandem mass spectrometry
Authors: Snyder A. Peter , Liebman Shirley A. , Schroeder Michael A. , Fifer Robert A.
Published: May 2005

The oxidative pyrolysis‐atmospheric‐pressure chemical ionization tandem mass Spectrometry (Py‐APCI MS/MS) of Cyclotrimethylenetrinitramine (RDX) was investigated under various sample introduction conditions. Subambient (0.97 atm) as opposed to ambient (0.98 atm) pressure (1 atm = 101325 kPa) facilitated the appearance of new pyrolysis mass spectral ions, including m/z 44. Deuterated decomposition products from [2H]RDX contained amide groups and, depending on the ion source pressure, significant differences in the degree of proton‐deuterium exchange occurred on the amide groups. The D2O Py‐APCI MS/MS method also confirmed and extended the analogous H2O APCI information from RDX, [2H]RDX and pure standards. The m/z 44 decompositon species was identified as protonated dimethylimine, [H3CNCH2]+ as opposed to its primary amine isomer, [H3CC(H)NH]H+, which contains an acidic proton. It was determined that m/z 60 is due to protonated N‐methylformamide and acetaldoxime, [H3CC(H)NOH]H+.
Low‐energy fragmentations of five isomeric [H3, C, N, O2] +· ions
Authors: Sirois M. , Holmes J. L. , Hop C. E. C. A.
Published: May 2005

The low‐energy fragmentation characteristics of the [H3,C,N,O2]+· isomers [H3CNO2]+· (a), [H2CN(O)OH]+· (b), [H3CONO]+· (c), [HC(O)NHOH]+· (d) and [HC(OH)NOH]+· (e) were studied in detail by metastable ion mass spectrometry. In agreement with most earlier observations, appearance energy measurements established the potential energy surface of the isomers a, b and c, showing the intricate interrelations between them. It was concluded that a isomerizes into b prior to fragmentation by loss of ·OH and H2O and into c before loss of ·H and H3CO· moreover, the reverse reactions do not take place on the metastable time‐frame. The dominant metastable process for isomers d and e (obtained via HCN loss from glyoxime) was generation of [H2NOH]+·. For isomer e this process was proposed to involved a rate‐determining isomerization into d. It was concluded that isomers d and e do not intercommunicate with ions a, b and c prior to fragmentation. Neutralization‐reionization mass spectrometry indicated that the enol form of formohydroxamic acid as well as the keto counterpart are stable in the gas phase.
Interlaboratory comparison of limits of detectic in negative chemical ionization mass spectrometry
Authors: Arbogast B. , Budde W. L. , Deinzer M. , Dougherty R. C. , Eichelberger J. , Foltz R. D. , Grimm C. C. , Hites R. A. , Sakashita C. , Stemmler E.
Published: May 2005

Data are presented on the limits of detection for a series of nine compounds in negative chemical ionization (NCI) mass spectra obtained in five different mass spectrometers: Finnigan 4000 with a 4500 ion source, Kratos MS‐80, Hewlett‐Packard 5985 and two Finnigan 4500s. The nine compounds undergo either resonance capture or dissociative capture of an electron at optimum energies ranging from 0.0 to 1.1 eV. The limits of detection generally increased with increasing optimum electron energy. The limit of detection as a function of optimum electron capture energy is expected to provide information about the electron energy distribution in the ion sources. The data showed scatter within and between instruments. The scatter is believed to be due primarily to reactions with low levels of adventitious gases such as oxygen in the ion source. The data also suggested wide variations in electron energies between the instruments. The variation in the electron energy distribution is thought to have been caused by variations in the ion optical fields within the instruments. These results suggest that the requirements for reproducibility in NCI mass spectra at the limit of detection are rigorous control of trace gases in the ion source, control of the electric fields within the source including ion optical fields that penetrate the source aperture control of pressure, temperature and other factors that influence NCI mass spectra.
Flash‐vacuum pyrolysis of 1‐acylbenzotriazole: Direct observation of cyclopenta‐2,4‐dienylidenemethaneimines by tandem mass spectrometry and low‐temperature infrared spectrometry
Authors: Maquestiau André , Beugnies Didier , Flammang Robert , Freiermuth Beat , Wentrup Curt
Published: May 2005

A real‐time analysis of the flash‐vacuum pyrolysis products of 1‐acetylbenzotriazole (1) and 1‐benzoylbenzotriazole (2) was performed by tandem mass Spectrometry. In the temperature range 500‐600 °C, these compounds lose nitrogen, yielding N‐acetyl‐ and N‐benzoylcyclopenta‐ 2,4‐dienylidenemethaneimines (10 and 17, respectively). At higher pyrolysis temperatures, 1 gives 2‐methylbenzoxazole, cyanocyclopentadiene, methylcyanocyclopentadiene(s), benzonitrile and ketene, which were identified by collision‐activated dissociation mass Spectrometry. Low‐temperature infrared experiments confirmed the pyrolytic transformation 1(2) → 10(17) at mediated temperatures.
Collisional activation spectra of the adduct ions of 1,2,3‐triarylpropenones under ammonia chemical ionization conditions
Authors: Madhusudanan K. P. , Durani S. , Kapil R. S. , Fraisse D.
Published: May 2005

Under ammonia chemical ionization (CI) conditions triarylpropenones undergo hydrogen radical‐induced olefinic bond reduction on metal surfaces, resulting in [M + 2H + NH4]+ ions corresponding to the ammonium adduct of the saturated ketone. The decomposition of the adduct ions, [MNH4]+ and [M + 2H + NH4]+, was studied by collision‐induced dissociation mass‐analysed ion kinetic energy (CID‐MIKE) spectroscopy in a reverse geometry instrument. From the CID‐MIKE spectra of the [MNH4]+, [M + 2H + NH4]+, [MND4]+ and [M + 2D + ND4]+ ions it is clear that the fragmentation of the adduct ions involves loss of NH3 followed by various cyclization reactions resulting in stable condensed ring systems. Elimination of ArH and ArCHO subsequent to the loss of NH3 and formation of aroyl ion are characteristic decomposition pathways of the [MNH4]+ ions, whereas elimination of ArCH3 and formation of [ArCH2]+ are characteristic of the [M + 2H + NH4]+ ions of these propenones.
Nitromethane as a substitute for nitric oxide in chemical ionization
Authors: Vairamani M.
Published: May 2005

Nitromethane chemical ionization mass spectra of some alkenes, alkynes and alcohols show that the spectra are similar in many respects to those obtained under nitric oxide chemical ionization conditions.
Evidence for metal surface catalysed reduction of unsaturated carboxylic acids under negative ion chemical ionization
Authors: Vairamani M. , Saraswathi M. , Rao G. K. Viswanadha
Published: May 2005

Unsaturated carboxylic acids have been found to undergo hydrogenation in the presence of methanol prior to ionization. Z‐Isomers are found to undergo more facile reduction than the E‐isomers in the case of two pairs of dicarboxylic acids studied.
Rearrangements and fragmentations of metastable [C13H9S]+, [C14H11]+, [C13H11]+ and [C8H7S]+ ions
Authors: Rietjens Marcel , Oudman Drewes , Weringa Wilke D.
Published: May 2005

[C13H9S]+, [C14H11]+, [C13H11]+ and [C8H7S]+ ions with unknown structures were generated from two [C14H12S]+·precursor ions by fragmentation reactions that must be preceded by extensive rearrangements. Ions with the same compositions, each with several initial structures, were prepared by simple bond‐breaking reactions. Metastable characteristics were compared for each of the four types of ions. It was found than in all cases fast isomerization reactions occur prior to fragmentation, so that no information about the unknown ion structures could be obtained by comparison of the observed fragmentations of metastable ions.