The mass spectrometry of four acyl derivatives of 2,2′-difurylmethane

Low resolution mass spectra and high resolution data for selected important peaks are presented and discussed for the following compounds: [(5-acetyl-2-furyl)-(2′-furyl)]methane (I), [(5-acetyl-2-furyl)-(5′-methyl-2′-furyl)]methane (II), [(5-formyl-2-furyl)-(2′-furyl)]methane (III) and [(5-formyl-2-furyl)-(5′-methyl-2′-furyl)]methane (IV). The fragmentation of II has been clarified by examining the mass spectrum of its d3-acetyl analog; the fragmentation of III and IV by examining the spectra of their carbonyl 13C-labeled analogs.     

Electron impact mass spectra of 3,5,7-triaryl-and -trialkyl-4H-1,2-diazepines

Electron impact mass spectra of 3,5,7-trisubstituted 4H-1,2-diazepines indicate that aryl substituents lead to N₂ expulsion while alkyl substitutents do not. A common fragmentation pattern is observed and discussed for all alkyldiazepines, most of which are newly reported compounds. Assignments are based on electron impact mass spectra of deuteriated substrates and high resolution mass spectra. A previous interpretation of N₂ expulsion is corrected.     

The modern mass spectrometer—a complete chemical laboratory

Using nitrobenzene as an example, various ways in which a contemporary mass spectrometer can be utilized to yield a wealth of information about the compound studied are reviewed. Applying a variety of different techniques and procedures, in addition to the conventional low resolution mass spectrum, the following nitrobenzene spectra have been obtained: collision induced dissociation mass spectrum, mass analysed ion kinetic energy spectra, collision induced dissociation mass analysed ion kinetic energy spectra, spectra obtained at constant B/E, spectra obtained at constant B²/E, high voltage scans of metastable ion fragmentation processes, consecutive fragmentations in different field free regions, charge exchange mass spectra, charge stripping mass spectra, doubly charged ion mass spectra, chemical ionization mass spectra, negative ion mass spectra, negative ion mass analysed ion kinetic energy spectra, negative ion mass analysed ion kinetic energy collision induced dissociation spectra, charge inversion spectra, etc. The complementary types of information available from the above studies are discussed to show the unique versatility of mass spectrometry as a technique for the examination of organic compounds.     

Discrimination of diastereoisomers by electron ionization mass spectrometry in the 1,3‐dioxane series

Important differences in the electron ionization (EI) mass spectra of cis,cis and trans,trans isomers of some 2(r)‐R‐2,4(R),6(S)‐trimethyl‐1,3‐dioxanes are recorded and the significant preference for the fragmentation with the cleavage of the axial substituents at C(2) position is revealed. The fragmentation pattern is elucidated by high‐resolution mass measurements, metastable ion spectra and linked‐scan mass spectrometry. Copyright © 2005 John Wiley & Sons, Ltd.     

Mass spectra of Sceletium alkaloids

The mass spectra of several naturally occuring Sceletium alkaloids, as well as several synthetic variants, have been investigated. Techniques used include high resolution mass measurements, analysis of metastable transitions by the technique of ion defocusing and specific deuterium labeling in conjunction with low resolution mass spectrometry. Characteristic major fragmentation pathways have been determined for four basic structure types studied: the cis-3a-aryl-octahydroindole ring system, exemplified by mesembrine; its 2,3-pyrido analog, Sceletium A₄; the seco series of pyridine bases, represented by tortuosamine; and the seco analog of the mesembrine-type represented by tortuosamine; and the seco analog of the mesembrine-type represented by dehydrojoubertiamine.     

High resolution mass spectrometry in molecular structure studies—V: The fragmentation of amaryllis alkaloids in the crinine series

High resolution mass spectral fragmentation patterns of Amaryllidaceae alkaloids in the crinine series have been investigated as well as those of certain isotopically labeled derivatives. Attention is directed to the particular importance of employing entire high resolution mass spectra in attempting the elucidation of structural correlations in compounds which display few significant relative intensity variations in a complex fragmentation pattern.     

Wavelength-dependent fragmentation in resonance-enhanced multiphoton ionization mass spectrometry

Wavelength-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.     

C2对称的手性双噁唑啉和双噻唑啉的质谱碎裂机理研究

The electron impact mass spectra of ten new C2-symmetric chiral bis(oxazoline) and bis(thiazoline) have been studied. Bis(thiazoline) and bis(oxazoline) possess the same fragmentation mechanism under EI conditions. An unusual fragmentation pathway has been found in the compounds studied. Due to the presence of phenyl group, compounds 6 and 10 undergo a new fragmentation pathway except for the common way as the other eight compounds.Mass analyzed ion kinetic energy spectra experiments and high resolution accurate mass measurement were conducted to confirm the proposed fragmentation pathways.     

Derivatization and mass spectrometric investigations of substituted benzeneboronic acids. The use of linked scanning during gas chromatography mass spectrometry

Substituted benzeneboronic acids are important intermediates in the synthesis of support matrices for affinity chromatography but their analysis by mass spectrometry is hindered by thermal reactions in the ion source. A simple derivatization with 1,2- or 1,3-diols removes this difficulty and imparts sufficient volatility for application of gas chromatography/mass spectrometry. The mass spectra of the resulting boronate esters are discussed with reference to high resolution measurements, isotope labelling studies and observation of metastable ions. ortho Substituents are shown to interact strongly during fragmentation. Linked scanning at constant B/E was used to characterize fragmentation pathways and the compatibility of linked scanning and GC/MS is reported.     

Electron-impact fragmentation of N-substituted 2-aminobenzoxazoles and 2-aminobenzothiazoles

The mass spectra of twenty-eight N-substituted 2-aminobenzoxazoles and 2-aminobenzothiazoles have been recorded and the identity of various ions in the mass spectra have been established by high resolution measurement. The molecular compound of series, benzoxazole, undergoes loss of hydrogen cyanide and carbon monoxide from the molecular ion as the most important decomposition pathways. The mass spectra of N-substituted benzothiazoles showed a similar fragmentation as that of N-substituted benzoxazoles.     

Mass spectrometric studies of structural isomers—II: Mono-and bicyclic C6H10 molecules

The electron-impact-induced ionization and fragmentation of six C₆H₁₀ structural isomers have been studied in order to determine the effect of isomerism upon their mass spectrometric behavior. The 70 eV mass spectra, metastable transitions and appearance potentials of the principal ions are reported. Significant differences between the mass spectra of the six isomers were observed; however, metastable transition and appearance potential data indicate that the fragmentation path-ways are the same for all the C₆H₁₀ molecules. Experimentally determined ionization potentials for the structural isomers are presented and compared to ionization potentials calculated by the bond orbital method. Utilizing fragmentation pathways deduced from general features in the mass spectra and from observed metastable transitions, we calculated heats of formation (ΔH_f) for the observed principal ions and compared these values to ΔH_f values for isomeric ions from other molecules.     

Basic mass spectrometric investigations of amino acid-fluorescamine derivatives

Mass spectra of fluorescamine derivatives of seventeen of the naturally occuring amino acids have been studied using electron ionization mass spectrometry. Seven fragmentation processes of fluorophors are proposed and elucidated by accurate mass measurements, stable isotope incorpor-ation and low resolution spectra. Two major pathways contributed most of the predominant peaks in the mass spectra. All fragmentations involve specific rearrangements of the spiro ring, while some lose the amino acid sidechain. Identification of the amino acid is unequivocally established in the low resolution spectra. Loss of one oxygen atom from the dehydrated fluorophors was found for all compounds studied. The site of this loss was determined by incorporation of ¹⁸O into the carbonyl groups of fluorescamine.     

The mass spectra of small-ring heterocycles. III. Aroylaziridines

The mass spectra of cis-trans isomeric aroylaziridines are presented. Attempts to extend the method for distinguishing between cis and trans isomers previously established for aroylazetidines are described and the results rationalized. A simple fission of the 1-alkylnitrogen bond is described and detailed fragmentation mechanisms are presented and discussed.     

Electron ionization mass spectrometric study of substituted alloxazine‐5‐oxides and iso‐alloxazine‐5‐oxide

The fragmentation pathways in electron ionization (EI) mass spectra of a series of new N(5)‐oxides of alloxazines and iso‐alloxazine are presented, and compared with those of substituted alloxazines and iso‐alloxazine. The EI mass spectra of these compounds showed characteristic fragmentation pathways A, B and C, started by the ejection of atomic oxygen, a HNCO molecule and an OH ^. radical, respectively. On the basis of B/E and B²/E spectra, the mechanism of elimination of the OH ^. radical is discussed. The influence of the methyl substituent in the benzene ring of alloxazine on the mass fragmentation pathways is described. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of electrospray ionization conditions on low-energy tandem mass spectra of peptides

The effect of electrospray ionization (ESI) conditions on low-energy tandem mass spectra of peptides in the relative molecular mass range 400–1200 was examined. For singly charged peptide ions the source skimmer potential (which determines the degree of acceleration of the ions through the intermediate pressure region in the source) can strongly influence the extent of fragmentation observed in tandem mass spectra, especially at low collision energies. For each peptide there is an optimum skimmer potential which represents a balance between generating ions with sufficient internal energy for subsequent tandem mass spectrometric experiments and inducing the onset of other processes such as source fragmentation. The fragmentation which can be achieved in tandem mass spectra with high skimmer potentials differs from ESI source fragmentation for the same peptides. We have found that fragmentation in ESI mass spectra depends both on skimmer potential and on solvent pH, presumably because the latter determines the proportion of doubly charged species generated from a given peptide. Low-energy tandem mass spectra of peptides following ESI are equally as sensitive to peptide structure and the type of adduct studied (e.g. [M + H]⁺ vs. [M + NH₄]⁺) as tandem mass spectra obtained following older ionization methods such as fast atom bombardment.     

The mass spectra of alkyl- and phenyl-4-imidazolin-2-ones

The mass spectra of a variety of alkyl- and aryl-4-imidazolin-2-ones have been determined and the fragmentation mechanisms have been analyzed by deuterium labelling, high resolution and metastable transitions allowing certain differentiations of positional isomers. In contrast to the benzoid systems the mass spectra of isomeric alkyl-4-imidazolin-2-ones are distinctive. The influence of the position of substituents is demonstrated by phenyl-4-imidazolin-2-ones establishing an exact prediction of fragmentation pathways. Fragment ions (e.g. [M-HNCO].⁺) which are the result of rearrangement processes were excluded for structure determinations. The ion structures involved were elucidated by collisional activation comparing model ions. Alkyl-phenyl-4-imidazolin-2-ones give almost identical mass spectra, but the positional isomers can easily be distinguished by different fragmentation patterns in both metastable and collisional activation spectra of the molecular ions.     

Fragmentation of dihydro- and tetrahydro-1,5-benzothiazepines under electron impact

The fragmentation mechanisms of dihydro- and tetrahydro-1,5-benzothiazepines under electron impact have been studied in detail using high resolution mass spectrometry, metastable decompositions and deuterium labelling techniques. Both kinds of the benzothiazepines possess high stability. The [M? SH]⁺ and the cyclic benzothiazole ions derived from the fragmentation and severe skeletal rearrangement of the molecular ion comprised the main features of the spectra. Some doubly charged ions were noticed in the low resolution electron impact mass spectra.     

Electron impact mass spectra of synthetically modified milbemycins

The potent endectocide 23-(O-methyloximino)-F28249α and related compounds were identified and characterized by mass spectrometry. The fragmentation pathway of 23-(O-methyloximino)-F28249α was identified by its high-resolution mass spectrum and the electron impact unit mass spectra of its homologs. This fragmentation pathway is presented and discussed.     

Nachbargruppeneffekte bei der Massenspektrometrischen Fragmentierung von Amino-benzylalkoholen

The fragmentation behaviour of a series of substituted aminobenzylacohols has been investigated by means of high and low resolution mass spectroscopy, metastable defocusing and deuterium labelling. o-, m- and p-Isomers exhibit characteristic fragmentation patterns that are of considerable diagnostic value in the structure determination of this class of compounds.     

Mass spectra of pyridazines and their N-oxides

The mass spectra of pyridazines and their N-oxides is reported. Previous electron impact studies on aromatic amine N-oxides reported the appearance of M-16 and/or M-17 ion peaks as the usual feature of the fragmentations. In our experiments, the representative fragmentation of the pyridazine N-oxides involves formation of a M-30 (M-NO) ion. The fragmentation patterns substantiated by extensive high resolution studies and the analysis of the appropriate metastable ions.