Mass spectrometry of nitroazoles: 2—the mass spectra of methyl substituted nitroimidazoles

The mass spectra of six isomers of methylnitroimidazoles are reported and discussed. All compounds exhibit strong molecular ions, along with the characteristic fragmentations of aromatic nitro compounds. In some cases ortho effects—losses of OH^., H₂O, CHO^., CH₂O—are observed, due to interactions of adjacent substituents.     

Multiply charged ions of ruthenium(II), rhodium(III) and cobalt(III) complexes in electrospray ionization mass spectrometry

Multiply charged ions from electrospray ionization (ESI) were observed for ruthenium-bidentate ligand complexes, such as [RuL₂B]X₂ and [(RuL₂)₂B]X₄, where L is 2,2′-bipyridine, B are tetradentate ligands of 2,2′-bis(2′-pyridyl)bibenzimidazole and 2,6-bis(2′-pyridyl)benzodiimidazole, bidentate ligand of 2-(2′-pyridyl)benzimidazole and related compounds and X is CIO₄^- or CI^-. ESI mass spectra showed a simple mass pattern for easy structural assignment and detecting impurities. The mass spectra for binuclear complexes provide a charge state distribution ranging from 4+ to 2+ for Ru(II)—Ru(II) compounds and 5+ to 2+ for Ru(II)—Rh(III) compounds. It was found that different multiply charged ions are generated by loss of counterions and by protonation/deprotonation at the proton site of ligands B. The abundances of these ions are qualitatively explained in terms of the acidity of metal complexes depending on the bridging ligand structures and the charge of the metal ions. Ions produced by removal of ligands were hardly observed.     

Mass spectra of methylsubstituted monosila- and monogermacyclobutanes, 1,2- disila- and 1,2-digermacyclopentanes. Some correlations with thermal decomposition

The mass spectra of 1,1-dimethyl-1-silacyclobutane (I—as reported by Cherniak et al.),⁶, 1,1-dimethyl-1-germacyclobutane (II), 1,1,2,2-tetramethyl-1,2-disilacyclopentane (III) and 1,1,2,2-tetramethyl-1,2-digermacyclopentane (IV) are compared and some correlations between electron-impact fragmentation and thermal decomposition are derived. The mass spectra of the germanium compounds with respect to the silicone compounds are enriched by light fragment ions and exhibit lower intensities of odd-electron ions. The composition of some ions and apparently of neutral fragments coincides with that of the unstable intermediates which are suggested in the thermal decomposition mechanism of some related compounds. The loss of C₂H₄ is more characteristic under electron-impact as well as in thermal decomposition of Si-compounds, while C₃H₆ is preferable eliminated by the Ge-compounds.     

Characterization of five [C4H7O]+ Ion structures. Fragmentation of the 2-pentanone molecular ion

The [C₄H₇0]⁺ ions [CH₂?CH? C(?OH)CH₃]⁺ (1), [CH₃CH?CH? C(?OH)H]⁺ (2), [CH₂?C(CH₃)C(?OH)H]⁺ (3), [Ch₃CH₂CH₂C?O]⁺ (4) and [(CH₃)₂CHC?O]⁺ (5) have been characterized by their collision-induced dissociation (CID) mass spectra and charge stripping mass spectra. The ions 1–3 were prepared by gas phase protonation of the relevant carbonyl compounds while 4 and 5 were prepared by dissociative electron impact ionization of the appropriate carbonyl compounds. Only 2 and 3 give similar spectra and are difficult to distinguish from each other; the remaining ions can be readily characterized by either their CID mass spectra or their charge stripping mass spectra. The 2-pentanone molecular ion fragments by loss of the C(1) methyl and the C(5) methyl in the ratio 60:40 for metastable ions; at higher internal energies loss of the C(1) methyl becomes more favoured. Metastable ion characteristics, CID mass spectra and charge stripping mass spectra all show that loss of the C(1) methyl leads to formation of the acyl ion 4, while loss of the C(5) methyl leads to formation of protonated vinyl methyl ketone (1). These results are in agreement with the previously proposed potential energy diagram for the [C₅H₁₀O]⁺˙ system.     

N-Alkyl-5,5-diorganodiptychoxazstannolidone,intramolekular pentakoordinierte stannylester der N-methylminodiessigsäure

The reaction of dialkyltin oxides or dialkyltin dialkoxides with N-methyliminodiacetic acid in dimethylformamide/toluene gives the title compounds in 80–90% yields. The structure of these compounds was investigated by NMR, IR and mass spectroscopy. The temperature dependent ¹H and ¹³C NMR spectra are interpreted by a dissociation—inversion mechanism.     

The mass spectra of the trimethylsilyl esters of acetyl,schiff base and isothiocyanate derivatives of some aminoalkylphosphonic acids

Trimethylsilyl esters of acetyl, Schiff base and isothiocyanate derivatives of a series of aminoalkylphosphonic acids were prepared for the purpose of characterizing these phosphorus compounds by combined gas chromatography and mass spectrometry. The mass spectra of these derivatives were investigated by means of high resolution mass measurments and deuterium labeling. Ions characteristic of the presence of the trimethylsilylphosnate group were observed at m/e 121, 195, 211 and 225 to 227 in the spectra of all the derivatives. Several ions produced by interaction between the trimethylsilyl group and the derivatized amino function were present, particularly in the spectra of the acetate derivatives ([M — 56]⁺, [M — R]⁺ and [M — 153]⁺, where R is the side chain attached to C-1), and the isothiocynate derivatives (m/e 268, 253, 241, 190 and 116).     

Mass spectrometric behaviour of simple oxazolidines under electron impact and chemical ionization

The 70 eV electron impact mass spectra of 33 differently substituted oxazolidines were studied to determine the effect of substituents and the existence of the ring—chain tautomeric equilibrium on the decomposition of the molecular ions. Most of the fragmentations can be rationalized to start as the cleavage initiated by the radical site at nitrogen. Isomeric compounds showed different spectra and were easily differentiated. The position of the ring—chain equilibrium could be located only roughly. The chemical ionization mass spectra of the compounds were also recorded, with ammonia, isobutane, acetone or methane as reagent gas. Methane was the only reagent gas that promoted extensive fragmentation of the protonated molecules. However, no information about the position of ring-chain tautomerism was obtained under these conditions. Analogously to other related five-membered heterocycles, the oxazolidines reacted under acetone chemical ionization conditions to afford [M + CH₃CO]⁺ adduct ions. These adducts were stable, however, and unlike those of 1,3-dioxolanes and 1,3-oxathiolanes, they did not decompose and form stable oxonium ions.     

New isomeric 2‐ortho‐(meta‐ and para‐) chloro‐(bromo and nitro‐)benzylthio‐6‐aminouracils

Ten new ortho, meta and para substituted derivatives of 2‐benzylthio‐6‐aminouracils have been prepared. Electron Impact (EI) induced mass spectral fragmentation of these compounds was investigated. Fragmentation pathways are proposed on the basis of accurate mass and metastable transition measurements. The correlation between the intensities of the M⁺‐ and the selected fragment ions of these compounds is discussed. The data obtained create the basis for dinstinguishing isomers. The ¹H and ¹³C NMR spectra of these compounds were assigned unambiguously using a combination of heteronuclear (HETCOR) spectra the chemical shifts. The data derived from these spectra can be used to differentiate the isomers.     

Synthesis and physicochemical characterization of fluorescent (E)‐2‐stilbenyloxyalkylthiouracils and isomer differentiation using EIMS

Twelve new fluorescent (E)‐2‐stilbenyloxyalkylthiouracils and 6‐methyluracils 5a‐51 were prepared. EI induced mass spectral fragmentation of these compounds was investigated. Fragmentation pathways are proposed on the basis of accurate mass and metastable transition measurements. Correlation between the intensities of the M⁺ and the selected fragment ions of these compounds is discussed. The data obtained permit a distinction of the metamers. The ¹H and ¹³C NMR spectra of these compounds were assigned unambiguously using a combination of heteronuclear (HETCOR) spectra and the chemical shifts. The data derived from these spectra can be used to differentiate the isomers.     

A mass spectrometric study of substituted furfuryl compounds

The mass spectra of six furfuryl compounds — namely, furfuryl alcohol, 5-furfuryl-furfuryl alcohol, difurfuryl ether, difurylmethane, 2,5-difurfurylfuran, and 4-furfuryl-2-pentenoic acid-γ-lactone — have been studied. Their fragmentation mechanisms are discussed in detail with particular emphasis on the modes that lead to the formation of aromatic fragments. The majority of the fragment ions are formed by elimination of CO and C₂H₂ from even-electron precursor ions and HCO from odd-electron precursor ions. Molecules containing two furan rings linked by a methylene group give mass spectra that exhibit large abundances of aromatic fragment ions.     

Negative and positive secondary ion mass spectra of organic acids

Organic compounds can be ionized by sputtering the solid sample. The resulting negative and positive secondary ions provide mass spectra which characterize both the molecular weights and the structures of the compounds. Ionization occurs either by direct ejection of charged species from the solid into vacuum or by electron or proton transfer. The sputtered secondary ions dissociate unimolecularly to give fragment ions. These reactions are identical to those which occur when the secondary ions are independently generated by chemical ionization, selected by mass and dissociated in a high energy gas phase collision. The negative ion SIMS spectra show molecular ions (M^?.) or (M-H)^? ions as the dominant high mass species together with fragments due to decarboxylation, dehydration and losses of other simple molecules. Stronger acids show larger (M-H)^?/M^?.abundance ratios. The positive ion spectra are complementary and also useful in characterizing molecular structures. Attachment of cations to organic molecules (cationization) occurs much more readily than anion attachment and this makes negative SIMS spectra simpler than these positive ion counterparts.     

Mass spectra of 1,19-epoxycardenolides

The mass spectra of six 18,19-epoxycardenolides (ECs) with various substituents in the 3β position — OH, OAc, rhamnosyloxy have been studied. In all the spectra the contribution of the fragments characteristic for cardenolides formed by the cleavage of the bonds of rings C and D were lowered. The strongest peaks were those of ions with m/z 259, 272, and 285 arising on the splitting out of the elements of rings A and B. It was established by the metastable defocusing (MD) method that in the case of the 3β-ols these fragments are formed from M⁺ and in the case of acetyl derivatives, in addition, they may arise from the (M — AcOH)⁺ ions. The spectra of the epoxycardenolides were compared with the structure of 3β,5β,14β,19-tetrahydroxycardenolide (strophanthidol) and its 3β-monoacetate and 3β,19-diacetate. The MD spectra and elementary compositions of the ions showed that other mass-spectrometric conditions strophanthidol monoacetate decomposes partially in the form of the 8,19-epoxycardenolide.     

Chemical ionization of aliphatic nitro compounds

The chemical ionization mass spectra of various aliphatic nitro compounds have been studied. Almost all the nitro compounds form stable [M+H]⁺ ions in sharp contrast to the instability of their ions produced by other ionization techniques. The fragmentations of the [M+H]⁺ ions are discussed, the mechanisms and structures of fragment ions proposed being supported by quantum mechanical calculations. Correlations between chemical ionization mass spectra and structures of compounds studied are outlined. Reliable identification of mono- and polynitroalkanes and some related derivatives may be made by analysis of their chemical ionization mass spectra.     

Reduction of trinitroaromatic compounds in water by chemical ionization mass spectrometry

A reduction process was found to occur in the ion source when observing the chemical ionization mass spectra of a series of trinitroaromatic compounds, using water as reagent. The [MH–30]⁺ ions in the CI mass spectra were due mainly to the reduction of the compounds to their corresponding amines. This was proved by using D₂O as reagent: the [MH–30]⁺ ions were shifted to [MD–28]⁺ ions. The trinitroaromatic compounds investigated included 1,3,5-trinitrobenzene, 2,4,6-trinitrotoluene, 2,4,6-trinitro-m-cresol, 2,4,6-trinitroaniline (picramide) and 2,4,6-trinitrophenol (picric acid).     

Characterization of gem-difluoropropargyl synthons through HF loss from protonated molecules in methane chemical ionization mass spectra

Electron impact and methane chemical ionization mass spectra were obtained following gas chromatography/mass spectrometry for several gem-difluoropropargyl compounds, which had been synthesized as potential intermediates for synthesis of gem-difluoromethylene-containing C-3 acetylenes. EI spectra were variable with respect to the presentation of molecular ions, depending on substituent functional groups present. Methane-CI spectra were characterized by loss of 19 mass units from molecular weight with all compounds examined. These [M − 19]⁺ ions often presented as base peaks of the CI spectra, and were more reliably present and abundant than [M + 1]⁺ ions for these compounds. These ions could have been formed by elimination of HF from the protonated molecules under conditions of methane chemical ionization.     

σ,π-Conjugation in organomercurial systems

Two series of compounds LHgL′ are described. In series I, L = L′ = organic radical with an oxo group in position 2 or L′ = Cl, in series II, L = L′ = allylic radical or L′ = Cl. Some compounds possess chemical properties of conjugated systems (IA, IIA), while others do not possess such properties (IB, IIB). The J¹³C¹⁹⁹Hg) coupling constants and chemical shifts in ¹³C NMR spectra, the integral intensities of multiple bond vibrations in Raman spectra, the half-wave reduction potentials and the resonance energies in mass spectra of the negative ions are compared. The main contribution to the observed differences between compounds A and B is made by interaction of the CHg σ-bond with the π-system (σ, π-conjugation).     

Raman Spectra of Carbohydrates

The Raman spectra of 13 different carbohydrates are investigated by a laser Raman spectrometer. It is found the C—H stretching vibrations around 3000 cm⁻¹ is the best region for qualitative analysis of these compounds. All compounds show an O—H stretching vibration around 3370 cm⁻¹ which was not mentioned by earlier works. Excessive background noise appears in many spectra, probably due to their amorphous stuctures.     

Electron and chemical ionization mass spectrometry in stereochemical differentiation of some 1,3-amino alcohols

Mass spectral fragmentations of two cyclopentane, eight cyclohexane and four norbornane/one 1,3-amino alcohols were studied under electron ionization (EI) by low-resolution, high-resolution, metastable ion analysis and collision-induced dissociation (CID) techniques. All stereoisomeric compounds gave rise to identical 70 eV EI mass spectra. However, the spectra of positional isomers clearly differed. The main fragmentation pathway for the saturated compounds began as an α-cleavage reaction with respect to the nitrogen atom. For the norbornene compounds a retro-Diels—Alder reaction was favoured. Relative to the aminomethyl-substituted compounds the fragmentation patterns for the compounds having the amino group connected directly to the ring were more complicated. The chemical ionization (CI) mass spectra were recorded using ammonia, isobutane, methane, dichloromethane and acetone as reagent gas. From the norbornane/One compounds the di-exo isomers decomposed more easily than the di-endo isomers with most of the reagent gases used. Differences between stereoisomers were observed directly only under methane CI. The decomposition products of the [M + H]⁺ ions generated under ammonia and isobutane CI were studies by recording their CID mass spectra. These spectra allowed the differentiation of the stereoisomers, at least to some extent.     

Fast atom bombardment (FAB) mass spectrometry of piperidine N-oxide derivatives and free radical quenching under FAB conditions

Mechanisms are proposed for the formation of M⁺, [M + 2H]⁺ and [M + 3H]⁺ ions in the fast atom bombardment (FAB) mass spectra of 4-(2,2,6,6-tetramethyl-1-oxyl)-piperidol and its carboxylates. Free radical quenching induced by the fast atom beam has been observed. The effects of temperature on the radical quenching and of acid on the FAB mass spectra are discussed. The experiment showed that the volatile liquid samples with vapour pressures higher than that for glycerol produced M⁺ even-electron molecular ions, and the FAB mass spectra were similar to the corresponding electron ionization mass spectra. For the solid samples, it was found that the free radicals were quenched during the FAB process so that the mononitroxide and dinitroxide compounds produced [M + 2H]⁺ and [M + 3H]⁺ ions, respectively. Further experiments showed that the intensities and stabilities of [M + 2H]⁺ and [M + 3H]⁺ ions could be improved by addition of acids.     

Comparative study of [Xe]+ and [Ar]+ bombardment in secondary ion mass spectrometry for bioorganic compounds

Secondary ion mass spectra obtained by [Xe]⁺ bombardment are compared with those obtained by [Ar]⁺ bombardment. Although [Ar]⁺ ions are commonly used as primary ions in secondary ion mass spectrometry for organic compounds, [Xe]⁺ ions seem better as primary ions because they give a larger sputtering yield for a metal substrate than [Ar]⁺ ions. Cationized molecular intensities of sucrose, raffinose and stachyose, and quasimolecular ion intensities of tuftsin and eledoisin related peptide are investigated using [Xe]⁺ and [Ar]⁺ bombardments. The observed molecular species are 2–4 times more intense for [Xe]⁺ bombardment than for [Ar]⁺ bombardment, although the secondary ion mass spectra are almost the same in both cases.