MASS SPECTRA OF SELECTED SULFONES SULFONE-SULFINATE REARRANGEMENT

Abstract

Mass spectra of three sulfones selected to reveal certain structural features were obtained and interpreted. The mass spectra of the two isomeric sulfones, benzyl methyl sulfone and methyl p-tolyl sulfone, were quite different. The spectra showed that rearrangement of sulfones to the isomeric sulfinates occurs under the influence of the electron impact and that the migration of the aryl group is preferred over migration of the alkyl group.     

Site of protonation in the chemical ionization mass spectra of olefinic methyl esters

The H₂ and CH₄ chemical ionization mass spectra of the olefinic esters methyl acrylate, methyl methacrylate, methyl crotonate, methyl 3-butenoate, methyl 2-methyl-2-butenoate, methyl 3-methyl-2-butenoate and methyl cinnamate have been determined. In addition to the expected loss of CH₃OH from [MH]⁺, in many cases the protonated molecules also show loss of CO or CH₂CO with methoxy group migration to the positive ion centre, indicative of protonation at the double bond. These rearrangement reactions, which have analogies in electron impact mass spectra, result in chemical ionization mass spectra of isomeric molecules which show more substantial differences than the electron impact mass spectra. In the case of methyl cinnamate, isotopic labelling experiments show considerable interchange of the added proton with the ortho and meta phenyl hydrogens prior to CH₃OH or CH₂CO loss, although the extent of interchange is not the same for both cases.     

Strukturspezifischer Massenspektrometrischer Abbau von mono- und di-n-alkyl-substituierten Acetylenverbindungen

Mass spectra of mono-n-alkyl substituted acetylenes of type 1 containing at least 7 carbon atoms in the alkyl group, R, are characterized by key fragments of mass 82 and 96 which decompose by loss of a methyl radical. Spectra of di-n-alkyl substituted acetylenes of type 2 show these key fragments shifted by an appropriate mass number. These ions suffer further decomposition by loss of a C₅H₁₀ particle of mass 70. In addition key fragments of mass 67, 81, 82 and 95, 96 are again observed.     

Identification of esters by collisional charge inversion of their enolate ions

The collisional charge inversion and neutralization-reionization (^?NR) mass spectra of the enolate ions of m/z 115 derived from the four butyl acetates, the two propyl propionates, ethyl butyrate, ethyl isobutyrate, methyl valerate, methyl 2-methylbutyrate and methyl 3-methylbutyrate were recorded. The major primary fragmentation reactions of the unstable carbenium ion formed by charge inversion involve elimination of an alkoxy radical to form a ketene or alkylketene molecular ion and formation of an alkyl ion consisting of the R¹ group of RCOOR¹. A minor fragmentation reaction involves elimination of an alkyl radical by cleavage of a C? C bond α to the ether oxygen. The alkylketene ions fragment by β-cleavage eliminating an alkyl radical to form an olefinic acylium ion. In most cases the charge inversion mass spectra of the enolate ions allow identification of the ester.     

Mass spectral fragmentation of thioesters. 1—Identification of low energy,slow processes

A variety of thiol and thion esters, including acetates and benzoates with n-butyl and β-phenethyl alkyl groups, have been studied by electron impact mass spectrometry. Several rearrangement ions were documented and their persistence in low voltage and field ion spectra demonstrated. Among the significant ions found in the rich thion spectra, the most general requires O to S rearrangement of the alkyl group and subsequent cleavage to yield acyl ions (CH₃CO or PhCO). This process is more important in longer chain compounds than in the methyl and ethyl homologues studied previously.     

Mass spectra of some high molecular weight tetra(alkyl)silanes

The mass spectra of tetra(alkyl)silanes having one short and three long alkyl groups were recorded (C₂₀? C₃₂). These silanes did not give molecular ions, and the dominant course of fragmentation involved the loss of alkyl groups, followed by elimination of alkenes. The loss of alkyl groups, followed by elimination of alkenes. The loss of alkyl groups followed the sequence iso-Pr>Et, Pr,… C₁₄H₂₉» Me. With alkyl groups longer than hexyl, a rearrangement was observed in which n-alkanes were eliminated from the base ion. When the alkyl group was hexyl, the alkane eliminated was methane; when the alkyl group was heptyl, ethane was expelled; with octyl, it was propane; and with decyl, n-pentane was expelled. Deuterium labeling showed that at least two modes of rearrangement were occurring.     

THE MASS SPECTRA OF ALKYL ARYL SULPHIDES,DIARYL DISULPHIDES AND ALKYL ARYL SULPHONES

Abstract

The electron impact mass spectra of a comprehensive series of each of the titled compounds have been studied. The spectra of the sulphides show that one or two methyl groups at the ortho position of the aryl rings markedly increase the intensity of the M-R and/or M-SR ions. The abundance of the latter also increase with increases in the size of the alkyl group. The fragmentation patterns of the diaryl disulphides are also sensitive to steric effects. The presence of an ortho methyl group inhibits the appearance of M-S₂ ions but leads to the formation of new ions at M-SH₃ mu. The spectra of ethyl phenyl sulphone and the ethyl tolyl sulphones are unique in that their spectra contain peaks at M–14 mu. Deuterium labelling indicates that these ions arise by the loss of a methyl radical from protonated (or deuteriated) molecular ions.     

Mass spectra of alkenyl methyl ethers

Electron impact ionization mass spectra of numerous alkenyl methyl ethers C_nH_2n-1OCH₃ (n = 3–6) recorded under normal (4 kV, 70 eV, 175°C) and low-energy, low-temperature (8 kV, 12 eV, 75 °C) conditions are reported. The influence of the position and stereochemistry of the double bond on the dissociation of ionized alkenyl methyl ethers is discussed. The mechanisms by which these ethers fragment after ionization have been further investigated using extensive ²H-labelling experiments and by studying the energy dependence of the reactions. Ethers of allylic alcohols show spectra that are distinct from those of the isomeric species in which the double bond is separated by one or more sp³ carbon atoms from the carbon atom carrying the methoxy group. Three principal primary fragmentations are observed. The most common process, especially for ionized ethers of allylic alcohols, is loss of an alkyl group. This reaction often occurs by simple α-cleavage of radical-cations of the appropriate structure; however, alkyl groups attached to either end of the double bond are also readily lost. These formal β- and γ-cleavages are explained in terms of rearrangements via distonic ions and, at least in the case of γ-cleavages, ionized methoxycyclopropanes. Ionized homoallyl methyl ethers tend to eliminate an allylic radical, particularly at high internal energies, with formation of an oxonium ion (CH₃ ⁺O?CH₂ or CH₃ ⁺O?CHCH₃). The ethers of linear pentenols and hexenols show abundant [M - CH₃OH]⁺? ions in their spectra, especially when a terminal methoxy group is present Methanol loss also takes place from ionized ethers of allylic alcohols in which there is a Δ-hydrogen atom; this process is significantly favoured by cis, rather than trans, stereochemistry of the double bond.     

An intermolecular methyl transfer during field desorption mass spectrometry. Identification of the [M + 14] peak in choline chloride

Choline chloride, bromide and iodide all contain a peak at m/e 118 [M + 14] in their field desorption mass spectra. Comparison with choline(methyl-d₉) chloride demonstrates that this peak arises from an intermolecular transfer of a methyl group. Partial exchange of hydroxyl hydrogen on choline chloride does not displace the m/e 118 peak, suggesting that this hydrogen is lost during methyl transfer. The structure of the [M + 14] species should then correspond to choline methyl ether.     

Mass spectrometry of hydroxyammoniocarboxylates: Lactonization of thermally rearranged molecules during field desorption

The field desorption mass spectral behavior of several hydroxyammoniocarboxylates was studied at both low and high emitter heating currents. The molecular weights of these thermally unstable compounds can be determined directly from the low emitter current (<10 mA) field desorption mass spectra, which are dominated by [xM+H]⁺ and [xM+H? CO₂]⁺ ions (1?x?4). At higher emitter currents (～20 mA), pyrolytic processes become important. These include intermolecular transfer of a single alkyl group yielding [M+alkyl]⁺ ions, intermolecular isomerization producing a hydroxyaminoester as the rearranged form of the molecule, and elimination of alcohol from the rearranged molecule, producing γ or δ lactones. The distribution of pyrolysis products does not depend significantly on the length of the carboxylate chain, but does appear to depend upon the chain length of the alkyl substituent on nitrogen. The spectra of molecules containing a long alkyl substituent (e.g. C₁₄H₂₉, C₂₂H₄₅) exhibit relatively high levels of [M+alkyl]⁺ ions, unlike the spectra of compounds which contain only methyl or ethyl substituents on the quaternary nitrogen. These latter compounds exhibit a relatively greater tendency toward lactone formation.     

Site specificity in the H–D exchange reactions of gas-phase protonated amino acids with CH3OD

H–D exchange reactions of methanol-d₁ with protonated amino acids were performed in an external-source Fourier transform mass spectrometer. Absolute rate constants were determined for the group which included glycine, alanine, valine, leucine, isoleucine and proline. By comparing reactivities with selected methyl esters, it was found that exchange on the carboxylic acid occurs 3–10 times faster than exchange on the amino group. No simple correlation is observed between the rates of H–D exchange on the acid group and the size of the alkyl group. However, the rates of exchange on the amine decrease with increasing gas-phase basicity. Glycine, the least basic amino acid, exchanges its amine hydrogens the fastest. These results are useful for determining the interaction of methanol with protonated amino acids and can provide insight into the H–D exchange reactions observed with polyprotonated proteins produced by electrospray ionization.     

Characterization of the Retro-[3+2] Reaction of Protonated 2,3′-Bisindolylmethanes by Electrospray Ionization–Tandem Mass Spectrometry

Twelve 2,3′-bisindolylmethanes with various substituents were investigated using electrospray ionization quadrupole time-of-flight tandem mass spectrometry in positive ion mode. A retro-[3+2] reaction was observed in the collision-induced dissociation spectra of protonated 2,3′-bisindolylmethanes for the first time. The mechanism of retro-[3+2] reaction was concerted or stepwise. For the concerted pathway, carbon–carbon bonds of a protonated compound simultaneously cracked and the m/z 208 ion ([C₁₅H₁₀D₂N]⁺) was observed with hydrogen–deuterium exchange labeling. The stepwise pathway goes through 1,3-hydrogen migration twice and the m/z 208 ion ([C₁₅H₁₀D₂N]⁺) and m/z 207 ion ([C₁₅H₁₁DN]⁺) were detected with deuterium labeling. In the deuterium-labeled tandem mass spectrum for one compound, only the peak at m/z 208 was present at high abundance, suggesting that the concerted pathway is more likely. In addition, the substituents have no obvious trends on the ratios of the product intensity to the base intensity, further supporting the concerted pathway.     

Unimolecular reactions of ionized methyl allyl ether

The fragmentation of CH₂?CHCH₂OCH₃^+· cation-radicals has been investigated by means of ²H- and ¹³C-labelling experiments and by analysis of collision-induced dissociation spectra. Metastable C₄H₈O^+· species decompose via one of three main channels which involve loss of (a) a hydrogen atom, (b) a methyl radical or (c) a formaldehyde molecule. Extensive, but not complete, exchange of the hydrogen and deuterium atoms in specifically labelled C₄H₈-_nD_nO^+· analogues precedes each of the three fragmentation pathways. The role of distonic ions in the rearrangement steps which bring about hydrogen exchange is discussed. The influence of isotope effects on the relative rates of the major reactions and the associated kinetic energy releases is examined. Only loss of a hydrogen atom is subject to a substantial isotope effect. Elimination of a methyl radical releases a large amount of kinetic energy, as is shown by the broad and dish-topped appearance of the corresponding metastable peak (T_1/2 ≈ 42 kJ mol^?1). The carbon atom of the original methoxy group is specifically expelled in this process. Both the large T_1/2 value and the unusual site selectivity are atypical of methyl and other alkyl radical losses from ionized alkenyl methyl ethers. The carbon atom of the methoxy group also participates specifically in formaldehyde elimination, but the two hydrogen atoms are not always selected from the three contained in the initial methoxy group. The implications of these labelling results for the synchronicity of concert of formaldehyde loss, which can be formu lated as a pericyclic process, is analysed.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen.—I Methoxygruppenwanderungen in den Massenspektren der Methyläther von linearen und verzweigten aliphatischen Polyalkoholen

The formation of 1,1-dimethoxy-alkyl rearrangement ions in the mass spectra of methyl ethers of linear diols, 1,2,3-triols and of derivatives of pentaglycerol and pentaerythritol has been investigated by deuterium labelling and mass measurements. Methoxy group migrations do not occur, or at least only to a small amount, in the mass spectra of the diol-dimethyl ethers. The mass spectra of methyl ethers of 1,2,3-triols exhibit characteristic peaks of the rearrangement ions +CH(OCH₃)₂ and +CR(OCH₃)₂. These ions arise by a 1,3-migration of a methoxy group, probably during a one step degradation of the molecular ion to give a molecule methyl alkenyl ether and a H-atom or alkyl radical as neutral fragments. Large peaks of the rearrangement ion +CH(OCH₃)₂ are observed in the mass spectra of compounds of the following type: A radical ion, formed by loss of HY from the molecular ion, rearranges by methoxy group migration in the mass spectra of these substances. Rearrangement by migration of hydroxy and acetoxy groups are also observed, but no migration of a Cl-atom.     

Microsynthesis and mass spectral study of Chemical Weapons Convention related 2‐alkyl‐1,3,6,2‐dioxathiaphosphocane‐2‐oxides

Chemical Weapons Convention (CWC)‐related compounds where the phosphorus atom is part of a ring have very limited representation in mass spectral libraries and the open literature. Here we report electron ionization (EI), chemical ionization (CI) and electrospray ionization tandem mass spectrometry (ESI‐MSⁿ) spectra and retention indices for 2‐alkyl‐1,3,6,2‐dioxathiaphosphocane‐2‐oxides (alkyl C₁ to C₃) which are new cyclic chemicals covered under the CWC. The EI mass spectra show a pattern of ion fragmentation that is similar to that of other cyclic phosphonates in that loss of the alkylphosphonic acid as a neutral loss is more important than the presence of the protonated alkylphosphonic acid. In contrast to other cyclic phosphonates, the 2‐alkyl‐1,3,6,2‐dioxathiaphosphocane‐2‐oxides show almost no protonated alkylphosphonic acid and as a result the spectra do not carry the same distinctive signature of the phosphorus–carbon bond that is required for the chemical to be covered under the CWC. Copyright © 2008 John Wiley & Sons, Ltd.     

The mass spectra of 1-arylanthra-9,10-quinones

The mass spectra of 1-phenylanthra-9,10-quinone and 20 substituted l-phenylanthraquinones having alkyl methoxyl, nitro and halogen substituents in the o-, m- and p-positions of the phenyl ring have been measured. The spectra are discussed with particular reference to the loss from the molecular ion of a hydrogen atom and of the substituent.     

The effect of para-substitution on the mass spectra of arylsulphonate esters of neopentanol

The mass spectra of several para-substituted benzenesulfonic and benzoic esters of unlabelled and 1,1-d₂-neopentyl alcohol are examined and compared. Evidence is presented of migration of the aryl group from the sulfur to an oxygen atom in the molecular ions of the sulfonic esters. The nature of the fragmentation processes and the occurrence of metastable ions for these processes are both much more dependent upon the polarity of the para substituent in the case of the sulfonates than for the benzoates. Elimination of C₅H₁₀ occurs from the molecular ion of the p-methoxysulfonate with transfer to the residual ion of a hydrogen atom selected randomly from the alkyl fragment, while in the case of the p-aminosulfonate, incomplete randomization is demonstrated.     

The electron-impact-induced fragmentation of some alkyl isocyanides and α-branched alkyl cyanides

The HRP mass spectra of some alkyl isocyanides (R? NC in which R equals CH₃, C₂H₅, n-C₃H₇, n-C₄H₉ and t-C₄H₉) and two methyl branched alkyl cyanides (R? CN in which R equals i-C₃H₇ and t-C₄H₉) have been studied. Using metastable ion transitions and appearance potentials, the fragmentation patterns and spectral characteristics of the isocyanides can be given. A comparison has been made with the mass spectral data of the corresponding cyanides. Although the mass spectra of alkyl cyanides and isocyanides show close relationship, evidence could be obtained that this resemblance is not caused by rearrangement of the isocyanide into cyanide molecules. The main difference between the spectra of both compounds is caused by the strength of the α-bond, being weaker in the case of the isocyanides. The abundance of ions formed by α-bond cleavage decreases with increasing size of the alkyl group.     

Fragmentation of trimethylsilyl derivatives of 2-alkoxyphenols: A further violation of the ‘even-electron rule’

The mass spectra of trimethylsilyl (TMS) ethers of 2-methoxyphenols show abundant [M–30]⁺˙ ions originating from consecutive loss of two methyl radicals. This is illustrated by comparison of the accurate mass-measured and linked-scan spectra of the TMS derivatives of 2-methoxyphenol (guaiacol), 4-hydroxy-3-methoxybenzaldehyde (vanillin) and 3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid methyl ester (ferulic acid methyl ester) with those of the TMS derivatives of phenol, 4-hydroxybenzaldehyde, 3-(4-hydroxyphenyl)-2-propenoic acid methyl ester (p-coumaric acid methyl ester), 3-methoxyphenol and 4-methoxyphenol. This distinctive ortho effect is valuable in the identification of isomeric phenolic compounds. In the spectra of the TMS derivatives of 2-ethoxyphenol and 2-propoxyphenol the sequential loss of two radicals is less pronounced, because elimination of the side-chain and a methyl group with rearrangement and hydrogen migration is competitive.     

Substituent Effects on Phosphate-Catalyzed Proton Exchange in Amides

The kinetics of phosphate-catalyzed proton exchange have been measured with NMR lineshape analysis for a series of amides, ureas, and carbamates. A hypothetical energy profile for transition structures of concerted phosphate catalysis ?_c and of stepwise phosphate catalysis ?₁, ?₂ is used for discussion of the substituent effects. The concerted mechanism of phosphate catalysis operates only for amides RCONHCH₃ for which the electron-donating ability of the substituent R is comparable with, or better than, that of the methyl group. We conclude that concerted phosphate catalysis is inoperative in proteins or polypeptides.