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.     

The Mass Spectrometric Fragmentation of 1-Alkyl Ions derived from Halides

The mass-spectrometric fragmentation of n-pentyl, n-hexyl, n-octyl and n-nonyl ions has been studied using ¹³C- and D-labelling. The ions were produced from the corresponding halide ions. The loss of an olefin as a neutral fragment is the main reaction. The elimination of this fragment must be by a complex mechanism, since the terminal carbon atoms have the smallest probability of being lost with the neutral fragment. On chains with five to six carbon atoms, hydrogen scrambling seems to preceed the fragmentation; this is not true for hydrogen on terminal positions of longer chains. Ring formation prior to the fragmentation could explain some of the results; but no reasonable conclusion could be reached.     

Mass spectrometric study of organocyclosiloxanes

Organocyclosiloxanes of various chemical structures were studied by mass spectrometry using different ionization methods. The electron ionization mass spectra contain no peaks of molecular ions, and the main fragment ions are formed due to complicated rearrangements in a molecular ion, which provides no comprehensive view about the molecular structure. The desorption spectra exhibit peaks of quasimolecular and fragment ions, which characterize both molecular weights and chemical structures of the compounds under study. Dedicated to Academician G. A. Abakumov on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1746–1749, September, 2007.     

Mass spectrometry of substituted 1,3-dihydro-2H-imidazole-2-thiones

The electron impact mass spectra of the 4-formyl-1, 3-dihydro-2H-imidazole-2-thione, its six 1-methyl(n-propyl, n-hexyl)-3-methyl(phenyl)-disubstptuted derivatives, and the 1,3-dihydro-1-phenyl-2H-imidazole-2-thiome are discussed. The fragmentation pattern is strongly influenced by the alkyl or phenyl N-substituents, as well as by the length of the alkyl chain. The odd-electron ions containing an N-phenyl substituent, but not a propyl or hexyl group, eject a hydrogen atom from the phenyl ring, while the presence of a long alkyl chain greatly enhances the loss of the sulphyhydryl radical and facilitates the expulsion of several alkenes, and alkyl and alkenyl radicals.     

Mass spectral identification of isomeric fluoronitroanilines

The electron impact and methane chemical ionization mass spectra of 2-fluoro-5-nitroaniline and 4-fluoro-3-nitroaniline have been investigated. Similar fragment ions were observed for both compounds under electron impact conditions, but the difference in the relative ion intensities of two key fragment ions allowed the positive identification of these two isomers. The difference can be attributed to an ortho effect involving fluorine and the amino substituent. A fragmentation pathway consistent with this observation is suggested.     

Mass and kinetic energy spectrometries of alkylketenes

Real-time analysis of alkylketenes synthesized by flash vacuum thermolysis (FVT) of acid chlorides has been performed by mass spectrometric techniques using a triple sector (E/B/E) spectrometer. The major high energy reactions of the molecular ions consist of β-cleavages relative to the C?C bond of the ketene function, followed by decarbonylation. The unimolecular reactions of the metastable molecular ions are often different as shown by collisional activation of the fragment ions produced in a field-free region of the spectrometer and also from estimated heats of formation.     

Mass spectra of germacranolide type sesquiterpene dilactones and derivatives

The low and high resolution mass spectra of five germacranolide sesquiterpene dilactones together with the d₀-, d₃-acetates, d₀-, d₉-TMSi derivatives and deuterium exchange products were obtained. The mass spectra of the five dilactones revealed several kinds of common fragment ions. The germacranolides containing a 4,5 double bond exhibit characteristic ions at m/e 274, 256, 228, 165 and 147, while the 4,5-dihydro compound exhibits fragment ions at m/e 276, 258, 230 and 162.     

Mass spectra of azetidines and 2-azetidinones

The mass spectra of twenty three azetidines and seventeen 2-azetidinones are reported and discussed. The majority of spectra contain molecular ions. Fragmentation modes are largely dependent upon the substitution pattern and involve specific cleavage of the four-membered ring. Several hydrogen rearrangement reactions occur, and in some cases, cis and trans isomers may be distinguished by large differences in the relative abundances of certain fragment ions.     

Mass spectral studies of some carbohydrate enones

A study of the mass spectra of ethyl 2,3-dideoxy-α-d-glycero-hex-2-enopyranosid-4-ulose1a, its 6-0-acetyl derivative1b, and methyl 3,4-dideoxy-α-d-glycero-hex-3-enopyranosidulose2a indicates that the main fragmentation of these compounds occurs by a retro-Diels-Alder type cleavage. Recognition of the ions produced by this cleavage provides a means of allocating the position of the α,β-unsaturated chromophore in carbohydrate enones. Fragmentation of molecular ions of1a and1b also occurred by direct loss of individual ring substituents to give ions which retain the pyranoid nucleus. The fragmentation pathways suggested are supported by accurate mass measurements of the salient fragment ions of1a.     

Mass Spectra of Substituted N-Alkylpyrroles and Related Compounds

Under El conditions, substituents on N-alkylpyrroles affect fragmentation of the alkyl groups. The electron-withdrawing character of the cyano group retarded formation of a N-methylene cation, and a labile substituent, i.e., -CHO, -CH₂COOMe, competed with an N-alkyl group to be a initial fragmentation center, MNDO calculation of the heat of formation of possible fragment ions showed that protonated pyridine is the most stable among the isobaric ions.     

Mass spectrometric differentiation of huperzinine,N,N-dimethylhuperzine A and N-methylhuperzine B

The newly isolated Lycopodium alkaloids huperzinine, N,N-dimethylhuperzine A and N-methylhuperzine B were studied by high-resolution and tandem mass Spectrometry. Differentiation of these subtly different isomers from each other and from the general class of Lycopodium alkaloids can be achieved by taking advantage of the structure-specific fragment ions formed upon electron ionization (El) or collisional activation of these alkaloids. β-Obscurine was identified as an impurity component in the N-methylhuperzine B sample and contributed to the El mass spectrum. The previously published El mass spectrum of N-methylhuperzine B is likely to be that of a mixture of N-methylhuperzine B and β-obscurine. The presence of the impurity can now be readily recognized on the basis of two characteristic fragment ions of m/z 215 and 187.     

Mass spectra of ferrocenophanes. Cyclic peroxide derivatives

The electron impact mass spectra of two 6-membered cyclic peroxides of ferrocenophane depend sensitively on the temperature of the sample heater of the direct inlet system. It is confirmed that their spectra contain some fragment ion peaks arising from thermal decomposition or rearrangement products, but that the ion at m/e (M–28) is due to elimination of ethylene from the molecular ions of the peroxides by a retro Diels-Alder type reaction.     

Tandem Time-of-Flight Mass Spectrometer with High Precursor Ion Selectivity Employing Spiral Ion Trajectory and Improved Offset Parabolic Reflectron

In this study, we have developed a tandem time-of-flight mass spectrometry (TOF/TOF) technique involving the use of a matrix-assisted laser desorption/ionization ion source that exhibits high precursor ion selectivity. An ion optical system with a 17 m spiral ion trajectory was used in the first time-of-flight mass spectrometer. High precursor ion selectivity was achieved by realizing a 15 m flight path, which is considerably longer than that of the conventional MALDI-TOF/TOF before the precursor ion selection by an ion gate; monoisotopic ions could be selected properly up to m/z 2500. Furthermore, the first time-of-flight mass spectrometer was composed of electrostatic sectors and could eliminate post-source decay (PSD) ions. Precursor ions with 20 keV kinetic energy were selected and injected into a collision cell, leading to the generation of fragment ions by high-energy collision-induced dissociation (HE-CID). The optimized second time-of-flight mass spectrometer included a post-acceleration region and an offset parabolic reflectron to record product ion spectra in the entire mass range. Our system could generate a simple HE-CID product ion spectrum because each fragment pathway could be observed as a single peak by the selection of monoisotopic ions of all precursor ions and HE-CID fragment pathways could be predominantly observed by the PSD ion elimination.     

应用高效液相色谱和电喷雾质谱联用技术分析鉴定车前草中的苯乙醇苷化合物

本文采用高效液相色谱与电喷雾质谱联用技术在线分析鉴定了车前草提取物中的三种苯乙醇苷化合物。实验采用反相C18色谱柱,0.2%的醋酸水溶液和乙腈梯度洗脱,车前草中的苯乙醇苷化合物得到很好的分离。在电喷雾质谱负离子条件下,获得了三种苯乙醇苷化合物的分子离子峰,分子量信息,进一步通过质谱的源内CID技术得到相应化合物的结构信息。通过得到的这些信息与文献中的已知化合物或标准品对照从而推断出化合物的结构。     

Mass spectral fragmentation patterns of various 6-substituted 2,4-bis-(m-aminoanilino)-s-triazines

The mass spectra of 6-substituted-2,4-bis-(m-aminoanilino)-s-triazines were determined and found to be dominated by intense molecular ions (base peak), strong M-H peaks and fragment ions diagnostic for the arylamino and C-6 substituents. Mechanisms have been proposed for the formation of the major peaks in the spectra. Fragmentation pathways involving hydrogen transfer reactions have been rationalized by concomitant opening of the triazine ring rather than an amino-imino tautornerization as previously suggested. Fragmentation reactions accompanied by rearrangement modes have also been encountered in some of the compounds studied.     

Mass spectra of chloro-, aminochloro- and ethylaminochloro-s-triazines

The mass spectra of cyanuric chloride and all of the possible amino-and ethylaminochloro-s-triazines have been investigated. The simpler compounds provide evidence for the structures of low mass fragment ions found in the spectra of the more complex derivatives. Fragmentation patterns are presented which account for most of the major ions. These pathways involve either the initial loss of a chlorine atom (important for the simple aminochlorotriazines) or ring cleavage with expulsion of cyanogen chloride (important for cyanuric chloride). The ethyl derivatives undergo cleavage of the side chain prior to either ring cleavage or chlorine expulsion.     

Mass spectra of isomeric monoalkylanisoles and monoalkylphenols with straight alkyl chains

An analysis of the mass spectra of ortho, meta and para isomers of monoalkylanisoles and monoalkylphenols, the alkyls being n-amyl, n-octyl and n-undecyl, has been performed. The regularities found allow determination of the structure of such compounds by inspection of the mass spectrum only. It was shown that Hammett's equation may be used for evaluating a ration of intensities of peaks originating formally from the simple β-cleavage and that accompanied by hydrogen migration of the resulting fragment ions.     

Mass spectra of nucleoside derivatives

The mass spectra of derivatives of uridine, adenosine, cytidine and guanosine are recorded. Derivatization techniques include permethylation, acetylation, trifluoroacetylation, trimethylsilylation and the synthesis of 2′,3′-O-isopropylidenes and 2′,3′-O-phenylboronic esters. Sequential derivatization by a selective combination of some of these procedures results in nucleosides which are blocked with a characteristic group at the cis 1,2 diol position, and, which contain other substituent groups that enhance the volatility of the compound. The specific substitution at the cis-glycol region has been shown to be particularly useful in asymmetrically derivatizing dinucleoside phosphates since certain fragment ions from their mass spectra indicate the sequence of the two nucleoside components. Sequence isomers such as adenylyl-(3′-5′)-uridine and uridylyl-(3′-5′)-adenosine can be unambiguously distinguished.     

Mass spectrometry of bis-quinolizidine alkaloids: Lactams of sparteine and α-isosparteine

The mass spectral fragmentations of 2-oxosparteine (lupanine), 2-oxo-α-isosparteine (α-isolupanine), 15-oxosparteine, 17-oxosparteine and 10-oxosparteine (aphylline) are reinvestigated and discussed. Fragmentation pathways, elucidation of which were assisted by accurate mass measurements and metastable transitions, are proposed. The fragmentation assignments are not consistent with those previously reported in the literature. Many fragment species of the same m/z (low-resolution spectra) are composed of two or three fragment ions of different elemental compositions (high-resolution spectra). The obtained data create a safe basis for distinguishing structural and stero isomers.     

Mass spectra of trimethylsilyl Di-O-methyloximes of aldosuloses and dialdoses

Aldosuloses and dialdoses are readily converted to the corresponding acyclic methyloxime trimethylsilyl derivatives which give characteristic mass spectra. The gas chromatography mass spectrometry coupling technique also allows trace analysis since full information can be derived from the most intense fragment ions of the mass spectra which mainly arise from fragmentations β and δ to the O-methyloxime group.