The mass spectrometric fragmentation of n-heptane

The electron impact fragmentation of n-heptane has been investigated using ¹³C labelled derivatives. A mechanism is proposed for the loss of alkyl radicals where the cleavage of a C? C bond is coupled with the rearrangement of a hydrogen atom, thus yielding a secondary alkyl ion that eventually fragments further by a subsequent loss of olefin. For alkyl ions with less than six carbon atoms this consecutive pathway is in competition with formation directly from the molecular ion. The consecutive pathway contributes about 15% to the intensity of the alkyl ions with four and five carbon atoms and 80% for smaller ions. The electron energy dependence was studied.     

Isomerization of hydrocarbon ions. VIII—The electron impact induced decomposition of n-dodecane

The literature on the mass spectrometry of ²H and ¹³C labelled higher alkanes is reviewed and the decomposition behaviour of both the molecular and the fragment ions of n-dodecane, n-dodecane-1, 12-[¹³C₂] and n-dodecane-1,1,1,12,12,12-[²H₆] studied with special emphasis on metastable decompositions. It is shown that the elimination of alkane molecules and alkyl radicals from the n-dodecane molecular ion occurs primarily by simple splitting of the C? C bond. In addition, both small alkane molecule and alkyl radicals are eliminated with low probability from centreal parts of the molecular ion. The alkane elimination is less specific than the alkyl elimination. The methyl elimination shows an exceptionally high non-specificity, but is of negligible abundance in the 70 e V electron impact spectrum. The metastable ion spectra suggest, but do not prove unambiguously, that those small alkyl ions (with up to four carbon atoms) originating directly from the molecular ion, may be formed both by direct cleavage of the terminal groups and from central parts of the molecular ion. However, the majority of the small alkyl fragment ions in the 70 eV spectrum are formed by secondary decomposition explaining their apparent non-specific formation. The strikingly different fragmentation behaviour of even electron, [C_nH_2n+1]⁺, and odd electron fragment ions, results from differences in the product stabilities. Using collisional activation and metastable ion spectra it is shown that the odd electron fragments have the structure of the linear alkene (most probably the 1-alkene) molecular ion. In contrast to the molecular ions, alkyl fragment ions decompose with complicated skeletal rearrangements, which lead to substantial, but not complete, carbon randomization. The terminal hydrogen atoms, however, show little scrambling.     

Organic cluster ions from continuous-flow fast atom bombardment

Cluster ions from fast atom bombardment of liquid alcohols and nitriles were examined using a continuous-flow technique. Protonated molecular M_nH⁺ species are the dominant cluster ions observed in molecules of formula M. The abundances of the M_nH⁺ cluster ions decrease monotonically with increasing n, and within a homologous series the M_nH⁺ abundance diminishes more rapidly for higher molecular mass compounds. Reaction products (ROH)_n(H₂O)H⁺ and (ROH)_n(ROR)H⁺ are observed also in the case of alcohols, and the ion abundances decrease with increasing n. Radiation damage yields fragment ions and ionic alkyl reaction products which are captured in solvent clusters. Semi-empirical molecular orbital methods were used to examine the energetics of cluster ion formation and decomposition pathways. Metastable decomposition processes exhibit only evaporative loss of monomers, with the probability of loss increasing sharply with n. The evaporative ensemble model of Klots was used to predict the cluster size-dependent trends of metastable dissociation processes observed for alcohol and nitrile cluster ions.     

Electron-impact induced fragmentation of alkylthiopyridines. The extent of ring nitrogen involvement

The fragmentation patterns obtained upon electron impact of 2-, 3-, and 4-alkylthio-pyridines were examined to determine the extent that the ring nitrogen is involved. The effect of the size of the alkyl group, as well as the ring position of the sulfide on the fragmentation of the methyl-, ethyl-, n-propyl-, n-butyl- and t-butylthiopyridines, is discussed. The mass spectrum of 2-n-octylthiopyridine is also recorded. The molecular ions decomposed with rupture of the bonds α, β, γ, and δ from the hetero-aromatic ring. The molecular ions also exhibited the loss of alkenes via a number of mechanisms, notably the loss of C_n-1 H_2n from the alkyl group of 2-alkylthiopyridines by a transition involving the rupture of a C-C bond γ to the ring and the transfer of a proton on a carbon ? to the ring. The role of the pyridine sp² nitrogen in this and other fragmentations is discussed. An elimination of HS radical is also observed in a number of these alkylthiopyridines.     

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.     

A detailed analysis of the complete hydrogen and carbon scrambling and fragmantation reactions of boron-containing cationic species in the mass spectrometer

A detailed analysis of the mass spectral behavior of trialkylboranes and deuterium and ¹³C labeled tri-n-butylborane, and comparison with the ion cyclotron resonance behavior, have revealed several unique features and herefore unrecognized rearrangement and fragmantation reactions:(1)the occurrence of intial fragmentation of the parent ion with apparently nearly exclusive loss of one complete alkyl radical; (2) the extensive formation of boron-containing spiecies (>% of the total ion current)in the mass spectrometer relative to the virtual absence (<5%) of such spieces in the ion cyclotron resonance spectrum; (3) the dominant formation of boron-containing species of the composition [CⁿH²ⁿ⁺²B]⁺ with [C²H⁶B]+ generally being the base peak; (4) the formation of appreciable quantities of alkane molecular ions (5 to 10% of the total ion current) up to [R²]+. (from R³B); (5) the characterization of a fragmentation reaction involving the loss of CH²; (6) the observation that very extensive hydrogen and carbon scrmbling occurs in the ions formed after the initial fragmentation of the parent ion. A mechanism which satisfactorily accounts for the fragmentation and rearrangement reactions is proposed which involves the reversible formation and rearrangement of protonated boracyclopropanes and cyclopropanes formed by cationic insertions in ß-C? H bonds.     

Degradation of poly(acrylates) under SF5+ primary ion bombardment studied using time‐of‐flight secondary ion mass spectrometry. 2. Poly(n‐alkyl methacrylates)

Polyatomic primary ions offer low penetration depth and high damage removal rates in some polymers, facilitating their use in the molecular depth profiling of these polymers by secondary ion mass spectrometry (SIMS). This study is the second in a series of systematic characterizations of the effect of polymer chemistry on degradation under polyatomic primary ion bombardment. In this study, time‐of‐flight SIMS (ToF‐SIMS) was used to measure the damage of ～90 nm thick spin‐cast poly(methyl methacrylate), poly(n‐butyl methacrylate), poly(n‐octyl methacrylate) and poly(n‐dodecyl methacrylate) films under extended (～2 × 10¹⁴ ions cm^?2) 5 keV SF₅⁺ bombardment. The degradation of the poly(n‐alkyl methacrylates) were compared to determine the effect of the length of the alkyl pendant group on their degradation under SF₅⁺ bombardment. The sputter rate and stability of the characteristic secondary ion intensities of these polymers decreased linearly with alkyl pendant group length, suggesting that lengthening the n‐alkyl pendant group resulted in increased loss of the alkyl pendant groups and intra‐ or intermolecular cross‐linking under SF₅⁺ bombardment. These results are partially at variance with the literature on the thermal degradation of these polymers, which suggested that these polymers degrade primarily via depolymerization with minimal intra‐ or intermolecular cross‐linking. Copyright © 2004 John Wiley & Sons, Ltd.     

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 spectrometry of volatile derivatives—II: N-alkyl trifluoracetamides

Under electron-impact, N-alkyl trifluoracetamides exhibit peaks due to [CF₃]⁺ and [M ? CF₃]⁺. Ions corresponding to [COCF₃]⁺ are absent. The base peak in many straight chain derivatives occurs at m/e 126 due to alkyl radical loss from the molecular ion; the mass of this ion rising to m/e 140 in the α-substituted N-sec-butyltrifluoracetamide and to m/e 154 in the tert-butyl derivative. High resolution measurements on a number of peaks indicate that they originate by loss of HF from other fragment ions.     

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.     

Spectres de masse des composés organiques. 5e communication. Perte de méthane et d'éthylène à partir des ions butyle

The loss of methane and ethylene in the mass-spectrometric fragmentation of different isomeric butyl ions which originate from butyl halides has been studied. The different carbon atoms in n-butyl are already equivalent after 10^?7 s, whereas the statistical distribution of the hydrogen atoms within the molecule can only be observed for the metastable peaks. A protonated cyclobutane structure is proposed as an intermediate product in the fragmentation of the n-butyl ion. The fragmentation of this model has been simulated by a computer. This allows prediction as to the time-scale of fragmentation. The comparison of this model fragmentation with that of isomeric butyl ions shows that, even in the decay of the tertiary butyl ion, the formation of the proposed rearranged cyclic structure competes favorably with the direct fragmentation.     

FTMS studies of mass-selected,large cluster ions produced by direct laser vaporization

A method is described for the production of large cluster ions by direct laser vaporization in a low-pressure FTMS. Production of high-mass carbon cluster ions (C_n⁺; 40 <n < 180) and bismuth-antimony (Bi_xSb_y⁺) cluster ions containing up to five metal atoms are reported. The observed distributions are compared with those obtained previously by both direct laser vaporization and molecular beam sources. Details of the mechanism for formation of these larger cluster ions by direct laser vaporization are discussed. The mass selectivity and long ion residence times obtainable in the FTMS may now be utilized in the study of these cluster ions. Results are presented from a limited study of the ion/molecule reactions and collision induced dissociation of the high-mass carbon cluster ions.     

Mass spectrometry in structural and stereochemical problems—CXCIX Contrasting fragmentations of singly- and doubly-charged o-, m- and p-hydroxyalkylphenones and their trimethylsilyl ethers

High resolution mass spectrometry, metastable defocusing and deuterium labeling of trimethylsilyl (TMS) ethers have been used to study the electron-impact induced fragmentations of o-, m- and p-hydroxyalkylphenones and their TMS ether derivatives. These derivatives have proven useful in contrasting the fragmentation patterns of singly- and doubly-charged ions because of the competing fragmentations: α-cleavage and a McLafferty rearrangement from the ketone moiety and methyl cleavage from the TMS group. A proximity effect was responsible for a markedly increased methyl radical loss from the o-TMS ether. This fragmentation was minor with the m- and p-isomers. Significantly intense doubly-charged ions were formed from ketonic cleavage and by the loss of a TMS methyl radical. The sequence of fragmentation depended on the size of the alkyl group attached to the ketone carbonyl. There was no evidence found for a McLafferty rearrangement occurring from the doubly-charged molecular ion of the TMS ethers of the hydroxyalkylphenones but the rearrangement occurred from the doubly-charge molecular ion of bis-3-(1-oxopentyl)-4-hydroxy-phenyl-methane and, of course, from the singly charged [M]^+. The bis-p-hydroxyphenylmethane derivatives were studied in an effort to increase the intensity of the doubly-charged ions as it was expected that the charges would be separated by a longer distance.     

The mass spectra of diaziridinones

The electron-impact induced fragmentation of four N,N′-di-t-alkyl-substituted diaziridinones (I to IV) has been studied by both conventional and high resolution mass spectrometry. All diaziridinones exhibit weak molecular ions. Ejection of an alkyl isocyanate, corresponding to the N-alkyl substituent, from the molecular ion, is a dominant and general fragmentation process. Isocyanate-type odd-electron fragment ions occur only in III and IV (where at least one R group is phenyl) and are of low abundance. Elimination of a hydrocarbon radical from the tertiary alkyl substituents is observed in all compounds investigated. McLafferty rearrangement with elimination of a neutral alkene occurs in all compounds. Abundant even-electron hydrocarbon ions corresponding to the mass of the N-alkyl substituent are prevalent. The complete absence of elimination of carbon monoxide is noted. Loss of oxygen from the [M ? RCH²]+ species has been confirmed by accurate mass measurement. Several remarkable rearrangement reactions have been uncovered by high resolution studies and deuteration experiments.     

Determination of the fragmentation mechanisms of organophosphorus ions by H2O and D2O atmospheric-pressure ionization tandem mass spectrometry II—dialkyl alkylphosphonate ions

Diethyl methylphosphonate (DEMP), diisopropyl methylpbosphonate (DIMP), diethyl isopropylphosphonate (DEIP) and diethyl ethylphosphonate (DEEP) were characterized by H₂O and D₂O atmospheric pressure ionization tandem mass Spectrometry (API-MS/MS). Collision-induced dissociation (CID)/fragmentation pathways included alkyl ions by direct cleavage, alkyl radical and water loss processes and McLafferty and McLafferty-type rearrangements by six- and five-membered ring transition states, respectively. D₂O API proved particularly useful in that certain decomposition pathways (i.e. water and methanol neutral losses) had a statistical distribution as to the loss of an acid deuteron and proton(s). This phenomenon was manifested by two pairs of ions in the D₂O API daughter-ion mass spectrum for each phosphonate compound (e.g. both m/z 79/80 and 65/66 for DEMP and DIMP). The observed ion intensity ratios for these pairs of ions served as guides in the determination of their predicted ion relative abundance ratios and CID decomposition pathways. Water neutral losses as opposed to ether and alcohol neutral losses were favored for most of the protonated organophosphonate molecular ion decomposition schemes.     

Benzyl vs. tropylium ions in the decomposition of some alkylnitrobenzenes

The [NO₂C₇H₆]⁺ ions generated from m-alkylnitrobenzenes have been shown to be different in their decomposition from those generated from p-alkylnitrobenzenes, even when the alkyl group is methyl and the departing fragment a hydrogen radical. Thus, in these cases even molecular ions of relatively high internal energy do not reversibly ring-expand to cycloheptatriene structures. In addition, the [NO₂C₇H₆]⁺ ions, assumed to be benzylic, do not ring-expand to nitrotropylium ions at internal energies sufficient to cause subsequent loss of NO or NO₂ from the p- and m-isomers, respectively.     

Studies in organic mass spectrometry. XXIII-Evidence for the occurrence of cyclic sulphonium ions in the gas phase

Functional group interaction occurs in the loss of a phenoxy radical from the molecular ions of ω-phenylthio alkylphenylethers ØO(CH₂)_nSØ (n = 2–6). Labelling data provide evidence for the S-phenyl thietanium, S-phenyl tetrahydrothiophenium and S-phenyl tetrahydrothiopyranium structures of the decomposing (lifetimes between 10^?6 and 10^?5 s) [M - ØO]⁺ ions in the case of n = 3, 4 and 5, respectively.     

Repeller-Induced dissociation of alkyl alcohols in thermospray mass spectrometry

Six alkyl alcohols were studied using thermospray mass Spectrometry. Whereas the dominant ion in the spectrum up to a repeller potential of 120 V was [M + NH₄]⁺, above that potential [M + H]⁺ and fragment ions appeared. The fragments observed were largely due to hydrogen release from alkyl ions ([C_nH_2n+1]⁺ – H2 → [C_nH_2n-1]⁺) and loss of water or some other stable molecule from the same species. The results are compared with those from ionization of the same alcohols under electron impact and photoionization conditions and with results obtained for methanol under thermospray conditions.     

Origin of the [C4H9O]+ ions in the mass spectrum of n-butyl ethyl ether

The mechanisms of formation of m/z 73 ions in the mass spectrum of the ionized title compound were investigated by deuterium substitution and by examining the decompositions of metastable ions. Two routes to the [C₄H₉O]⁺ ions were found in the normal spectrum. The ethyl lost by the major pathway contains the α- and β-hydrogens and a γ-hydrogen from the butyl group. The minor route involves the loss of ethylene from the [M? H]⁺ ion. There were metastable peaks for losses of ethyl, ethanol and methyl from the molecular ion. The ethyl contains the α- and β-methylenes and a γ-hydrogen, while the methyl is the δ-methyl of the butyl group. The labeling data rule out a previous mechanistic proposal for the loss of ethyl and support a mechanism involving stepwise isomerization to the sec-butyl ethyl ether molecular ion. However, the metastable ion chemistries of the molecular ions from the n- and sec-butyl ethyl ethers are highly dissimilar, perhaps due to decompositions from different electronic states. The n-pentyl methyl ether ions loses both ethyl and propyl, apparently following rearrangements to the 3-pentyl and 2-pentyl ether ions. Di n-butyl and n-butyl methyl ethers also give metastable peaks for loss of methyl, ethyl and the shorter chain alcohol.     

Isomerization and fragmentation of methyl benzofuran ions and chromene ions in the gas phase

The rearrangement of the molecular ions of the isomeric 2- and 3-methyl benzofurans (1 and 2), 2H-chromene (3) and 4H-chromene (4) has been studied as a further example of the isomerization of oxygen-heteroaromatic radical cations via a ring expansion/ring contraction mechanism well documented for molecular ions of alkyl benzenes. The ions 1⁺˙?4⁺˙ fragment mainly by H loss into identical chromylium ions a. The process exhibits consistently a large kinetic energy release and an isotope effect k_H/k_D, which arise from a rate-determining energy barrier of the last dissociation step. Differences of the kinetic energy releases, the isotope effects and the appearance energies of the methyl benzofuran ions and the chromene ions indicate a large energy barrier also for the initial hydrogen migration during the rearrangement of the methyl benzofuran ions. This is substantiated by an MNDO calculation of the minimum energy reaction path. In contrast to the behaviour of alkyl benzene ions, a unidirectional isomerization of the methyl benzofuran ions by ring expansion takes place but no mutual interconversion of the molecular ions of methyl benzofurans and chromenes.