Mass spectrometry of steroid systems. XXIV–The origin of [M – 28]+ and [M – 43]+ ions in equilenin and its 14β-isomer

The formation of the [M? 43]⁺ ion in equilenin is due mainly to elimination of Me radical from the [M? CO]⁺ ion and, to a lesser extent, to CO loss from the [M? Me]⁺ ion. In 14β-isoequilenin the [M? CO]⁺ ion is absent, and the formation of [M? 43]⁺ occurs via the [M? Me]⁺ ion. This makes the determination of the mode of junction of the rings C and D in the equilenin series possible, using high resolution mass spectra, even when only one stereoisomer is available.     

Electron impact mass spectra of some dioximes of o-diacyl benzenes

The electron impact mass spectra of the new synthesized dioximes of o-diacyl benzenes (2) are reported. In addition to the molecular ion, characteristic peaks appear at values corresponding to the [M ? OH] ⁺, [M ? NOH]⁺ and [M ? NHOH]⁺ ions. No initial dehydration of the molecular ion has been observed.     

Steric strain and electron-impact. The behaviour of some n,n'-dimethyl-1,1-binaphthyls,some n,n'-dimethylbiphenyls and model compounds

For the title compounds I(M), A[M ? H]⁺ and A[M ? Me]⁺ have been determined. In the biaryl compounds, unlike the xylene and the two dimethylnaphthalenes studied, A[M ? Me]⁺ is noticeably dependent on the position of the methyl group. Deuterium labelling of the methyl groups in the biaryl compounds reveals that more than one process is involved in the formation of the [M ? Me]⁺ ion. In contrast, only in the case of 2,2′-dimethylbiphenyl is the appearance potential of the [M ? H]⁺ ion position dependent. The labelling results suggest complete scrambling of the hydrogen atoms before formation of the [M ? H]⁺ ions. Some comments are made on the problem of trying to relate differences in strain energies and ionisation and appearance potentials in alkyl substituted aromatics.     

A mass spectral study of 1-(aryldimethylsilyl)-2-propanones and their isomeric 2-(aryldimethylsiloxy)-1-propenes

The mass spectra of a series of β-ketosilanes, p-Y? C₆H₄Me₂SiCH₂C(O)Me and their isomeric silyl enol ethers, p-Y? C₆H₄Me₂SiOC(CH₃)?CH₂, where Y = H, Me, MeO, Cl, F and CF₃, have been recorded. The fragmentation patterns for the β-ketosilanes are very similar to those of their silyl enol ether counterparts. The seven major primary fragment ions are [M? Me·]⁺, [M? C₆H₄Y·]⁺, [M? Me₂SiO]⁺˙, [M? C₃H₄]⁺˙, [M? HC?CCF₃]⁺˙, [Me₂SiOH]⁺˙ and [C₃H₆O]⁺˙ Apparently, upon electron bombardment the β-ketosilanes must undergo rearrangement to an ion structure very similar to that of the ionized silyl enol ethers followed by unimolecular ion decompositions. Substitutions on the benzene ring show a significant effect on the formation of the ions [M? Me₂SiO]⁺˙ and [Me₂SiOH]⁺˙, electron donating groups favoring the former and electron withdrawing groups favoring the latter. The mass spectral fragmentation pathways were identified by observing metastable peaks, metastable ion mass spectra and ion kinetic energy spectra.     

Structural and substituent effects on [M]+. vs. [MH]+ formation in fast atom bombardment mass spectra of simple organic compounds

The peak intensity ratios of [M]^+. vs. [MH]⁺ were measured in the fast atom bombardment (FAB) mass spectra of readily available test compounds with 3-nitrobenzyl alcohol as the matrix. For simple aromatic amines, the ratio increases as the ionization energy of the substrate decreases. 4-Substituted benzophenones showed preferential formation of [MH]⁺ ions, regardless of the nature of the substituents. This is probably due to the fact that the benzophenoes have carbonyl groups which can form hydrogen bonds with the matrix molecule. The peak intensity ratio is roughly proportional to the Hammett σ^+. Among 4-substituted biphenyls, both bromo and chloro substituents afforded abnormally high peak intensity ratios. The effects of the substituents in these compounds are discussed semi-quantitatively in terms of the Hammett correlation and the hard and soft acids and bases principle. The mechanism of ion formation in FAB and chemical ionization (CI) ion sources appeared to be different because some of the compounds studied showed an intense [M]^+. peak with a relatively weak [MH]⁺ peak in FAB spectra but exhibited a strong [MH]⁺ peak in ordinary CI spectra.     

Mass spectra of chlorinated esters: 2—Methyl mono- and dichlorobutanoates

The mass spectral fragmentations of methyl mono- and dichlorobutanates have been studied. Deutrium labelling and metastable ion analysis were used to elucidate the fragmentation mechanisms. The molecular ion peaks of the esters are weak and show only in the spectra of the monochloro isomers. A McLafferty rearrangement gives the base peaks in the spectra of methyl 2-chloro-, 4-chloro- and 4,4-dichlorobutanoate; α-cleavage, [COOCH₃]⁺, in methyl 2,2- and 2,4-dichlorobutanoate; [M? Cl]⁺, in methyl 3-chlorobutanoate; [M? Cl? HCl]⁺, in methyl 3,4-dichlorobutanoate; [M? Cl? CH₂CO]⁺, in methyl 3,3-dichlorobutanoate and [M? Cl? COOCH₃]^+˙, in methyl erythro- and threo-2,3-dichlorobutanoate. The mass spectra of the stereoisomers are nearly identical, the loss of a chlorine atom and the McLafferty rearrangement giving the higher peaks in the spectrum of the threo form.     

Low-energy mass spectra of some aliphatic ketones

The charge exchange mass spectra of a selection of C₅-C₇ ketones have been measured using [CS₂]⁺˙, [COS]⁺˙ and [N₂O]_+. as reagent ions. The low energy charge exchange with [CS₂]⁺˙ or [COS]⁺˙ provides simple primary ion mass spectra, which readily permit structure elucidation in contrast to metastable ion spectra. In several cases, isomer distinction is easier from the charge exchange mass spectra than from the electron impact mass spectra. The energy transfer from [N₂O]⁺˙ is sufficiently high for complex spectra resembling electron impact mass spectra to be obtained.     

Deuterium labelling and thermochemical studies of dissociative ionization of methyl substituted monosilacyclobutanes. Ring expansion in the molecular ion of 1,1,3-trimethyl-1-silacyclobutane

Dissociative ionization of 1,1-dimethyl-1-silacyclobutane, 1,1-bis(trideuteromethyl)-1-silacyclobutane, 1,1,3-trimethyl-1-silacyclobutane, 1,1-bis(trideuteromethyl)-3-methyl-1-silacyclobutane and 1,1,2-trimethyl-1-sila-cyclobutane have been studied. Low energy electron impact fragmentation of 1,1-bis(trideuteromethyl)-3-methyl-1-silacyclobutane results mainly in the loss of ethene with the involvement of the C-methyl group from the rearranged molecular ion. No fragment ions indicating rearrangement of the molecular ion have been detected in mass spectra of 1,1-bis(trideuteromethyl)-1-silacyclobutane. The ionization energies for 1,1,3-trimethyl-1-silacyclobutane, 1,1,2-trimethyl-1-silacyclobutane and 1,1-dimethyl-1-silacyclopentane, and also the appearance energies for the [M ? 28]^+˙ and [M ? 42]^+˙ ions, have been measured by photoioniza-tion mass spectrometry. The heats of formation of these ions and of 1,1,3-trimethyl-1-silacyclobutane and 1,1-dimethyl-1-silacyclopentene molecules have been calculated as have the enthalpies of the transformation processes.     

Formation of M+. ions under matrix fast atom bombardment conditions: Does charge exchange reaction occur?

The formation of molecular ions, M^+., under fast atom bombardment (FAB) conditions using a liquid matrix was examined by using a new type of synthesized compounds in which preferential M^+. peaks appear in their FAB spectra. The FAB spectra were compared with the corresponding mass spectra obtained by the electron impact (EI) ionization, chemical ionization (CI) and charge-exchange ionization (CEI) methods. All of the spectra showed preferential peaks of M^+. ion and a characteristic intense fragment ion peak originating from a β-fission. The FAB spectra were similar in the fragment ions appearing in the EI spectra and were very similar in the fragmentation pattern to the CEI spectra using Ar^+. and Xe^+. as the reagent ions. Further, the FAB spectra did not show any doubly charged ion peaks, while the 70 eV EI spectra showed the peaks of doubly charged molecular and/or fragment ions. The isobutane CI spectra of the synthesized compounds suggested that the formation of M^+. ions occurred through the CE reaction with isobutane ion, C₄H₁₀^+., and the CI spectra showed a marked intense fragment ion peak originating from the β-fission which seemed to occur characteristically in CEI processes. The results obtained suggested that the formation of M^+. ions under matrix FAB conditions occurred mainly by CE reactions between the analytes M and matrix molecular ions B^+. and/or fragment ions b^+..     

Ion-molecule reactions observed for nitroaromatic compounds in negative ion fast atom bombardment mass spectrometry

Analyses of a series of nitroaromatic compounds using fast atom bombardment (FAB) mass spectrometry are discussed. An interesting ion-molecule reaction leading to [M + O ? H]^? ions is observed in the negative ion FAB spectra. Evidence from linked-scan and collision-induced dissociation spectra proved that [M + O ? H]^? ions are produced by the following reaction: M + NO₂^? → [M + NO₂]^? → [M + O ? H]^?. These experiments also showed that M^?˙ ions are produced in part by the exchange of an electron between M and NO₂^? species. All samples showed M^?˙, [M ? H]^? or both ions in their negative ion FAB spectra. Not all analytes studied showed either [M + H]⁺ and/or M⁺˙ in the positive ion FAB spectra. No M⁺˙ ions were observed for ions having ionization energies above ～9 eV.     

Gas-phase ion-molecule bimolecular and clustering reactions in dilute solutions of acetonitrile in xenon,krypton, argon,neon and helium

Gas-phase bimolecular and clustering reactions of acetonitrile in Xe, Kr, Ar, Ne and He were studied at high chemical ionization pressures in the new coaxial ion source at Auburn. With electron energies near the ionization threshold, the mass spectra are exceedingly simple and are comprised of [CH₄CH]⁺ and clusters of [CH₄CN]⁺ with various ligands such as H₂O and CH₃CN. At higher electron energies many other peaks appear. The intensities of the new peaks depend upon the ionization potential of the charge transfer gas, the ionizing electron energy and the ion source conditions, and are due to reactions of fragment ions. Residence time distributions at electron energies above the ionization threshold (∼ 30 eV) demonstrate that two molecular structures are present in the ion beam at m/z 42, one presumably is protonated acetonitrile ([CH₃CNH]⁺) while the evidence indicates that the second species does not contain acidic hydrogens. With ionizing electron energies near threshold (∼ 10. 5 eV) only one structure is observed. Studies with electron energies near the ionization threshold under high-pressure chemical ionization conditions result in greatly simplified mass spectra and are possible only because of the coaxial geometry of the ion source.     

The [M?OH]+ ions of m- and p-ethylnitrobenzene: A common structure?

The structures of the [M? OH]⁺ ions of m- and pethylnitrobenzene have been compared by measurements of metastable ion spectra, collisional activation spectra, kinetic energy releases and critical energies for the formation of these ions and their subsequent decomposition. Normalized rates of fragmentation of metastable molecular ions and metastable [M? OH]⁺ ions have been compared for ion lifetimes up to 30 μs. The energy measurements fail to distinguish between the structures of the [M? OH]⁺ ions, but the normalized fragmentation rates and the collisional activation spectra show their structures to be different.     

In-beam electron impact mass spectrometry of aliphatic alcohols

The characteristics of the in-beam electron impact mass spectra of six isomers of undecanol as well as several 1-alkanols have been examined. In addition to the characteristic ions observed in the conventional electron impact spectra, the [2M+1]⁺, [2M+1-H₂O]⁺, [2M+1-2H₂O]⁺, [2M-R or R′]⁺, [2M-H₂O? R or R′]⁺, [2M? 2H₂O? R or R′]⁺ and [M+1? H₂O]⁺ peaks are common in the in-beam electron impact mass spectra of the undecanol isomers of structure RCH(OH)R′. Deuterium labelling experiments have shown that the extra proton in the protonated dimer ions, [2M+1]⁺, originates from the hydroxy group. The processes which produce the important peaks in the high m/e regions are discussed.     

Studies on the mechanism of electron impact induced SH radical loss from the diastereoisomeric 2-dimethylamino-2-thiono-4-methyl-1,3,2-dioxaphosphorinanes

The intensity differences between the [M? HS]⁺ peaks in the mass spectra of diastereoisomeric 2-dimethylamino-2-thiono-4-methyl-1,3,2-dioxaphosphorinanes have been rationalized in terms of fragmentation mechanisms and ion structures. Metastable characteristics together with D labelling experiments support these rationalizations.     

Positive electron impact and chemical ionization mass spectra of some nitramine nitrates

The positive electron impact (EI) and isobutane chemical ionization (CI) mass spectra of six nitramine nitrates were studied with the aid of some accurate mass measurements. In the EI spectra, β fission relative to both the nitramine and nitrate ester is important. In the CI spectra a major ion occurs at [MH – 45]⁺ and was found to be mainly due to [M + 2H ? NO₂]⁺. All of the compounds except N-(2 hydroxyethyl)-N-(2′,4′,6′-trinitrophenyl)nitramine nitrate gave an [MH]⁺ ion. The [MH – 45]⁺ ion in the isobutane CI mass spectra of tetryl is also due to [M + 2H ? NO₂]⁺.     

Evaluation of nitric oxide chemical ionization mass spectra of substituted benzenes

Nitric oxide chemical ionization mass spectra of substituted benzenes obtained with the Townsend discharge technique were studied. There were four kinds of base peaks in the mass spectra, i.e. [M + NO]⁺˙, M⁺˙, [M ? H]⁺ and [M ? OR]⁺ (R = H, CH₃). The formation of the specific ion [M + NO]⁺˙ was highly dependent on the kind of substituent, and it was produced more abundantly in the case of substitutions involving electron-accepting groups. The measure of [M + NO]⁺˙ production was evaluated from the value of the ratio [M + NO]⁺˙/M⁺˙. In mono-substitutions, it was clarified that the ratios of [M + NO]⁺˙/M ⁺˙ were correlated with the Hammett substituent constant s?_p or the electrophilic substituent constant s?_p⁺. Monosubstitutions (C₆H₅R) and toluene substitutions (CH₃C₆H₄R) could be classified into six groups in terms of base peak species, [M + NO]⁺˙/M⁺˙ ratios and substituents. In disubstitutions, the mass spectral patterns were governed by the combination of substituents. Mass spectral distinctions among ortho, meta and para isomers could be made in many cases.     

Mass spectra of chlorinated aromatics formed in pulp bleaching: I—chlorinated catechols

A systematic study on the electron impact mass spectra of all nine chlorinated catechols in presented. Metastable ion analysis was used to elucidate the fragmentation pathways. The influence of the position of the chloro substituents can be used to distinguish the structural isomers. In this respect the most characteristic fragment ions are [M? CHl]⁺˙, [M? HCOOH]⁺˙, [M? COCl]⁺, [M? HCl? CO]⁺˙, [M? CHOCl]⁺˙ and [M? HCl? HCl]⁺˙.     

天然有机化合物的质谱研究 黄酮和黄酮甙与碱金属离子K+和Na+的快原子轰击质谱

在作黄酮和黄酮甙的快原子轰击质谱时,如果同时加入氯化钠和氯化钾两种碱金属盐,就会在谱图中同时出现两个强峰,由于这两个峰相差１６个原子质量单位,很容量辨认,并且发现在其谱图中找不到明显的碎片离子和Ｎａ＾＋和Ｋ＾＋的加合离子,而只有其分子和Ｋ＾＋和Ｎａ＾＋的加合,所以很容易在ＦＡＢ谱中识别它们,因而就可以快速,准确地确定样品的分子量。     

Electrospray mass analysis of chemical oxidation of 1,2-diarylcyclopropanes by Cu(II) salt

Electrospray ionization mass spectrometry was used to study chemical electron-transfer reactions of 1,2-diarylcyclopropanes by Cu(II) salt in acetonitrile. The ion [M ? H]⁺ with a hydrogen atom loss and the solvent adduct ions, [M+42]⁺, etc., were detected as the initial reaction products, where [M+42]⁺ represents the ion whose mass is 42 u greater than the parent molecule M. From the study of deuterated derivatives, the hydrogen abstraction was revealed to occur at the 3 position of the cyclopropanes, and the mechanism of the hydrogen abstraction reaction and of the solvent addition were discussed.     

The observation of broad metastable ion peaks in the surface-induced dissociation mass spectra of protonated and cationated peptides

The product ion mass spectra of protonated and cationated peptides of relative molecular mass (RMM) 555–574 Da have been obtained by surface-induced dissociation of MH⁺ and [M + Cat]⁻ ions in a four-sector tandem mass spectrometer equipped with a specially designed collision cell. A linked scan of the electric and magnetic sector field strengths of the second mass spectrometer was used to transmit the fragment ions arising from collisions with a stainless steel surface. The resulting mass spectra contained broad metastable ion peaks produced by the dissociation of MH⁺ and [M + Cat]⁺ ions before the second magnetic sector, in the fourth field-free region of the instrument.