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1.
Structure differentiation between [C5H8]+˙ ions, formed by electron ionization of various precursors, has been used as a test case for comparison of three experimental techniques involving collision-induced dissociation (CID). Low-energy CID in an rf-only quadrupole collision cell has been studied in the range 1–150 eV laboratory collision energy. These data have been compared with those obtained using mass-analyzed ion kinetic energy spectroscopy at 8 keV energy, and with results from dissociative charge-stripping (DCS) coupled with a second electron capture collision (EC) in order to remove intense interferences (DCS/EC). The greatest degree of structure differentiation was possible using the DCS/EC technique. The other two methods were comparable in this regard, although effects of pre-collision internal energy was apparent for collision energies much below 30 eV. Day-to-day reproducibility of spectra was most difficult to obtain for the low-energy CID technique. Of the [C5H8]+˙ ions thus tested, the isoprene molecular ion was clearly the best match to the fragment ion formed from limonene.  相似文献   

2.
The mass spectra of 5,6,6a,7,12,12a-hexahydrobenzo[a]anthracene and 2-methoxy, 3-methoxy-, 4-methoxy and 1-methyl-4-methoxy derivatives are reported. Among the fragment ions observed under electron impact ionization, [C8H8] and [M? C8H8] can be generated by a retro-Diels-Alder process. Studies of metastable ion reactions show these ions to be formed by fragmentation directly from the molecular ion. The CA spectra of the [C8H8] ions from the subject compounds were compared with spectra from ions of the same composition from various sources. From these data, kinetic energy release measurements and stereochemical considerations, it is concluded that these ions are formed by a stepwise, rather than a concerted mechanism.  相似文献   

3.
The mechanism of the formation of [C7H8]+ ions by hydrogen rearrangement in the molecular ions of 1-phenylpropane and 1,3-diphenylpropane has been investigated by looking at the effects of CH3O and CF3 substituents in the meta and para positions on the relative abundances of the corresponding ions and on the appearance energies. The formation of [C7H8]+ ions from 1,3-diphenylpropane is much enhanced at the expense of the formation of [C7H7]+ ions by benzylic cleavage, due to the localized activation of the migrating hydrogen atom by the γ phenyl group. A methoxy substituent in the 1,3-diphenylpropane, exerts a site-specific influence on the hydrogen rearrangement, which is much more distinct than in 1-phenylpropane and related 1-phenylalkanes, the rearrangement reaction being favoured by a meta methoxy group. The mass spectrum of 1-(3-methoxyphenyl)-3-(4-trideuteromethoxyphenyl)-propane shows that this effect is even stronger than the effect of para methoxy groups on the benzylic cleavage. From measurements of appearance potentials it is concluded that the substituent effect is not due to a stabilization of the [C7H7X]+ product ions. Whereas the [C7H7]+ ions are formed directly from molecular ions of 1-phenylpropane and 1,3-diphenylpropane, the [C7H8]+ ions arise by a two-step mechanism in which the s? complex type ion intermediate can either return to the molecular ion or fragment to [C7H8]+ by allylic bond cleavage. Obviously the formation of this s? complex type ion, is influenced by electron donating substituents in specific positions at the phenyl group. This is borne out by a calculation of the ΔHf values of the various species by thermochemical data. Thus, the relative abundances of the fragment ions are determined by an isomerization equilibrium of the molecular ions, preceding the fragmentation reaction.  相似文献   

4.
Appearance energies for [C7H7]+ and [C6H5]+ fragment ions obtained from methylphenol isomers were measured at the threshold using the electron impact technique. Different processes for the formation of the ions are suggested and discussed. Metastable peaks were detected and the kinetic energies released were determined. The results indicate that [C7H7]+ ions are formed from metbylpbenois with both benzyl and tropylium structures, whereas [C6H5]+ ions are formed with the phenyl structure at the detected thresholds. Kinetic energies released on fragmentation of reactive [ C7H7]+ and [C6H5]+ ions were used as a probe for the structure of the ions at 70 eV.  相似文献   

5.
The ion [C3H5]+ generated in a chemical ionization source by a variety of methods, including protonation and charge exchange, exhibits a metastable peak for H2 loss which is two orders of magnitude weaker than that formed in an electron impact source. The stable [C3H5]+ ions generated by electron impact and chemical ionization undergo collision-induced dissociation to a comparable extent, both losing H2 by only one of the two competitive mechanisms observed for metastable ions. In contrast to the behavior of [C3H5]+, the molecular ions of p-substituted nitrobenzene, generated by charge exchange at high source pressure, yield composite metastable peaks for NO loss which are very similar in shape and intensity to those generated by electron impact. The contrasting behavior of the metastable ions extracted from high pressure ion sources in the two systems may be due to differences in the efficiencies of quenching of the ionic states responsible for fragmentation as metastable ions. It is noteworthy that the NO loss reactions require considerably lower activation energies than does the H2 loss reaction.  相似文献   

6.
The mass spectra of spiropentane, cyclopentene, 1,3-pentadiene (piperylene), isoprene and some deuterium labelled analogues have been studied. The heats of formation of the molecular ions and the major daughter ions [C5H7]+ and [C4H5]+ have been estimated. Fragmentation of these molecules mostly proceeds via common intermediates in which hydrogen atoms have lost their positional identity. Only spiropentane displays any specific fragmentation behaviour related to its ground state geometry.  相似文献   

7.
The appearance energies for the [C7H7]+ and [C8H9]+ fragment ions produced in the fragmentation of the C-1? C-4 monosubstituted alkyl benzenes have been measured by photon impact. The mean heat of formation calculated for [C7H7]+ is 205.3 ± 1.9 kcal mol?1 which is consistent with a threshold tropylium structure. For [C8H9]+ the mean heat of formation is calculated to be 199.2 ± 1.3 kcal mol?1 which can be equated with either a methyl tropylium or α-phenylethyl structure at threshold. Some evidence is provided for the existence of the α-phenylethyl ion.  相似文献   

8.
Additional evidence for the rearrangement of the 1- and 3-phenylcyclobutene radical cations, their corresponding ring-opened 1,3-butadiene ions and 1,2-dihydronaphthalene radical cations to methylindenetype ions has been obtained for the decomposing ions by mass analysed ion kinetic energy spectroscopy (MIKES). The nature of the [C9H7]+ and [C10H8] daughter ions arising from the electron ionization induced fragmentation of these [C10H10] precursors has been investigated by collisionally activated dissociation (CAD), collisional ionization and ion kinetic energy spectroscopy. The [C9H7]+ produced from the various C10H10 hydrocarbons are of identical structure or an identical mixture of interconverting structures. These ions are similar in nature to the [C9H7]+ generated from indene by low energy electron ionization. The [C10H8] ions also possess a common structure, which is presumably that of the maphthalene radical cation.  相似文献   

9.
Metastable ion peak shapes, dimensions and relative abundances have been measured for the three fragmentations [C3H6]+· → [C3H4]+· + H2, [C3H6]+· → [C3H5]+ + H· and [C3H6]+· → [C3H3]+ + H2 + H·. [C3H6]+· ions were derived from propene, cyclopropane, tetrahydrofuran, cyclohexanone, 2-methyl but-1-ene and cis-pent-2-ene. Activation energies for these fragmentations have been evaluated. Three daughter ion dissociations ([C3H5]+ → [C3H3]+ + H2, [C3H5]+ → [C3H4]+· + H· and [C3H4]+· → [C3H3]+ + H·) have been similarly examined. Ion structures have been determined and the metastable energy releases have been correlated with the thermochemical data. It is concluded that the molecular ions of propene and cyclopropane become structurally indistinguishable prior to fragmentation and that differences in their metastable ion characteristics can be ascribed wholly to internal energy differences; the latter can be correlated with the photoelectron spectra of the isomers. The pathway for the consecutive fragmentation which generates the metastable ion peak (m/e 42 → m/e.39) has been shown to be It is likewise concluded that fragmentating [C3H6]+· ions generated from the various precursor molecules are also structurally indistinguishable and cannot be classified with either molecular ion of the isomeric C3H6 hydrocarbons.  相似文献   

10.
A detailed energy-resolved study of the fragmentation of CH2?CHCH(OH)CD2CD3 (1-d5) has been carried out using metastable ion studies and charge exchange techniques, combined with collision-induced dissociation studies to establish the structures of fragment ions. At low internal energies (metastable ions) the molecular ion of 1-d5 rearranges to the 3-pentanone structure and fragments by loss of C2H5 or C2D5 leading to the acyl structure, [CH3CH2C?O]+ or [CD3CD2C?O]+, for the fragment ion. However, with increasing internal energy of the molecular ion this rearrangement process decreases rapidly in importance and loss of C2D5 by direct cleavage, leading to [CH2?CHCH?OH]+, becomes the dominant fragmentation reaction. As a result the [C3H5O]+ ion seen in the electron impact mass spectrum of 1-penten-3-ol has predominantly the protonated acrolein structure.  相似文献   

11.
For two competing decompositions of the same molecular ion to give products [A1+] and [A2+], the ratio [A1+]/[A2+], is equal to the ratio of rate constants for the formation of the stable ions. Thr ratios, [Y C7H4O+]/[C7H5O+], were determined for several benzophenones for electron energies from 15 to 70 eV. Plots of log [Y C7H4O+]/[C7H5O+] vs.[ω+] gave good straight lines at all energies. Similar correlations have been reported for log [Y C7H6+]/[C7H7+] from substituted diphenyl ethanes and are also true for substituted acetophenones, log [YøCO+]/[CH3CO+]. A few charge exchange data were obtained which showed the same general trend as the electron-impact data and emphasize the contribution of low energy ions in the 70 eV mass spectra. Relatively poor correlations were obtained for the [Y C6H4+] and [C6H5+] ions that are formed by both one-step and two-step decompositions.  相似文献   

12.
Composite metastable peaks are generated in the unimolecular fragmentations (i) [C3H5]+ → [C3H3]+ + H2 (flat-top upon flat-top) and (ii) [C4H9]+ → [C3H5]+ + CH4 (flat-top and gaussian). The measurement of appearance potentials and kinetic energy releases lead us to conclude, in agreement with earlier proposals, that in (i) the components can arise from the generation of the isomeric cyclopropenium and propargyl daughter cations. In (ii) the components are proposed to arise from the fragmentation of tert- and sec-butyl cations yielding allyl as the common daughter ion. The composite peak observed in the fragmentation (iii) [C3H4]+· → [C3H3]+ + H· is shown to be present only if the decomposing molecular ion is large enough to also produce [C6H8]2+ ions. The second component in (iii) then arises from the reaction [C6H8]2+ → [C6H6]2+ + H2.  相似文献   

13.
The laser-ion beam photodissociation for [C4H4]+˙ ions produced from a variety of precursors has been studied. Based on the data it is apparent that two structurally distinct forms of the [C4H4]+˙ ion are produced by fragmentation of larger systems. The relative population of the various structural forms is very dependent on the internal energy of the fragmenting ion, with 1-buten-3-yne [C4H4]+˙ ions being favored at low internal energies. As the internal energy of the reactant ion is increased, the relative population of butatriene [C4H4]+˙ ions increases. The laser-ion beam photodissociation technique is able to selectively sample these two structural forms.  相似文献   

14.
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.  相似文献   

15.
The mass spectra of norbornene, nortricyclene and deuterium labeled derivatives thereof have been studied. The appearance potentials of the ions [C7H10], [C7H9]+, [C6H7]+ and [C5H6] have been determined for both compounds and heats of formation of the hydrocarbons have been estimated. Detailed fragmentation schemes are proposed for the molecular ions and it is concluded that they dissociate by essentially different mechanisms which do not involve common intermediates. The structures and energy contents of the primary fragment ions are discussed in detail by comparing energetics, labeling experiments and metastable ion abundances.  相似文献   

16.
The mass spectrum of propene-2-[13C] shows 81% retention of C-2 in the [C2H3]+ fragment ion at 70 eV electron energy, decreasing to 75% C-2 retention at low electron energies. The mass spectra of propene-2-d1, propene-1,1,3,3,3-d5, propene-1,1,2-d3 and propene-3,3,3-d3 also have been examined at a resolution sufficient to resolve H2-D doublets. The results at 70 eV electron energy show complete H/D randomization prior to fragmentation to form [C3(H, D)5]+ but, in agreement with the 13C labelling data, incomplete H/D interchange prior to fragmentation to form [C2(H, D)3]+. The results are interpreted in terms of a reversible isomerization of the propene molecular ion to a cyclopropane structure in competition with fragmentation.  相似文献   

17.
We report the first positive chemical ionization (PCI) fragmentation mechanisms of phthalates using triple‐quadrupole mass spectrometry and ab initio computational studies using density functional theories (DFT). Methane PCI spectra showed abundant [M + H]+, together with [M + C2H5]+ and [M + C3H5]+. Fragmentation of [M + H]+, [M + C2H5]+ and [M + C3H5]+ involved characteristic ions at m/z 149, 177 and 189, assigned as protonated phthalic anhydride and an adduct of phthalic anhydride with C2H5+ and C3H5+, respectively. Fragmentation of these ions provided more structural information from the PCI spectra. A multi‐pathway fragmentation was proposed for these ions leading to the protonated phthalic anhydride. DFT methods were used to calculate relative free energies and to determine structures of intermediate ions for these pathways. The first step of the fragmentation of [M + C2H5]+ and [M + C3H5]+ is the elimination of [R? H] from an ester group. The second ester group undergoes either a McLafferty rearrangement route or a neutral loss elimination of ROH. DFT calculations (B3LYP, B3PW91 and BPW91) using 6‐311G(d,p) basis sets showed that McLafferty rearrangement of dibutyl, di(‐n‐octyl) and di(2‐ethyl‐n‐hexyl) phthalates is an energetically more favorable pathway than loss of an alcohol moiety. Prominent ions in these pathways were confirmed with deuterium labeled phthalates. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

18.
The kinetics of formation of [C3H5]+[M ? CH3]+, [C3H4]+·[M ? CH4]+· and [C2H4]+·[M ? C2H4]+· from but-1-ene, cis- and trans-but-2-ene, 2-methylpropene, cyclobutane and methylcyclopropance following field ionisation have been determined as a function of time 20 (or 30) picoseconds to 1 nanosecond and at two points in the microsecond time-frame. The results are consistent with the supposition that at the shortest accessible times (20 to 30 picoseconds) the structure of the [C4H8]+· molecular ion qualitatively resembles that of its neutral precursor, but suggest that prior to decomposition within nanoseconds the various molecular ions (excepting cyclobutane where the processes are slower) attain a common structure or mixture of structures. Reaction pathways of the presumed known ion structures are delineated from the nature of decompostion at the shortest times.  相似文献   

19.
The [CH3O?CHCH3]+ ions observed in the mass spectra of ethers of formula CH3OCH (CH3)R(R = H or alkyl) undergo two rearrangement fragmentation reactions to form [C2H5]+ and [CH2OH]+. The scope of the rearrangements has been investigated and it is shown that enlargement of the alkyl group on either side of the ether linkage leads to alternative fragmentation routes. From a study of metastable intensities it is concluded that the fragmentations probably occur directly from the [CH3O?CHCH3]+ structure through four centred rearrangements rather than through the intermediacy of the [C2H5O?CH2]+ ion.  相似文献   

20.
In ab initio calculations, we determined the most probable routes of decomposition of the [CF3Cl]+, [CF2Cl2]+, [CFCl3]+, [CCl4]+ molecular ions of freons and [C3H8]+ ions of hydrocarbons formed by collision of neutral molecules with protons with energies of the order of 10 keV. The calculated potential surface sections are compared on a qualitative level with the probability of various ion fragments in experiments on fragmentation. The role of the charge transfer dynamics between the proton and the molecule is discussed.  相似文献   

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