Mass-analyzed Ion Kinetic Energy Research Articles

Do Nitrogen-Atom-Containing Endohedral Fullerenes Undergo the Shrink-Wrap Mechanism?

The endohedral nitrogen-atom-containing fullerenes, N@C60 and N@C70, were studied mass spectrometrically. The neutral endohedrals are thermally unstable. This instability can be followed by mass spectrometry as well as by EPR spectroscopy. The corresponding ions were studied by using mass-analyzed ion kinetic energy (MIKE) spectroscopy. Unlike metal-containing and noble-gas-containing endohedral fullerenes studied earlier, the ions N@C60+ and N@C70+ do not lose C2 groups unimolecularly but lose the nitrogen atom instead. This indicates that the activation energy for nitrogen-atom loss is lower than that for the C2 elimination from the cage. This was verified through the analysis of kinetic energy release distribution (KERD) measurements. The results are discussed in light of the thermal instability of nitrogen-atom-containing endohedral compounds compared to noble-gas-containing analogues.

Photodissociation dynamics of the propargyl bromide molecular ion

Photodissociation of the propargyl bromide molecular ion has been investigated using mass-analyzed ion kinetic energy spectrometry (MIKES). The MIKE spectra for the bromine loss has been measured as a function of the laser polarization angle at 357, 488.0, 514.5, and 607.5 nm. A convenient analytic method to treat the MIKE profile has been developed and the anisotropy parameters and kinetic energy release distributions (KERDs) have been determined by analyzing the experimental data. At all the wavelengths used, an anisotropic dissociation in the repulsive excited electronic state(s) has been observed. In addition, the statistical dissociation in the ground electronic state has been observed at 357 and 607.5 nm. Results from quantum chemical calculations carried out at the TDDFT/UB3LYP level using the 6-311++G** basis set will be discussed in relation to the nature of the repulsive excited electronic state involved.

Metastable decompositions of gem‐dialkoxyalkanes upon electron impact. III. Diethoxymethane (CH2(OCH2CH3)2)

Unimolecular metastable decomposition of diethoxymethane (CH2(OCH2CH3)2, 1) upon electron impact has been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and theD-labeling technique in conjunction with thermochemistry. The m/z 103 ion ([M − H]+ : CH(OCH2CH3) = O+CH2CH3) decomposes into the m/z 47 ion (protonated formic acid, CH(OH) = O+H) by consecutive losses of two C2H4 molecules via an m/z 75 ion. The resulting product ion at m/z 47 further decomposes into the m/z 29 and 19 ions by losses of H2O and CO, respectively, via an 1,3-hydroxyl hydrogen transfer, accompanied by small kinetic energy release (KER) values of 1.3 and 18.8 meV, respectively. When these two elimination reactions are suppressed by a large isotope effect, however, another 1,1-H2O elimination with a large KER value (518 meV) is revealed. The m/z 89 ion ([M − CH3]+ : CH2(OCH2CH3)O+ = CH2) decomposes into the m/z 59 ion (CH3CH2O+ = CH2) by losing CH2O in the metastable time window. The source-generated m/z 59 ion ([M − OCH2CH3]+ : CH2 = O+CH2CH3) decomposes into the m/z 41 (CH2 = CH+CH2) and m/z 31 (CH2 = O+H) ions by losses of H2O and C2H4, respectively, with considerable hydrogen scrambling prior to decomposition. Copyright © 2000 John Wiley & Sons, Ltd.

Tandem mass spectrometric study of 1,3,5-trinitrobenzene molecular ion: an unusual ortho effect involving a hydrogen atom from the aromatic ring

Unimolecular (metastable) and collision-induced dissociation of 1,3, 5-trinitrobenzene molecular ion was studied using linked scans and mass-analyzed ion kinetic energy spectrometry on a hybrid instrument of EBEqQ geometry. An unusual ortho effect leading to the loss of OH radical from the parent molecular ion is observed as a unimolecular dissociation process only in the first-field free region between the ion source and the electric sector, although corresponding dissociation in the ion source is of negligible abundance (<0.1%). This unimolecular process is taken over by other dissociation pathways when the parent ion is collisionally activated, suggesting that this dissociation process occurs only in a very narrow energy window with a rate constant of the order of 2 x 10(5) s(-1). Copyright 2000 John Wiley & Sons, Ltd.

Gaseous [ t-C 4H 9+ α,ω-diphenylalkane] complexes: methyl substituent effects on the intracomplex proton transfer and regioselective hydride abstraction

The loss of isobutane and isobutene from a series of long-lived protonated 1-(4- tert-butylphenyl)-3-phenylpropanes bearing two or three methyl substituents on the aromatic rings have been studied by mass-analyzed ion kinetic energy spectrometry and deuterium labelling. Both processes occur via intermediate ion/neutral complexes consisting of the tert-butyl cation and the corresponding di- or trimethyl-substituted 1,3-diphenyl-propanes, and the competition between these intracomplex proton and hydride transfer processes is governed by the (overall) gas-phase basicity of the 1,3-diphenylpropane, on the one hand, and by the localized activation of the benzylic C–H bonds acting as hydride donors, on the other. In addition to the benzylic α- and ω-methylene groups, ortho-sited methyl substituents are also involved in the hydride transfer, in contrast to those at the meta and para positions, and their fractional contributions to the overall isobutane loss strongly depends on the electronic influence of the other methyl group(s) at the same ring. Regioselectivities of the intracomplex hydride abstraction by t-C 4H 9 + from the α and γ or α, γ and ortho positions of the 1,3-diphenylpropanes have been evaluated assuming the kinetic isotope effect k H/ k D that is found to operate ubiquitously in these complexes. While acting as hydride donors on their own, ortho-methyl groups attenuate the hydride donor ability of the adjacent benzylic methylene group by steric hindrance, in particular in the case of twofold (2,6-dimethyl) substitution. Comparison with the fragmentation behavior of the 2-methyl- and 2,6-dimethyl-substituted mononuclear 4-( tert-butyl)ethylbenzenium ions corroborate the kinetic isotope effect and, in particular, the activating effect of the second (“spectator”) ring on the hydride abstraction within the [ t-C 4H 9 + α,ω-diphenylalkane] complexes.

Mass spectrometric study of six cyclic esters

A series of cyclic esters, which are optically active as a consequence of their helical structures, were synthesized to investigate the relationships between their structures and their optical activities. This paper reports the electron impact fragmentation mechanisms of these six cyclic esters. Accurate mass measurements and mass analyzed ion kinetic energy spectrometry confirmed fragmentation patterns. The stability of the fragment ions has a great influence on the fragmentation pathways, but no correlation with the optical activity was found.

Electron impact mass spectral fragmentation patterns of 2a,4-disubstituted 5-benzoyl-2-chloro-2a,3,4,5-tetrahydroazeto[1,2-a][1,5]benzodiazepin-1( 2H)-ones.

The mass spectrometric behaviour of six 2a,4-disubstituted 5-benzoyl-2-chloro-2a,3,4,5-tetrahydroazeto[1,2-a][1,5]benzodia zepin-1(2H)-ones has been studied with the aid of mass-analysed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization. All compounds show a tendency to eliminate a chlorine atom, a chlorine atom plus benzaldehyde, benzoyl radical, chloroketene or chlorine atom plus CO and H2O molecules to yield, respectively, [M-Cl]+ ions, 2a,4-disubstituted 2a,3-dihydroazeto[1,2-a][1,5]benzodiazepin-1(2H)-one ions, [M-PhCO]+ ions, 2,4-disubstituted 1-benzoyl-2,3-dihydro-1H-1,5-benzodiazepine ions, or 1,2,4-trisubstituted 1H-1,7-benzodiazonine ions, which could also be formed from [M-Cl]+ ions by loss of CO and H2O molecules simultaneously. The [M-Cl]+ ions could further lose benzoyl radical to form [M-Cl-PhCO]+ ions, or lose benzoyl amide and undergo a rearrangement to form 4,6-disubstituted 1-benzoazocine-2(1H)-one ions. The [M-PhCO]+ ions could eliminate NH to produce 2a,4-disubstituted 2,2a,3,4-tetrahydroazeto[1,2,-a]quinolin-1-one ions, which could further eliminate chloroketene, CO and/or HCl to produce some important ions, respectively. 2,4-Disubstituted 1-benzoyl-2,3-dihydro-1H-1,5-benzodiazepine ions could lose benzoyl radical to yield 2,4-disubstituted 2,3-dihydro-1H-1,5-benzodiazepine ions, which could further yield other small fragment ions by loss of propene/styrene or small fragments.

Electron impact mass spectral fragmentation of 2a,4-disubstituted 2-chloro/2,2-dichloro-2,2a,3,4-tetrahydro-1H-azeto[2,1-d][1,5]benzothia zepin-1-ones.

The mass spectrometric behaviour of nine 2a,4-disubstituted 2-chloro/2,2-dichloro-2,2a,3,4-tetrahydro-1H-azeto[2,1-d][1,5]b enzothiazepin-1-ones has been studied with the aid of mass-analysed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization. All compounds show a tendency to eliminate a neutral chlorine atom, or a chloroketene, or neutral propene, or styrene or substituted styrene molecule, plus Cl and/or H (or Cl) atom(s), to yield [M-Cl]+ ions, 2,3-dihydro-1,5-benzothiazepine derivative ions, 4,5-dihydro-5H-1,5-benzothiazepin-4-one ions which can further lose CO to give 1,4-benzothiazine ions. Both molecular ions and [M-Cl]+ ions show a tendency to eliminate an ethyl or benzyl/substituted benzyl radical to produce 2,2a-dihydro-1H-azeto[2,1-c][1,4]benzothiazin-1-one ions. The [M-Cl]+ ions could undergo rearrangement to yield 2,2a-dihydro-1H-azeto[2,1-d][1,5]benzothiazepin-1-one ions, 2,2a,3,4-tetrahydro-1H-azeto[1,2-a]quinoline ions or 1,1a,2,3-tetrahydro-azirino[2,1-d][1,5]benzothiazepine ions by loss of an ethane or a benzene/substituted benzene, a SH radical or a CO molecule. The molecular ions could also undergo rearrangement reactions to form other small fragment ions.

The Unimolecular Loss of HF by Simple Inorganic Ions: A Computational Dynamic Reaction Path Study

The metastable dissociations of a series of simple inorganic gaseous ions of general formula (H,F,M)+, including NF3H+ (M = NF2), N2F2H+ (M = FN2), F2NOH2+ (M = FNOH) and FNOH+ (M = NO) have been investigated using classical dynamics in order to rationalise their mass-analysed ion kinetic energy (MIKE) spectra which, from earlier experimental studies, showed the systematic loss of neutral hydrofluoric acid, HF, accompanied by a fairly large release of translational energy ( T). All the simulations were initiated in correspondence of the transition structures involved in the above decompositions and expanded on the related Hartree–Fock potential energy surfaces, calculated “on the fly” during the evolution of the trajectories according to the Dynamic Reaction Path methodology. The initial associated momenta were deduced by performing a standard kinetic analysis of the above dissociations taking into account the specific features of the MIKE experiments. For all the above ions, the resulting computed Ts are in reasonable agreement with the experimental values. In addition, from a more accurate analysis of the trajectories, it could be possible to appreciate qualitatively both the specific role of the internal degrees of freedom of the decomposing ions and the dynamic and energetic influence of the HF–M+ ion–neutral complex located between the dissociation products and the tight transition structure involved in the above decompositions.

Unimolecular metastable decompositions of gem-dimethoxyalkanes (RR'C(OCH(3))(2)) upon electron impact. I. Dimethoxymethane and 1, 1-dimethoxyethane

The unimolecular metastable decompositions of dimethoxymethane (CH(2)(OCH(3))(2), 1) and 1,1-dimethoxyethane (CH(3)CH(OCH(3))(2), 2) upon electron impact have been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry, collision-induced dissociation (CID) spectrometry and D-labeling techniques. Both molecular ions are formed at extremely low abundance. Sequential transfers of a methyl group and a hydrogen atom to an ether oxygen are observed during the decomposition of [M - H](+) ions from 1 and 2. The [M - H](+) ion from 2 also decomposes into the m/z 43 ion by the loss of dimethyl ether. Almost complete hydrogen exchange is observed prior to the loss of CH(4) from the m/z 45 ion ([M - OCH(3)](+)) of 1. The m/z 59 ions ([M - OCH(3)](+)) of 2 decompose competitively into the m/z 31 and 29 ions by the losses of C(2)H(4) and CH(2)O, respectively. The former loss occurs via two different fragmentation pathways. The relative abundances of the ions in the MIKE spectra increase with decreases in the total heat of formation (Sigma DeltaH(f)) of the ion plus the neutral fragment. Copyright 2000 John Wiley & Sons, Ltd.

Mass spectrometric study of ?-aryl-?,�?-diethoxy-?, ?-butenolides

Electron impact mass spectra of eight of the title compounds are reported. Abundant fragment ions were produced under electron impact (EI) conditions and, with one exception, the ($¿hbox¿ArC¿¿equiv$ O) ions were the base peaks. The EI fragmentation mechanisms of two representative compounds were studied with the aid of high-resolution and mass-analyzed ion kinetic energy spectrometry (MIKES) data. The M(+) ions fragment to give both an odd-electron ion and an even-electron fragment ion. Two H-atom rearrangements proceeding via four-membered ring intermediates and three losses of CO through i- and alpha-fragmentations were observed under EI. On comparing fragmentations under EI conditions with those under FAB conditions for two of the compounds, the fragmentation mechanisms were reasonably similar, with additional fragmentations rationalized in terms of the ionization proton being located on the oxygen atom of the beta-ethoxy group.

Metastable Ion Study of Organosilicon Compounds. Part XII. 2-Propoxytrimethylsilane, (CH3)3SiOCH(CH3)2.

Unimolecular decomposition of 2-propoxytrimethylsilane, (CH3)3SiOCH(CH3)2 (Mw: 132, 1), upon electron impact ionization, has been studied by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling technique. The abundance of the molecular ion of 1 is too low to measure the MIKE spectrum. A methyl radical loss occurs from two different groups in 1+.; i.e., from either trimethylsilyl group or 2-propyl group. The ion at m/z 117 formed via the latter reaction decomposes into the m/z 73 ion, trimethylsilyl ion, by the loss of acetaldehyde, whereas the ion via the former reaction decomposes exclusively into the m/z 75 ion, hydroxydimethylsilyl ion, by the loss of propene. The methyl loss from the trimethylsilyl group prevails over that from the 2-propyl one.

Methyl loss from conventional and distonic isomers of C 3H 7N ·+

The unimolecular reactions of a number of C 2H 5N ·+ and C 3H 7N ·+ isomers have been investigated. Mass-analyzed ion kinetic energy (MIKE) and collision-induced dissociation (CID) experiments as well as ab initio calculations show that the N-methylaziridine molecular ion and the distonic radical cation CH 2N(CH 3)CH 2 ·+ can interconvert below the threshold for unimolecular fragmentation, whereas the N-ethylmethylenimine molecular ion is structurally distinct. Likewise, the aziridine molecular ion can undergo ring opening below the threshold for unimolecular fragmentation. Methyl loss from the N-methylaziridine radical cation proceeds via isomerization to the distonic ion, whereas methyl loss from the imine radical cation is a straightforward simple cleavage. The heats of formation of the C 3H 7N ·+ and C 2H 5N ·+ isomers determined with four different composite methods, G2(MP2), G3, CBS-Q, and CBS-RAD, agree reasonably well, and reasonably well with the available experimental data. The distonic ions are found to be the more stable isomers on the C 2H 5N ·+ and C 3H 7N ·+ potential energy surfaces.

Electron impact fragmentation mechanisms of some cyclic esters with helical structures

The electron impact mass spectra of several cyclic esters with helical structures have been studied. Their fragmentation pathways were proposed and confirmed by mass-analyzed ion kinetic energy (MIKE) and high-resolution data. In general, the dominant fragmentation pathways in the spectra of these compounds originate from a alpha-cleavage with loss of a hydrogen or methyl group. The difference between hydrogen and methyl group loss greatly affects the subsequent fragmentations. Although, due to their helicity, these cyclic esters are optically active no stereo-related fragmentation pathway was observed. Copyright 2000 John Wiley & Sons, Ltd.

Metastable Decompositions of gem-Dimethoxyalkanes, (RR'C(OCH3)2), upon Electron Impact. II. 1,1- and 2,2-Dimethoxypropanes.

Unimolecular metastable decompositions of 1,1-dimethoxypropane (CH3CH2CH(OCH3)2, 1) and 2,2-dimethoxypropane ((CH3)2C(OCH3)2, 2) upon electron impact have been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling technique. The molecular ions of 1 and 2, even at 20eV electron energy, are formed with extremely low abundance. Sequential transfers of a methyl group and a hydrogen atom to an ether oxygen are observed during the decomposition of [1-H]+ (CH3CH2C(OCH3)=O+CH3, m/z 103), [2-CH3]+ (CH3C(OCH3)=O+CH3, m/z 89), and [1-C2H5]+ (CH(OCH3)=O+CH3, m/z 75) to protonated dimethyl ether (m/z 47). The relative importance of this process increases with decreasing the length of the alkyl chain, R, in RC(OCH3)=O+CH3 (R=C2H5, CH3, and H). On the other hand, the abundance of the ion (RCO+) generated by the loss of CH3OCH3 from these ions with transfer of only a methyl group increases with increasing R. The relative abundance of the ions in the MIKE spectra is rationalized with energetic consideration, i.e, the total heat of formation (ΣΔHf) of the ion plus the neutral fragment, or proton affinity (PA) of two components in an intermediate proton-bridged complex.

Metastable decompositions of gem-dialkoxyalkanes upon electron impact. III. Diethoxymethane (CH(2)(OCH(2)CH(3))(2))

Unimolecular metastable decomposition of diethoxymethane (CH(2)(OCH(2)CH(3))(2), 1) upon electron impact has been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and theD-labeling technique in conjunction with thermochemistry. The m/z 103 ion ([M - H](+) : CH(OCH(2)CH(3)) = O(+)CH(2)CH(3)) decomposes into the m/z 47 ion (protonated formic acid, CH(OH) = O(+)H) by consecutive losses of two C(2)H(4) molecules via an m/z 75 ion. The resulting product ion at m/z 47 further decomposes into the m/z 29 and 19 ions by losses of H(2)O and CO, respectively, via an 1,3-hydroxyl hydrogen transfer, accompanied by small kinetic energy release (KER) values of 1.3 and 18.8 meV, respectively. When these two elimination reactions are suppressed by a large isotope effect, however, another 1,1-H(2)O elimination with a large KER value (518 meV) is revealed. The m/z 89 ion ([M - CH(3)](+) : CH(2)(OCH(2)CH(3))O(+) = CH(2)) decomposes into the m/z 59 ion (CH(3)CH(2)O(+) = CH(2)) by losing CH(2)O in the metastable time window. The source-generated m/z 59 ion ([M - OCH(2)CH(3)](+) : CH(2) = O(+)CH(2)CH(3)) decomposes into the m/z 41 (CH(2) = CH(+)CH(2)) and m/z 31 (CH(2) = O(+)H) ions by losses of H(2)O and C(2)H(4), respectively, with considerable hydrogen scrambling prior to decomposition. Copyright 2000 John Wiley & Sons, Ltd.

Secondary structure of peptide ions in the gas phase evaluated by MIKE spectrometry. Relevance to native conformations.

AbstractThe recent explosive growth of studies of noncovalent interactions of biomolecules using mass spectrometry (1) has greatly stimulated interest in biomolecular conformations in the absence of solvent (2). The two most widely used approaches to study the three-dimensional structure of biomolecules in the gas phase are hydrogen/deuterium exchange (2,3) and measurement of collisional cross-sections of bio-ions (4). These experiments characterize biomolecular conformations in the gas phase on the global level, with little or no detail at the local level. In this chapter we describe the application of mass-analyzed ion kinetic energy (MIKE) spectrometry (5) to produce local information on the gas phase conformations of peptide ions, i.e., to evaluate secondary structures of metastable peptide ions produced by electrospray ionization. Secondary structure is evaluated based on the intercharge distances for metastable, multiply charged, peptide ions. Unimolecular dissociation of such ions results in significant increases of fragment ion kinetic energy if the fragmentation proceeds via charge-separation channels as in Eq. 1).1" />KeywordsPeak ProfileKinetic Energy ReleaseUnimolecular DissociationMetastable PeakMetastable DecompositionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Electron impact mass spectral fragmentation of 3a,5-disubstituted 1,3-diphenyl-3a,4,5,6-tetra-hydro-3H-1,2,4-triazolo[4,3-a][1,5]benzo-diazepines

The mass spectrometric behaviour of six 3a,5-disubstituted 1, 3-diphenyl-3a,4,5,6-tetrahydro-3H-1,2,4-triazolo[4,3-a][1, 5]benzodiazepines has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization. All compounds show a tendency to eliminate (substituted) styrene molecules, aryl radicals, arylmethyl radicals or phenylnitrene (PhN:). All of the resulting fragment ions, except [M - PhN:](+.), could further undergo a reverse [2 + 3] cycloaddition. The [M - PhN:](+.) ions could further lose styrene derivatives and undergo a ring enlargement rearrangement. The molecular ions also show a tendency to eliminate a phenyl radical, and the [M - Ph](+) ions could eliminate styrene derivatives. The [M - R(1)CH = CH(2)](+.) ions could further lose NH(2) to yield stable tetracyclic 1,3-diphenyl-1,2,4-triazolo[4,3-d]phenanthridine ions, which could further lose benzonitrile, or undergo a reverse [2 + 3] cycloaddition. The molecular ions could also undergo a reverse [2 + 3] cycloaddition to produce N-phenylbenzonitrile imine ions and 2, 4-disubstituted 2,3-dihydro-1H-1,5-benzodiazepine ions, whose further fragmentations were also investigated.

Regioselective synthesis and electron impact mass spectral fragmentation differentiation of 2- or 3-isopropylthio, phenylthiosuccinic acid 4-methyl/isopropyl ethers.

2- and 3-isopropylthio/phenylthiosuccinic acid 4-methyl/isopropyl esters were synthesized regioselectively by Michael additions of isopropylthiol/thiophenol to maleic anhydride, followed by alcoholysis with methanol/isopropanol. Their mass spectrometric behavior has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization. All compounds show a tendency to eliminate an alkoxy group, an alcohol, or an alkoxycarbonyl group from the respective molecular ion. Some molecular ions also show a tendency to eliminate a molecule of water, propene or CO(2). The [M - MeCH = CH(2)](+) ions could further lose H(2)O to form substituted succinic anhydride ions, or lose CO(2) to form 3-substituted propionic acid ions. Both of these ions could further yield other small fragment ions by loss of CO(2)H, CO or other small fragments. It has been found that 2-isopropylthio- and 3-phenylthiosuccinic acid 4-esters show more abundant [M - H(2)O](+) peaks than their 3-isopropylthio and 2-phenylthio isomers in their mass spectra.

Tandem mass spectrometric study of 1,3,5-trinitrobenzene molecular ion: an unusualortho effect involving a hydrogen atom from the aromatic ring

Unimolecular (metastable) and collision-induced dissociation of 1,3, 5-trinitrobenzene molecular ion was studied using linked scans and mass-analyzed ion kinetic energy spectrometry on a hybrid instrument of EBEqQ geometry. An unusual ortho effect leading to the loss of OH radical from the parent molecular ion is observed as a unimolecular dissociation process only in the first-field free region between the ion source and the electric sector, although corresponding dissociation in the ion source is of negligible abundance (<0.1%). This unimolecular process is taken over by other dissociation pathways when the parent ion is collisionally activated, suggesting that this dissociation process occurs only in a very narrow energy window with a rate constant of the order of 2 x 10(5) s(-1). Copyright 2000 John Wiley & Sons, Ltd.