Metastable Ion Peak Research Articles

The gas phase Smiles rearrangement of anions PhO(CH2)nO− (n = 2–4). A joint theoretical and experimental approach

A combination of experimental data [using (18)O labelling fragmentation data together with metastable ion studies in a reverse sector mass spectrometer (from a previous study)] and ab initio reaction coordinate studies at the CCSD(T)/6-31++G(d,p)//B3LYP/6-31++G(d,p) level of theory, have provided the following data concerning the formation of PhO(-) in the gas-phase from energized systems PhO(CH(2))(n)O(-) (n = 2-4). All DeltaG values were calculated at 298 K. (1) PhO(CH(2))(2)O(-) effects an ipso Smiles rearrangement (DeltaG(r) = +35 kJ mol(-1); barrier to transition state DeltaG(#) = +40 kJ mol(-1)) equilibrating the two oxygen atoms. The Smiles intermediate reverts to PhO(CH(2))(2)O(-) which then undergoes an S(N)i reaction to form PhO(-) and ethylene oxide (DeltaG(r) = -24 kJ mol(-1); DeltaG(#) = +54 kJ mol(-1)). (2) The formation of PhO(-) from energized PhO(CH(2))(3)O(-) is more complex. Some 85% of the PhO(-) formed originates via a Smiles intermediate (DeltaG(r) = +52 kJ mol(-1); DeltaG(#) = +61 kJ mol(-1)). This species reconverts to PhO(CH(2))(3)O(-) which then fragments to PhO(-) by two competing processes, namely, (a) an S(N)i process yielding PhO(-) and trimethylene oxide (DeltaG(r) = -27 kJ mol(-1); DeltaG(#) = +69 kJ mol(-1)), and (b) a dissociation process giving PhO(-), ethylene and formaldehyde (DeltaG(r) = -65 kJ mol(-1); DeltaG(#) = +69 kJ mol(-1)). The other fifteen percent of PhO(-) is formed prior to formation of the Smiles intermediate, occurring directly by the S(N)i and dissociation processes outlined above. The operation of two fragmentation pathways is supported by the presence of a composite metastable ion peak. (3) Energized PhO(CH(2))(4)O(-) fragments exclusively by an S(N)i process to form PhO(-) and tetrahydrofuran (DeltaG(r) = -101 kJ mol(-1); DeltaG(#) = +53 kJ mol(-1)). The Smiles ipso cyclization (DeltaG(r) = +64 kJ mol(-1); DeltaG(#) = +74 kJ mol(-1)) is not detected in this system.

The unexpected decarbonylation of ionized 2,3-pentanedione and related diketones

The fragmentation of the 2,3-pentanedione radical cation gives rise to an unexpected composite metastable ion peak, m/ z 72, resulting from the isobaric losses of CO and C 2H 4. These two fragmentation channels are energetically competitive (i.e., the transition states have similar energies). The two processes yield [CH 3C(O)CH 2CH 3] + and [CH 3C(OH) C O] +, respectively. The latter new ion, which is produced by a McLafferty rearrangement, has Δ f H° = 604 kJ/mol, obtained from G3 calculations. The four competing processes for metastable [CH 3COCOCH 2CH 3] +, the (intense) losses of CH 3CO , CH 3CH 2CO and the weak losses of C 2H 4 and CO and their transition states were placed on a potential energy surface computed at the G3 level of theory. The homologous ionized diketone 2,3-butanedione also displays the decarbonylation channel and 3,4-hexanedione loses CO and C 2H 4.

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.

Determination of kinetic energy release distributions by metastable peak shape analysis in an ion trap reflectron time-of-flight instrument

A procedure for extraction of the kinetic energy release distribution from the metastable fragment ion peak shape in an ion trap reflectron time-of-flight mass spectrometer is presented. Peak shapes for a set of discrete values of kinetic energy releases are calculated and their contributions to the measured peak shape determined. Information about the temporal evolution of each component is required in order to get the correct kinetic energy release distribution. This is obtained by carrying out the measurements at different times following ion formation. The method is demonstrated for the metastable decay of aniline ions formed by multiphoton ionization.

Metastable dissociation dynamics of molecular cluster ions

Metastable uni-cluster dissociation for several hydrogen-bonded and van der Waals cluster ions are observed via resonance-enhanced two-photon ionization reflectron time-of-flight (TOF) mass spectrometry. All of the cluster ions studied show evaporation of a single molecule from the respective parent cluster ions as dominant metastable decay processes. Furthermore, the averaged metastable evaporation rate constants (kevap) of these cluster ions in a fixed time domain of 0.2–50 µs are obtained by analyzing the relative intensity of metastable ion peaks due to evaporation in the acceleration and the field-free drift regions of the TOF mass spectrometer. An intensity anomaly in some of the observed metastable ion peaks, indicative of magic number stability of the cluster ion, is also presented.

Translational energy release and stereochemistry of steroids. 14. Epimeric dihydroxy steroids of the androstane series

The stereochemistry of dihydroxy steroids, both the mode of the A/B ring junctions and the configuration of OH groups, may be determined from translational energy (T50%) measurements for the loss of a CH.3 radical, from the ratios of metastable-ion peak heights to those of the main beam (determined for the dehydration reactions), and by comparing unimolecular and collision-induced, mass-analysed ion kinetic energy spectra of the new main beam of [M-H2O]+ ions (i.e. those formed via dehydration of metastable molecular ions of epimeric hydroxy steroids in the first field-free region of a double-focusing mass spectrometer.

Tandem mass spectrometry by time-resolved Ion momentum spectrometry

Superposition of ion momentum analysis by magnetic field scanning with time-of-flight analysis through ion source pulsing and time-resolved detection affords a unique tandem mass spectrometric method, developed by Enke and coworkers, called time-resolved ion momentum spectrometry. This method has been applied by our group to study unimolecular fragmentations, metastable ion peak shapes and kinetic energy release distributions for long-lived (up to ˜ 1 ms) ions. This has been achieved through ion trapping in an electron space charge. Special attention was given to unimolecular fragmentations which are slow at threshold energies and demonstrate large ‘kinetic shifts’, as are expected for large polyatomic molecules of biomedical importance. The time dependence of bimodal distributions was studied for formaldehyde loss from anisole. Extensions of the method to collisional activation tandem mass spectrometry as well as experimental and computer handling of the data have been achieved.

Multiphoton metastable ion spectra and ion dissociation kinetics: Analysis of the decay channels of the aniline cation with a reflectron time-of-flight instrument

Multiphoton mass spectrometry when carried out in conjunction with a reflecting field time-of-flight mass spectrometer is shown to yield rich new metastable ion spectra of polyatomic molecules, here aniline. The reflecting field acting also as an energy analyzer is shown to be particularly suitable for sensitive metastable ion detection without interference from stable fragment ions. With this technique several new metastable ion decay channels of the aniline cation have been observed. On the basis of the measured metastable ion spectrum a reasonable fragmentation pathway for the aniline cation C6H7N+ after multiphoton excitation is presented. The prominent intensity of the metastable ion peak C5H+6 points to a narrow energy distribution within the aniline cation typical for a ladder switching multiphoton excitation mechanism.

An investigation of fragmentation mechanism by metastable peak shapes

From the shape of metastable ion peak, the mechanism of the fragmentation reaction C 7H 7I +·(m/z 218) → C 7H 7 +(m/z 91) + I· for m- and p-iodotoluenes has been estimated. Further, the fragmentation mechanism of several substituted nitrobenzenes has been studied by measuring the kinetic energy release.

An investigation of structure of intermediate ions by kinetic energy release. C7H7+ ions from several derivatives of toluene.

The amount of kinetic energy release accompanying the metastable ion reaction C7H7+ (m/z 91) → C5H5+(m/z 65) + C2H2 has been determined for 24 derivatives of toluene by scanning electric sector voltage of a double focusing mass spectrometer.Except for the case of benzyl methyl ether and benzyl chloride, the determined values were nearly equal to those for the C7H7+ ions from toluene and isopropyl o-toluate, which are considered to produce the C7H7+ ion of tropylium ion structure.From the profile of metastable ion peak, the mechanism of the fragmentation reaction C7H7I+· (m/z 218) → C7H7+ (m/z 91) + I · for m- and p-iodotoluenes has been estimated.

Mechanisms of water elimination and other degradations of 2‐hydroxy‐5‐ketobornane cations in the gas phase

AbstractThe mechanisms of water elimination in 2‐hydroxy‐5‐ketobornanes have been investigated and compared with those in borneol and isoborneol in order to ascertain whether and how such processes are affected by incorporation of a remote second functional group of lower ionization energy. Analysis of metastable ion peak shapes and specific labelling with 2H and 18O are employed as methods for differentiating between individual reaction paths. In addition, those degradation reactions which give rise to the most prominent fragments and pattern differences in the mass spectra of the epimeric keto alcohols were scrutinized. The elimination of water is shown to exhibit entirely different features due to the presence of the carbonyl group. The other most important degradation reactions all proceed by hydrogen transfer from the hydroxyl group onto the charged product and concomitant loss of the hydroxyl oxygen in a neutral entity. The results are interpreted in terms of initial charge radical localization on the carbonyl group initiating the relevant chemical reactions. Peak shape analysis proves a very valuable, and at times the only useful way to distinguish individual reactions.

Metastable ion analysis by automatic repetitive scanning

An offset amplifier has been designed which allows detection of metastable transitions (in a Hitachi RMU-7E Double Focusing Mass Spectrometer) by the Acceleration Voltage Scan using signal averaging to give peak enhancement. Using a second mode of operation the amplifier allows Ion Kinetic Energy Spectra to be collected through repetitive scanning. The offset amplifier is extremely versatile and is readily adaptable to other makes of mass spectrometers. A set of composite metastable ion peaks obtained by the Acceleration Voltage Scan are presented to demonstrate the efficiency of the unit and for comparison with previously published data.

Metastable ion studies. IV—thermochemistry and ion structures among fragmenting [C3H6]+· ions

AbstractMetastable 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 equation image 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.

Mass Spectra of α-Substituted o-Methoxybenzyl Alcohols

The mass spectra of various α-alkyl and α, α-dialkyl-o-methoxybenzyl alcohols were measured, and their fragmentation pathways were discussed. With the aid of metastable ion peaks and of the spectra of some deuterated derivatives, the mechanism of the fragmentation was deduced and compared with those of simple benzyl alcohols and anisole. Considerable effects of the α-substituents on the fragmentation pathway is observed and interpreted by taking into accounts the easiness of the Cα-Calkyl bond fission.

Thermochemistry and metastable energy release for the dissociation [C3H7]+ → [C3H5]+ + H2

AbstractThe reverse activation energy, Erev, for the dissociation [C3H7]+ → [C3H5]+ + H2 has been determined as 0.24 ± 0.06 eV from measurements of the AP of [C3H5]+ produced by electron‐impact from thermally generated sec‐C3H7 radicals. The energy release observed in the corresponding metastable dissociation is 0.21 ± 0.01 eV, indicating that virtually all of Erev is partitioned as translational Kinetic energy of the fragmentation products. The metastable ion peak shape is also discussed with respect to the evaluation of the energy release.

Degenerate skeletal rearrangement and energy partitioning in the fragmentation of the norbornyl cation

AbstractThe electron‐impact‐induced fragmentation of a 13C labelled exo‐2‐norbornyl chloride has been studied. When the norbornyl cation, [C7H11]+, [M – Cl], dissociates by elimination of C2H4, carbon atoms are randomly lost showing that extensive skeletal rearrangement (in addition to the complete hydrogen atom scrambling reported earlier) has taken place in the norbornyl cation prior to dissociation.The metastable ion peak associated with the above fragmentation consists of superimposed gaussian and flat‐top components; it is proposed that these correspond to the formation of isomeric [C5H7]+ daughter ions whose heats of formation differ by ∼0.4 eV.

Mass spectra of some alkynylmercury compounds

The fragmentation, under electron impact, of eleven halogenated, aliphatic, and aromatic alkynylmercury compounds of general structure (XCC)2Hg has been examined. The dominant fragmentation of organomercury compounds, namely the fission of the carbon–mercury bond, has been observed. The organic fragments formed either by direct fission or by extrusion of mercury fragmented in ways familiar from metal-free systems; losses of C2H4, C2H2, and H2 were commonly seen, the proposed fragmentation pathways being supported by the observation of numerous metastable ions. Comparison of the relative metastable ion intensities suggests that the mercury extrusion products from the compounds with X = Et, Pr and But decompose through the same structures as are formed from the higher alkylbenzenes. Fragmentation of the organic portion competes effectively with mercury–carbon bond cleavage in the mass spectra of the compounds with X = But and p-MeOC6H4. The loss of HgCl2 from bis-(p-chlorophenylethynyl)mercury is also observed. In the mass spectrum of bis-(p-methoxyphenyl)mercury and bis-(4-phenylbutadiynyl)mercury metastable ion peaks were found corresponding to fragmentations from doubly charged parent ions to doubly charged daughter ions.

Mass-spectral Fragmentation of 3-(5-Nitro-2-furyl)-1,2,4-triazoles

Abstract The mass spectra of 3-(5-nitro-2-furyl)- (I), 5-methyl-3-(5-nitro-2-furyl)- (II), 5-ethyl-3-(5-nitro-2-furyl)-(III), and 5-trideuteriomethyl-3-(5-nitro-2-furyl)-l,2,4-triazole (IV) and their 1-acetyl, 1-trideuterioacetyl, and 1-propionyl derivatives were examined. The mother compounds, I-IV, gave the most prominent molecular ions and showed the same fragmentation pattern. The molecular ions undergo two modes of cleavages; one is the elimination of NO (1) and CO (2), followed by the loss of C2H2 (3), thus giving the M−84 ions which include the 5-substituents of the triazole ring, while the other is the loss of NO2–CO, which directly gives rise to the fragment M−74 ions. The 1-acyl derivatives gave extremely weak molecular ion peaks. In the first step of the fission of the molecular ions, the elimination of an acyl group occurred, with the rearrangement of a hydrogen atom, thus giving rise to prominent ions. These ions correspond to the molecular ions of the mother compounds, I–IV, and are expected to show a similar fragmentation pathway. The fragmentation of these compounds was also considered to occur in view of the existence of meta-stable ion peaks. The meta-stable peaks appearing at the transition of the loss of NO are flat-topped, releasing kinetic energy of 0.78±0.04 eV.

Field ionization mass spectrometry of long chain fatty methyl esters

AbstractMass spectrometry is particularly useful for identifying lipid materials. One primary factor in the interpretation of mass spectra is the recording of the molecular ion peak giving the molecular weight of the compound. Regrettably many compounds, including hydroxy compounds, do not give significant molecular ion peaks; consequently their identification is difficult. A mass spectrometer equipped with a field ionization source produces a greatly different mass spectrum consisting almost entirely of the molecular ion peak. This new source was used to measure the mass spectra of methyl esters of saturated, unsaturated and hydroxy fatty acids. Saturated esters gave the molecular ion peak almost exclusively; unsaturated esters yielded molecular plus metastable ion peaks; whereas the hydroxy esters had molecular, M‐18, meaastable and fragment ion peaks.

Charge Migration through Sigma Bonds in a Fragment Ion from 5,5′-Divaleryl-2,2′-spirobiindan under Electron Impact

Abstract In order to examine the nature of charge and radical electron migration in mass spectral fragment ions, 5,5′-di-n-valeryl-2,2′-spirobiindan (3) was prepared and its mass spectrum was measured. The successive loss of two propylene molecules from the molecular ion to give peaks at m⁄e 346 (b) and 304 (c) was clearly indicated by observing appropriate metastable ion peaks. As the radical electron is one of the prerequisites for these fragmentations to occur, the results demonstrate that the charge and radical electron can migrate through sigma bonds in fragment ion b, since the two n-valerophenyl groups cannot be brought into close proximity even with C–C bond fission of the spiro ring. While the strongest peak at m⁄e 331 (a) is ascribable to the ordinary α-fission of the carbonyl group in 3, a moderately intense peak at m⁄e 289 (d) can adequately be explicable by the similar α-fission of the carbonyl group in ion b, the process of which again involves radical electron migration prior to fragmentation.