Energy Release Data Research Articles

Multiple ionization, charge separation and charge stripping reactions involving polycyclic aromatic compounds

AbstractThe 70 eV electron ionization mass spectra of polycyclic aromatic compounds are characterized by the presence of relatively stable multiply charged molecular ions [M]n+ (n=2–4). When generated from the compounds benzene, napthalene, anthracene, phenanthrene, 2,3‐benzanthracene, 1,2‐benzanthracene, chrysene, 9,10‐benzophenanthrene and pyrene, the relative abundances of the multiply charged ions increase dramatically with the number of rings. These compounds form multiply charged molecular ions (n=2, 3) which undergo unimolecular decompositions indicative of considerable ionic rearrangement. The main charge separation processes observed here [M]2+→m1++m2+, [M]3+˙→m3++m→+m42+) involve, in almost every case, one or more of the products [CH3]+, [C2H3]+˙ and [C3H3]+. This suggests the existence of preferred structures amongst the metastable parent ions. Information on the relative importance of the various fragmentation pathways is presented here along with translational energy release data. Some tentative structural information about the metastable ions has been inferred from the translational energy release on the assumption that the released energy is due primarily to coulombic repulsion within the transition state structure. For the triply charged ions these interpretations have necessitated the use of a coulombic repulsion model which takes account of an extra charge. Vertical ionization energies for the process [M]n++G→[M](n+1)+G+e− (charge stripping) have also been determined where possible for n=1 and 2 and the results from these experiments allow the derivation of simple empirical equations which relate successive ionization energies for the formation of [M]2+ and [M]3+˙ to the appearance energy of [M]+˙.

Mass spectrometric investigation of phenylthiovinyl chlorides

AbstractThree isomeric phenylthiovinyl chlorides (β‐, E and Z, and α‐) have been examined under EI and FI conditions with particular attention paid to the fragmentation leading to [C8H7S]+ ions via loss of Cl˙. Kinetic energy release data, MIKE and CAD MIKE spectra obtained with these precursors, as well as with model compounds, suggest the thioacylium ion PhCH2CS+ as the predominant structure observed for the [C8H7S]+ species in all cases examined. A comparison is made with the solution behaviour of incipient β‐thiovinyl carbenium ions, which are known to rearrange to the S‐bridged thiirenium structure.

Further examples of skeletal rearrangements of the Wagner‐Meerwein type in chemical ionization mass spectrometry: The case of [C6H9]+ ions

AbstractThe [C6H9]+ ions produced either via unimolecular H2O loss from 13 [C6H11O]+ precursors or direct protonation of 1,3‐ and 1,4‐cyclohexadiene have identical collisional activation mass spectra. The kinetic energy release data for the process [C6H11O]+→[C6H9]++H2O are also very similar (on average T0.5=24 meV) irrespective of the constitution of the precursor. From the proton affinities of 1,3‐cyclohexadiene (PA=837.2 kJ mol−1) using ion cyclotron resonance mass spectrometry the heat of formation of the [C6H9]+ ion is determined to 804.6 kJ mol−1. This value taken together with the results of molecular orbital calculations (MNDO) and the structure indicative losses of CH3. and C2H4 upon collisional activation suggest that the [C6H9]+ ion has the structure of the 1‐methylcyclopentenylium ion f and not that of the slightly less stable cyclohexenylium ion g. The generator of an easily interconverting system of isomeric [C6H9]+ ions is unlikely to be due to the high barrier separating the various isomers.

The elimination of carbon monoxide from molecular ions of phenol and other [C6H6O]+˙ isomers

AbstractPhenol and five acyclic isomeric compounds have been investigated using electron impact and field ionization techniques, mass analysed ion kinetic energy spectrometry and collision induced dissociation mass analysed ion kinetic energy spectrometry. The kinetic energy release data corresponding to the elimination of carbon monoxide from the molecular ions show that at least two structures of the reactant ion are involved. The electron impact and field ionization collision induced dissociation mass analysed ion kinetic energy spectra indicate that under electron impact conditions, the phenol ion partially isomerizes to another ion structure. An isomerization of about 40% to the structure of cis‐2‐hexen‐4‐yn‐1‐al is in good accordance with the spectral data.

Kinetic energy release in unimolecular reactions of the type CH 3XH + → CH 2X + + H 2

Kinetic energy release data on metastable vicinyl H 2 loss from CH 3XH + is reported for X = NH 2, OH, F, SH and Cl. The energy release - E t is a strong function of X and varies from 1.75 eV for X = NH 2 to 0.12 eV for F. Metastable loss of HX from CH 3XH + is observed only for X = OH and F. In both cases - E t is extremely small (≈0.003 meV).

Gas phase equilibration of triphenyl cyclohexenyl and diphenyl cyclopentenyl benzyl cations

AbstractEvidence for the gas phase equilibration of triphenyl cyclohexenyl and diphenyl cyclopentenyl benzyl cations directly or through common intermediates has been obtained from chemical ionization/collisional activation and kinetic energy release data.

Gaseous alkyne ions

AbstractUsing the relative abundance of metastable ions, collisional activation spectra, field ionization kinetic measurements, isotopic labelling, appearance energy and kinetic energy release data, it is shown that linear alkyne radical cations with more than six carbon atoms do not isomerize to equilibrating structures prior to decomposition. At the shortest ion lifetimes the molecular ions of linear alkynes decompose mainly by simple β‐bond fission which allows an unequivocal localization of the triple bond. At medium ion lifetimes fragmentation occurs predominantly via a McLafferty rearrangement, while at long ion lifetimes competing alkyl losses prevail. These alkyl losses occur via cyclic intermediates leading to thermochemically stable cycloalkenyl ions. All these reactions occur with a high specificity with respect to the carbon and hydrogen atoms involved and are preceded by little or no hydrogen exchange reactions.

Kinetic energies released on dissociation of photoexcited aromatic ions of high translational energy

An experiment is described in which the kinetic energy released upon photodissociation of organic ions in a modified, reverse-geometry mass spectrometer is measured. Linear dependence of the kinetic energy released on photon energy is observed for most of the ions studied, as predicted by statistical theories. The results indicate that only a fraction of the available degrees of freedom are involved. Non-linear dependence is observed for some of the ions studied and the significance of this is discussed. The kinetic energy release data for isomeric structures exhibit variation in their dependence upon photon energy, and it is suggested that the technique can be used for structural characterisation.

Deconvolution of the effect of apparatus broadening and the kinetic energy spread of the parent ion beam from metastable peak shapes

A procedure is described, based on fast Fourier transformations, which allows the effect of the velocity spread of the parent ion beam to be deconvoluted out of an experimentally obtained metastable peak. The fast Fourier deconvolution procedure is described and details concerning the computer programming involved are given. Examples of the techniques are shown where considerable enhancement of fine structure in metastable peaks is demonstrated. Also it is shown that in the case of narrow metastable peaks, this technique provides the only precise means of obtaining kinetic energy release data.

The scope of metastable peak observations

AbstractThis short review focuses attention upon the present status of metastable ion studies with emphasis upon the relationship between metastable peak shapes, ion structur and fragmentation mechanisms. Some recommendations are made concerning nomenclature and the reporting of observations on Gaussian‐type metastable peaks. Experimental methods for recording relative abundances of metastable peaks are critically appraised. The relationship between metastable ion phenomena and isomerization of gaseous ions is reviewed with particular attention drawn to the effect of rate‐determining isomerizations. The shapes of Gaussian‐type metastable peaks are discussed in some detail and selected examples from recent studies are used to show that such peaks may, by appropriate experiments, be separated into two Gaussian‐type components thus revealing new features of the fragmentation reaction. The magnitude and significance of released kinetic energies, T, are considered and it is stated that few conclusions can be drawn from the evaluation of T alone; the importance of accurate thermochemical data as an aid to understanding and interpreting kinetic energy release data is emphasized. Other topics discussed include composite metastable peaks, metastable peaks produced in chemical ionization and field ionization and the partitioning of internal energy of the fragmenting ion into translational degrees of freedom of the products, for reactions with and without a reverse energy barrier.

Kinetic energy release and ion structure: [C6H6O]+˙

AbstractIt is shown that, contrary to a recent report, the kinetic energy release for CO loss from phenol does not decrease as the ion accelerating voltage is decreased. In fact, an increase is observed which is attributed, at least in part, to greater discrimination against off‐axis ions at low voltage. The kinetic energy release data do not demand the tautomerization of ionized phenol before decarbonylation.

Vergleichende Untersuchung von Reaktivitäten und Ionenstrukturen beim monomolekularen Abbau isomerer Hydroxyzimtsäuremethylester unter Elektronenbeschuss

Comparative investigation of reactivities and ion structures upon electron impact induced degradations of the isomeric hydroxy cinnamic acid methyl estersBy analysis of energy release data in metastable peaks it is shown that m‐ and p‐hydroxy cinnamic acid methylesters retain specific structural features upon degradation under electron impact, although their spectra are very similar and the shapes of the metastable peaks due to the initial methoxy cleavage are identical. A pronounced charge radical localization in the molecular ion is indicated. The o‐isomer is found to yield a coumarin type structure by elimination of methanol, whose further degradation by decarbonylation involves at least three different mechanisms. The resulting ion structures are different from benzofuran.

The benzoyl ion. Thermochemistry and kinetic energy release

AbstractThe thermochemistry of benzoyl ion formation from a variety of sources has been examined by using the measured kinetic energy release for metastable ions to estimate the excess energy of the activated complex. A correlation is observed between this estimated excess energy and literature values of the heat of formation of the benzoyl ion. From this relationship and the observed correlation between the uncorrected heat of formation and the difference between the appearance potential of [C6H5CO]+ and the ionization potential of the parent compound, the large range of reported values for ΔHf[C6H5CO]+ is seen to be due, at least in part, to variation in the kinetic shift with the critical energy of the reaction. With the exception of the ion generated from trifluoroacetophenone and possibly that from benzaldehyde, the fragmenting [C7H5O]+ ions are shown, from kinetic energy release data, to be structurally identical. The approach adopted here may have general merit in improving or testing the accuracy of thermochemical data based on appearance potential measurements.