The fragmentation of metastable NH+3 ions and isotopic analogs: an example of tunneling through a rotational barrier

The fragmentation of metastable NH⁺₃ ions and isotopic analogs via the reaction NH⁺₃ → NH⁺₂ + H has been investigated using mass analysed ion kinetic energy spectrometry (MIKES). Kinetic energy release distributions and the metastable intensity were measured as a function of ion source temperature. Both the average kinetic energy release and the metastable intensity increase with ion source temperature. The data are consistent with the metastable reaction arising from tunneling through a rotational barrier. The experimental data are compared with the predictions of a tunneling model.     

The dependence of collision induced fragmentation pattern on the internal energy of the precursor ion

A Method is presented whereby product organic ions formed as the result of fragmentation of metastable ions can be selected on the basis of their internal energies. The method requires requires angular collimation of the beam of reactant ions issuing from the ion source and that the fragmentation of the metastable ions is studied in the first field free region of a reversed geometry double focusing mass spectrometer. The product ions that make up the metastable peak are allowed to fall on the intermediate resolving slit and, by adjusting the magnet current over a small range, ions contained in different regions of the peak can be allowed into the second field free region. It is shown that the position of an ion within the metastable peak correlates with its internal energy, ions near the edges of the peak being the least excited. The ions entering the second field free region can be investigated by collisional activation. This has been done for molecular ions of p-chlorophenol, methylbenzoate, benzaldehyde, m-chlorotoluene and n-butane. The in which the collision induced fragmentation pattern varies with internal energy of the ions is illustrated.     

Studies of consecutive reactions of organic ions in a reversed geometry mass spectrometer

Over thirty consecutive reactions of a type m₁⁺ → m₂⁺ → m₃⁺ have been studied in a variety of organic compounds using a reversed geometry mass spectrometer. Two field free regions allow the separation of the two steps that make up the consecutive reaction sequence. Translational energy release measurements are used in the comparison of m₂⁺ ions formed as a result of a high energy process in the ion source with m₂⁺ ions formed as a product of a metastable decomposition. In some cases the structures of such ions have been found to be different. Examples have also been found where consecutive fragmentations of metastable ions do not occur although, when higher energy ions within the source are studied, the two-step reaction does take place. Furthermore, it has been found that a control over ion internal energy may be achieved by selecting portions of a peak due to the fragmentation of a metastable ion. Unimolecular reactions may then be used to study the reactivity of such ‘energy-selected’ ions; collision induced reactions can be used to study the structure of both the reactive and unreactive energy selected ions.     

Kinetic studies in mass spectrometry—IV. The [M — Cl] reaction in substituted chlorobenzenes and the question of molecular ion isomerization.

The [M]⁺˙ → [M ? Cl]⁺ reaction in a series of m- and p-X substituted chlorobenzenes has been studied, utilizing a simple kinetic approach, comparison of metastable ion relative abundances, and by measurement of ionization and appearance potentials. All evidence obtained is consistent with rearrangement prior to cleavage in the molecular ions, in which substituent position becomes effectively randomized. These findings are related to known hydrogen randomization reactions occurring in either the molecular ion or [M ? Cl] ion of chlorobenzenes. Mechanisms involving carbon scrambling via such species as ionized benzvalenes or prismanes, or ring-opening to isomeric acyclic molecular ions in which hydrogen randomization might occur can be entertained, but mechanisms involving simple hydrogen shifts in the intact benzene ring appear less likely.     

A study of the electron impact induced retro-diels-alder process with selected 5,6,6a, 7,12,12a-hexahydrobenzo[a]anthracenes

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.     

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.     

Intramolecular aromatic substitution reactions in substituted N,N-dimethylthiobenzamide ions

The molecular ions of N,N-dimethylthiobenzamide and its ortho substituted derivatives (substituents CH₃, Cl, Br, I) lose a hydrogen atom and/or the ortho substituent. The mechanism of this process has been studied by measurements of the ionization energies, appearance energies of the product ions m/z 164 and the kinetic energy release during this process. The structure of the product ions m/z 164 and relevant reference ions have been investigated by mass analysed ion kinetic energy spectra, B/E linked scan spectra and collision induced decompositions. The results show clearly the formation of two different kinds of product ions m/z 164 depending on the substituent lost. Type a ions are formed by loss of a H atom or the CH₃ substituent and correspond to protonated 3,4-benzo-N-methylpyroline-2-thione. The formation of these ions occurs by a hydrogen rearrangement followed by an intramolecular substitution via a 5-membered cyclic intermediate and is associated with a large release of kinetic energy. In contrast, the loss of the halogeno substituents to give type b ions probably occurs via a direct displacement reaction by the sulfur atom of the thioamide group giving rise to Gaussian shaped peaks mass analysed ion kinetic energy spectra.     

Kinetic studies in mass spectrometry—VI: The [M ? CH3] reaction in substituted ethylbenzenes

The [M] → [M ? CH₃] reaction in a series of m- and p-X substituted ethylbenzenes has been studied by wide range electron energy kinetics and metastable ion characteristics techniques. By this approach, qualitative measures of activation evergy differences between [XC₆H₄CH₂]⁺ ions derived from m- and p-X isomer substrates have been secured, for both their formation and further decomposition. These evergy differences are consistent with (but do not prove) ion structures that have been suggested by previous work in this area, involving the use of isotope labeling, and ionization and appearance potential methods.     

Consecutive reactions in the vitamin D series. A New insight into the mechanism of vitamin D and provitamin D isomerizations in the mass spectrometer

The individual steps of the consecutive reactions arising from metastable molecular ions, derived from vitamin D₃, vitamin D₂ and their respective provitamins (7-dehydrocholesterol, ergosterol), were examined in different field-free regions of a triple-sector mass spectrometer of B/E/E geometry. The comparison of the translational energy release (T) and the metastable peak shapes corresponding to these reactions, as well as unimolecular and collision-induced dissociation mass-analysed ion kinetic energy spectra, showed that there are probably two structures of the [M – H₂O]⁺˙ and [M – CH₃˙]⁺ ions depending upon whether the respective ions are formed in the ion source through high-energy reactions, or from the fragmentation of metastable molecular ions through slow, low-energy processes which occur in the first field-free region.     

Gas phase substitution under ammonia chemical ionization

Evidence for the isomerization of the [M + NH₄]⁺ adducts of nitrobenzene and triphenylnitrocyclohexenes to the isonitroso derivatives in the first field free region has been obtained. The isomerized adducts undergo successive loss of ?O and ?H or NH₃ to give ions corresponding to the substitution products. Mass analysed ion kinetic energy and collisionally activated dissociation/mass analysed ion kinetic energy data reveal that the adduct decomposes by similar pathways in the second field free region. In the ion source an addition elimination reaction and nucleophilic substitution contribute to the formation of the substituted product ion.     

Etude Structurale de Cations-Radicaux

Mass analysed ion kinetic energy spectrometry has been used to study the structure of radical cations from 26 precursor molecules. Metastable ion characteristics, viz. kinetic energy releases, abundance ratios, and isotope distribution in labelled compounds, show that all the metastable ions are isomerized into a common structure or similar mixture of structures before fragmentation. Collision induced dissociations and collisional ionization to doubly charged ions have been used to study stable ions whose lifetimes are greater than the time-of-flight from the source to the collector, and the results are interpreted on the basis of the occurrence of mixtures of initial and isomerized species.     

Kinetic energy release during CO loss by rearrangement of α-benzoylcarbenium ions

Primary α-benzoylcarbenium ions (a) and tertiary α-benzoyldimethylcarbenium ions (b) are obtained by chemical ionization of ω-hydroxyacetophenone and its dimethyl derivative, respectively. Both α-acylcarbenium ions decompose by a rearrangement reaction and subsequent loss of a CO molecule. The kinetic energy released during this process by metastable ions a and b has been determined under different experimental conditions. The kinetic energy released during the CO elimination from the tertiary α-benzoyldimethylcarbenium ions b is independent of the experimental conditions and gives rise to dish-topped peaks with T₅₀=440±20 meV in the MIKE spectra of b. In contrast to this the kinetic energy releases and the peak-shapes in the MIKE spectra of the primary α-benzoylcarbenium ion a varies with the experimental conditions. The mechanism of this rearrangement reaction is discussed, and it is shown by a MNDO calculation of the heats of formation of the relevant ions that the different characteristics of the kinetic energy release during the fragmentation of primary and tertiary carbenium ions can be attributed to different types of reaction energy profiles.     

Metastable ion and induced stable ion fragmentation of isomeric 5-methyl-3-tolyl-1,2,4-oxadiazoles

The electron ionization fragmentation patterns of 5-methyl-3-(o-, m- and p-tolyl)-1,2,4-oxadiazoles (1a—c) have been examined by metastable ion and high resolution mass spectrometry. The o-tolyl isomer loses CO and C₂H₂O from the metastable molecular ion whereas the m- and p-tolyl isomers lose only CH₃CN thus indicating a strong ortho effect in directing the fragmentation in 1a. Slight differences between o-, m- and p-tolyl isomers in the collisional activation fragmentation of stable [C₇H₆N]⁺ ions suggest that structural differences exist even after a series of extensive rearrangements of the molecular ions. Metastable ion kinetic energy (MIKE) and collisional activation (CA) spectra were very helpful in providing valuable information about many fragments.     

Metastable ions in chemical ionization

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.     

The scope of metastable peak observations

This 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.     

Mass analysis of overlapping ion kinetic energy peaks arising from charge separation in dications

A triple-sector instrument of reversed geometry, BEQQ, has been employed to resolve overlapping ion kinetic energy peaks arising from charge separation of metastable dications. Separation was achieved through mass resolution of the dication in the magnetic sector and of the singly charged product ion of greater mass in the quadrupole mass filter accompanied by energy resolution with the electric sector; the electric sector was scanned to cover the complete range of each charge separation peak. Two overlapping ion kinetic energy peaks arising from charge separation of the diphenylenimine dication and up to four overlapping ion kinetic peaks arising from charge separations of dications arising from benzene-1,3,5-d₃ have been resolved. The kinetic energy releases have been calculated in each case. Enhanced z-discrimination is observed with the final stage of mass analysis.     

Charge localization—I The ion kinetic energy spectra of ortho-, meta- and para-phenylenediamine

The ion kinetic energy spectra of o-,m-and p-phenylendiamine are presented. Decompositions of doubly-charged ions are given for the three isomers. From the width of the peaks, the energy released is measured and the equivalent intercharge distance is calculated. The results are discussed in terms of possible structures of the various ions. Also, metastable transitions which could give rise to peaks corresponding to energies below that of the mainion beam are listed and the results discussed. Significant differences were observed among the energy spectra of the three isomers.     

Kinetic energy release in metastable ions from β-keto esters

The kinetic energy release, T, in metastable ion transitions accompanying the main fragmentation reaction by electron impact has been determined for methyl-, ethyl- and propyl-pivaloyl acetates. The measurements have been made using a MAT 311 mass spectrometer with inverse Nier–Johnson geometry, by high voltage and mass-analysed ion kinetic energy methods. The peak width at 50% height has been used in the calculations. The T values and the shape of the metastable peaks are correlated to reaction types and to the alcohol radical.     

13C labelling study of the interconversion of ethylbenzene, 7-methylcycloheptatriene and p-xylene ions

The isomerization of the molecular ions of ethylbenzene, 7-methylcycloheptatriene and p-xylene by skeletal rearrangement prior to the formation of [C₇H₇]⁺ ions has been investigated by using ¹³C labelled compounds. The results obtained for ions generated by 70 eV and 12 eV electron impact, and fragmenting in the ion source, the 1st field free region and the 2nd field free region, respectively, are compared with those obtained from D labelled derivatives. It is shown that at long reaction times metastable p-xylene ions lose a methyl radical after scrambling of all C atoms and H atoms, while the unstable molecular ions in the ion source react by specific loss of one of the methyl substituents. Both unstable and metastable ethylbenzene ions fragment by two competing mechanisms, one corresponding to specific loss of the terminal methyl group, and the other involving scrambling of all C and H atoms. These results are discussed by use of a dynamic model developed for the mutual interconversion and fragmentation of the molecular ions of ethylbenzene, methylcyclo-heptatriene and p-xylene. The experimental results can be explained by an equilibrium between metastable methylcycloheptatriene ions and p-xylene ions with sufficient energy for skeletal rearrangement, while about 40% of the metastable ethylbenzene ions fragment after rearrangement to methylcycloheptatriene ions and about 60% of the ethylbenzene ions rearrange further to xylene ions before fragmentation. Metastable methylcycloheptatriene ions, mainly lose a methyl group without a skeletal rearrangement, however, because the rearranged ions are kinetically trapped as ‘stable’ xylene ions or ethylbenzene ions.     

Structural correlations with kinetic energy release during loss of chlorine from nineteen selected chlorinated biphenyls

Unimolecular and collision induced decompositions of the major ions of selected polychlorinated biphenyls in the field free region between the magnetic and electric sectors of a reversed Nier-Johnson instrument were studied. Loss of a single chlorine atom is associated with a wide range of kinetic energy releases but still can be correlated by a single reaction mechanism. Loss of two chlorines is interpreted as a rapid sequential loss from isomerized molecular ions for all but one compound. The decompositions which metastable ions undergo are not always the same as those of high energy ions in the source. Correlations between substituent positions and kinetic energy release can be made for the [M]^+·→[M? Cl]⁺ and [M? Cl₂]^+· processes.