Kinetic Energy Release Values Research Articles

Unimolecular reactions of ionized methyl acetate and its hydrogen‐rearranged isomers

AbstractThe proposed formation of [CH3C(OH)OCH2]+˙ (b) as the intermediate in the isomerization [CH2C(OH)OCH3]+˙ (c)⇄b⇄[CH3COOCH3]+˙ (c has been confirmed by preparation of b from CH3COOCH2OCH3. For the three isomers a–c the dominant metastable ion (MI) dissociation, CH3O˙ loss, involves identical kinetic energy release values. The kinetic barriers for a⇄b and b⇄c must be nearly as high as that for CH3O˙ loss from c, as shown by the insensitivity of the mass spectra from collisionally activated dissociation (CAD) of a–c to ionizing electron energy. The H/D scrambling of metastable [CH2C(OD)OCH3]+˙ and c–D3 ions confirm this, indicating that the barrier for a⇄b is slightly below that for b⇄c. Minor low‐energy dissociations include losses of CH4 and CH3OH from a and losses of ˙CHO and CH2O from b. Comparison of MI and CAD spectra of a–c with those from [CH3(OH)CH2O]+˙ (d) and [CH3COCH2OH]+˙ (e) give no evidence for skeletal rearrangement of a–c to d or e.

Predissociation processes in N2+2, O2+2, and NO2+ studied by ion kinetic energy spectroscopy

Experimental values of kinetic energy release T, for predissociation processes of diatomic di-cations N2+2, O2+2, and NO2+ have been obtained using techniques of ion-kinetic-energy spectroscopy on a double-focusing mass spectrometer. Both unimolecular and collision-induced processes were studied. As a result, the present data confirm earlier work but include additional transitions not reported previously. Values of T obtained by entirely different technqiues are shown to be consistent with the present work when differences in energy resolution and experimental time window are considered. Attempts to correlate all available information on these diatomic di-cations (Auger, double-charge-transfer and kinetic-energy loss spectra, as well as appearance energy measurements) are hampered by lack of high-level a priori calculations of the potential curves.

Accurate determination of kinetic energy release values and metastable peak shapes with a commercial varian MAT‐731 mass spectrometer of Mattauch‐Herzog geometry

Accurate determination of kinetic energy release values and metastable peak shapes with a commercial varian MAT‐731 mass spectrometer of Mattauch‐Herzog geometry

Structure and formation of gaseous [C4H5O]+ ions. III—Ions derived from unsaturated ethers

AbstractThe abundant [C4H5O]+ (m/z 69) ions found in the 70 eV mass spectra of a series of acetylenic, allenylic and unsaturated cyclic ethers are shown to have the following structures: HCC\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}HOCH3 (e), H2CC—OCH3 (f), (g) and HCCCH2—O\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}H2 (h). Of these, the cyclic ion g is the most stable: its ion enthalpy (≥ 165 kcal mol−1) is close to that found for the acyclic C3H5\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}O isomers identified in a previous study. Evidence that these four isomeric [C4H5O]+ ions are stable species with lifetimes ≥ 10−5 s is obtained from their collisional activation spectra, the shape of the metastable peaks and the associated kinetic energy release values for the common loss of CO, thermochemical information and analysis of deuterium and carbon‐13 labelled precursor molecules. It is further shown that loss of X⋅ from ethers of the type XCCCH2OCH3 involves isomerization into energy rich allenyl type ions [(X)HCCCHOCH3]+˙ . These ions undergo loss of X⋅ by simple bond cleavage, yielding, e type product ions, when the CX bond strength is relatively low (XI, Br). When XCl and especially CH3 or H, X⋅ is only lost after rearrangement yielding the cyclic product ion g. The mechanism for this cyclization reaction is related to that proposed in a previous study for the ester→ acid isomerization in the molecular ions of the esters of α, β‐unsaturated carboxylic acids.

The use of a mass spectrometer of mattauch‐herzog geometry for the determination of the kinetic energy release in ionic decompositions

AbstractA double focusing mass spectrometer of Mattauch‐Herzog geometry has been modified to allow the evaluation of the kinetic energy (T) released during ionic fragmentations within the first field free region. The data obtained are consistent with, though less accurate than those determined on Nier‐Johnson type mass spectrometers. Kinetic energy release values for the decomposition of metastable [C2H3O]+ ions formed from a series of compounds containing the acetyl group are reported. A ‘degrees‐of‐freedom’ effect on T was not detected.

ChemInform Abstract: UNIMOLECULAR FRAGMENTATION OF ENERGY SELECTED ALLENE MOLECULAR CATIONS

The unimolecular fragmentations of internal energy selected allene molecular ions (1+) have been investigated by photoelectron-photoion coincidence spectroscopy. The energy dependence of the branching ratio between the two most important fragmentation channels is reported, together with values for the kinetic energy release during fragmentation. Loss of an hydrogen atom from 1+ is interpreted to proceed via two paths, yielding different isomeric forms of C3H with different kinetic energy release values.

Unimolecular Fragmentation of Energy Selected Allene Molecular Cations

AbstractThe unimolecular fragmentations of internal energy selected allene molecular ions (1+) have been investigated by photoelectron‐photoion coincidence spectroscopy. The energy dependence of the branching ratio between the two most important fragmentation channels is reported, together with values for the kinetic energy release during fragmentation. Loss of an hydrogen atom from 1+ is interpreted to proceed via two paths, yielding different isomeric forms of C3H with different kinetic energy release values.

Electron impact studies. Part 119. The nature of the transition state for loss of keten from quinone acetate molecular anions. A kinetic isotope effect study

A combination of relative rates of reaction, primary deuterium isotope effects, and kinetic energy release values (for metastable decompositions) suggests that the unimolecular elimination of keten from quinone acetate radical anions proceeds via a rate-determining step which possibly involves hydrogen atom transfer to a phenoxide radical centre. The reaction is ‘endothermic’, i.e. the unsymmetrical transition state resembles the product of the reaction.

Electron impact studies. CXXI. Release of kinetic energy during negative-ion fragmentations. The loss of NO+ from substituted nitrobenzene molecular anions

Kinetic energy release values [T(50%) values] between 0.05 and 0.8 eV have been noted for the unimolecular decomposition M- → (M-NO.)- of the molecular anions of variously substituted nitrobenzenes. Large T values are observed when the substituent is strongly electron-withdrawing. Composite peaks are obtained for m-nitrobenzoic acid and m-nitroacetophenone, showing the occurrence of two distinct processes in these cases.

The structure of the ion: An ion kinetic energy spectrometry study

AbstractThe kinetic energy release (T) values for the loss of CO from ions from five compounds have been obtained and are consistent with at least two different structures for the ion. This result is supported by the T value obtained for the decomposition of the molecular ions.

Electron impact studies. CXV. Release of kinetic energy during negative ion decompositions. C-O bond cleavage reactions

Kinetic energy release values T(50%) for the collision-induced formation of the p-nitrobenzoate and p-nitrophenoxide anions from the molecular anions of p-O2NC6H4COOC6H4R-p and p-O2NC6H4- OC6H4R-p respectively are small and relatively insensitive to the nature of the substituent R.T (50%) values for the elimination of R? (R = alkyl) from O2NC6H4OR molecular anions range from 0.4 to 0.01 eV, and are dependent upon the nature of R and upon its number of active oscillators.

Electron impact studies. CVII—Negative ion mass spectra of the oxime group. Monoximes derived from α‐dicarbonyl systems

AbstractSpecific losses of water are observed from the molecular anions of monoximes of α‐diketones. Labelling studies, kinetic energy release values and the +E spectra of magnified image ions have been used to aid in the elucidation of the fragmentation pathways. It is proposed that the majority of magnified image ions have α‐keto nitrile structures.

A photoion–photoelectron coincidence (PIPECO) study of fragmentation rates and kinetic energy release in energy selected metastable ions

The absolute fragmentation rates and kinetic energy releases associated with the unimolecular dissociation reactions of energy selected halo-hydrocarbon ions have been investigated by photoion–photoelectron coincidence (PIPECO) spectroscopy in which nominally zero kinetic energy electrons are detected in coincidence with their corresponding fragment ions. The reactions investigated were HCl loss from C2H5Cl+ and ClCH2CH2Cl+, Cl loss from C3H3Cl+ (propargyl chloride) and Br loss from C3H3Br+ (propargyl bromide). Although the kinetic energy released in the HCl loss from C2H5Cl+ reaction is small, the other three reactions have large energy release values associated with them. We found that X loss from C3H3X+ proceeds via two paths yielding different isomeric forms of C3H3+ with markedly different kinetic energy release values. The most unusual feature in these complex dissociation reactions is that the rate of fragmentation cannot be described by a single exponential decay function but rather must be expressed as the sum of at least two exponentials. Possible explanations for two-component decay rates and their implications for the statistical theory of unimolecular decay are discussed.

Mass and Energy Distributions in the Spontaneous Fission of Some Heavy Isotopes

A back-to-back semiconductor counter system was used to study the energy and mass distributions of the fission fragments in the spontaneous fission of the isotopes, ${\mathrm{Fm}}^{254}$, ${\mathrm{E}}^{253}$, ${\mathrm{Cf}}^{254}$, and ${\mathrm{Cm}}^{248}$. The results are compared to the fission fragments produced by the spontaneous fission of ${\mathrm{Cf}}^{252}$, whose fission properties are well known. All distributions (including those of the odd-mass isotope ${\mathrm{E}}^{253}$) are rather similar, but not identical with the standard ${\mathrm{Cf}}^{252}$. The mean prompt kinetic energy of the fragments increases with $Z$, and this trend is compared with the results of other related experiments. The asymmetry of the mass distributions shows only small differences between the isotopes studied here. Some theoretical aspects of the asymmetries are discussed. The variance (widths) of nearly all the distributions increases with $Z$ and seems to increase with $A$. A new semiempirical correlation between the variances and $\frac{{Z}^{2}}{A}$ is proposed and interpreted on a qualitative basis. The effects of neutron emission on the results are briefly discussed and the values of the mean total kinetic energy release are corrected for the effect of neutron emission.