Abstract:Laser-ion beam photodissociation is used to study the C4H+6 radical cation formed by electron impact (70 eV nominal energy) ionization of 1,3-butadiene. The kinetic energy release value obtained from C4H+6→C3H+3+CH3 does not agree with the distribution reported by Farrar [J. Chem. Phys. 77, 263 (1982)]. The photodissociation studies of C4H+6 suggest the existence of a long-lived (>0.05×10−6 s) photoexcited C4H+6 ion. Also, results of photodissociation studies of C4H+6 trapped in the electron beam space … Show more
“…The methyl loss reaction of C 4 H 6 + ions has been extensively investigated. − Werner and Baer 1 have shown that a number of isomers (1,3- and 1,2-butadiene, 1- and 2-butyne, and cyclobutene) dissociate with identical rate constants, thus demonstrating that these ions rapidly isomerize to the lowest energy 1,3-butadiene ion prior to dissociation. The barrier for isomerization from the butyne ions to 1,3-butadiene ion was found to lie 0.5 eV below the dissociation limit …”
The kinetics of the metastable dissociation of energy-selected 1,3-butadiene and 3-methylcyclopropene cations to form C 3 H 3 + (cyclopropenyl cation) and CH 3 have been investigated by threshold photoelectron photoion coincidence (TPEPICO) time-of-flight mass spectrometry, ab initio molecular orbital calculations, and RRKM statistical theory. Both the experimental results and the molecular orbital calculations indicate that 1,3-butadiene ions lose CH 3 by first isomerizing to a higher energy structure (3-methylcyclopropene cation) which can rapidly lose CH 3 or isomerize back to the 1,3-butadiene cation. A complete kinetic model of the twowell potential and its three rate constants is necessary to account for the measured dissociation rates as a function of the ion internal energy. At low energies, the dissociation rate is limited by the bond cleavage step, while at higher energies, the bottleneck shifts to the lower energy isomerization step. A fit of calculated RRKM rate constants to the experimental data yields a 0 K isomerization barrier (relative to the 1,3-butadiene ion) of 2.02 ( 0.03 eV and an activation entropy at 600 K of -4 cal K -1 mol -1 . The entropy of activation for the dissociation step from 3-methylcyclopropene was found to be +7 cal K -1 mol -1 . 3-Methylcyclopropene was found to have an adiabatic ionization energy of 9.28 ( 0.05 eV and a neutral heat of formation of 273( 2 kJ mol -1 at 0 K (257 ( 2 kJ mol -1 at 298 K). This is the first experimental determination of this value.
“…The methyl loss reaction of C 4 H 6 + ions has been extensively investigated. − Werner and Baer 1 have shown that a number of isomers (1,3- and 1,2-butadiene, 1- and 2-butyne, and cyclobutene) dissociate with identical rate constants, thus demonstrating that these ions rapidly isomerize to the lowest energy 1,3-butadiene ion prior to dissociation. The barrier for isomerization from the butyne ions to 1,3-butadiene ion was found to lie 0.5 eV below the dissociation limit …”
The kinetics of the metastable dissociation of energy-selected 1,3-butadiene and 3-methylcyclopropene cations to form C 3 H 3 + (cyclopropenyl cation) and CH 3 have been investigated by threshold photoelectron photoion coincidence (TPEPICO) time-of-flight mass spectrometry, ab initio molecular orbital calculations, and RRKM statistical theory. Both the experimental results and the molecular orbital calculations indicate that 1,3-butadiene ions lose CH 3 by first isomerizing to a higher energy structure (3-methylcyclopropene cation) which can rapidly lose CH 3 or isomerize back to the 1,3-butadiene cation. A complete kinetic model of the twowell potential and its three rate constants is necessary to account for the measured dissociation rates as a function of the ion internal energy. At low energies, the dissociation rate is limited by the bond cleavage step, while at higher energies, the bottleneck shifts to the lower energy isomerization step. A fit of calculated RRKM rate constants to the experimental data yields a 0 K isomerization barrier (relative to the 1,3-butadiene ion) of 2.02 ( 0.03 eV and an activation entropy at 600 K of -4 cal K -1 mol -1 . The entropy of activation for the dissociation step from 3-methylcyclopropene was found to be +7 cal K -1 mol -1 . 3-Methylcyclopropene was found to have an adiabatic ionization energy of 9.28 ( 0.05 eV and a neutral heat of formation of 273( 2 kJ mol -1 at 0 K (257 ( 2 kJ mol -1 at 298 K). This is the first experimental determination of this value.
“…The absence of photodissociation products from BTY and DA, reported by Preuninger and Farrar, was explained later by Bunn and Baer , and by Russell et al , …”
Section: Introductionmentioning
confidence: 70%
“…The absence of photodissociation products from BTY and DA, reported by Preuninger and Farrar, 11 was explained later by Bunn and Baer. Dissociation of the BD radical cation has also been investigated by Dannacher et al 12,13 and by Russell et al 14,15 The most comprehensive study on the C 4 H 6 •+ PES by ab initio calculations is contained in the above-mentioned recent paper by Keister et al 8 (other work concerned mainly the ring opening of CB .+ , which is treated in detail in our other paper 7 ). However, this study was based entirely on the UMP2 methodology, which is known to be prone to artifacts due to UHF spin contamination, 16 especially also in the case of weakly bound ion-molecule complexes.…”
The reaction of the ethylene radical cation (Et•+) with acetylene (Ac) to form stable C4H6
•+ intermediates and
the subsequent fragmentation of these to C3H3
+ + CH3
• or to C4H5
+ + H• have been studied by the UMP2,
RMP2, and B3LYP methods with the 6-31G* basis set, as well as by single-point calculations at the RCCSD(T)/cc-pVTZ level of theory. The aim of this study was to identify all stationary points that might be relevant
to explain the course of the observed reactions. According to their stability to dissociation, we distinguish
three classes of C4H6
•+ structures: weakly bonded complexes, structures of medium stability, and tightly
bonded complexes. Methylcyclopropene radical cation seems to be the most likely ultimate precursor for the
formation of the observed fragmentation products.
“…The excited 1,3-butadiene molecule are scrambled by structural isomerization reactions leading to an excited cyclobutene molecule. And then it would be deactivated to a stable cyclobutene molecule through a collision [ 40 ]. Fig.…”
The reaction process of gaseous 1,3-butadiene following ultraviolet irradiation at the temperature range from 298 to 323 K under nitrogen atmosphere was monitored by UV–vis spectrophotometry. A gaseous mini-reactor was used as a reaction vessel and could be directly monitored in a UV–vis spectrophotometer. We investigated the reactivity and kinetics of 1,3-butadiene under non-UV and UV irradiation to evaluate its photochemical stability. A second-order kinetic model with 50.48 kJ·mol–1 activation energy fitted the reaction data for non-UV irradiation, whereas a first-order kinetic model was appropriate in the case of UV irradiation with activation energies of 19.92–43.65 kJ mol–1. This indicates that ultraviolet light could accelerate the photolysis reaction rate of 1,3-butadiene. In addition, the reaction products were determined using gas chromatography-mass spectrometry (GC–MS), and the reaction pathways were identified. The photolysis of 1,3-butadiene gave rise to various volatile products by cleavage and rearrangement of single C–C bonds. The differences between dimerization and dissociation of 1,3-butadiene under ultraviolet irradiation were elucidated by combining experimental and theoretical methods. The present findings provide fundamental insight into the photochemistry of 1,3-butadiene compounds.
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