The fate of excited unsaturated radicals produced in the vacuumuv photolysis of gaseous olefins (original) (raw)
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The vacuum UV photolysis of 1-hexene and 1-hexyne
Journal of Photochemistry, 1983
A comparative study was made of the photolysis of l-hexene and l-hexyue at 184.9 and 147 run. Three primary processes were observed in each system. They are, in decreasing order of importance, the rupture of the p(C-C) bond, the rupture of the y(C-C) bond and the retro-ene process. Thus the behaviour of both photoexcited molecules is similar to a first approximation. However, there are meaningful differences. In particular, the rupture of the y(C-C) bond is relatively more important in the l-hexyne case and leads to the formation of vinylacetylene at 147 nm. This observation suggests that the r(C-C) rupture may be the result of isomerization of the photoexcited molecule (a l,3 hydrogen shift) which is followed by the rupture of the @(C-C) bond.
Vacuum UV photolysis of n-1-hexene and 4-methyl-1-pentene
Journal of Photochemistry, 1980
We studied the vacuum UV photolysis of n-l-hexene and 4-methyll-pentene at 147,163 and 174 nm. In both systems the use of NO or 06 and DI indicates that the main fragmentation process (a = 0.8) of the 147 nm photoexcited molecule is the breaking of the C-C bond located at the p position. This process leads to the formation of ally1 and propyl radicals. The propyl radicals decompose further at low pressure, giving rise to the formation of ethylene or propylene. By following the pressure effect on the ethylene or propylene quantum yields, and using RRKM results, it is shown that the n-propyl radicals formed in the n-1-hexene photolysis carry less energy than would be expected from a statistical distribution of the excess energy. The situation seems to be more complex for 4-methyl-l-pentene, and the isopropyl radicals have an energy content not far from the statistical distribution.
Canadian Journal of Chemistry, 1978
A systematic study of the pressure effects on the quantum yields of some products between 0.1 and 600 Torr (13 and 80 000 N m−2) was carried out in the 7.6 and 8.4 eV photolysis of normal, iso- and cis-2-butenes. The propylene quantum yield (s-C4H9* → C3H6 + CH3) decreased with the increase in the n-butene pressure and a good linearity of S/D (stabilization/decomposition) vs. pressure plot, over a broad pressure region, was observed. It is concluded that hydrogen atoms involved in the s-C4H9* radical formation are produced with a relatively narrow energy distribution. The slope of S/D vs. pressure lines decreased with the increase in photon energy, indicating the trend in the kinetic energy of the H-atoms.In the case of isobutene and cis-2-butene photolysis, the Stern–Volmer plots for allene formation were nonlinear. It is concluded that the formation of two different allene precursors is needed to account for this result. By the use of a simple RRK-type formalism we also conclude t...
Journal of Photochemistry, 1974
The vacuum ultra-violet photolysis of cyclobutanone in the gas phase has been carried out at 147.0 and 123.6 nm. Two primary photochemical processes which produce vibrationally excited products have been shown to be important at these wavelengths: ,O+ CH,CO* + CH, LY =CH2* (1) + -CH,CH,CH;* + CO (2) Radical processes are unimportant, the main reaction products arising either from the primary molecular reactions or subsequent isomerization or decomposition of vibrationally excited molecules. The ratio of the quantum yields of reactions (1) and (2) is a function of the waveIength of the exciting light and has a high pressure limiting value of 0.95 f 0.05 and 0.45 & 0.05 at 147.0 and 123.6 nm respectively. Detailed RRKM calculations have indicated that the cyclopropane molecule has, on average, 17 and 30 kJ/mol excess vibrational energy from the photochemical reactions at 147.0 and 123.6 nm respet tivel y. Fluorescence of electronically excited CO has not been observed and the implications of this fact are discussed.
Gas phase photolysis of 1-pentene and 1-pentene- d 10 at 7.1 and 7.6 eV. Kinetic considerations
Canadian Journal of Chemistry, 1978
The gas phase photolysis of n-pentene was carried out in a static system using nitrogen resonance lines at [Formula: see text] and the bromine line at [Formula: see text] The mechanism for the photolysis was proposed and compared to what was concluded at 8.4 eV (147 nm, the xenon resonance line). The kinetics of the decomposition of the excited C3H5* radicals formed in the primary photochemical process and the C5H11* radicals formed by the addition of hydrogen atoms to the parent molecules were discussed. The investigations were extended to the n-C5D10 photolytic System. The observed decomposition rate constants of the excited pentyl radicals as well as the secondary non-equilibrium isotope effects agree with the data published earlier. It is concluded from these experiments that, at least at 7.6 eV, hot hydrogen atoms are produced.Only a small fraction of the C3H5* radicals décompose and yield aliène. At the same time the combined primary–secondary non-equilibrium isotope effects a...
Photolysis of n-butene and isobutene at 174.3 – 174.5 nm (7.10 eV)
Journal of Photochemistry, 1978
The photolysis of n-butene and isobutene was carried out in a static system using nitrogen resonance lines at 174.3 -174.5 nm (7.11 -7.10 eV). The main fragmentation process of the photoexcited n-butene molecule is the C-C split in the fl position to the double bond. The primary quantum yield Cp is 0.66. The Q, value for the (x C-C split of isobutene is equal to 0.78.
The Journal of Chemical Physics, 2003
The visible fluorescence of CH fragments (A 2 ⌬→X 2 ⌸ and B 2 ⌺ Ϫ →X 2 ⌸ transitions͒ formed in the vacuum ultraviolet photodissociation of ethylene in the 11.7-21.4 eV energy region, was recorded. Two formation thresholds for each excited fragment, CH* ͑A͒ or CH* ͑B͒, were identified and associated with two dissociation channels namely CH*ϩCH 3 and CH*ϩCHϩH 2 . Unlike previous studies of the dissociation dynamics on the ground-state potential energy surface, neither of these channels exhibit an energy barrier within the experimental uncertainty, even in the latter case of molecular H 2 elimination. It is proposed that both channels pass via an ethylidene intermediate (H 3 CCH:), an isomer never previously experimentally detected and whose existence has been debated in theoretical publications. The observed behavior, at the excitation energies used in the present work, also suggests that fast isomerization and internal conversion to excited states of ethylene precede fragmentation. Above 18.5 eV, that is around the ionization limit C 2 H 4 ϩ (D 2 B 1u ), dissociative ionization starts to compete with neutral dissociation into excited CH fragments giving rise to a substantial decrease in the neutral fragment signal.
UV Photolysis of α-Cyclohexanedione in the Gas Phase
The Journal of Physical Chemistry A, 2011
Ultraviolet absorption spectrum of R-cyclohexanedione (R-CHD) vapor in the wavelength range of 220À320 nm has been recorded in a 1 m long path gas cell at room temperature. With the aid of theoretical calculation, the band has been assigned to the S 2 r S 0 transition of largely ππ* type. The absorption cross section at the band maximum (∼258 nm) is nearly 3 orders of magnitude larger compared to that for the S 2 r S 0 transition of a linear R-diketo prototype, 2,3-pentanedione. The photolysis was performed by exciting the sample vapor near this band maximum, using the 253.7 nm line of a mercury vapor lamp, and the products were analyzed by mass spectrometry as well as by infrared spectroscopy. The identified products are cyclopentanone, carbon monoxide, ketene, ethylene, and 4-pentenal. Geometry optimization at the CIS/6-311þþG** level predicts that the carbonyl group is pyramidally distorted in the excited S 1 and S 2 states, but the R-CHD ring does not show dissociative character. Potential energy curves with respect to a ring rupture coordinate (CÀC bond between two carbonyl groups) for S 0 , S 1 , S 2 , T 1 , T 2 , and T 3 states have been generated by partially optimizing the ground state geometry at DFT/B3LYP/6-311þþG** level and calculating the vertical transition energies to the excited states by TDDFT method. Our analysis reveals that the reactions can take place at higher vibrational levels of S 0 as well as T 1 states.
Vacuum Ultraviolet Photolysis of Trimethylethylene
Canadian Journal of Chemistry, 1974
The vacuum u.v. photolysis of trimethylethylene (2-methyl-2-butene) was carried out in a static system using rare gas resonance lamps: xenon (147.0 nm) and krypton (123.6 nm). The main hydrocarbon products were isoprene, 1,3-butadiene, propyne, allène, ethylene, and other minor products. Identification and measurements of the yields of hydrogen atoms, methyl, and ethyl radicals were carried out quantitatively by the use of radical–radical reactions. Because of the high yield of isoprene, the effect of conversion was studied. At a high conversion (i.e. 0.1%) the isoprene quantum yield decreases. Hydrogen atoms add mainly to the secondary carbon of the monomer (≥90%). The Δ(CH3,tert-C5H11) value was calculated to be 1.32 ± 0.14. With the krypton line (10.0 eV) no evidence was found for the participation of ionic reactions in the formation of the measured products except for the formation of 2-methyl-1-butene in a low yield. At this wavelength the ion quantum yield is 0.224 ± 0.005.