- Photolysis of Gaseous Tetramethylethylene between 185 and 230 nm
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The far-UV photolysis of gaseous tetramethyletylene was carried out at 184.9 (mercury line), 202,4-213.8 (zinc lines), and 214.4-228.8 nm (cadmium lines).The photoproducts may be divided int two groups.In the first one, the quantum yield of each product decreases with increase in the total pressure.The proposed mechanism involves the α(C-CH3) or the β(C-H) fragmentation of the photoexcited molecule.Excited (CH3)2CC(CH3)CH2* radicals decompose further at 184.9 nm, and 2,3-dimethyl-1,3-butadiene and 3-methyl-1,2-butadiene are the products.Excited (CH3)2C=CCH3* radicals decompose further, and 2-butyne and isoprene are the products.Isomers 2,3-dimethyl-1-butene, 1,1,2-trimethylcyclopropane, and 3,3-dimethyl-1-butene are products of the second group; its quantum yield increases with an increase in the total pressure.The results support a collisionally induced preisomerization mechanism.Finally, it seems that both groups of products are formed from a different set of excited states
- Collin, Guy J.,Deslauriers, Helene,Wieckowski, Andrzej
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- Rate Constants and Arrhenius Functions for Rearrangements of the 2,3-Dimethyl-3-butenyl and (2,2-Dimethylcyclopropyl)methyl Radicals
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Rate constants for the rearangement of the 2,2-dimethyl-3-butenyl radical (1) to the 1,1-dimethyl-3-butenyl radical (3) via the intermediate (2,2-dimethylcyclopropyl)methyl radical (2) were measured over the temperature range 74 to -78 deg C by the competition method using the reaction of radical 1 with Bu3SnH as the basis reaction.Rate constants for ring opening of radical 2 to both 1 and 3 were measured over the temperature range 50 to -78 deg C by competition against reaction of 2 with PhSH.Arrhenius functions for the 1 to 3, 1 to 2, 2 to 1, and 2 to 3 conversions were calculated; at 25 deg C, the rate constants for these conversions are 4.8*106, 5.6*106, 1.2*108, and 7.7*108 s-1, respectively.At room temperature, cyclization of gem-dimethyl-substituted radical 1 is accelerated by about 3 orders of magnitude over its parent, 3-butenyl radical.Ring opening of 2 is about 1 order of magnitude faster than ring opening of its parent, cyclopropylcarbinyl radical.For the 1 to 2 conversion, K12 varies from 0.05 at 25 deg C to 0.02 at -78 deg C, ΔH12 is 1.0 kcal/mol, ΔS12 is -2.7 eu, and ΔG12 at 25 deg C is 1.8 kcal/mol.
- Newcomb, Martin,Glenn, Anne G.,Williams, William G.
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p. 2675 - 2681
(2007/10/02)
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- The high pressure photochemistry of alkenes. III. The 184.9 nm photoisomerization processes in acyclic alkenes
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We have made a systematic study of the 184.9 nm photoisomerization of the gaseous acyclic alkenes.Apart from the cis-trans isomerization (geometric isomerization), we have also observed the formation of products arising from the 1,3-hydrogen and methylene shifts (structural isomerization). 1-Alkenes do not show evidence of structural isomerization.This kind of isomerization increases with an increase in the number of alkyl substituents around the double bound.These observations, combined with those from the literature, may be explained on the basis of the following: (a) the 1?,?* state is involved in cis-trans isomerization process; (b) the 1?,R(3s) state is responsible for the methylene shifts; (c) another singlet state is required for the 1,3-hydrogen shift; (d) this last state is either at an energy level higher than that of the Rydberg state or the hot ground state.Finally, the photoexcited molecules, through internal conversion, may convert from one state to another, and their lifetime is long enough to be stabilized by collision.
- Collin, Guy J.,Deslauriers, Helene
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p. 1424 - 1430
(2007/10/02)
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- WAVELENGTH-DEPENDENT PHOTOCHEMISTRY OF 2,3-DIMETHYL-2-BUTENE AND 2-OCTENE IN SOLUTION
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Direct irradiations of the title compounds were performed at 185, >200, >220, and 228 nm in hydrocarbon solvents.The evident wavelength dependence of the product distributions observed is of importance in specifying the excited state involved in the formation of each photoproduct, and suggests that more than two excited singlet states of an alkene undergo distinctly different photoreactions rather than suffer mutual conversions between the states.
- Inoue, Yoshihisa,Mukai, Toshio,Hakushi, Tadao
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p. 1665 - 1668
(2007/10/02)
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- Photoisomerization of gaseous tetramethylethylene in the far ultraviolet
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We have studied the gaseous photoisomerization of tetramethylethylene at 184.9 nm in the presence of various quenchers.The quantum yield of isomers is a function of the pressure, such that =a+b.It is shown that the isomerization efficiency of a quencher M correlates with the intermolecular well depth between the photoexcited molecule A* and the quencher M, ε.The log of the efficiency of isomerization is a linear function of this well depth.Moreover, molecular oxygen, nitric oxide, and temperature have distinctive effects on the photoisomerization quantum yield measured at 213.8 nm: oxygen has a reverse effect, whereas nitric oxide induces a very strong isomerization.
- Collin, Guy J.,Deslauriers, Helene
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p. 1510 - 1515
(2007/10/02)
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- REARRANGEMENT OF CYCLOPROPYL, SUBSTITUTED VINYL AND ALKYL GROUPS TO DIVALENT CARBON.
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The relative carbenic migratory abilities of cyclopropyl, β,β-dimethylvinyl, methyl,ethyl and isopropyl groups have been determined.
- Kraska, A. R.,Cherney, L. I.,Shechter, H.
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p. 2163 - 2166
(2007/10/02)
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