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ABSTRACT: The temperature- and pressure-dependent kinetics of the reaction between propargyl and molecular oxygen have been studied with a combination of electronic structure theory, transition state theory, and the time-dependent master equation. The stationary points on the potential energy surface were located with B3LYP density functional theory. Approximate QCISD(T,Full)/6-311++G(3df,2pd) energies were obtained at these stationary points. At low temperatures the reaction is dominated by addition to the CH2 side of the propargyl radical followed by stabilization. However, addition to the CH side, which is followed by one of various possible internal rearrangements, becomes the dominant process at higher temperatures. These internal rearrangements involve a splitting of the O2 bond via the formation of 3-, 4- or 5-membered rings, with the apparent products being CH2CO + HCO. Rearrangement via the 3-membered ring is found to dominate the kinetics. Rearrangement from the CH2 addition product, via a 4-membered ring, would yield H2CO + HCCO, but the barrier to this rearrangement is too high to be kinetically significant. Other possible products require H transfers and, as a result, appear to be kinetically irrelevant. Modest variations in the energetics of a few key stationary points (most notably the entrance barrier heights) yield kinetic results that are in good agreement with the experimental results of Slagle and Gutman (I. R. Slagle and D. Gutman, Proc. Combust. Inst., 1986, 21, 875) and of Atkinson and Hudgens (D. B. Atkinson and J. W. Hudgens, J. Phys. Chem. A, 1999, 103, 4242).
Faraday Discussions 02/2001; · 5.00 Impact Factor
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ABSTRACT: The time-resolved production of HO2 in the Cl-initiated oxidation of iso- and n-butane is measured using continuous-wave (CW) infrared frequency modulation spectroscopy between 298 and 693 K. The yield of HO2 is determined relative to the Cl2/CH3OH/O2 system. As in studies of smaller alkanes, the branching fraction to HO2 + alkene in butyl + O2 displays a dramatic rise with increasing temperature between about 550 and 700 K (the "transition region") which is accompanied by a qualitative change in the time behavior of the HO2 production. At low temperatures the HO2 is formed promptly; a second, slower production of HO2 is responsible for the bulk of the increased yield in the transition temperature region. In contrast to reactions of smaller alkyl radicals with O2, the total HO2 yield in the butyl radical reactions appears to remain significantly below 1 up to 700 K, implying a significant role for OH-producing channels. The slower HO2 production in butane oxidation displays an apparent activation energy similar to that measured for smaller alkyl + O2 reactions, suggesting that the energetics of the HO2 elimination transition state are similar for a broad range of R + O2 systems. A combination of QCISD(T) based characterizations of the propyl and butyl + O2 potential energy surfaces and master equation based characterization of the propyl + O2 kinetics provide the framework for explanation of the experimentally observed HO2 production in Cl-initiated propane and butane oxidation. These calculations suggest that the HO2 elimination channel is similar in all reaction systems, and that hydroperoxyalkyl (QOOH) species produced by internal H-atom abstraction in RO2 can provide a path to OH formation. However, the QOOH formed by the energetically favorable 1,5 isomerization (via a six-membered ring transition state) generally experiences significant barriers (relative to the radical + O2 reactants) to the production of an oxetane + OH. In contrast, the barriers to forming OH + an oxirane or an oxolane, via 1,4 or 1,6 isomerizations, respectively, are generally below reactants.
Faraday Discussions 02/2001; · 5.00 Impact Factor
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ABSTRACT: Fifteen consecutive eyes in 14 patients (six men and eight women, ranging in age from 43 to 75 years) underwent anterior vitrectomy for chronic cystoid macular edema documented by fluorescein angiography. All eyes had either vitreous adhesions to the cataract incision with pupillary distortion, or adherence of vitreous strands to the iris. Anterior vitrectomy via the corneoscleral limbus was performed in each case to remove vitreous from anterior segment structures. Thirteen of the 15 eyes showed visual improvement, with 12 of the 15 showing improvement of two Snellen lines or more. There was no direct correlation between the duration of aphakic cystoid macular edema and final visual acuity.
American Journal of Ophthalmology 07/1983; 95(6):811-7. · 4.22 Impact Factor
