Kinetics and mechanisms of the allyl + allyl and allyl + propargyl recombination reactions

The kinetics and mechanisms of the self-reaction of allyl radicals and the cross-reaction between allyl and propargyl radicals were studied both experimentally and theoretically. The experiments were carried out over the temperature range 295-800 K and the pressure range 20-200 Torr (maintained by He or N(2)). The allyl and propargyl radicals were generated by the pulsed laser photolysis of respective precursors, 1,5-hexadiene and propargyl chloride, and were probed by using a cavity ring-down spectroscopy technique. The temperature-dependent absorption cross sections of the radicals were measured relative to that of the HCO radical. The rate constants have been determined to be k(C(3)H(5) + C(3)H(5)) = 1.40 × 10(-8)T(-0.933) exp(-225/T) cm(3) molecule(-1) s(-1) (Δ log(10)k = ± 0.088) and k(C(3)H(5) + C(3)H(3)) = 1.71 × 10(-7)T(-1.182) exp(-255/T) cm(3) molecule(-1) s(-1) (Δ log(10)k = ± 0.069) with 2σ uncertainty limits. The potential energy surfaces for both reactions were calculated with the CBS-QB3 and CASPT2 quantum chemical methods, and the product channels have been investigated by the steady-state master equation analyses based on the Rice-Ramsperger-Kassel-Marcus theory. The results indicated that the reaction between allyl and propargyl radicals produces five-membered ring compounds in combustion conditions, while the formations of the cyclic species are unlikely in the self-reaction of allyl radicals. The temperature- and pressure-dependent rate constant expressions for the important reaction pathways are presented for kinetic modeling.     

Systematic computational study on the unimolecular reactions of alkylperoxy (RO2), hydroperoxyalkyl (QOOH), and hydroperoxyalkylperoxy (O2QOOH) radicals

Unimolecular isomerization and decomposition reactions of alkylperoxy (RO(2)), hydroperoxyalkyl (QOOH), and hydroperoxyalkylperoxy (O(2)QOOH) radicals play important roles in the low-temperature oxidation of hydrocarbons. In this study, these reactions have been investigated by the CBS-QB3 quantum chemical method, and the variation of the rate parameters by the structural change of alkyl groups has been studied systematically for the rule-based construction of the low-temperature oxidation mechanisms of arbitrary noncyclic alkanes. The results can be well-interpreted in terms of the group additivity and the ring-strain effect of the cyclic transition states. To extract the important processes needed for the chemical kinetic modeling, the competing reaction channels were compared in detail by steady-state analysis with the high-pressure limiting rate constants. The importance of some reactions of O(2)QOOH radicals, which have not been considered in the previous modeling studies, such as the hydrogen exchange reactions between -OOH and -OO(?) groups and hydrogen shift reactions from non-OOH sites, is suggested.     

Hypervalent phenyl-λ3-iodane-mediated para-selective aromatic fluorination of 3-phenylpropyl ethers

Exposure of 3-phenylpropyl ethers to an activated iodosylbenzene monomer·18-crown-6 complex [PhI(OH)BF(4)·18C6] in the presence of BF(3)-Et(2)O and water results in the para-selective monofluorination of benzene ring via neighboring alkoxy group participation and directly affords 3-(4-fluorophenyl)propyl ethers regioselectively in good yields.     

X-ray absorption, photoemission spectroscopy, and Raman scattering analysis of amorphous tantalum oxide with a large extent of oxygen nonstoichiometry

The electronic structure and modification of the local interatomic structure of a reactive sputtered amorphous tantalum oxide (a-TaO(x)) thin film with the variation of oxygen nonstoichiometry, x in a-TaO(x) have been investigated by X-ray absorption spectroscopy (XAS), X-ray photoemission spectroscopy (XPS), Raman scattering spectroscopy, and Rutherford back scattering spectroscopy. A parallel chemical shift of Ta4f(7/2) and O1s core levels observed with the variation of x indicates the Fermi level shift by reduction and oxidation in the framework of the rigid band model. Extended X-ray absorption fine structure (EXAFS) suggests both the increase of average coordination number of the first Ta-O shell in polyhedra and a considerable reduction of the average Ta-O bond length with the increase of x. The relative intensity of Raman shift peaks at 670 cm(-1) and 815 cm(-1), corresponding to Ta-O stretching of TaO(6) octahedra and TaO(5) probably with a pyramidal form, respectively, drastically changes between x = 2.47 to 1.86, suggesting the change in the predominant polyhedron from TaO(6) to TaO(5) with a modification in multiplicity of oxygen by the reorganization of the polyhedral network.     

On foliated circle bundles over closed orientable 3-manifolds

We show that there exists a family of smooth orientable circle bundles over closed orientable 3-manifolds each of which has a codimension-one foliation transverse to the fibres of class C ⁰ but has none of class C ³ . There arises a necessary condition induced from the Milnor-Wood inequality for the existence of a foliation transverse to the fibres of an orientable circle bundle over a closed orientable 3-manifold. We show that with some exceptions this necessary condition is also sufficient for the existence of a smooth transverse foliation if the base space is a closed Seifert fibred manifold. Received: May 13, 1996     

Carbon-13 NMR spectra of retinal1 and its related compounds

Carbon-13 NMR spectra of all-trans retinal₁ and vitamin A₁ were measured by the pulse Fourier transform method in CCl₄. All peaks in these spectra were assigned from considerations of chemical shifts, half proton-decoupled spectra, spin-lattice relaxation times and induced chemical shift by the addition of shift reagent. The carbon-13 NMR spectrum was also measured for all-trans retinal₁ which had been exposed to the sunlight for three hours, and the induced isomer was proposed to be 11-s-cis retinal₁.