Lewis acid tuned facial stereodivergent HDA reactions using beta-substituted N-vinyloxazolidinones

The [4 + 2] acido-catalyzed heterocycloaddition between new beta-substituted N-vinyl-1,3-oxazolidin-2-ones (with R' = Me, Ar, CH2 Ar) and beta,gamma-unsaturated alpha-ketoesters (R = Ar) afforded heteroadducts with high levels of endo and facial selectivities. A complete reversal of facial differentiation was achieved by varying the Lewis acid, leading to the stereoselective formation of either endo-alpha or endo-beta adducts. [reaction: see text].     

Interaction of hydrogen chloride with ice surfaces: the effects of grain size, surface roughness, and surface disorder

Characterization of the interaction of hydrogen chloride (HCl) with polar stratospheric cloud (PSC) ice particles is essential to understanding the processes responsible for ozone depletion. The interaction of HCl with ice was studied using a coated-wall flow tube with chemical ionization mass spectrometry (CIMS) between 5x10(-8) and 10(-4) Torr HCl and between 186 and 223 K, including conditions recently shown to induce quasi-liquid layer (QLL) formation on single crystalline ice samples. Measurements were performed on smooth and rough (vapor-deposited) polycrystalline ice films. A numerical model of the coated-wall flow reactor was used to interpret these results and results of studies on zone-refined ice cylinders with grain sizes on the order of several millimeters (reported elsewhere). We found that HCl adsorption on polycrystalline ice films typically used in laboratory studies under conditions not known to induce surface disordering consists of two modes: one relatively strong mode leading to irreversible adsorption, and one relatively weak binding mode leading to reversible adsorption. We have indirect experimental evidence that these two modes of adsorption correspond to adsorption to sites at crystal faces and those at grain boundaries, but there is not enough information to enable us to conclusively assign each adsorption mode to a type of site. Unlike what was observed in the zone-refined ice study, there was no strong qualitative contrast found between the HCl uptake curves under QLL versus non-QLL conditions for adsorption on smooth and vapor-deposited ices. We also found indirect evidence that HCl hexahydrate formation on ice between 3x10(-7) and 2x10(-6) Torr HCl and between 186 and 190 K is a process involving hydrate nucleation and propagation on the crystal surface, rather than one originating in grain boundaries, as has been suggested for ice formed at lower temperatures. These results underscore the dependence of the HCl-ice interaction on the characteristics of the ice substrate.     

The oxidation of oleate in submicron aqueous salt aerosols: evidence of a surface process

We have studied the oxidation of submicron aqueous aerosols consisting of internal mixtures of sodium oleate (oleic acid proxy), sodium dodecyl sulfate, and inorganic salts by O3, NO3/N2O5, and OH. Experiments were performed using an aerosol flow tube and a continuous flow photochemical reaction chamber coupled to a chemical ionization mass spectrometer (CIMS). The CIMS was fitted with a heated inlet for volatilization and detection of organics in the particle phase simultaneously with the gas phase. A differential mobility analyzer/condensation particle counter was used for determining aerosol size distributions. The oxidation of oleate by O3 follows Langmuir-Hinshelwood kinetics, with gammaO3 approximately 10(-5) calculated from the observed loss rate of oleate in the particle phase. The best fit Langmuir-Hinshelwood parameters are kImax=0.05+/-0.01 s-1 and KO3=4(+/-3)x10(-14) cm3molec-1. These parameters showed no dependence on the ionic composition of the aerosols or on the presence of alkyl surfactants. Several ozone oxidation products were observed to be particle-bound at ambient temperature, including nonanoic acid. We observed efficient processing of oleate by OH (0.1相似文献   

Samarium diiodide mediated cascade radical cyclisations—synthesis of the eudesmane tricyclic framework

Radical cascade cyclisation of methylenecyclopropyl cyclohexanone adducts, using samarium diiodide to generate an initial ketyl radical, provide a short route to tricyclic ethers, and the stereochemical outcome can be influenced by the solvent used for the reaction.     

Ozone oxidation of surface-adsorbed polycyclic aromatic hydrocarbons: role of PAH-surface interaction

The heterogeneous chemistry of surface-adsorbed polycyclic aromatic hydrocarbons (PAHs) plays key roles in nanoscience, environmental science, and public health. Experimental evidence shows that the substrate can influence the heterogeneous oxidation of surface-bound PAHs, however, a mechanistic understanding of the role of the surface is still lacking. We examine the effects of the PAH-substrate interaction on the oxidation of surface-adsorbed anthracene, pyrene, and benzo[a]pyrene by ozone (O(3)) using density functional theory. We find that some O(3) oxidation mechanisms for these planar PAH molecules lead to nonplanar intermediates or products, the formation of which may necessitate partial desorption or "lift-off" from a solid substrate. The energy penalty for partial desorption of each PAH from the surface is estimated for four different substrate types on the basis of literature data and accounted for in the thermodynamic analysis of the reaction pathways. We find that the attractive PAH-substrate interaction may render oxidation pathways involving nonplanar intermediates or products thermodynamically unfavorable. The influence of the PAH-substrate interaction could contribute in part to the variations in PAH oxidation kinetics and product distributions that have been observed experimentally. Our choice of test molecules enabled us to identify trends in reactivity and product formation for four types of potentially reactive site (zigzag, armchair, bridge, and internal), allowing us to infer products and mechanisms of O(3) oxidation for PAHs of larger sizes. Implications for atmospheric chemistry and the stability of graphene in the presence of O(3) are discussed.     

Development of a standardized sequential extraction protocol for simultaneous extraction of multiple actinide elements

Sequential extraction is a useful technique for assessing the potential to leach actinides from soils; however, current literature lacks uniformity in experimental details, making direct comparison of results impossible. This work continued development toward a standardized five-step sequential extraction protocol by analyzing extraction behaviors of ²³²Th, ²³⁸U, ^239,240Pu and ²⁴¹Am from lake and ocean sediment reference materials. Results produced a standardized procedure after creating more defined reaction conditions to improve method repeatability. A NaOH fusion procedure is recommended following sequential leaching for the complete dissolution of insoluble species.     

Energies and electron correlation effects in two-electron doubly excited (nl)2 states

Energies of singlet doubly excited states 2p^{2 1}D, 3d^{2 1}G, 4f^{2 1}I of He isoelectronic series are calculated in the framework of the variational method by using Hylleraas-type wave functions and a real Hamiltonian. The results obtained show quantitatively the importance of electron correlation effects in the doubly excited states and they are in good agreement with some experimental data and other theoretical results. Received 30 March 2000 and Received in final form 4 July 2000     

Modifications to the lidar equation due to nonlinear propagation in air

We study how the well-known lidar equation is affected by the use of ultra-short, high-power laser pulses. Because of the self-focusing and self-guiding, the overlap function ξ, representing the reduction fraction of the signal resulting from geometrical effects inside the experimental system, needs to be reconsidered. The losses due to multi-photon ionisation in the filament entail a heavy weakening of the return signal. We also investigate the contribution of the white-light components generated by self-phase modulation. Received: 2 January 2001 / Revised version: 8 June 2001 / Published online: 18 July 2001