Evidence is reported for a nonrandom process by which laser-produced plasmas emit suprathermal electrons. Emission is dominated by a 1 to 2 psec monoenergetic burst, during which the electron energy decreases rapidly. The suprathermal tail on the energy distribution is due to the integrated temporal variation of the electron energy, not to statistical processes. The hot-electron temperature thus produced is practically independent of laser pulse energy.
The reaction kinetics of cyclohexene epoxidation using aqueous H2O2 oxidant and the highly selective epoxidation catalyst Bu(cap)TaSBA15 were studied. The reaction was determined to be first-order in Ta(V) surface coverage. The reaction rate exhibited saturation with respect to increasing concentrations of cyclohexene and H2O2. An Eley-Rideal mechanism and rate equation may be used to describe the epoxidation kinetics, which are similar to those for Ti(IV)SiO2-catalyzed epoxidations. The observed kinetics may also be modeled by a double-displacement mechanism typically associated with saturation enzyme catalysts. In addition, (1)H NMR spectroscopy was employed to investigate H2O2 decomposition by Bu(cap)TaSBA15 and the unmodified TaSBA15 catalysts. Little decomposition occurred over the surface-modified material, but the unmodified material catalyzed a 30% conversion of H2O2 after 6 h. UV-visible absorbance and diffuse reflectance UV-visible (DRUV-vis) spectroscopy were used to investigate the structure of the Ta centers on the TaSBA15 catalysts. DRUV-vis spectroscopy was also used to identify a Ta(V)-based epoxidation intermediate, proposed to be a Ta(V)(eta(2)-O2) species, which forms upon reaction of the TaSBA15 and Bu(cap)TaSBA15 materials with H2O2.
The reaction of ZnEt2 with HO(O)P(OtBu)2 gives the insoluble polymer {Zn[O2P(OtBu)2]2}n (1). In the presence of slight amounts of water, this reaction produces good yields of the oxo-centered tetranuclear cluster Zn4(μ4-O)[O2P(OtBu)2]6 (2), which has been characterized by X-ray crystallography. Compound 2 is thermally labile and eliminates isobutene and water over the temperature range 130−220 °C. The ceramic yield at 900 °C corresponds to the theoretical yield for a Zn4P6O19 material, and the observed products at this temperature are α-Zn2P2O7 and β-Zn(PO3)2 (by XRD). When heated in ethanol at 85 °C for 30 h, 2 converts to polymer 1 and ZnO. This transformation is facilitated by acids, which allow the conversion to occur at room temperature. Polymer 1, characterized by X-ray crystallography, adopts a zigzag structure with zinc atoms linked alternately by one and then three bridging phosphate groups. This structure is therefore different from that adopted by the other two organozincophosphate {Zn[O2P(OR)2]2}n polymers that are known, which exist as linear chains with the zinc atoms bridged by two phosphate groups. Polymer 1 undergoes a quantitative pyrolytic conversion to β-Zn(PO3)2. Diffusion of a toluene solution of 2 into a dichloromethane solution of 1,6-hexanediamine produces a coordination network with the formula {Zn[O2P(OtBu)2]2[H2N(CH2)6NH2]}n (3), with elimination of ZnO. The network structure of 3 consists of {Zn[O2P(OtBu)2]2[H2N(CH2)6NH2]}n polymer strands interconnected via hydrogen bonds between the N−H and PO groups to form layers stacked along the crystallographic b axis. Each polymer chain contains four-coordinate zinc atoms bonded to two monodentate di-tert-butylphosphate ligands and linked by 1,6-hexanediamine groups. Slabs of the layered structure are held together by a dense array of hydrogen bonds involving the N−H and PO functionalities. These layers possess zinc phosphate/1,6-hexanediamine cores and are coated with tert-butyl groups such that there are only van der Waals interactions between layers. Thermogravimetric analysis and XRD studies show that 3 undergoes thermolysis to a mixture of crystalline α-Zn2P2O7 and β-Zn(PO3)2.
In this second paper in a series we present measurements of spectral features\nof 432 low-redshift (z < 0.1) optical spectra of 261 Type Ia supernovae (SNe\nIa) within 20 d of maximum brightness. The data were obtained from 1989 through\nthe end of 2008 as part of the Berkeley SN Ia Program (BSNIP) and are presented\nin BSNIP I (Silverman et al. 2012). We describe in detail our method of\nautomated, robust spectral feature definition and measurement which expands\nupon similar previous studies. Using this procedure, we attempt to measure\nexpansion velocities, pseudo-equivalent widths (pEW), spectral feature depths,\nand fluxes at the centre and endpoints of each of nine major spectral feature\ncomplexes. We investigate how velocity and pEW evolve with time and how they\ncorrelate with each other. Various spectral classification schemes are employed\nand quantitative spectral differences among the subclasses are investigated.\nSeveral ratios of pEW values are calculated and studied. The so-called Si II\nratio, often used as a luminosity indicator (Nugent et al. 1995), is found to\nbe well correlated with the so-called "SiFe" ratio and anticorrelated with the\nanalogous "SSi ratio," confirming the results of previous studies. Furthermore,\nSNe Ia that show strong evidence for interaction with circumstellar material or\nan aspherical explosion are found to have the largest near-maximum expansion\nvelocities and pEWs, possibly linking extreme values of spectral observables\nwith specific progenitor or explosion scenarios. [Abridged]\n
Abstract not Available.
