The electronic spectrum of Si3 I: triplet D(3h) system

We report the measurement of a jet-cooled electronic spectrum of the silicon trimer. Si(3) was produced in a pulsed discharge of silane in argon, and the excitation spectrum examined in the 18 000-20 800 cm(-1) region. A combination of resonant two-color two-photon ionization (R2C2PI) time-of-flight mass spectroscopy, laser-induced fluorescence/dispersed fluorescence, and equation-of-motion coupled-cluster calculations have been used to establish that the observed spectrum is dominated by the 1(3)A(1)" - a? (3)A(2)' transition of the D(3h) isomer. The spectrum has an origin transition at 18,600 ± 4 cm(-1) and a short progression in the symmetric stretch with a frequency of ～445 cm(-1), in good agreement with a predicted vertical transition energy of 2.34 eV for excitation to the 1(3)A(1)" state, which has a calculated symmetric stretching frequency of 480 cm(-1). In addition, a ～505 cm(-1) ground state vibrational frequency determined from sequence bands and dispersed fluorescence is in agreement with an earlier zero-electron kinetic energy study of the lowest D(3h) state and with theory. A weaker, overlapping band system with a ～360 cm(-1) progression, observed in the same mass channel (m/z = 84) by R2C2PI but under different discharge conditions, is thought to be due to transitions from the (more complicated) singlet C(2v) ground state ((1)A(1)) state of Si(3). Evidence of emission to this latter state in the triplet dispersed fluorescence spectra suggests extensive mixing in the excited triplet and singlet manifolds. Prospects for further spectroscopic characterization of the singlet system and direct measurement of the energy separation between the lowest singlet and triplet states are discussed.     

Temporary thio-derivatization in the synthesis of (+)-4-acetylbromoxone

A stereocontrolled synthesis of the marine natural products (+)-bromoxone (1) and (+)-4-acetylbromoxone (2) is reported. The sequence features the enzymatic kinetic resolution of 4-hydroxycyclohexenone (6) via its S-benzyl adduct. Thereafter, a base-mediated elimination-silylation generated an optically active (−)-4S-4-tert-butyldimethylsilyoxycyclohexenone (5), which then underwent diastereoselective epoxidation. Saegusa-Ito oxidation enabled formation of the corresponding α,β-unsaturated ketone 13. Bromination-elimination and subsequent removal of the silicon protecting group afforded (+)-bromoxone (1) which was converted into (+)-(4S,5R,6R)-4-acetoxy-2-bromo-5,6-epoxycyclohex-2-enone (2) [(+)-4-acetylbromoxone]. Using a luciferase gene reporter assay ED₅₀ for NFκB inhibition of 9 μM was determined.     

INFRARED RADIATION OF SF6 AND ITS APPLICATION TO GAS-FILLED DOUBLE-PANE WINDOWS

Interest in using sulfur hexafluoride (SF₆ as a gas-fill in multipane windows has raised questions concerning the calculation of heat transfer rates through such windows. The infrared absorption characteristics of this gas make the heat transfer analysis much more complicated. In order to account for the absorption effect, we measured the spectral absorptivity of several infrared-active bands of sulfur hexafluoride at low resolution and a temperature of 298 K. We correlated the spectral absorption data with the Edwards exponential wide-band model and with the Elsasser narrow-band model, and incorporated the wide-band model into a one-dimensional, finite-element heat transfer model. The finite-element heat transfer model considered combined conduction and radiation effects in a double-pane window, and was used to evaluate the overall heat transfer coefficients of double-pane windows filled with SF₆; CO₂, or air. The numerical results show good agreement with the experimental results     

Regular principal factors in free objects in Rees-Sushkevich varieties

In a previous paper, the author showed how to associate a completely 0-simple semigroup with a connected bipartite graph containing labelled edges. In the main theorem, it is shown how these fundamental semigroups can be used to describe the regular principal factors of the free objects in certain Rees-Sushkevich varieties, namely, the varieties of semigroups that are generated by all completely 0-simple semigroups over groups in a variety of finite exponent. This approach is then used to solve the word problem for each of these varieties for which the corresponding group variety has solvable word problem.     

Control of reflectivity and stop bands in a Bragg stack with a four-layer period

We provide a general expression for the band dispersion in a Bragg stack with a four-layer period, and show how the expression is considerably simplified in the case of a perturbed two-layer Bragg stack, where every second layer has refractive index n₁ and n₂, respectively. We present a range of analytical expressions to describe key features of the band dispersion, including the dependence of the band gap and of the evanescent decay length on variations in layer thickness for TE and TM polarized light incident perpendicular and at an angle to the Bragg stack.     

Synthesis of a constrained polyfunctional bicyclic iminocyclitol scaffold from l-sorbose via a tandem sequence including stereoselective intramolecular Huisgen cycloaddition

The synthesis of a functionalized (azido, amino, and hydroxy) 8-oxa-3-azabicyclo[3.2.1]octane framework and its conversion into a protected sugar amino acid and a tricyclic framework is described. The sequence includes a one-pot Huisgen 1,3-dipolar cycloaddition, with decomposition to an aziridine and subsequent ring opening by azide. The stereoselectivity observed in the Huisgen cycloaddition reaction is attributed to minimization of allylic strain.     

A systematic approach to the study of multicomponent polymerization kinetics: The butyl acrylate/methyl methacrylate/vinyl acetate example. IV. Optimal Bayesian design of emulsion terpolymerization experiments in a pilot plant reactor

A systematic study of the terpolymerization of butyl acrylate/methyl methacrylate/vinyl acetate (BA/MMA/VAc) is being conducted. In this stage of the study, emulsion terpolymerizations were performed in a 5 L stainless steel pilot plant reactor. The experimental trials were of the two-level factorial type and were designed optimally using a Bayesian method. The design procedure allowed us to improve our knowledge about the process using our prior knowledge and our subjective judgement. The polymers produced were characterized for conversion, composition, molecular weight, and particle size. The Bayesian design of experiments is shown to have several advantages over conventional factorial designs. © 1996 John Wiley & Sons, Inc.     

Decomposition of the lattice of pseudovarieties of finite semigroups induced by bands

It has been shown by the authors that the mapping,where B is the pseudovariety of finite bands, is a complete retraction of the lattice L(F) of pseudovarieties of finite semigroups onto the lattice of pseudovarieties of bands. It follows that the classes of the induced congruence on L(F), or on the lattice of subpseudovarieties L(W) for any subpseudovariety W of F, are intervals. In this paper we solve the membership problem for the upper limit of the classes of restricted to L(W) for various , including F itself, and provide bases of pseudoidentities for certain cases. Received April 7, 1999; accepted in final form April 30, 2000.