Electronic properties of Si surfaces and side reactions during electrochemical grafting of phenyl layers

The electrochemical grafting process of 4-nitrobenzene and 4-methoxybenzene (anisole) from diazonium salt solutions has been investigated in situ by monitoring the current density, the band bending, and the nonradiative surface recombination during grafting at different potentials and different concentrations of the diazonium salt in the solution. Ex situ infrared spectroscopic ellipsometry has been used to inspect the Si surface species before and after the grafting process. The band bending decreases with either increasing concentration of diazonium salt or when the redox potential of the diazonium compound (anisole) is nearer to the competing H+/H2 couple. The surface recombination increases at more cathodic potentials if an electron donor group is present at the phenyl ring (nitrobenzene) and vice versa for the electron acceptor group (anisole). The influence of side reactions can be reduced by use of moderate concentration and moderate or strong cathodic potential, depending on the redox potential of the diazonium compound.     

Enantioselective synthesis of an all-syn four vicinal fluorine motif

Alkanes bearing multiple vicinal fluorine atoms at adjacent stereocenters may be considered intermediate between alkanes and perfluoroalkanes, and as a class, their chemistry and behavior remain to be explored. We report here a stereoselective synthesis of an all-syn four vicinal fluorine motif as a single enantiomer. The four vicinal fluorine compound was amenable to single-crystal X-ray analysis, and the resulting structure displays gauche relationships between all four fluorines, consistent with the fluorine gauche effect, and CF...HC hydrogen bonding between adjacent fluoroalkyl chains.     

Photoionization-induced dynamics of ammonia: ab initio potential energy surfaces and time-dependent wave packet calculations for the ammonia cation

An analytical anharmonic six-dimensional three-sheeted potential energy surface for the ground and first excited states of the ammonia cation has been developed which is tailored to model the ultrafast photoinduced dynamics. Selected ab initio cuts, obtained by multireference configuration interaction calculations, have been used to determine the parameters of a diabatic representation for this Jahn-Teller and pseudo-Jahn-Teller system. The model includes higher-order coupling terms both for the Jahn-Teller and for the pseudo-Jahn-Teller matrix elements. The relaxation to the ground state is possible via dynamical pseudo-Jahn-Teller couplings involving the asymmetric bending and stretching coordinates. The photoelectron spectrum of NH3 and the internal conversion dynamics of NH3+ have been determined by wave packet propagation calculations employing the multiconfigurational time-dependent Hartree method. Three different time scales are found in the dynamics calculations for the second absorption band. The ultrafast Jahn-Teller dynamics of the two excited states occurs on a 5 fs time scale. The major part of the internal conversion to the ground state takes place within a short time scale of 20 fs. This fast internal conversion is, however, incomplete and the remaining excited state population does not decay completely even within 100 fs.     

Enantioselective total syntheses of (-)-clasto-lactacystin beta-lactone and 7-epi-(-)-clasto-lactacystin beta-lactone

An alkylidene carbene 1,5-CH insertion has been used as a key step in an efficient enantioselective total synthesis of (-)-clasto-lactacystin beta-lactone, and its C7-epimer. An additional noteworthy feature of the synthesis is the use of a novel oxidative deprotection procedure, utilizing DMDO, for the conversion of a late-stage benzylidene acetal into a primary alcohol and a secondary benzoate ester.     

Adsorption of methanol on zeolites X and Y. An atomistic and quantum chemical study

The adsorption of methanol on basic zeolites X and Y was investigated with both atomistic and quantum chemical methods. The Monte Carlo docking method was used to localize preferred adsorption sites within the framework. Sites were found adjacent to the interstitial alkali cations in the sites SI, SII, and SIII. We investigated the influence on adsorption behavior of all possible interstitial alkali metal cations, i.e., Li(+), Na(+), K(+), Rb(+), and Cs(+), and in the case of site SII also the influence of varying the Si/Al ratio and distribution. Clusters were cut from the periodic framework in a way that the topological character of the different sites was preserved. DFT calculations yielded geometries and energetic data, which are analyzed with respect to the nature of the cation and to the Si/Al ratio. Adsorption of the methanol molecule is influenced mainly by the identity of the alkali metal cation. Other factors, including Si/Al ratio, are of secondary importance, though there is evidence of weak hydrogen bonding between methanol hydrogen and framework. Cation positions are displaced only slightly by interaction with methanol, although somewhat more at the SIII sites than the SII. We propose that the SIII sites may be a more likely location for methanol activation, particularly in the reaction with toluene, which favors the SII site.     

Analytical scale purification of zirconia colloidal suspension using field programmed sedimentation field flow fractionation

Sedimentation field flow fractionation was used to obtain purified fractions from a polydispersed zirconia colloidal suspension in the potential purpose of optical material hybrid coating. The zirconia particle size ranged from 50/70 nm to 1000 nm. It exhibited a log-Gaussian particle size distribution (in mass or volume) and a 115% polydispersity index (P.I.). Time dependent eluted fractions of the original zirconia colloidal suspension were collected. The particle size distribution of each fraction was determined with scanning electron microscopy and Coulter sub-micron particle sizer (CSPS). These orthogonal techniques generated similar data. From fraction average elution times and granulometry measurements, it was shown that zirconia colloids are eluted according to the Brownian elution mode. The four collected fractions have a Gaussian like distribution and respective average size and polydispersity index of 153 nm (P.I. = 34.7%); 188 nm (P.I. = 27.9%); 228 nm (P.I. = 22.6%), and 276 nm (P.I. = 22.3%). These data demonstrate the strong size selectivity of SdFFF operated with programmed field of exponential profile for sorting particles in the sub-micron range. Using this technique, the analytical production of zirconia of given average size and reduced polydispersity is possible.     

Synthesis of primary arylselenoamides by reaction of aryl nitriles with Woollins' reagent

[Structure: see text] The reaction of aryl nitriles with Woollins' reagent followed by water affords a variety of primary arylselenoamides in 60-100% yield. The first crystal structures of two primary selenoamides are reported.     

Using aggregation to optimize long-term production planning at an underground mine

Motivated by an underground mining operation at Kiruna, Sweden, we formulate a mixed integer program to schedule iron ore production over multiple time periods. Our optimization model determines an operationally feasible ore extraction sequence that minimizes deviations from planned production quantities. The number of binary decision variables in our model is large enough that directly solving the full, detailed problem for a three year time horizon requires hours, or even days. We therefore design a heuristic based on solving a smaller, more tractable, model in which we aggregate time periods, and then solving the original model using information gained from the aggregated model. We compute a bound on the worst case performance of this heuristic and demonstrate empirically that this procedure produces good quality solutions while substantially reducing computation time for problem instances from the Kiruna mine.     

GENERAL APPROACH TO THE SYNTHESIS OF SYMMETRICAL 1,3,2-DIAZAPHOSPHOCANES

Abstract

Synthesis of eight-membered phosphorus-nitrogen heterocycles has been performed by phosphocyclization of the corresponding diamines using different derivatives of phos horous acid. The structures of diazaphosphocanes were supported by elemental analysis; ¹H, ¹³C, and ³¹PNMR spectroscopy; and x-ray diffraction analysis.     

A posteriori estimation of the linearization error for strongly monotone nonlinear operators

We investigate the a posteriori estimation of the modeling (or linearization) error which arises when a nonlinear problem is replaced by a linear model. Using the context of strongly monotone operators, we construct a computable upper estimator for this error, and also provide an estimator that gives a lower bound. Several numerical results illustrating our theory are provided.