An open design microfabricated nib-like nanoelectrospray emitter tip on a conducting silicon substrate for the application of the ionization voltage

This paper describes a novel emitter tip having the shape of a nib and based on an open structure for nano-electrospray ionization mass spectrometry (nanoESI-MS). The nib structure is fabricated with standard lithography techniques using SU-8, an epoxy-based negative photoresist. The tip is comprised of a reservoir, a capillary slot and a point-like feature, and is fabricated on a silicon wafer. We present here a novel scheme for interfacing such nib tips to MS by applying the ionization voltage directly onto the semi-conductor support. The silicon support is in direct contact with the liquid to be analyzed at the reservoir and microchannel level, thus allowing easy use in ESI-MS. This scheme is especially advantageous for automated analysis as the manual step of positioning a metallic wire into the reservoir is avoided. In addition, the analysis performance was enhanced compared with the former scheme, as demonstrated by the tests of standard peptides (gramicidin S, Glu-fibrinopeptide B). The limit of detection was determined to be lower than 10(-2) microM. Due to their enhanced performance, these microfabricated sources might be of great interest for analysis requiring very high sensitivity, such as proteomics analysis using nanoESI-MS.     

Enantiospecific approach toward pentalenolactone

[reaction: see text] The enantiospecific assembly of the pentalenolactones' carbon skeleton was achieved in 17 steps and 16% overall yield from methyl alpha-D-glucopyranoside. The synthetic strategy relies on two highly efficient key steps: an exo-diastereoselective Diels-Alder reaction and a nonsymmetric ozonolysis.     

Highly diastereoselective Diels-Alder reaction using a chiral auxiliary derived from levoglucosenone

[reaction: see text] A new chiral auxiliary derived from levoglucosenone is reported. The compound is obtained by a cycloaddition reaction with 9-methoxy methylanthracene followed by a diastereoselective reduction of the C-2 keto function. The auxiliary has been used as a chiral template in an asymmetric Diels-Alder reaction of the corresponding acrylic ester derivative with cyclopentadiene. The results showed excellent diastereomeric excess even at room temperature when the reaction was promoted by Et(2)AlCl as the Lewis acid.     

Stationary isothermic surfaces and some characterizations of the hyperplane in the N-dimensional Euclidean space

We consider the entire graph S of a continuous real function over RN−1 with N?3. Let Ω be a domain in RN with S as a boundary. Consider in Ω the heat flow with initial temperature 0 and boundary temperature 1. The problem we consider is to characterize S in such a way that there exists a stationary isothermic surface in Ω. We show that S must be a hyperplane under some general conditions on S. This is related to Liouville or Bernstein-type theorems for some elliptic Monge-Ampère-type equation.     

Prediction and Experimental Characterization of the Molecular Architecture of FRP and ATRP Synthesized Polyacrylate Networks

Summary: This work reports experimental and modeling studies concerning the conventional (FRP) and atom transfer radical polymerization (ATRP) of acrylate/diacrylate monomers. In the framework of a recently developed general approach, kinetic models including crosslinking reactions and branching by chain transfer to polymer are discussed for FRP and ATRP polymerization systems. Besides molecular weight distribution (MWD), fairly good predictions of the z-average radius of gyration could be obtained for these non-linear polymers. A set of experiments was performed at 1 L scale in a batch reactor using n-butyl acrylate (BA) or methyl acrylate (MA) as monovinyl monomers and 1,6-Hexanediol diacrylate (HDDA) or bisphenol A ethoxylate diacrylate (BEDA) as crosslinkers. In FRP experiments, AIBN was used as initiator and ATRP polymerizations were initiated by ethyl 2-bromopropionate (EBrP) and mediated by CuBr using PMDETA (N,N,N′,N″,N″-pentamethyldiethylenetriamine) as ligant. Polymerizations were carried out in solution at 60 °C with different dilutions using toluene and DMF as solvents. Products formed at different polymerization times were analyzed by SEC/RI/MALLS yielding average MW, MWD, z-average radius of gyration and monomer conversion. Important differences in the molecular architecture of the synthesized FRP and ATRP highly branched polyacrylates have been identified. Comparisons of experimental results with predictions have put into evidence the important effect of intramolecular cyclizations at all dilutions, even with ATRP polymerizations.     

Translation planes admitting many homologies

The translation planes with spreads inPG(3,2 ^r ) that admit >2 ^r +1 axes (coaxes) of homologies of orderu1 are classified.Partially supported by FONDECYT Project No. 890-1061     

Syntheses, structures, and photoisomerization of (E)- and (Z)-2-tert-butyl-9-(2,2,2-triphenylethylidene)fluorene

"Sterically geared" 9-(2,2,2-triphenylethylidene)fluorene (1) is of potential interest as a photoactive moiety in molecular devices, and the 2-tert-butyl derivative (6) has been synthesized to investigate photoisomerization. E and Z stereoisomers of 6 were separated and identified by X-ray crystallography. The tert-butyl group does not introduce additional strain, and its close proximity to the trityl group in the Z isomer suggests an attractive van der Waals interaction. The UV spectra of (E)-6 and (Z)-6 are nearly identical, showing absorption bands that are similar to those of fluorene occurring at wavelengths longer than 240 nm. Photoisomerization of 6 was investigated at 266, 280 and 320 nm. Solutions initially containing only (E)-6 or (Z)-6 were irradiated with pulsed laser light, monitoring isomerization by 1H NMR spectroscopy. Negligible photodecomposition was observed when the solutions were agitated by N2 ebullition. Experimental data were fitted to theoretical curves to obtain photoisomerization quantum yields (phi(ZE) and phi(EZ)) ranging from 0.04 to 0.09. This first photoisomerization study of a dibenzofulvene reveals significant quantum yields, despite theoretical prediction of inefficient or negligible isomerization of the parent hydrocarbon, fulvene. Thermal isomerization of 6 at 270 degrees C (t(1/2) = 120 min) was also followed by 1H NMR spectroscopy, resulting in an estimated activation energy (deltaG(double dagger)) of 43 kcal/mol.     

Diameter dependence of second-order Raman features of graphene tubes

The second-order Raman band (ω_G^′) for single wall carbon nanotubes (SWNTs) in the range 2600–2700 cm⁻¹ depends on the excitation energy and the diameter. However, the diameter dependence is not well established and understood. Here we propose that the two different physical dependencies are separable and can be taken into account by a phenomenological here derived equation. Using this methodology we have been able to find consistency between data for double-WNTs and large diameter SWNTs. Further, the determined diameter dependence of G′ correctly predicts the frequency for an infinitely large tube diameter, which is equivalent to a planar graphene sheet.