Direct sample fraction deposition using electrospray in narrow-bore size-exclusion chromatography/matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for polymer characterization

A direct sample fraction deposition method was developed for off-line size-exclusion chromatography (SEC)/matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. By using electrospray, the SEC eluent, together with a suitable matrix solution added coaxially, was directly deposited on the MALDI plate. Owing to the formation of very small droplets in electrospray, solvent evaporation is much faster. The fractionation volume in narrow-bore SEC, which can directly be collected in one MALDI spot, can easily be optimized in the range of a few microlitres. In addition, fairly homogeneous sample spots were obtained. The possible influence of composition variation of the SEC effluent on the analytical results using direct fraction deposition was investigated; no substantial effects were observed. The applicability of the method was demonstrated by characterizing a broad poly(methyl methacrylate) sample. Copyright 2000 John Wiley & Sons, Ltd.     

Investigation of corannulene for molecular hydrogen storage via computational chemistry and experimentation

Molecular simulations for hydrogen physisorption with corannulene molecules arranged according to their crystal structure result in good agreement with the weight-percent hydrogen stored as determined experimentally employing a 3-g sample of highly crystalline corannulene at ambient temperatures and 72 bar of pressure. Calculated enthalpies of adsorption for corannulene/hydrogen molecular systems obtained from ab initio calculations which take into account electron correlation via second-order M?ller-Plesset perturbation theory are in good agreement with literature experimental enthalpies of adsorption for activated carbons interacting with molecular hydrogen. Ab initio results also show that corannulene molecules arranged in a sandwich structure are important for approximately doubling the binding energy of corannulene interacting with molecular hydrogen through a cooperative interaction. To test the effects of finite temperatures and pressures, stack arrays were used as input for molecular dynamics simulations and indicate that physisorption mechanisms including van der Waals forces and dipole-induced dipole interactions may yield enhanced adsorption capacity in relation to other carbon-based materials. These results will be instrumental in identifying interlayer separations of an array of corannulene or related molecules that may provide a high weight percent of physisorbed hydrogen.     

Time domain investigation on vibrational dephasing and spectral diffusion in CO-doped solid N2

Infrared picosecond accumulated photon echo experiments have been performed for the first time, using the Orsay Free Electron Laser, on the v = 0-->v = 1 transition of CO in solid nitrogen. The vibrational dephasing time is found to be exceptionally long ( T2>/=120 ns) at low temperature. The analysis of the observed spectral diffusion leads one to assume different energy transfer mechanisms depending on the CO concentration.     

Diels-alder reactions with 2-(Arylsulfinyl)-1,4-benzoquinones: effect of aryl substitution on reactivity, chemoselectivity, and pi-facial diastereoselectivity

Diels-Alder reactions of (SS)-2-(2'-methoxynaphthylsulfinyl)-1, 4-benzoquinone (1b), 2-(p-methoxyphenylsulfinyl)-1,4-benzoquinone (1c), and 2-(p-nitrophenylsulfinyl)-1,4-benzoquinone (1d) with cyclopentadiene are reported. These cycloadditions allowed the highly chemo- and stereoselective formation of both diastereoisomeric endo-adducts resulting from reaction on the unsubstituted double bond C(5)-C(6) of quinones working under thermal and Eu(fod)(3)- or BF(3).OEt(2)-catalyzed conditions. The synthesis of endo-adduct [4aS,5S,8R,8aR,SS]-9d resulting from cycloaddition on the substituted C(2)-C(3) double bond was achieved in a chemo- and diastereoselective way from quinone 1d in the presence of ZnBr(2). The reactivity and selectivity of the process proved to be dependent on the electron density of the arylsulfinyl group.     

High stereocontrol in aldol reaction with ketones: enantioselective synthesis of beta-hydroxy gamma-ketoesters by ester enolate aldol reactions with 2-acyl-2-alkyl-1,3-dithiane 1-oxides

The asymmetric aldol reaction of 1,2-diketones, masked as nonracemic 2-acyl dithiane oxides, with lithium enolates derived from several esters and lactones, proceeds with a high degree of stereocontrol at both carbonyl and enolate prochiral centers, the stereocontrol mainly determined by the configuration of the sulfoxide sulfur atom. The sense of induced stereochemistry observed for ester enolates is different from that seen for lactone enolates. Hydrolysis of the dithiane oxide units of the major diastereoisomerically pure aldol products affords enantiomerically pure tertiary alpha-substituted beta-hydroxy-gamma-ketoesters.