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.     

Vibrational dynamics of 9-fluorenemethanol using infrared-ultraviolet double-resonance spectroscopy

Vibrational spectroscopy of jet-cooled 9-fluorenemethanol and its clusters 9-fluorenemethanol-H2O, 9-fluorenemethanol-CH3OH, 9-fluorenemethanol-C2H5OH, and 9-fluorenemethanol-C3H7OH has been carried out using an IR-UV double-resonance method. The spectrum of the OH stretching vibration, v(OH), has been measured for the 9-fluorenemethanol monomer and for each of the clusters. Two conformers of 9-fluorenemethanol, symmetric (sym) and unsymmetric (unsym), have been identified using a combination of spectroscopy and quantum chemical calculations with B3LYP and HF methods using the 6-31G(d) basis set. Vibrational dynamics resulting from IR excitation has also been studied using the S0-S1 transition probed by a nanosecond-time-delayed UV laser. The data suggest that isomerization occurs as a result of the IR excitation, but the breadth of the probe spectra makes an unequivocal conclusion difficult. The effect of hydrogen bonding on the v(OH) of 9-fluorenemethanol has also been studied in clusters with water, methanol, ethanol, and propanol by measuring the IR spectra. Cluster dissociation dynamics have also been studied following IR excitation. It is observed that upon excitation of the cluster of a particular conformation the monomer product is generally produced in both conformer forms. Energetic considerations indicate that isomerization occurs before dissociation.     

Binding energies and dissociation pathways in the aniline-Ar2 cation complex

Mass analyzed threshold ionization spectroscopy is used to measure the Ar binding energy for the cationic aniline-Ar (An(+)-Ar) and aniline-Ar(2) (An(+)-Ar(2)) complexes. Since the experiments begin with the neutral species, photoexcitation creates the cations in the pi-bonding configuration with the Ar located above the phenyl ring. The binding energy in this conformation of the An(+)-Ar complex is determined to be 495+/-15 cm(-1). Measurements of An(+)-Ar(2) revealed the production of a lower energy dissociation product which is assigned to the An(+)-Ar H-bonding configuration. Combinations of measurements allow determination of the dissociation energy of this complex to be 640+/-20 cm(-1). The observation of a more stable H-bonded conformer is consistent with recent infrared experiments on An(+)-Ar complexes created by complexing An(+) with Ar, rather than creation through the neutral complex. Calculations are presented which closely reproduce the binding energy of the pi bound Ar but underestimate the stability of the H-bonded species.     

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.     

Communication: spectroscopic measurement of the binding energy of a carboxylic acid-water dimer

Infrared-ultraviolet two color pump-probe spectroscopy is used to measure the binding energy, D(0,) of a carboxylic acid-water dimer where the acid is 9-hydroxy-9-fluorenecarboxylic acid. The acid-water configuration presents a standard structure for the general acid-water interaction where the water bonds to the carboxylic acid group through two intermolecular hydrogen bonds. Photodissociation studies with product vibrational state resolution have enabled an accurate determination of the binding energy for this acid-water system to be D(0) = 2975 ± 30 cm(-1). Quantum chemical calculations are performed to compare with the experimental observations and a recent measurement on the water dimer (D(0) = 1105 ± 10 cm(-1)).