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991.
992.
The conformational preference of calix[4]pyrrole and its fluoride and chloride anion-binding properties have been investigated by density functional theory calculations. Geometries were optimized by the BLYP/3-21G and BLYP/6-31G methods, and energies were evaluated with the BLYP/6-31+G method. To model the effect of medium, the SCIPCM solvent model was also employed. Four typical conformations of the parent substituent-free calix[4]pyrrole were studied. Both in the gas phase and in CH(2)Cl(2) solution, the stability sequence is predicted to be 1,3-alternate > partial cone > 1,2-alternate > cone. The cone conformation is predicted to be about 16.0 and 11.4 kcal/mol less stable in the gas phase and CH(2)Cl(2) solution, respectively. This is mainly due to electrostatic repulsions arising from the all-syn pyrrole/pyrrole/pyrrole/pyrrole arrangement present in this conformer. The existence of possible 1:1 and 1:2 anion-binding modes were explored in the case of fluoride anion, and the factors favoring the 1:1 binding mode are discussed. The calculated binding energy for fluoride anion is about 15 kcal/mol larger than that for chloride anion. The calculated binding energy for chloride anion agrees with the experimental value very well. The presence of meso-alkyl substituents destabilizes the cone conformer with respect to the 1,3-alternate conformer and, therefore, reduces the anion-binding affinity by 3-4 kcal/mol. The strength of N-H- - -anion hydrogen bonds in the various structures subject to study were estimated on the basis of the calculated anion-binding energies and the predicted structural deformation energies of substituent-free calix[4]pyrrole. 相似文献
993.
The structure and stability of endohedral X@Si20H20 complexes (X = Li0/+, Na0/+, K0/+, Be0/2+, Mg0/2+, Ca0/2+) have been studied at the B3LYP/6-31G* level of density functional theory. It is found that complexes with X = Na0/+, K0/+, Mg and Ca0/2+ are energy minimum structures with X at the cage center in Ih symmetry, while those with X = Li0/+, Be0/2+, Mg2+ have off-centered structures with X towards one pentagon face in C5v symmetry. Large electron or charge transfer between the Si20H20 cage and the encapsulated X has been observed. 相似文献
994.
Multilayer poly(vinyl alcohol)-adsorbed coating on poly(dimethylsiloxane) microfluidic chips for biopolymer separation 总被引:1,自引:0,他引:1
A poly(dimethylsiloxane) (PDMS) microfluidic chip surface was modified by multilayer-adsorbed and heat-immobilized poly(vinyl alcohol) (PVA) after oxygen plasma treatment. The reflection absorption infrared spectrum (RAIRS) showed that 88% hydrolyzed PVA adsorbed more strongly than 100% hydrolyzed one on the oxygen plasma-pretreated PDMS surface, and they all had little adsorption on original PDMS surface. Repeating the coating procedure three times was found to produce the most robust and effective coating. PVA coating converted the original PDMS surface from a hydrophobic one into a hydrophilic surface, and suppressed electroosmotic flow (EOF) in the range of pH 3-11. More than 1,000,000 plates/m and baseline resolution were obtained for separation of fluorescently labeled basic proteins (lysozyme, ribonuclease B). Fluorescently labeled acidic proteins (bovine serum albumin, beta-lactoglobulin) and fragments of dsDNA phiX174 RF/HaeIII were also separated satisfactorily in the three-layer 88% PVA-coated PDMS microchip. Good separation of basic proteins was obtained for about 70 consecutive runs. 相似文献
995.
Fresh grapes and grape products, such as grape wine and grape juice, were analyzed for proanthocyanidins (PACs) using liquid chromatography with electrospray ionization mass spectrometric (MS) detection. PACs were successfully separated and analyzed on the basis of their protonated molecules, allowing the identification of PACs in different degrees of polymerization from monomers to oligomers (up to 7 units), and in various isomeric forms. Using reversed-phase high-performance liquid chromatography (HPLC) combined with MS detection, the PAC monomers, (+)-catechin (C), (-)-epicatechin (EC), (-)-catechin gallate (CG), and (-)-epicatechin gallate (ECG), were successfully quantified using selected ion monitoring (SIM) mode. Standard curves were fitted for each PAC ranging from 43.8 to 5600 ng/mL for C, from 42.2 to 5400 ng/mL for EC, from 36.7 to 4700 ng/mL for CG, and from 39.8 to 5100 ng/mL for ECG. Good linearity (r2>0.999) was achieved for each analyte. The accuracy and precision (RSD) were within 10% (n=8) at the limit of detection. This method allows direct quantification of monomeric PACs in fresh grapes and grape-derived products. Additionally, flow injection analysis (FIA) was applied to estimate the concentration levels of PAC oligomers by comparing their FIA-MS peak areas, which were well correlated (r2=0.936) to the total concentrations of PAC monomers. 相似文献
996.
Liu L Qiu S Wang B Zhang W Lu P Xie Z Hanif M Ma Y Shen J 《The journal of physical chemistry. B》2005,109(49):23366-23370
We have investigated the thermal degradation in air by Fourier transform infrared spectroscopy of a ladder-type copolymer containing fluorene units in the backbone (Me-LPF), to reveal the formation of the ketonic defects. As thermal treatment of Me-LPF film at 200 degrees C in air proceeds, a new group of complex absorption bands due to degradation products arises in the range between 1800 and 1600 cm(-1). The observed overlapping bands were separated and assigned by utilizing the second-derivative IR spectral analysis, which can narrow the peak width to one-third of the originals and thereby eases the analysis. The degraded products were assigned as fluorenone (1718 cm(-1)) and benzophenone (Ar-(C=O)-Ar) (1665 cm(-1)), formed by the oxidation of the backbone, and acylphenone (Ar-(C=O)-R) (1685 cm(-1)) from the side chain. The fluorenone was found to be the major component among the degraded products in the main chain, and the time and temperature dependence indicated that the oxidation is a kind of autocatalytic radical-chain process. The oxidation can reach a very high degree (approximately 30% for 6 h oxidation at 240 degrees C estimated by absorption of the alkyl). Our results suggest the possibility of the oxidation of the 9-bialkylfluorene sites. We propose that the degradation of the alkyl in the side chain can help the radicals to propagate in the chain reaction. 相似文献
997.
Fu L Wu NQ Yang JH Qu F Johnson DL Kung MC Kung HH Dravid VP 《The journal of physical chemistry. B》2005,109(9):3704-3706
Supported gold catalysts have drawn worldwide interest due to the novel properties and potential applications in industries. However, the origin of the catalytic activity in gold nanoparticles is still not well understood. In this study, time-of-flight secondary ion mass spectroscopy (TOF-SIMS) has been applied to investigate the nature of gold in Au (1.3 wt %)/gamma-Al2O3 and Au (2.8 wt %)/TiO2 catalysts prepared by the deposition-precipitation method. The SIMS spectrum of the supported gold catalysts presented AuO-, AuO2-, and AuOH- ion clusters. These measurements show direct evidence for oxidized gold on supported gold catalysts and may be helpful to gaining better understanding of the origin of the catalytic activity. 相似文献
998.
Rotationally resolved infrared emission spectra of HCl(v=1-3) in the reaction of Cl+CH3SH, initiated with radiation from a laser at 308 nm, are detected with a step-scan Fourier-transform spectrometer. Observed rotational temperature of HCl(v=1-3) decreases with duration of reaction due to collisional quenching; a short extrapolation to time zero based on data in the range 0.25-4.25 micros yields a nascent rotational temperature of 1150+/-80 K. The rotational energy averaged for HCl(v=1-3) is 8.2+/-0.9 kJ mol(-1), yielding a fraction of available energy going into rotation of HCl, fr=0.10+/-0.01, nearly identical to that of the reaction Cl+H(2)S. Observed temporal profiles of the vibrational population of HCl(v=1-3) are fitted with a kinetic model of formation and quenching of HCl(v=1-3) to yield a branching ratio (68+/-5):(25+/-4):(7+/-1) for formation of HCl(v=1):(v=2):(v=3) from the title reaction and its thermal rate coefficient k(2a)=(2.9+/-0.7)x10(-10) cm(3) molecule(-1) s(-1). Considering possible estimates of the vibrational population of HCl(v=0) based on various surprisal analyses, we report an average vibrational energy 36+/-6 kJ mol(-1) for HCl. The fraction of available energy going into vibration of HCl is f(v)=0.45+/-0.08, significantly greater than a value fv=0.33+/-0.06 determined previously for Cl+H2S. Reaction dynamics of Cl+H(2)S and Cl+CH3SH are compared; the adduct CH3S(Cl)H is likely more transitory than the adduct H(2)SCl. 相似文献
999.
Time-dependent density-functional theory(TDDFT)has been applied to calculate the electronic structure and second-order nonlinear optical(NLO) properties of some organic molecules.The two-dimensional(2-D)charge transfer charateristics of calculated molecules were studied and compared with corresponding experimental results.All the theoretical results agree well with the measurement.For 2-D molecule with two-fold symmetry,the dominant charge transfer is off-diagonal,while for three-fold symmetry 2-D molecule,the dominant charge transfer is not only between branches and central group but also among branches. 相似文献
1000.