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121.
Rudolf F. X. Bauer Ikhlas A. Khan Hermann Lotter Hildebert Wagner Victor Wray 《Helvetica chimica acta》1985,68(8):2355-2358
The structure elucidation of four new constituents from the roots of Echinacea purpurea is described. They are shown to be cinnamoyl esters of sesquiterpene alcohols with a germacrane or a guaiane skeleton. First pharmacological results indicate immunological activities. 相似文献
122.
Bedir E Calis I Piacente S Pizza C Khan IA 《Chemical & pharmaceutical bulletin》2000,48(12):1994-1995
A new flavonol glycoside, isorhamnetin 3-O-beta-D-apiofuranosyl-(1-->2)-alpha-L-rhamnopyranosyl-(1-->6)]-beta-D-galactopyranoside, and the known diglycoside, isorhamnetin 3-O-alpha-L-rhamnopyranosyl-(1-->6)-beta-D-galactopyranoside were isolated from the aerial parts of Astragalus vulneraria. Characterization of the two compounds was done by spectroscopic methods (1D and 2D NMR, and FAB-MS). 相似文献
123.
A novel transesterification reaction between acetylated maritime pine sapwood (Pinus pinaster Soland) and methyl benzoate (MB), in the presence of dibutyltin oxide (DBTO) as a catalyst, was performed. Reaction was confirmed by Fourier-transform infrared spectroscopy (FTIR) and 13C nuclear magnetic resonance cross-polarisation with magic-angle spinning (NMR CP MAS) analysis: signals corresponding to benzoylated wood were identified and, when transesterification was performed with ethyl trimethylsilylacetate (ETMSA), characteristic trimethylsilyl groups were detected. The acetyl/benzoyl exchange rate was found to increase with increasing amount of DBTO, temperature and reaction time and a concurrent deterioration of the woody material was evidenced. 相似文献
124.
The solvation shell structure and dynamics of a single Cu2+ ion in a periodic box with 32 water molecules under ambient conditions has been investigated using Car-Parrinello molecular dynamics simulations in a time-window of 18 ps. Five-fold coordination with four equidistant equatorial water molecules at 2.00 A and one axial water molecule at 2.45 A from the Cu2+ ion is found. A "hole" without water molecules is found on the opposite side of the axial water. The ion-water bonding character for the equatorial water molecules is different from that of the axial water molecules, as shown by a localized orbital analysis of the electronic structure. Moreover, the calculated OD stretching vibrational band for the equatorial water molecules lies ca. 175 cm-1 below the axial-water band, in good agreement with experimental data. The equatorial-water band lies below, and the axial-water band above, the pure liquid D2O band, also in agreement with experimental data. 相似文献
125.
Theoretical elucidation of kinetic and thermodynamic control of radical addition regioselectivity 总被引:1,自引:0,他引:1
Leach AG Wang R Wohlhieter GE Khan SI Jung ME Houk KN 《Journal of the American Chemical Society》2003,125(14):4271-4278
The cyclizations of two structurally similar 2-oxo-5-hexenyl-type radicals have been investigated by ab initio and density functional (UB3LYP/6-31+G**//UHF/6-31G* and UB3LYP/6-31G*//UB3LYP/6-31G*) calculations. The origin of apparently contradictory reports of 6-endo and 5-exo cyclizations is determined. Kinetic control favors 6-endo cyclization, while thermodynamic control gives 5-exo cyclization, and the observation of different products from different research groups arises from the difference in experimental conditions used by the two groups. The outcome of a new cyclization reaction was predicted by using these theoretical techniques. Kinetic control is predicted to yield exclusively the products of 6-endo cyclization, while thermodynamic control would lead to an approximately equal mixture of one 6-endo and one 5-exo cyclized product. Experimental studies revealed that the reaction yields only the products of 6-endo cyclization through kinetic control. 相似文献
126.
A novel kinetic method for the determination of trace amounts of Co(II) has been developed. The proposed method based on the catalytic effect of Co(II) on the oxidation of xylenol orange tetra sodium salt by H2O2 in the presence of cationic surfactant (N‐dodecylpyridinium chloride). Co(II) at μg.mL?1 was determined spectrophotometrically by measuring the decrease in the absorbance of xylenol orange at 577 nm by the differential method. The method is precise, selective, and sensitive. The detection limit of the procedure was 0.058 μg.mL?1. The relative standard deviation for the replicate determination (n = 6) of 0.7 μg.mL?1 was 1.285%. The results compared satisfactorily with those of atomic absorption spectrometry. The method was successful for the analysis of Co(II) in veterinary and synthetic samples. 相似文献
127.
S. B. Halligudi M. M. Taqui Khan B. L. Moroz A. L. Chuvilin I. P. Prosvirin V. A. Likholobov 《Reaction Kinetics and Catalysis Letters》1991,44(1):139-146
Ru/SiO2 catalysts prepared by reduction of supported RuCl3·xH2O are active in gas-phase hydroformylation of propylene at low pressure (ca. 0.3 MPa) of H2+CO+C3H6 mixtured and show unexpectedly high selectivity towards unbranched oxo-products. Data concerning the effect of electronic state and dispersity of Ru on their catalytic behavior have been obtained.
Ru/SiO2, RuCl3·xH2O, (0,3 ) - . .相似文献
128.
M. Rafi K. Ahmed I. A. Khan M. R. Husain 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,18(4):379-382
The ultraviolet spectra of Na2 and K2 molecules have been investigated. These studies were made in absorption in the second order of a 3.4 m Ebert Spectrograph with a reciprocal dispersion of 2.6 Å/mm. A number of new bands in the spectra of both the molecules not previously reported have been observed. Computer methods have been used to calculate the term values and to evaluate molecular constants. 相似文献
129.
130.
Mehmood A Malik A Anis I Khan PM Riaz M Makhmoor T Choudhary MI 《Natural product letters》2002,16(6):371-376
Two new oleanane triterpenes; 2alpha,3alpha,24-trihydroxyolean-12-ene-28,30-dioic acid ([structure: see text]) and 2alpha,3alpha,24,28-tetrahydroxyolean-12-ene ([structure: see text]) have been isolated from the roots of Atropa acuminata. Anti-oxidant p-hydroxyphenethyl trans-ferulate ([structure: see text]), beta-sitosterol-3-O-beta-D-glucopyranoside ([structure: see text]) and oleanolic acid ([structure: see text]) have also been reported for the first time from this species. The structures were determined by spectroscopic studies including 2D-NMR. 相似文献