摘要:

The synthesis of a novel 12-crown-4 analogue (cis-p-dioxano-12-crown-4 orcis-3,6,9,13,15- pentaoxabicyclo[9.3.1]pentadecane) is described. The (1:1) LiNCS complex crystallizes in the monoclinicCcspace group;a= 8.4973(2) Å,b= 18.0659(4) Å,c= 8.9168(1) Å, and &bgr; = 90.713(1)° withZ= 4. The crown adoptsC1symmetry to which the Li+is coordinated in a rectangular pyramid with an apical-NCS.Key words: crown ethers, structure, complexation.

ISSN:0008-4042    

DOI:10.1139/v97-220

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Thetransform of octahydro-1-methyl-1H-2,1-benzothiazine 2,2-dioxide, in which a six-membered sultam unit istrans-fused to cyclohexane, has been synthesized. The single-crystal X-ray structure shows the molecule to have theN-methyl group in theaxialorientation. The generalized anomeric effect believed to be responsible for the axial preference is estimated at =>2 kcal mol-1.Key words: generalized anomeric effect, negative hyperconjugation, sulfonamides, sultams, X-ray crystallography.

ISSN:0008-4042    

DOI:10.1139/v97-214

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The problem of measuring the kinetics of Si exchange between aqueous silicate species by29Si NMR has been revisited, using highly alkaline KOH solutions (2.8 mol SiIVper kg solvent, [SiIV]/K2O = 0.43) at 60-90°C to minimize the number of silicate species present. Longitudinal29Si relaxation timesT1and apparent rate constants estimated from line-shape analysis (LSA) varied markedly with the degree of purity of the KOH used, but rate constantskobtained by selective inversion-recovery (SIR) using the CIFIT data-fitting program were independent of the source of KOH and were smaller than those obtained from LSA by at least an order of magnitude. Although only four kinetically significant silicate anions (monomer M, dimer D, linear trimer L, and cyclic trimer C) were present, overlap of the D and L resonances prevented complete analysis of the SIR data. True rate constants could therefore be obtained only for the M-D exchange (for formation of D,k1(90°C) = 0.13 ± 0.01 kg mol-1s-1, &Dgr;H1dagger= 67.4 kJ mol-1, &Dgr;S1dagger= -78 J K-1mol-1; for dissociation of D,k-1(90°C) = 1.4 ± 0.1 s-1, &Dgr;H-1dagger= 64.7 kJ mol-1, and &Dgr;S1dagger= -66 J K-1mol-1). Models that included L as the precursor of C (MDLC mechanism) showed, within the limitations imposed by D-L band overlap, that the reactivities of M, D, L, and C in Si-O-Si link formation or dissociation were all roughly comparable. Good fits of the experimental data, however, and equally reliable rate constants for the M-D exchange, could be obtained with models that ignored the presence of L entirely (MDC mechanism). The simple MDC model also provides consistent apparent rate constantskCandk-Cfor theoverallformation of C from M + D and the reverse process, respectively, by SIR of either M or C ( &Dgr;HCdagger= 76.5 kJ mol-1, &Dgr;SCdagger= -57 J K-1mol-1; &Dgr;H-Cdagger= 88.6 kJ mol-1, and &Dgr;S-Cdagger= -7 J K-1mol-1).Key words: kinetics, silicates,29Si NMR.

ISSN:0008-4042    

DOI:10.1139/v97-229

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The capillary electrophoretic separation of iodide, nitrate, perchlorate, thiocyanate, bromate, iodate, and ethane-, butane-, pentane-, and octanesulphonate was examined in sodium chromate or potassium hydrogen phthalate electrolytes and in the presence of &agr; -, &ggr; -, (0-40 mmol/L) and &bgr; -cyclodextrin (0-10 mmol/L). Largest decreases in electrophoretic mobility were observed for iodide, perchlorate, and thiocyanate, probably due to inclusion of these anions in the cyclodextrin (CD) cavity. Changes in migration patterns and electroosmotic flow were observed, which depended on cyclodextrin type and concentration and on electrolyte-cyclodextrin interactions. Thus for larger cavity cyclodextrins ( &bgr;-or &ggr;-cyclodextrin) or in the presence of chromate, relatively small decreases in electrophoretic mobilities for all the anions were observed, indicating that a good match between analyte and cavity size and minimal cyclodextrin affinity for electrolyte ions are essential for different migration patterns. Separation efficiencies were between 50 000 and 400 000 theoretical plates, and calibration plots for iodide and octanesulphonate were linear;R2= 0.998 and 0.9999, respectively, in the concentration range (5 x 10-5)-(5 x 10-3) mol/L.Key words: capillary electrophoresis, inorganic and organic anions, cyclodextrins, inclusion complex, electroosmotic flow control.

ISSN:0008-4042    

DOI:10.1139/v97-230

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

trans-2,3-Dimethoxy-3-(phenylamino)flavanones (2), a new type of flavanone derivatives, are synthesized from flavanones or 2'-hydroxychalcones in two steps, the first one being a condensation with nitrosobenzenes (Ehrlich-Sachs reaction) and the second an oxidation with (diacetoxyiodo)benzene in methanol. Nitrone formation, an important side reaction of the Ehrlich-Sachs reaction, has also been exploited to synthesize2simply by use of an excess of nitrosobenzene in aqueous methanol. Syntheses of several analogues of2are also reported.Key words:trans-2,3-dimethoxy-3-(phenylamino)flavanones, 2'-hydroxychalcones, nitrosobenzenes, 3-(phenylamino)flavones, (diacetoxyiodo)benzene.

ISSN:0008-4042    

DOI:10.1139/v97-219

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The polynuclear manganese(II) complex of L-dihydroorotic acid has been synthesized and characterized by X-ray crystallography, UV-visible spectroscopy, and magnetic susceptibility measurements. Crystal data: triclinic, space groupP1 bar ,a= 4.7239(5) Å,b= 7.902(1) Å,c= 10.552(2) Å, &agr; = 105.80(1)°, &bgr; = 98.864(7)°, &ggr; = 104.69(1)°,V= 355.9(1) D 3,Z= 1,R= 0.035,Rw= 0.028. This complex consists of polymer chains of manganese atoms that are bound together via carboxylate bridges with a Mn...Mn distance in the chain of 4.724 D . The radical scavenging properties of the complex have been evaluated by ESR spectroscopy and cyclic voltammetry methods and compared to those of vitamin E and BHT (2,6-di-tert-butyl-4-methylphenol).Key words: orotic acid, L-dihydroorotic acid, manganese(II) complex, antioxidative properties.

ISSN:0008-4042    

DOI:10.1139/v97-234

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

A three-step synthesis of imidazolidine-4-one has been realized. Sodium dithionite was a very efficient reagent for the last step, the reduction of a ketone. The reaction is slow under the normal conditions of heating and its diastereoselectivity is poor. Under the influence of ultrasound, the selectivity and the yield are very good. The use of sodium dithionite makes it possible to work in water and to avoid the use of hydrides.Key words: sodium dithionite, ultrasound, imidazolidine-4-one.

ISSN:0008-4042    

DOI:10.1139/v97-232

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Gas phase thermolysis of 2,2-dimethoxy-5,5-dimethyl- &Dgr;3-1,3,4-oxadiazoline (1) in an ultraviolet photoelectron spectrometer by means of a CW CO2laser as directed heat source at 26 W gave a complex PE spectrum that included ionization bands belonging to acetone, tetramethoxyethylene (3), and dimethyl oxalate (4). Subtraction of the spectra of acetone,3, and4from the pyrolysis spectrum of1left a simple PE spectrum that is attributed to dimethoxycarbene (2) along with some ethane. ecke3LYP/6-31+G* calculations gave first adiabatic and vertical ionization potentials of2as well as orbital energies that are in perfect agreement with experimental values. From the available experimental and calculational data,2is assumed to adopt awconformation.Key words: dimethoxycarbene, 2,2-dimethoxy- &Dgr;3-1,3,4-oxadiazoline, tetramethoxyethylene, He(I) photoelectron spectroscopy, thermolysis.

ISSN:0008-4042    

DOI:10.1139/v97-233

出版商:NRC Research Press    

年代:1998

数据来源: NRC