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61.
Summary A general definition of reaction graphs is presented. For a pair of isomeric molecular graphs
and
, related by a chemical transformation
, the reaction graph
is determined using a maximal common subgraph defined for vertex mapping
. A binary operation defined for graphs constructed over the same vertex set enables us to decompose the reaction graph
into the sum of prototype reaction graphs. A decomposition of an overall reaction graph can be advantageously used for the construction of a reaction network. An oriented path in this network beginning at
and ending at
corresponds to a breakdown of the transformation
into a sequence of intermediates. 相似文献
62.
Ping-Yang Yeh Pavla Kopeckov Jindrich Kopecek 《Journal of polymer science. Part A, Polymer chemistry》1994,32(9):1627-1637
Novel pH-sensitive hydrogels containing azoaromatic crosslinks were synthesized by the crosslinking of polymeric precursors. First, a reactive polymeric precursor was synthesized by copolymerization of N,N-dimethylacrylamide, N-tert-butylacrylamide, acrylic acid, and N-methacryloylglycylglycine p-nitrophenyl ester. The hydrogel was prepared in the second step by the reaction of the polymeric precursor with N,N′-(ω-aminocaproyl)-4,4′-diaminoazobenzene. The hydrogels were characterized by the network structure, (that is, content of crosslinks, unreacted pendent groups, and cycles), the equilibrium swelling ratio as a function of pH, modulus of elasticity in compression, and the degradability in vitro. The results obtained indicated that the hydrogel network structure strongly depends on the reaction conditions such as polymer concentration, and the ratio of the reactive groups during the crosslinking reaction. The swelling and mechanical properties of hydrogels can be controlled by the modification of polymer backbone structure and/or the crosslinking density. The rates of hydrogel degradation depended on their degree of swelling. The higher the degree of swelling, the higher the degradability. The properties of the hydrogels suggest that they have a potential as carriers for colon-specific drug delivery. © 1994 John Wiley & Sons, Inc. 相似文献
63.
Olaf Heitzsch Karsten Gloe Alfréd Sabela Jiří Koryta Edwin Weber 《Journal of inclusion phenomena and macrocyclic chemistry》1992,13(4):311-319
The complex forming properties with alkali metal and ammonium ions of a series of oligo benzo-condensed 18-crown-6 ethers1–8 having a different gradation of lipophilicity and of molecular rigidity are investigated by voltammetry at the interface of two immiscible electrolyte solutions (ITIES) and by a liquid-liquid extraction technique. The experimental results obtained in the two phase system H2O/nitrobenzene are discussed in relation to the structure of the crown and the cation type. The stability constants for the 1 : 1 complexes of Na+, K+, Rb+, Cs+ and NH
4
+
in nitrobenzene have been determined and compared with the extraction constants for the 1: 1 complexes of Na+ and K+ and for the 1 : 1 and 1 : 2 complexes of Cs+, showing the effect of oligo benzo condensation for the 18-crown-6 system. 相似文献
64.
Liquid—liquid extraction of divalent palladium by solutions of the hexachloroderivate of cobalt dicarbolide (HBCl6) in the mixture of solvents (30 v/v % nitrobenzene+20 v/v % n-dodecane +50 v/v/ % toluene) from nitric acid medium has been studied. Besides HBCl6 the organic phase contained also 2,2′-dipyridyl (dipy). The yield of palladium extraction from 0.5M HNO3 is greater than 99.0%. The species extracted into the organic phase corresponds to the formula [Pd(dipy)2] (BCl6)2. 相似文献
65.
A simple on-column injection system for large volume of liquid samples for the GC-MS determination of traces of PAHs and PCBs has been investigated. A deactivated fused silica capillary 20 m × 0.53 mm I.D. and 2 meters of an HP5 column (0.53 mm,1 m film thickness) were used as retention gaps. Injection volumes of 80 L for PAHs and 90 L for PCBs, allow determination of 5–50 ng L–1 PAHs and 11–44 ng L–1 PCBs in hexane solution with an RSD of < 10%. The method has been used for the determination of PCBs and PAHs in soil sample. 相似文献
66.
Four 9H and four 7H tautomers of DNA base xanthine were studied by the ab initio LCAO-MO method at the MP2/6-311G**//HF/6-31G** and MP2/6-31G**//HF/6-31G** approximations. All calculated structures are minima at the HF/6-31G** potential energy surface with the dioxo 7H tautomer (A1) being the global minimum. The second most stable tautomer, dioxo-9H (B1) is by 9 kcal/mol less stable. For the A1 B1 transition the calculated MP2 energy gap corresponds to the equilibrium constant of 2 × 10–7. Therefore, only the major tautomeric form A1 is predicted to be detectable in the gas phase. The 7H and 9H groups of tautomers are discussed separately. Within both groups, the dioxo form (A1-7H, B1-9H) is the most stable one and is succeeded by the 2-dihydroxy (A2, B2) form. However, while the energy difference between A1 and A2 is 10 kcal/mol, the energy difference between B1 a B2 is only 2 kcal/mol. The effect of polar environment was estimated by the SCRF method, using a spherical cavity, at the HF/6-31G** level. These calculations did not change the gas phase stability order of the tautomers. However, the energy difference between A1 and B1 decreased from 9 kcal/mol at the HF/6-31G** level to 4 kcal/mol at the SCRF HF/6-31G** level. 相似文献
67.
Z. Řanda M. Vobecký J. Kuncír J. Benada 《Journal of Radioanalytical and Nuclear Chemistry》1978,46(1):95-107
The application of multielement standards (MES) in routine neutron activation analysis brings a whole range of advantages.
This paper deals with the experience obtained during many years of application of these MES. Nine of these MES contain a total
of 50 elements in suitable combinations and concentrations; thus, the determination of most of the common elements by NAA
can be carried out simultaneously. This refers to the following elements: Na, Mg, Al, Cl, K, Ca, Sc, V, Cr, Mn, Fe, Co, Ni,
Cu, Zn, Ga, As, Se, Br, Rb, Sr, Mo, Ag, Cd, In, Sb, Te, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf,
W, Re, Au, Hg, Th and U. For the determination of the remaining elements such as Zr, Ta, Ir etc., single element standards
(SES) are used. 相似文献
68.
Oxolane was fluoroalkylated by its photoadditions under atmospheric pressure. Monofluoro-alkylations were carried out with hexafluoropropene (1) and perfluorovinyl ethers C3F7O-[CF(CF3)CF2O]n-CF = CF2 (24, n = 0–2) by direct photoexcitation of the olefins to give high yields of addition products 9–12 (81–94%). The reactions were completely regioselecti ve at the oxolane molecule and almost completely regioselective (93–99%) at the double bond of fluoro-olefins; no bis-fluoroalkylated oxolanes were detected. The completely selective introduction of a second fluoroalkyl into position 5 of the oxolane molecule was accomplished by acetone-sensitised photoaddition of 2fluoroalkylated oxolanes 9,10 to fluoro-olefins 1 and 2. Byproducts from reactions of the dimethylketyl radical which is formed in the initiation step were isolated and have given some evidence about the reaction mechanism that is discussed. 相似文献
69.
Jiří Horský Josef Pitha 《Journal of inclusion phenomena and macrocyclic chemistry》1994,18(3):291-300
The stability constants were measured of inclusion complexes formed from aromatic amino acids and their oligopeptides with - and-cyclodextrin, hydroxypropyl-cyclodextrin, and partially methylated-cyclodextrin. The method of competitive spectrophotometry withp-nitrophenol as a competing reagent was used, and measurements were made at pH 7.4-Cyclodextrin formed complexes of higher stability than the other hosts. The stability of complexes of oligopeptides containing L-phenylalanine was invariably higher than that of L-phenylalanine itself. A model for interaction of proteins with cyclodextrins is proposed, in which the most stable complexes are formed when the native functional form of proteins is unfolded and the nonpolar residues that are buried inside the structure are exposed to water. The complexation of the unfolded structure favors its formation; thus thermal denaturation of proteins is easier in the presence of cyclodextrins. On the other hand, this complexation prevents the intermolecular association of unfolded structures by noncovalent hydrophobic bonding between the exposed nonpolar residues; furthermore, the unfolded complexed forms may revert to the native functional form. This prevention of intermolecular association may explain the stabilizing effect of cyclodextrins on solutions of proteins: a return to the native form is achieved more easily from the complexed, unfolded form than from the unfolded, aggregated forms.Dedicated to Professor József Szejtli. 相似文献
70.
Zusammenfassung Es wurden Komplexe des Typs YH2
TCl·2H2O, LaH2
TCl·3H2O und CeH2
TCl·3H2O isoliert. Die Individualität der Verbindungen wurde mit Hilfe der Thermoanalyse, IR-Absorptionsspektren und Röntgenstreuung geklärt.Complexes of the types YH2
TCl·2H2O, LaH2
TCl·3H2O and CeH2
TCl·3H2O were isolated, and the compounds characterized by thermogravimetric analysis, I. R. spectroscopy and X-ray diffraction.Mit 3 Abbildungen
Ln=Y, La, Ce; H4
T=C4H6O6. 相似文献