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111.
Celik H Ekmekci G Ludvík J Pícha J Zuman P 《The journal of physical chemistry. B》2006,110(13):6785-6796
Aromatic oximes are reduced in aqueous solution in a four-electron process. The reducible species in the pH range 5-8 is a diprotonated form of the oxime. This species is generated in the course of electrolysis in the vicinity of the electrode surface from the adsorbed neutral form of the oxime. The reduction is initiated by a cleavage of the N-O bond. The diprotonation facilitates the reduction process by the preformation of OH2+ as a good leaving group and by a positive charge on the azomethine nitrogen. Diprotonation has been proven based on shapes of i = f(pH) plots, by observed shifts of half-wave potentials with pH and by comparison with the reduction of nitrones. Some observed deviations from theoretical i = f(pH) plots were attributed to the role of adsorption on the rate of protonation. Adsorption is also responsible for dips on some of the i-E curves. Adsorption plays a role at concentrations as low as 1 x 10(-5) M, when the electrode surface is still not fully covered. This indicates that catalyzed protonation occurs on islets of adsorbed materials. At pH 2-5 the studied oximes in the vicinity of the electrode are predominately present in a protonated form, which is less strongly adsorbed. In this pH range the protonation takes place in a homogeneous reaction layer of the electrode. It yields a monoprotonated form, which is reduced. The separation of two two-electron waves observed for some oximes in acidic media serves as an experimental proof of the formation of imines as reduction intermediates. This separation is caused by the differences in pKa values of protonated forms of oximes and imines. The effects of substituents in the para position on the benzene ring are characterized by correlation with the Hammett substituent constant sigmax. This has been proven at pH 1.5 for substituted benzaldehyde oximes and at pH 5.0 for substituted acetophenone oximes. 相似文献
112.
Hana Dejmkova Matteo Scampicchio Jiri Zima Jiri Barek Saverio Mannino 《Electroanalysis》2009,21(9):1014-1018
A robust electrochemical method to measure the total phenol content in food samples is presented. Under optimal condition, BDD electrode showed excellent performance to detect the oxidation of several phenols and does not present the drawback due to electrode fouling. The analytical method used to perform such measurement has been optimized and successfully applied in different food samples. The results obtained were compared with the standard Folin–Ciocalteau method. 相似文献
113.
A Ramsey-Type Result for Convex Sets 总被引:1,自引:0,他引:1
Larman David; Matousek Jiri; Pach Janos; Torocsik Jeno 《Bulletin London Mathematical Society》1994,26(2):132-136
Given a family of n convex compact sets in the plane, one canalways choose n of them which are either pairwise disjoint orpairwise intersecting. On the other hand, there exists a familyof n segments in the plane such that the maximum size of a subfamilywith pairwise disjoint or pairwise intersecting elements innlog2/log5 n0·431. 相似文献
114.
Non empirical LCAO SCF MO investigations of electronic reorganizations accompanying core ionizations
Calculations have been carried out on an extensive series of molecules for both the neutral species and core ionized states. Substituent effects on C1s , N1s , O1s , and F1s levels have been investigated and where available comparison has been drawn with experiment. Comparison with Koopmans' theorem has allowed a relatively detailed study of change in relaxation energies as a function of substituent effect on a given core level. Whilst for C1s levels the computed shifts in core binding energies are approximately linearly related to differences in relaxation energies for the N1s , O1s , and F1s levels, the relative electronegativity of the substituent can invert this correlation. The empirical correction of Koopmans' theorem for differences in relaxation energies at different sites has been investigated for large molecules. The results compare well with direct hole state calculations. 相似文献
115.
Jiri KasparecJerry L. Adams Joseph SiskoDomingos J. Silva 《Tetrahedron letters》2003,44(24):4567-4570
A novel, highly efficient and scalable route for the synthesis of trisubstituted pyrido[2,3-d]pyrimidin-7-ones was developed. The target compounds were synthesized in five steps from readily available reagents in about 40% overall yield. 相似文献
116.
Charles W. Jefford Mehmet Acar Arlette Delay Jiri Mareda Ulrich Burger 《Tetrahedron letters》1979,20(21):1913-1916
The addition of difluorocarbene to benzobarrelene gives the syn and anti 1,2 mono-adducts. Heating of either of them at 200°C in hexadeuteriobenzene generates naphthalene and 3,3-difluorocyclopropene. 相似文献
117.
Schram K Miketova P Slanina J Humpa O Taborska E 《Journal of mass spectrometry : JMS》2004,39(4):384-395
Samples of 1,3- (1) and 1,5-dicaffeoylquinic acid (2) and their hexaacetate derivatives were examined using positive and negative electrospray ionization mass spectrometry and tandem mass spectrometry. Differences in the various spectra allow the discrimination of each of the isomers. Specific losses in the spectra of 2 also permit the identification of the site of substitution of one of the caffeic acid moieties as being at the 5-position. The spectra of 3,5- (3) and 4,5-dicaffeoylquinic (4) acids and their hexaacetate derivatives were compared with those of 1 and 2 and their derivatives, and differences in ion abundances or the presence/absence of specific ions can be used to identify uniquely each of the compounds. 相似文献
118.
Jrme Lacour David Monchaud Jiri Mareda France Favarger Grald Bernardinelli 《Helvetica chimica acta》2003,86(1):65-81
Aromatic ortho‐disulfone derivatives are readily accessible from diiodide precursors by CuI‐mediated reaction with sodium sulfinate salts (DMF, 110°). The sulfonyl substituents adopt in solution and in the solid state two enantiomeric conformations (λ and δ) as evidenced by 31P‐ and 1H‐NMR data of the chiral D3‐symmetric tris{4,5‐bis[(4‐methylphenyl)sulfonyl]benzene‐1,2‐diolato(2?)‐κO,κO′}phosphate(v) anion ( 3a ) and 1,2‐bis(camphor‐10‐sulfonyl)‐4,5‐dimethoxybenzene ((=1,2‐bis{{[(1S,4R)‐7,7‐dimethyl‐2‐oxobicyclo[2.2.1]hept‐1‐yl]methyl}sulfonyl}‐4,5‐dimethoxybenzene; 6c ). X‐Ray structure analysis of 1,2‐dimethoxy‐4,5‐bis(methylsulfonyl)benzene ( 6a ) and 1,2‐dimethoxy‐4,5‐bis(4‐methylphenyl)sulfonyl]benzene ( 6b ) confirmed in the solid state the preferred chiral orientation of the sulfonyl groups. Dynamic conformational isomerism was detected for 6c in its 1H‐NMR in the temperature range of 110°, the corresponding free energy being 19.8 kcal?mol?1. 相似文献
119.
Charles W. Jefford Maria G. H. Vicente Yvan Jacquier France Favarger Jiri Mareda Patricia Millasson-Schmidt Gerhard Brunner Ulrich Burger 《Helvetica chimica acta》1996,79(5):1475-1487
The treatment of artemisinin ( 1 ) and β-artemether ( 6 ) with Zn dissolving in AcOH for a few hours results in mono-deoxygenation giving deoxyartemisinin ( 5 ) and deoxy-β-artemether ( 7 ), respectively, as the sole product. In contrast, submission of 1 to FeCl2 · 4 H2O in MeCN at room temperature for 15 min causes only isomerization, (3aS,4R,6aS,7R,10S,10aR)-octahydro-4,7-dimethyl-8-oxo-2H-10H-furo[3,2-i] benzopyran-10-yl acetate ( 8 ) and (3R)-3-hydroxydeoxyartemisinin ( 9 ) being produced in 78 and 17% yield, respectively. The action of FeCl2 · 4 H2O in MeCN on 6 is similar. Under the same conditions, 6 gives products analogous to 8 and 9 accompanied by an epimeric mixture of 2-[4-methyl-2-oxo-3-(3-oxobutyl)cyclohexyl]propanaldehyde in yields of 32, 23, and 16%, respectively. No epoxide is formed on repeating the last two experiments in the presence of cyclohexene. The deoxygenation of 1 and 6 by Zn is rationalized in terms of its oxophilic nature. The catalyzed isomerization of 1 and 6 by Fe2+ is attributed to the redox properties of the Fe2+/Fe3+ system. 相似文献
120.
The stretched exponential relaxation modulus of regular and polymer modified asphalts is studied. It is shown that this relaxation function can generate the dynamic functions of these materials very well on any finite interval of the reduced frequencies (master curves). By continuation one can, in principle, cover the whole region of master curves of G′ and G″. The dispersive defect diffusion mechanism, which leads to the stretched exponential law, points to the stronger three-dimensional structure of modified asphalt at low temperatures. The method of calculating G′ and G″ from the stretched exponential relaxation modulus is proposed and tested on one regular and one modified asphalt. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1225–1232, 1997 相似文献