摘要:

AbstractActivation parameters were determined for the exchange between the two types of carboxylate groups which arecisandtransto the VO bonds in the bis(oxalato)dioxovanadate(V) ion, [VO2(C2O4)2]3−. Kinetic data were obtained from the13C NMR spectra at eleven temperatures between 268 and 306 K by the total line‐shape method. The coordination‐site exchange mechanism, involving chelate ring opening and closing in the ion, is proposed. The solid‐state13C NMR spectra of the oxalato complexes, (NH4)3[VO2(C2O4)2]·2H2O and K3[VO2(C2O4)2]·3H2O, are also discussed in relation to the

ISSN:0749-1581    

DOI:10.1002/mrc.1260290902

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractConformational studies were performed on the bicyclo [4.2.0] octane system of a series of related tetracyclic diterpenes, using molecular mechanics and1H NMR. Both methods gave compatible results, but did not permit the deduction of any quantitative relationship linking the vicinal coupling constants to the cyclobutane dihedral angles. Nevertheless, the Karplus relationship allowed a qualitative interpretation of the coupling constants in terms of conformation. A set of characteristic1H NMR coupling constants was obtained which allows the interpretation of the1H NMR spectrum of any compound presenting a similar system.

ISSN:0749-1581    

DOI:10.1002/mrc.1260290903

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY

摘要:

Abstract1H,13C and17O NMR spectra for 22 substituted nitropyridines were measured and their1H NMR spectra were analysed. The most significant variations in the NMR parameters are found for isomeric hydroxy derivatives, owing to the possibility of keto–enol tautomerism. The prevalence of the keto form is observed in 2‐ and 4‐hydroxy derivatives, while the 3‐hydroxy derivative exists in its enol form. Among the three nuclei studied,17O seems to be the best nucleus for probing the keto–enol tautomerism. No correlation is observed between the torsion angle of the nitro group and its17O NMR chemical shift. Molecular mechanics calculations were performed to clarify the torsional energetics of the nitro group and the preferences for keto–enol

ISSN:0749-1581    

DOI:10.1002/mrc.1260290904

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe structural determination of modified mononucleosides is important for understanding the chemistry, structure and functional changes that they impart to the nucleic acids in which they occur. Empirical analyses ofJ(HH)couplings have been used to describe nucleoside conformational equilibria for the glycosidic angle (syn‐anti), the C‐4′C‐5′ exocyclic rotamers (gauche+,gauche− andtrans) and the sugar pucker (N–S). Distance geometry calculations with distance constraints determined from nuclear Overhauser effects (NOE) should provide equally satisfactory structural information. Proton and carbon chemical shifts and coupling constants were used to describe structural parameters for the tricyclic nucleosides wyosine, 4,9‐dihydro‐4,6‐dimethyl‐9‐oxo‐3‐(β‐D‐ribofuranosyl)imidazo(1,2‐a)purine, and three of its isomers. Using distance geometry calculations with the 14‐19 observed NOEs per nucleoside as distance constraints, multiple structures could be produced with less than 1 Å relative mean standard deviation for the positioning of all atoms. The two methods of structural determination were consistent in their qualitative determination

ISSN:0749-1581    

DOI:10.1002/mrc.1260290905

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe assignments of the1H and13C NMR signals of 1‐substituted isatins and their corresponding 3‐(dicyanomethylidene)indol‐2‐ones were derived by analysis of the13C1H spin coupling patterns and by inspection of the two‐dimensional C, H shift correlation spectra. Erroneous literature assignments have been corrected. A significant difference between the5Δ‐deuterium isotope effects on the13C chemical shifts of the CN groupscisandtransto the heterocyclic amide fragment (D)HNCO was observed in partially deuteriated 3‐(dicyanomethylidene)indol‐2‐one. This geometric dependence was used to assign the two CN groups that are otherwi

ISSN:0749-1581    

DOI:10.1002/mrc.1260290906

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe17O NMR spectra of terminal oxygen atoms in esters, anions and amides of substituted arenesulphinic acids and in esters and amides of substituted arenesulphonic acids were measured. The signals of the terminal O appear close to those of the bridge O. Compared with carbonyl O, terminal SO shows (a) a lower sensitivity to the electronic influences of geminal groups, (b) only a low sensitivity to arene ring substituents and (c) small solvent effects. The difference between C‐ and S‐bound O is discussed in terms of π‐bond character. Dy3+complexation occurs with the terminal O in methyl arenesul

ISSN:0749-1581    

DOI:10.1002/mrc.1260290907

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY

摘要:

Abstract13C NMR spectra of six epimeric substituted 4‐hydroxypiperidines and two corresponding piperidines were recorded. Substituent parameters for equatorial methyl,gem‐dimethyl, equatorial hydroxy and axial hydroxy groups were calculated from the13C chemical shifts of these compounds and several other six‐membered ring compounds. The effects of nearby substituents on these parameters were examined. It is concluded that the magnitude and even the sign of the α effect is modified by the presence of nearby substituents. It was found thatgaucheinteractions play an important role in determining the magnitude of the α effect. However, the β effect is not influenced by the nearby substituents except for thegem‐dimethyl group. The magnitude of the γ effect is almost independent of the nearby s

ISSN:0749-1581    

DOI:10.1002/mrc.1260290908

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractSeveral NMR experiments, one proton and several carbon detected, were used for the determination of1J(C,H) values in saccharides. Couplings of anomeric carbons were measured from the13C satellites in1H NMR spectra. The simultaneous application of several purging schemes resulted in a very good suppression of the proton signals from12C molecules.1J(C,H) values of all saccharides were determined by 2DJ‐resolved spectroscopy using an INEPT‐type pulse sequence with a BIRD pulse for the suppression ofnJ(C,H) and folding in F1. Rules for the calculation of the correct values of1J(C,H) from the observed reduced splittings are proposed.1J(C,H) values were also obtained from a set ofJ‐modulated one‐dimensional13C spectra. A procedure for the determination of1J(C,H) values from only two one‐dimensional experiments is proposed. Mannose derivatives (mono‐ and poly‐saccharides) were used to test and compare the individual methods. Differences in1J(C,H) values for C‐2 and C‐3 were used for the chemical shift assignment

ISSN:0749-1581    

DOI:10.1002/mrc.1260290909

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe addition of β‐cyclodextrin to a solution containing β‐benzyloxy‐α‐propionic‐substituted DOTA‐ and DTPA‐Gd(III) complexes leads to the formation of inclusion compounds whose equilibrium constants have been determined by NMR titrations. The main effect associated with the formation of this ‘host‐uest’ interaction is a marked elongation of the molecular reorientational time, τr, which in turn results in increased solvent

ISSN:0749-1581    

DOI:10.1002/mrc.1260290910

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY

摘要:

Abstractβ‐Lactam‐condensed saturated thiaazabicycles underwent ring transformation to give a new type of alicycle‐fused 3‐phenyl‐2‐methoxycarbonyl‐4,5‐dihydro‐1,4‐thiazepine. Oxidation of thecisandtranscyclohexane derivatives with perbenzoic acid furnished sulphones. The configurations and conformations of the new compounds were confirmed by1H and13C NMR spectroscopy, making use of DR, DNOE, DEPT and

ISSN:0749-1581    

DOI:10.1002/mrc.1260290911

出版商:John Wiley&Sons, Ltd.    

年代:1991

数据来源: WILEY