摘要:

AbstractThe intramolecular Hbond strength incis‐enolic forms of β‐diketones was investigated. The correlations between the1H hydroxyl proton shieldings and the chemical shifts of the13C and17O nuclei involved in the chelate ring, and the H/D isotope shifts on the latter resonances, were determined. An increase in the hydrogen bond strength leads to increasing values of H/D isotopic shifts on the17O resonances. The results obtained are compared with quantum chemical calculations and x‐ray meas

ISSN:0749-1581    

DOI:10.1002/mrc.1260271002

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe dynamic NMR (DNMR) of the intramolecular acetate scramblings that occur in Ca(II)‐, Sr(II)–, Ba(II)– and La(III)–cyclohexanediaminetetraacetate complexes has been studied. The working temperature range was extended to 430 K by using a sealed thick‐walled capillary sample tube and new DNMR phenomena were thus observed. Information on activation energy, enthalpy, entropy and the free energy of nitrogen inversion of the chelates was obtained by computer

ISSN:0749-1581    

DOI:10.1002/mrc.1260271003

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractA convenient31P NMR method for determining the enantiomeric excess of free 1‐aminoalkylphosphonic acids is presented. It was found by NMR that the interaction of PdIIions with 1‐aminophosphonate ligands (L) yields, in a non‐diastereoselective manner, diastereoisomeric chelate pairs (PdL2) observable in alkaline D2O solutions. The chelate complexes give two peaks in the31P NMR spectra; one corresponds to the chiral species e (both ligands areRorSenantiomers) and the second to themesoforms (RandSligands). The shapes of these peaks and the differences in the chemical shifts for the species (Δδ) depend on the temperature and the resonance frequency, typical for a dynamic process. Under the experimental conditions used (273–293 K and 40.48 or 121.5 MHz) Δδ values in the range 0.03–0.18 ppm were obseved. The separation of the31P NMR signals of eight of the ten investigated racemic acids allowed the determination of the enantiomeric purity. The measured enantiomeric excess of an enantiomerically enriched sample of 1‐aminobenzylphosphonic acid (prepared by weighing) was in excellent agreement with the

ISSN:0749-1581    

DOI:10.1002/mrc.1260271004

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe unambiguous assignment of all the1H,13C and15N resonance signals in methyl β‐D‐glucopyranoside tetra‐nitrate and β‐cellobiose octanitrate was made using homonuclear and heteronuclear13C1H and15NOC1H two‐dimensional shift correlation NMR experiments. Solvent‐induced shifts were used to confirm the13C1H correlations in β‐cellobiose octanitrate. Based on these observations and selective decoupling experiments, the15N resonance signals in cellulose trinitrate

ISSN:0749-1581    

DOI:10.1002/mrc.1260271005

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

Abstract1H NMR data for isoreserpiline and tetraphylline and1H and13C NMR data for reserpiline hydrochloride are presented. New insights into the1H NMR coupling constants of the heteroyohimibines provide an easy means of assigning their configuration and conformation, allowing the elucidation of the stereochemical features of the two isomers of reserpiline hydrochloride.

ISSN:0749-1581    

DOI:10.1002/mrc.1260271006

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe principal elements of13C chemical shift tensors in some dicarboxylic acids were determined from powder samples by two methods: (i) from the theoretical fit of the experimental data and (ii) from the side‐band intensities of the CP‐MAS spectra using the graphical procedure of Herzfeld and Berger. Neither the methylene nor the carboxyl carbons are axially symmetrical. The methylene tensors corresponding to higher dicarboxylic acids were not analysed owing to severe overlap of the different carbon tensors. The influence of local molecular geometry on the carbon13C chemical shielding tensors in dicarboxylic acids and other related organic molecules are also reported. The σ22component of the tensor shows a linear correlation as a function of the structural parameter, the OCO bond angle, around this carboxyl carbon atom. The significance of the symmetry factor calculated for the carboxyl carbons is also prese

ISSN:0749-1581    

DOI:10.1002/mrc.1260271007

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe cluster11B chemical shifts in members of the important class ofcloso‐3,1,2‐metalladicarbaboranes were previously unassigned, and their cluster1H chemical shifts previously unmeasured. The use of [11B11B]‐COSY and1H‐{11B} NMR spectroscopy to measure and assign the cluster11B and1H chemical shifts of (previously unreported) [3‐(η6‐C6Me6)‐closo‐3,1,2‐RuC2B9H11] as a model compound is reported. The11B shielding patterns of this species and the nonmetalla simple carbaborane analogue [1,2‐C2B10H12] are compared

ISSN:0749-1581    

DOI:10.1002/mrc.1260271008

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe chemical shift (δ) of a xenon monoatomic molecule is very sensitive to the environment. The major contribution to this shift arises from the van der Waals interactions between the solvent and the xenon molecules. Usually, this solvent effect is interpreted via a reaction field model. This approach leads to a search for a correlation between δ and a function of the square of the refractive index of the solvent [f(n2)]. This paper illustrates the use of a theoretical model developed in our laboratory for the interpretation of the solvent effect on the xenon chemical shift. This model allowed the calculation of the xenon‐solvent dispersion energy (Edis) for different solvents, and the correlation ofEdiswith δ. TheEdis‐δ correlation is clearly better than the f(n2)‐δ c

ISSN:0749-1581    

DOI:10.1002/mrc.1260271009

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe 2‐pivaloyl‐1,4‐benzoquinone radical anion has been prepared by alkali metal reduction in tetrahydrofuran in the presence of dibenzo‐18‐crown‐6, 15‐crown‐5, or 12‐crown‐4, and its ESR spectrum in each system has been recorded over a range of temperatures. Line‐width variation in the ESR spectrum is observed in some systems. This line‐width variation has been attributed to cation migration between sites adjacent to the non‐equivalent carbonyl groups rather than to restricted rotation of the pivaloyl group. In some systems, however, the rate of migration is too fast and in others too slow, compared with the time‐scale of the ESR experiment, for line‐width variation to be observed. The influence of each crown ether upon the rate of cation migration has been found to depend upon the size of the crown ether ‘cavity’ compared with the siz

ISSN:0749-1581    

DOI:10.1002/mrc.1260271010

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractIsocyanate salts prepared fromcis‐2‐amino‐1‐cyclopentane‐ and ‐hexane‐carboxylate and the correspondingtrans‐cyclohexane and ‐cyclohexene derivates with KSCN were cyclized to cycloalkane/ene‐condensed 2‐thioxo‐4‐pyrimidinones, which were transformed to tricyclic 1,3‐thiazino[3,2‐a]pyrimidinones by addition of dimethyl acetylenedicarboxylate. The stereo structures of the new compounds were established by IR and1H and13C NMR spectroscopy, making use of the signal multiplicities in the

ISSN:0749-1581    

DOI:10.1002/mrc.1260271011

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY