摘要:

AbstractA linear relationship has been found to exist between the aluminum‐27 and gallium‐71 NMR chemical shifts of a series of structurally analogous aluminum and gallium compounds having only oxygen in the first metal coordination spheres; δ71Ga = 2.83(δ27Al) − 4.50. This relationship allows the prediction of71Ga chemical shifts for such gallium compounds from the27Al values already known for their aluminum analogues, and also a greater understanding of observed71Ga chemical

ISSN:0749-1581    

DOI:10.1002/mrc.1260311002

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractScleroglucan is a branched poiysaccharide, soluble in water. Solid‐state and solution13C NMR were used to follow the fixation of water molecules on anhydrous scleroglucan and the evolution of the scleroglucan—water complex from the solid state to the gel and solution state. Determination of both chemical shift values and relaxation times (T CHD,T1pHandT1C) for anhydrous and hydrated scleroglucans permit the observation of a transition between the two forms. The fixation of water leads to an increase in the solid‐state order, in the form of a triple‐helix association, and at the same time an increase in the interchain distance and in the molecular motion. The gel state obtained for a water‐scleroglucan composition of 90:10 is characterized by an important macromolecular rigidity which implies the use of the solid‐state NMR techniques. Solution13C NMR can be utilized to characterize the primary structure and the triple‐helix random‐coil conforma

ISSN:0749-1581    

DOI:10.1002/mrc.1260311003

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractA series of 2,6‐dihydroxy acylaromatic compounds were investigated to characterize the rotational and hydrogen bonding properties of the carbonyl group. Deuterium isotope effects on1H and13C chemical shifts due to deuteriation of OH groups were determined at both ambient and low temperature. In the latter case isotope effects on chemical shifts of the individual rotamers can be determined. Deuteriation of one of the OH groups may lead to isotopic perturbation of the tautomeric equilibrium of the carbonyl group and the two hydroxyl groups. The perturbation was found to be larger in ketones than in esters. Complete band shape analysis of the OH resonances of the esters and ketones in a temperature interval above and below the coalescence temperature led to ΔG≠,AH≠and ΔS≠values for various concentrations of added THF‐d8. ΔS≠was found to be strongly negative. Temperature coefficients for the shift of the OH resonances showed large variations for esters and ketones owing to the different hydrogen bond patterns. The esters have two intramolecular hydrogen bonds, one strong and an additional weaker one between the OH and OR groups. The second OH group of the ketones was shown to point primarily towards C‐5. Increasing amounts of THF‐d8increased the amount of this rotamer. The anisotropy of the XCO group at C‐2, C‐6 was shown to lead to a low‐field shift of C‐2, very different from that found for CO groups without hydrogen bonds. The anisotropy caused by OH groups can also be estimated. On the basis of the thermodynamic parameters, a model for the rotation of the ester group is suggested. The rate‐determining step involves both intramolcular hydrogen bonds, which are twisted out of the ring plane to form hydrogen bonds to the solvent

ISSN:0749-1581    

DOI:10.1002/mrc.1260311004

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractDeuterium isotope effects on13C NMR chemical shifts, (2H/1H)nΔ(13C), of 2‐methylene‐d2‐adamantane, 4‐methylene‐d2‐2‐adamantanone and 6‐methylene‐d2‐2‐adamantanone are presented. Long‐range effects (4Δ,5Δ and6Δ) in a range from 5 to – 23 ppb were observed. The results for5Δ and6Δ are explained in terms of cross‐ring field effects resulting from the change of dip

ISSN:0749-1581    

DOI:10.1002/mrc.1260311005

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractUtilizing 2D NMR spectroscopy (COSY, NOESY, TOCSY, inverse detected1H/13C correlation spectroscopy), the1H and13C NMR spectra of the ketone and epoxide precursors of β‐chamigrene were assigned. The dihedral angles and distances determined from NMR measurements provide input parameters to calculate the molecules' solution conformation by distance geometry. The factors which are important in influencing the conformation of these molecules are discuss

ISSN:0749-1581    

DOI:10.1002/mrc.1260311006

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractAll‐trans‐(9′Z,11′Z)‐(3R,3′S,5′R,6′R)‐Pyrrhoxanthin was isolated from a natural bloom of dinoflagellates (mainlyCeratiumspp.) and submitted to detailed1H NMR and13C NMR analysis, including homonuclear COSY (2D), ROESY (1D and 2D), TOCSY (1D) and1H‐detected one‐bond and multiple‐bond1H,13C COSY. All chemical shifts and coupling constants in the1H NMR spectrum and all carbons in the13C NMR spectrum were assigned. The result is consistent with the structure, previously deduced, including relative configuration. A recently proposed variant of the ROESY pulse sequence, called T‐ROESY, claimed to suppress undesired TOCSY transfer, provided excellent results free of TOCSY artifacts caused by coherent magnetization transfer viaJcoupling pathways independent of the choice of the position

ISSN:0749-1581    

DOI:10.1002/mrc.1260311007

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractIntermolecular effects are shown to induce a variation of about 5 ppm in the nitrogen NMR shieldings of aromatic nitro groups. The latter turn out to be comparable to those exerted by substituents in nitrobenzene derivatives in solutions in a given solvent. Substituent effects on the NO2nitrogen shielding in nitro benzenes, in a given solvent, seem to reflect contributions from the so‐called field‐inductive effects on the electron charge distribution in such systems, and are insensitive to the resonance effects concerned with the delocalized pπ‐electron systems in

ISSN:0749-1581    

DOI:10.1002/mrc.1260311008

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractReaction of the platinum complex K[PtCl3(NH3)] with NH3or NH2(CH2)nNH2in the presence of I−gives the mixed chloroiodoamine complexescis‐[PtCl3(NH3)2] and [{cis‐Pt(Cl)3(I)(NH3)2}2μNH2(CH2)nNH2], respectively. These findings prompted an examination of the nature of the reaction of K[PtCl3(NH3)] with I−in detail. Rapid displacement of two chlorides to give thetrans‐[Pt(I)2Cl(NH3)]−anion is observed with a subsequent slow displacement of the third chloride to give the [PtI3(NH3)]−anion. Concomitant with the formation of the triiodo anion, dimerization occurs, giving the iodo‐bridged compound [PtI2(NH3)]2. Correlation of δ(195Pt) and δ(15N) chemical shifts and1J(195Pt,15N) coupling constants was made with literature values, allowing a complete set of data for the series [PtXn(NH3)4 −n](2 − n)+with systematic variation of X

ISSN:0749-1581    

DOI:10.1002/mrc.1260311009

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractA systematic study of sensitivity enhancement in15N NMR spectra of phosphorus—nitrogen compounds by31P,15N polarisation transfer was performed using tris‐(dimethylamino)phosphine (1) as a model compound. The application of several multipulse techniques (refocussed and non‐refocussed INEPT, DEPT) and decoupling schemes shows that maximum enhancement was reached with the non‐refocussed INEPT sequence, whereas the performance of longer pulse sequences was deteriorated by relaxation effects. The practical application of the technique was demonstrated for the determination of15N data of low coordinate phosphorus—nitrogen compounds (iminophosphines and imidometaphosphate ions). In favourable cases, the sensitivity gain may be comparable to that of1H,15N INEPT measurements via long‐range couplings. In addition to the 1D experiments, the practicability of heteronuclear correlation spectroscopy was demonstrated by the measurement of 2D31P,15N shift‐correlated

ISSN:0749-1581    

DOI:10.1002/mrc.1260311010

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractAnomalous1H NMR spectra have been obtained for two related carcinogen‐nucleoside adducts in which a 1‐ or 3‐aminobenzo[a]pyrene ring is bound from its C‐6 position to the N−2 position of 2′‐deoxyguanosine. NOESY and COSY measurements at low temperature in methanol‐d4revealed chemical exchange between at least two forms. Large chemical shift differences exist between subspectra for all corresponding protons of the nucleoside moiety, whereas all such chemical shift differences are small for the protons of the benzo[a]pyrene moiety. Thus, at intermediate rates of exchange, the nucleoside resonances are broadened beyond recognition, whereas the aminobenzo[a]pyrene resonances appear normal. This is linked with the large anisotropic ring‐current field from the carcinogen ring system, and restricted internal rotation at the site of attachment of the carcinogen to the nucleic acid base. The possibility of an absence of resonances from a nucleoside moiety can complicate structure elucidation of unknown carcinogen‐nucleoside adducts and related compounds, especiall

ISSN:0749-1581    

DOI:10.1002/mrc.1260311011

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY