摘要:

AbstractFree energies of activation, ΔG 298≠, for the rotation of the phenyl group in the range of 46.06–63.25 kJ mol−1were observed in some model β‐hydroxyphosphonates. These barriers were found to depend on the type of substituents at the phosphorus atom and on the substituents at the β‐position with respect to the phenyl ring. The steric origin of this observation is discussed. Dynamic NMR studies were performed in order to determine the activation parameters for the exchange process. For that purpose, variable‐temperature1H and13C NMR spectra were treated by non‐iterative, iterative and iterative double‐fit procedures. MNDO, AM1 and PM3 theoretical calculations of the barriers for three of the compounds studied are reported and results are compared with the experiments. It is found that the PM3 results are very close to t

ISSN:0749-1581    

DOI:10.1002/mrc.1260321102

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractThe structure of the pentasubstituted pyridines obtained in the reaction of methyl 2‐isothiocyanato‐3‐phenylpropenoate with 1‐(N,N‐diethylamine)prop‐1‐yne were assigned based on 2D heteronuclear correlation experiments and NOE measurements. At 0°C a 2‐azabicyclo[2.2.2] octa‐2,7‐diene intermediate was isolated and spectroscop

ISSN:0749-1581    

DOI:10.1002/mrc.1260321103

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractThe1H and13C NMR spectra of three stereoisomeric 2‐(p‐chlorophenylthiomethyl)‐3‐(3‐methoxyphenyl)‐5‐carbomethoxy‐N‐methylpiperidines were analysed in terms of configuration and solute conformation. These studies together with 2DJ‐resolved, 2D‐COSY and 2D1H13C‐correlated spectra demonstrate the existence of 2a‐3a‐5e, 2e‐3a‐5

ISSN:0749-1581    

DOI:10.1002/mrc.1260321104

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractLong‐range1H13C coupling constants were measured for some simple, rigid model compounds containing hydroxyl, bromine or carbonyl groups. These were performed using the recently proposed selectiveJ‐resolved SPININEP and SDEPT‐2D experiments. The geminal and vicinal heteronuclear coupling constant values were analysed in terms of the nature of substituent, the position along the coupling path and the orientation of the substituent relative to the bounds. In some cases, through‐space interactions of the substituent group were also observed. Sizeable substituent effects are associated with the electronegativity and lone pair of the oxygen nucleus in the hydroxyl and carbonyl systems. In addition, the π‐system of the carbonyl group may also be important. Such substituent effects may be as large at ±5 Hz. SincenJCHconstants rarely exceed 10 Hz, these effects can produce important deviations innJCHmagnitudes and therefore are essential for applications to stereochemical and conformati

ISSN:0749-1581    

DOI:10.1002/mrc.1260321105

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractCoherence pathway selection with gradients in carbon‐detected experiments is demonstrated. Gradient selection (gs) offers excellent suppression of artifacts and unwanted magnetization, and in practice the signal‐to‐noise ratio is at least as good as for the equivalent phase cycled experiment. A selective gs‐1D‐13C‐COSY is used to correlate one carbon resonance with its neighbours. The gs‐INADEQUATE is used to select multi‐quantum coherences; this can be performed as a 1D or a 2D experiment. Complete elimination oft1noise is obtained in the gs version only. Applications to labelled glucose, 3‐phenylpropan‐1‐ol and natural abundance menthol are presented. Interference from high‐frequency stray radiation in the gradient cable is removed by inserting a low‐pass filter between the grad

ISSN:0749-1581    

DOI:10.1002/mrc.1260321106

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractEmpirical group electronegativities (substituent parameters λi), valid for3J(HH) in saturated HCCH fragments, were derived from the coupling to methyl in substituted ethanes and isopropyl derivatives according to the equation\documentclass{article}\pagestyle{empty}\begin{document}$$ \langle {}^3J({\rm{HH)}}\rangle = 7.660 - 0.596(\lambda _1 + \lambda _2) - 0.419(\lambda _1 \lambda _2) $$\end{document}In contrast to earlier work, it was found advantageous to differentiate between the λivalues of hydrogen acting as substituent in CH3as compared with H in CH2. Special attention was paid to solvent effects, in particular the influence of D2O, on the vicinal couplings and thus on λi. The previously derived λivalues remain valid in all common organic solvents but a special effect of D2O on λ is manifest in cases where the α‐substituent carries one or two non‐conjugated lone pairs of electrons that readily act as hydrogen bond acceptors: Δλ= −0.11 ± 0.03 for NH2, NHR, NR2, OH, OR, R = alkyl. Protonation of NH2to give NH3+lowers λiby 0.28 units. The λivalues for the nucleic acid bases (Ade, Gua, Ura, Thy, Cyt), as determined from theN‐isopropyl derivatives, are 0.56 ± 0.01 irrespective of the solvent. Secondary amides display similar values. The parameters of the Haasnoot equation, originally derived with the aid of a Pauling‐type electronegativity scale, were reoptimized on the basis of the present λiscale; the previous overall r.m.s. error of 0.48 Hz now drops to 0.36 Hz and separate parameterization of HCCH fragments with different substitution patterns

ISSN:0749-1581    

DOI:10.1002/mrc.1260321107

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

Abstract13C,29Si,119Sn and207Pb cross‐polarization magic angle spinning (CP/MAS) spectra for a series of closely related compounds R4−nM(CCR′)n, (n= 1−4; M = Si, Sn, Pb; R = alkyl; R′ = alkyl, SiMe3) are reported. Shielding tensor components for29Si,119Sn and207Pb as obtained from CP/MAS spectra are discussed in relation to the respective highest possible molecular symmetry group as a function of the degree of substitutionn= 1−4. For this class of compounds the orientation of the shielding tensor of M within the molecular frame can sometimes be derived from symmetry considerations. The limitations of such considerations, based on shielding tensor components obtained from polycrystalline powder samples under CP/MAS conditions,

ISSN:0749-1581    

DOI:10.1002/mrc.1260321108

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractThe versatility of multinuclear magnetic resonance techniques in the determination of stability constants of metal complexes is demonstrated for three cases in which potentiometry is impossible or less suitable. The complexation of cyclo‐trimetaphosphate to lanthanide aminopolycarboxylate complexes was investigated in a dilution experiment using31P NMR shifts. The stabilities of CdL and CdL2[L = 1‐(2‐aminoethylamino)‐1‐deoxy‐D‐galactitol] were determined by113Cd NMR speciation, in which the occurring species were in slow exchange. Competition experiments using both31P NMR shifts and13C and31P relaxation rate enhancements were performed to determine the stabilities of Nd(NTA)(PPP) and Nd(PPP)2(PPP = tripolyphosphate). Where possible, the results were compared with potentiometric data and found to be in g

ISSN:0749-1581    

DOI:10.1002/mrc.1260321109

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractThe15N chemical shifts of thirteen NH‐pyrazoles in the solid state and of seven NH‐pyrazoles in [2H8] THF solution at 170–175 K (with frozen annular tautomerism) are reported. The solid‐state values are discussed using an additive model. The differences in chemical shifts between the solid state and solution are due to the different intermolecular hydrogen bonds present in the various envir

ISSN:0749-1581    

DOI:10.1002/mrc.1260321110

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

ISSN:0749-1581    

DOI:10.1002/mrc.1260321111

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY