摘要:

AbstractThe two‐bond15N,H coupling constant in 3,5‐dimethylpyridine was determined in 30 solvents and shown to vary between −11 and −3 Hz. An increasing ability of the solvent to form hydrogen bonds causes a steady decrease in ∣2J(15N,H)∣, which is also observed with some selected pyridine derivatives with intramolecular hydrogen bonds. A linear correlation is found between the positive lone‐pair effect on ∣2J(15N,H)∣ and the decrease in n

ISSN:0749-1581    

DOI:10.1002/mrc.1260270602

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe pure absorption 2D NOE experiment can provide small molecule cross‐relaxation rate data suitable for quantitative conformational analysis even when the data are collected in a time‐saving manner using preparatory delays (PD) far short of the recommended values of 3–5 timesT1. In our experience, the relative NOE intensities and cross‐relaxation rates are most readily extracted from cross‐relaxation spectra which are sums of adjacent rows (or columns) of the 2D data matrix. Intensity anomalies associated witht1streaks can be removed by plotting cross‐relaxation difference spectra. PD truncation produces significant deviations from diagonal symmetry which must be accounted for in the data analysis. The influence of PD truncation on apparent auto‐peak decay rates and cross‐/auto‐peak intensity ratios is examined in both real and simulated spectra in order to develop appropriate quantitation strategies. These strategies, when applied to NOESY data for aqueous prostaglandin (PG) F2α and a PG analog in organic media, yield cross‐relaxation rates that are within experimental error of those calculated from structural models or determined by more time‐co

ISSN:0749-1581    

DOI:10.1002/mrc.1260270603

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractA method for observing long range1H;1H coupling pathways of polynuclear aromatics using the HOHAHA (TOCSY) experiment is described. The method affords a potentially superior alternative to the more established long range optimized COSY (LRCOSY) experiment. Hexahelicene derived phenanthro[3′,4′:3,4]phenanthro[2,1‐b]thiophene is employed as a model compound to demonstrate the

ISSN:0749-1581    

DOI:10.1002/mrc.1260270604

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe motion of a flexible molecule in solution can be described as a Brownian motion between the different conformations that the molecule can assume. Each conformation corresponds to a local minimum of the energy surface. A simple model is given for this type of motion in relation to nuclear magnetic resonance (NMR) relaxation measurements. This is based only on the assumption that the diffusion tensor for the overall tumbling is not appreciably affected by jumping between the different conformations. The limits of the model are discussed and an illustrative application to 2‐fluorobutane is give

ISSN:0749-1581    

DOI:10.1002/mrc.1260270605

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe13C chemical shifts ofcis‐ andtrans‐benzylidenepropylamine and substituted benzylideneanilines were analysed after additons of Eu(fod)3. Thecis/transratio increases as the amount of lanthanide shift reagent increased. Thecisisomers showed large lanthanide‐induced shifts, whereas thetransisomers showed little or no e

ISSN:0749-1581    

DOI:10.1002/mrc.1260270606

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe1H and13C NMR spectra of 23 quinic acid derivatives are reported. The complete assignment of the1H NMR spectra using selective proton decoupling experiments allowed the conformation of the six membered ring to be established from vicinal coupling constants and the calculation of dihedral angles. The13C spectral assignments were made by consideration of substituent effects and comparison within the series. In some cases the similarities in chemical shifts did not allow unambiguous assignments; the chemical shifts were then calculated from empirical parameters.

ISSN:0749-1581    

DOI:10.1002/mrc.1260270607

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractSome well known Karplus equations are discussed and their shortcomings are indicated. To remedy the problems associated with the experimentally observed non‐additive substituent effects on vicinal proton‐proton coupling constants, an extension given recently is discussed; this accounts for pairwise interactions between substituents through specific quadratic cross terms used to describe the electronegativity dependence of the coefficients in a truncated Fourier series. Since this formulation will be used in combination with simultaneous least‐squares optimization of both the Fourier coefficients and the electronegativity values, an invariance property of this formulation, and of similar ones, is discussed in order to elucidate problems associated with parameter redundancy. The procedure is then applied in a stepwise manner to the analysis of a set of 1404 coupling constants calculated by a repara‐meterized version of the Extended Hückel method for ethanes singly or multiply substituted with Cl, F, Me and OH, and ethane itself. This clearly reveals its essential features, and leads to a markedly improved des

ISSN:0749-1581    

DOI:10.1002/mrc.1260270608

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe electronegativity dependence of the torsion angle‐independent term in the Karplus equation, i.e. of the ‘constant’Ain the Fourier expansionA+Bcos ϕ +Ccos 2ϕ +…, was investigated. Experimental proton‐proton coupling constants of substituted ethanes and isopropanes appeared to be suitable for this purpose. A data set was constructed which contained 70 couplings, newly measured or remeasured at 300 MHz, and 25 couplings taken from the literature. The accuracy of each data point is estimated as ≤0.02 Hz, with a few exceptions. The actual analysis was carried out on 93 data points, i.e. onJvalues of 55 mono‐ and of 38 1,1‐di‐substituted ethanes, including 22 isopropyl derivatives. A total of 55 chemical groups is represented in the set; some of these were taken together, leaving 50 distinct groups. Regression analysis of the present data versus standard electronegativities did not yield acceptable results. Instead, substituent parameters λe, valid for3J(HH) in saturated HCCH fragments, were derived in a least‐squares procedure from the data set. The couplings from mono‐ and 1,1‐di‐substituted ethanes could be accounted for in a simple expression that contains an interaction termC012(λ1λ2). The best equation obtained is\documentclass{article}\pagestyle{empty}\begin{document}$$ {}^3J{\rm (HH) = 7}{\rm .84 - 0}{\rm .59(}\lambda _{\rm 1} + \lambda _{\rm 2}) - 0.42(\lambda _{\rm 1} \lambda _{\rm 2}) $$\end{document}The parameters are valid for λevalues scaled according to the Huggins electronegativities: λH= 0, λOR= 1.40. The equation fits 84 experimental couplings with a root‐mean‐square deviation of 0.018 Hz and a maximum deviation of 0.06 Hz. Some exceptions occur. (i) CH3CH3, CH3CHCl2and CH3CHF2appear to follow a different, but correlated, regression; (ii) C(O)H, C(O)R, SO2Cl and POCl2groups require different λevalues according to the substitution pattern, i.e. mono‐ or 1,1‐di‐substitution. The striking difference between the new λesubstituent‐effect scale and other empirical electronegativity scales lies in the inverse correlation of λewith increasing electronegativity of β‐substituents. The inverse relationship is not only found for α‐carbon atoms, but appears to represent a genera

ISSN:0749-1581    

DOI:10.1002/mrc.1260270609

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

Abstract13C NMR spectra of ten formamidines, five acetamidines and five butyramidines were recorded at low temperatures. The barrier to rotation around the CN1bond forN2‐alkylformamidines is approximately 50 kJ mol−1and forN2‐benzylformamidines approximately 52 kJ mol−1, and is thusca.10 kJ mol−1lower than forN2‐phenylformamidines. The height of the barrier is related to Taft's substituent constants atN2, δG* = 49.25 + 8.08σ* −3.21Es. In the acetamidines the steric hindrance between the alkyl substituent at N2with CFCH3contributed to the decrease in δ* (lower than 40 kJ mol−1). Thei‐C3H7substituent at CFin butyramidines forces isomerization and these amidines becomeZisomers. MNDO calculations indicate no significant changes of bond length onE/Zisomerization, and the low rotational barrier in butyramidines is probably caused by the steric interaction of the aromatic ring at N2

ISSN:0749-1581    

DOI:10.1002/mrc.1260270610

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY

摘要:

Abstract13C NMR spectroscopy can be used for the quantitative analysis of mixtures of glycerides and the enantiomeric purity can be determined by using Eu(hfbe)3‐d reagent. This technique was applied to the study of the reaction mixture obtained from the transesterification of tributyrin catalysed by My‐lipase (Candida cylindracea). A ‘preference’ of this lipase for the pro‐R ester function of tributyrin is dem

ISSN:0749-1581    

DOI:10.1002/mrc.1260270611

出版商:John Wiley&Sons, Ltd.    

年代:1989

数据来源: WILEY