Magnetic Resonance in Chemistry

Holzer, W.; Von Philipsborn, W.

doi: 10.1002/mrc.1260270602pmid: N/A

The two‐bond 15N,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 nitrogen shielding.

Andersen, Niels H.; Eaton, Hugh L.; Lai, Xiaonian

doi: 10.1002/mrc.1260270603pmid: N/A

The 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 times T1. 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 with t1 streaks 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) F2 α 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‐consuming 1D NOE methods.

Johnston, Milton D.; Martin, Gary E.

doi: 10.1002/mrc.1260270604pmid: N/A

A method for observing long range 1H;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 method.

Baldo, M.; Grassi, A.

doi: 10.1002/mrc.1260270605pmid: N/A

The 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 given.

Al‐Rawi, Jasim M. A.; Saleem, L. M. N.

doi: 10.1002/mrc.1260270606pmid: N/A

The 13C chemical shifts of cis‐ and trans‐benzylidenepropylamine and substituted benzylideneanilines were analysed after additons of Eu(fod)3. The cis/trans ratio increases as the amount of lanthanide shift reagent increased. The cis isomers showed large lanthanide‐induced shifts, whereas the trans isomers showed little or no effect.

Flores‐Parra, Angelina; Gutiérrez‐Avella, Dora Marina; Contreras, Rosalinda; Khuong‐Huu, Françoise

doi: 10.1002/mrc.1260270607pmid: N/A

The 1H and 13C NMR spectra of 23 quinic acid derivatives are reported. The complete assignment of the 1H 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. The 13C 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.

Donders, Lambertus A.; De Leeuw, Franciscus A. A. M.; Altona, Cornelis

doi: 10.1002/mrc.1260270608pmid: N/A

Some 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 description.

Altona, Cornelis; Ippel, Johannes H.; Hoekzema, Aldert J. A. Westra; Erkelens, Cornelis; Groesbeek, Michel; Donders, Lambertus A.

doi: 10.1002/mrc.1260270609pmid: N/A

The electronegativity dependence of the torsion angle‐independent term in the Karplus equation, i.e. of the ‘constant’ A in the Fourier expansion A + B cos ϕ + C cos 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. on J values 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 for 3J(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 term C012(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 λe values 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, CH3CHCl2 and CH3CHF2 appear to follow a different, but correlated, regression; (ii) C(O)H, C(O)R, SO2Cl and POCl2 groups require different λe values according to the substitution pattern, i.e. mono‐ or 1,1‐di‐substitution. The striking difference between the new λe substituent‐effect scale and other empirical electronegativity scales lies in the inverse correlation of λe with increasing electronegativity of β‐substituents. The inverse relationship is not only found for α‐carbon atoms, but appears to represent a general phenomenon, also seen for substituted α‐hetero atoms (O, N, S).