Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260260301pmid: N/A

Kövér, Katalin E.; Batta, Gy.

doi: 10.1002/mrc.1260260302pmid: N/A

A spin–echo sequence is proposed for the suppression of the generally unnecessary strong direct cross‐peaks in two‐dimensional heteronuclear NOE and relayed incoherent transfer spectroscopy. The procedure also increases the intensity of “remote peaks,” thereby enhancing the sensitivity of these techniques.

Gonnella, Nina C.; Chen, Jen

doi: 10.1002/mrc.1260260303pmid: N/A

The stereochemical and structural analysis of matrine was carried out using nuclear magnetic resonance spectroscopy. Several methods, including 13C1H two‐dimensional shift correlation spectroscopy, 1H1H two‐dimensional correlated spectroscopy and measurements of vicinal 1H1H coupling constants were used for chemical shift assignments and configurational analysis. Nuclear Overhauser effect difference spectroscopy was used to establish the sterochemistry. This investigation demonstrates the power of two‐dimensional NMR techniques and the nuclear Overhauser effect (NOE) in the structural determination of natural products.

Woodgate, P. D.; Herbert, J. M.; Denny, W. A.

doi: 10.1002/mrc.1260260304pmid: N/A

The 13C NMR spectra of some 1‐ and 2‐substituted perimidines and perimidinium salts are discussed and assigned. Although the spectra of most of the 2‐substituted perimidines are comparatively simple, several examples display 13C and 1H spectral characteristics indicative of inhibition of prototropic tautomerism.

Johnson, Mark R.; Colburn, Christopher A.; Ganion, Susan J.; Son, Byungki; Mosbo, John A.; Sousa, Lynn R.

doi: 10.1002/mrc.1260260305pmid: N/A

13C NMR studies of alkali metal cation complexation by six related crown ethers are described. The crown ethers (and ions studied) were 2,3‐naphtho‐20‐crown‐6 (Na+, K+, Rb+, Cs+), 2,3‐naphtho‐17‐crown‐5 (Na+, K+, Rb+, Cs+), 2,3‐naphtho‐14‐crown‐4 (Na+, K+, Rb+, Cs+), 1,8‐naphtho‐21‐crown‐6 (Na+, K+, Rb+, Cs+), 1,8‐naphtho‐18‐crown‐5 (Na+, K+, Rb+, Cs+) and 1,5‐naphtho‐22‐crown‐6 (K+, Rb+, Cs+). For each crown ethercation combination, 13C NMR spectra were obtained for a number of different ion concentrations in CD3OD. Data were analysed by a non‐linear least‐squares treatment to give complexation constants and chemical shifts that would result at the limit of complete 1:1 complexation. Analyses of the limiting chemical shifts for a number of carbons indicate regular, cation‐dependent trends (for Cs+ through Na+). Deviations from these trends are interpreted as reflecting differences in conformations of crown ether–cation complexes. Conformational differences between cation complexes that occur near the point of naphthalene attachment cause deviations from the trend in the aromatic carbon chemical shifts, while conformational differences further from the naphthalene cause trend deviations only in the crown ether carbon chemical shifts.

Kiehlmann, E.; Tracey, A. S.

doi: 10.1002/mrc.1260260306pmid: N/A

The 13C NMR spectra of 21 catechin derivatives substituted at C‐6/C‐8 by bromine and/or hydrogen and at oxygen by methyl, acetyl and/or hydrogen have been analysed in deuteriated acetone and chloroform. When a methoxy group is flanked by two bulky ortho substituents (6‐Br and C‐4), the MeO and ortho/para ring carbons (C‐4a, C‐6, C‐8) are deshielded owing to steric inhibition of resonance, which permits a distinction between 6‐ and 8‐substituted catechin methyl ethers. Hindered acetoxy groups do not give rise to this phenomenon.

Adcock, William; Iyer, V. Sankar

doi: 10.1002/mrc.1260260307pmid: N/A

Fluorine‐19 substituent chemical shifts (SCS) are reported for a series of 4‐substituted bicyclo(2.2.2)octane‐1‐carbonyl fluorides containing a limited but a representative number of dipolar substituents. The 19F SCS, which cover a very narrow range (<0.5 ppm), are found to correlate poorly against polar field parameters (σF). Polar field susceptibility parameters (ρF) for the fluorine probe were determined indirectly and their origin is briefly discussed in the light of the carbonyl 13C SCS which reflect π polarization of the COF group.

Hughes, Donald W.

doi: 10.1002/mrc.1260260308pmid: N/A

Relayed coherence transfer (RELAY) two‐dimensional NMR spectroscopy was demonstrated to be an effective technique for overcoming the problem of overlapping multiplets in the analysis of the 1H NMR spectra of steroids. Cross‐sections through appropriate proton resonances resulted in resolved spectra of the spin systems for each of the saturated rings of the estrogenic hormones 17β‐ and 17α‐estradiol. During RELAY experiments with a fixed delay time of 0.042 s it was possible to observe RELAY cross‐peaks between H‐11α and 18‐CH3 which resulted from magnetization transfer via long‐range coupling between 18‐CH3 and H‐12α. The difference in stereochemistry at C‐17 of these estradiol epimers allowed an examination of the effect of substituent configuration on the RELAY cross‐peak intensity in order to determine the ability of the RELAY method to provide qualitative structural information.

Perrin, Charles L.

doi: 10.1002/mrc.1260260309pmid: N/A

The valley‐to‐peak ratio of a 1:1 doublet undergoing chemical exchange can be calculated from the line shape equation. Numerical analysis provides a table that permits the evaluation of the rate constant from that ratio. This method is exact, without the approximations of previous equations that suffer from systematic errors. Statistical analysis shows how the experimental error depends on the rate constant and how to weight rate constants of unequal reliability for least‐squares analysis. From the tabulated errors it is possible to recognize conditions to optimize the evaluation of rate constants, so that it is realistic to obtain them with a precision of ± 1–2%.

Kruk, C.; Cox, H. C.; De Leeuw, J. W.

doi: 10.1002/mrc.1260260310pmid: N/A

Two‐dimensional 1H and 13C shift‐correlated NMR experiments have been used for the unambiguous assignments of all proton and carbon chemical shifts of two cadinanes and bicadinane.