NH Coupling constants

An HH COSY diagram can be shown in perspective as a stacked plot (Fig. 2.10a). Interpretation of this neat, three-dimensional representation, where the signal intensity gives the third dimension, can prove difficult because of distortions in the perspective. The contour plot can be interpreted more easily. This shows the signal intensity at various cross-sections (contour plots, Fig. 2.10b). However the choice of the plane of the cross-section affects the information provided by an HH COSY diagram if the plane of the cross-section is too high then the cross signals which are weak 

---

Table 2.9. Structural features and typical NH coupling constants (Hz) ...

Scheme2.10. Calculated bond lengths and 13C-nH coupling constants in cyclohexane and six-membered heterocycles [35],...

The hfc are either unchanged or increase moderately with increasing temperature. The NH coupling constant, however, decreases dramatically. For 1-hydropyridinyl... 

In asymmetrically substituted cations, the NMe2 groups are non-equivalent and they are observed as two doublets with different values of the /nmb,nh coupling constants (Table 8). It was suggested that the relationship between these constants can be used for estimating the IHB asymmetry via the proton localization index (PL)98, which is calculated in accordance with equation 6. 

Harman. Yamaguchi and co-workers next reported an extensive heteronuclear long-range correlation study of the pyridoindole alkaloid harman (15) involving both and The authors employed and compared several pulse sequences in their study including /-HMBC, selective /-HMBC, and doubly selective /-HMBC experiments. Their study is also atypical in that they report the measurements of the various "/nh coupling constants observed for harman. 

In general, in common solvent systems such as dimethylsulfoxide, methanol, and water, the /3-turn is characterized in terms of amide proton magnetic resonances by a high-field shifted resonance with a decreased temperature dependence for residue / - - 2. by a small CH—NH coupling constant for residue /, and by a large aCH—NH coupling constant for residue / -El. If the /3-turn is part of an antiparallel pleated sheet, then Ach will also be large, of the order of 7 cps, for the resonances of residues / — 1 and / -E 2 and the temperature dependence of chemical shift of the amide proton resonance of residue / — 1 will be lowered. 

---