Vicinal coupling constants and

It is difficult to decide whether the discrepancy between the calculated and experimental data is due to a different conformational preference of the thietane dioxides in the liquid and the solid phase, or to the crude approximations included in the Karplus-Barfield equation. However, the relationship between vicinal coupling constants and dihedral angles appears qualitatively valid in thietane oxides and dioxides, particularly if trends instead of exact values are discussed . At any rate thietane dioxides, 1,3-dithietane dioxides and tetroxides maintain either planarity or a slightly distorted average vibrating conformation with a low barrier to ring planarity . 

Fig. 10.18. IDR (Inverted Direct Response)—HSQC-TOCSY pulse sequence. The experiment first uses an HSQC sequence to label protons with the chemical shift of their directly bound carbons, followed by an isotropic mixing period that propagates magnetization to vicinal neighbor and more distant protons. The extent to which magnetization is propagated in the experiment is a function of both the size of the intervening vicinal coupling constants and the duration of the mixing period. After isotropic mixing, direct responses are inverted by the experiment and proton detection begins.

These conformer populations were confirmed by a number of n.O.e. measurements (the H2—H5 distance can act as a marker in identifying the ring form). The case of a hexasaccharide (compound 23 in Ref. 9) where the three IdoA residues yield different n.O.e. enhancements, is striking. Moreover, models computed with molecular mechanics have been put forward for the peculiar pentasaccharide of heparin. The models are in acceptable agreement with the observed vicinal coupling constants and n.O.e. values (10) (a full paper by Ragazzi, M. et al. Carbohvdr. Res.. is in press). 

Proton-Proton Vicinal Coupling Constants and the R-Value Method... 

A linear relationship between the ratio of the vicinal coupling constants and the resonance energy has been found, indicating that the electron delocalization of the indole 7r-system is more pronounced than that of isoindole and indolizine respectively (77TL535) (see Table 5 and Section 3.08.2.1 for theoretical considerations). 

Table 5 Ratio of Vicinal Coupling Constants and REPE Values of Nitrogen Heterocycles <77TL535) Compound /ab/- bc REPE Compound Ab//bc REPE...

The extent of deformation from the chair conformation of cyclohexane has been investigated by the R- value method (67JA1836). The R- value is based on vicinal coupling constants and is defined by the expression ... 

UEAITR, by using magnetic equivalence-factoring for the methyl protons, variation of only geminal and vicinal coupling-constants, and of chemical shifts, was allowed, and the refined, theoretical spectra agreed with the observed spectra about as well as when all of the parameters were varied. 

Altona C (1996) Vicinal Coupling Constants and Conformation of Biomolecules. In Grant DM, Harris RK (eds) Encyclopedia of Nuclear Magnetic Resonance. Wiley, New York, p 4909... 

C. Altona, Vicinal Coupling Constants and Conformation of Biomolecules. John Wiley London, 1996. 

The origins of the chemical shifts are probably not sufficiently well understood (as yet), to allow a quantitative discussion of aromatic character in the annulenes. If such a concept is considered meaningful it would probably best be defined in terms of the degree of bond alternation therein, which is of pivotal importance to the jr-electron properties (see Sections B and C). Apart from theoretical calculations, a number of physical methods have demonstrated their ability to estimate the extent of bond alternation in annulenes (crystallographic analysis, electronic/vibronic spectral analysis, diamagnetic anisotropy/susceptibility exaltation measurements and of course n.m.r.), see ref. > for a full discussion. (Furthermore the known correlation between n.m.r. vicinal coupling constants and carbon-carbon bond orders is of potential utility in any determination of bond alternation 65>). 

Solvent effects are more pronounced on geminal than on vicinal coupling constants and only a few investigations have been carried out on the latter." Some studies have also been made on the solvent dependence of Jisc—- si f- ... 

The correlation between vicinal coupling-constants and dipole moments has been applied to conformational problems concerning derivatives of 2-alkoxytetrahydropyran, the fundamental skeleton of all pyranoid sugar derivatives. The observation of a linear relationship between the squares of the electric-dipole moments and the vicinal coupling-constants, as the conformational equilibrium constant was varied by changing the alkoxyl substituent, indicated that the equilibrium mixture is composed of only two conformers in appreciable proportion. 

A combination of different homo- and heteronuclear 2D NMR techniques at 500 MHz have enabled complete assignment of the H and signals of the two conformers of RA-VII in CDCI3 (major conformer conformer A minor one conformer B) [94]. The structures of the two conformers (A and B) in CDCI3 were elucidated based on temperature effects on NH protons, deuterium exchange rate, vicinal coupling constants and NOE experiments (Fig. 27). These conformational analyses showed that the structure of these conformers is a result of geometrical isomerization and that the predominant conformer A exhibits a typical type II P-turn between Ala and Tyr, similar to the crystal structure as analyzed by X-ray diffraction. The minor conformer B exhibits a type VI... 

Figure 12.68 The conformational dependence of vicinal coupling constants and conformational analysis of cyclohexanes and heterocyclohexanes.

Table 14.2 shows the relationship between the vicinal coupling constant and the dihedral angle. 

---