Dihedral angle 3/hcch

The J(HH) coupling constants are within the narrow range of 5.75-5.97 Hz and tend to increase on increasing the value of Taft polar substituent constants . This was explained by a decrease in the dihedral angles HCCH on shortening the Si- -N bond. Precise determination of 7(HH) in the skeleton of some silatranes made it possible to estimate the NCCO torsion angles, which appeared to be comparable to the solid-state angles. This means that in solution the silatrane molecules have a Si- -N bond and a conformation similar to that in the solid state. 

For the above mentioned tricyclic quinolizidines, a comparison of the HCCH dihedral angles determined by H NMR J analysis and those predicted by molecular modelling or established by X-ray structures, when available, was also performed, which corroborates the previous stereochemical analysis [200]. The reason indicated for the similarity of conformations of these alkaloids in the solid state and in solution is the partial flattening of ring B, caused by the presence of a flat system in ring A, that diminishes the steric hindrance of ring C in an all chair conformation. 

Coupling between protons over three bonds provided the most important early stereochemical application of NMR spectroscopy. In 1961, Karplus derived a mathematical relationship between V(HCCH) and the H-C-C-H dihedral angle (f). The simple formula... 

Fig. 16.11a. When expressed as a series of terms involving cosn, this curve turns out to have significant contributions from only the cos<, cos2<, and cos3 terms. When the nine dihedral-angle contributions are added, the symmetry of <22 6 makes the contributions of the cos< and cos2< terms add to zero, leaving only the cos3 term (Fig. 16.11b). Because of insufficient experimental data, MM2 and MM3 take and V2 as zero for HCCH torsion (where C is a saturated carbon), but MMFF94 used results of ab initio conformational energy calculations to fit nonzero values for these quantities.

Figure 13 Calculated Kaiplus curve for dihedral variations of J[HCCH] spin-spin coupling in ethanol (idealized rigid-rotor methyl torsions), showing total Jhh (solid) and its Lewis (dashed) and non-Lewis (dotted) contributions (Hz) at each

The recent theoretical calculation of the dependence of the three-bond HH coupling on the dihedral angle in the HCCH fragment d compares very favorably with the empirical equation of the Karplus form which had been least-squares fitted to data from rigid systems of known geometry. In /(HH) = -I- 5 cos -I- C cos 2, ... 

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