Gauche position

Bonds 2 and 4 are rotated to gauche positions with respect to bond 3. All even-numbered bonds are in similar conformations with respect to the odd-numbered bonds. 

No Vg effect has to be considered for the CH3 carbon atoms. So it is reasonable to explain the splitting solely by the y-gauche effect. As can be seen from Fig. 14 the steric arrangement of the two CH3 carbon atoms within the segment differs by one y-gauche position of the carbons opposite of the CH—CH bond. With respect to the other carbons the steric situation is equivalent. Hence, the chemical shift... 

The picture is less clear when free electron pairs on other heteroatoms (O, S, etc.) are involved. There is only one clear-cut case providing evidence concerning endocyclic sulfoxides The unusually large shieldings of carbon atoms in gauche position relative to an axial oxygen atom in thiane oxides 79, 80, 82, and 83 (p. 252) (200,201) have been partially ascribed to an upfield-shifting effect of the equatorial antiperiplanar lone pair (cf. Section III-C). 

The electronic density in the radial direction (perpendicular to the cylinder axis) gets delocalized more and more with increasing segment length because there are more CH2 units in the gauche position included enlarging the cylinder diameter. 

The true conformation in the crystalline structure of a polymer is more complicated, the backbone chain often taking a helical shape in which alternate chain bonds take trans and gauche positions, such that the carbon chain is not in a plane. For example. 

Xi can be inferred from the measurement of 3H-3H couplings, and it generally corresponds to one of the three possible staggered rotamers, where each atom attached to is either in one of the two gauche positions relative to Ha or in the tram position (Figure 19). For unlabeled proteins, Xi determination relies on the measurement of (HqH ), whereas for 15N-labeled proteins the measurement of (NH ) is often more useful. 

In many ascorbate salts, the combined effects of metal co-ordination and hydrogen-bonding apparently cause the C-6-OH to adopt a synclinal (or gauche) position with respect to the C-5-OH. 

Fig. 1.2. Elementary configurations of four successive carbon atoms. The trans position and the two gauche positions are indicated.

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