In a chain of atoms A-B-C-D, the @[email protected] between the plane containing the atoms A,B,C and that containing B,C,D. In a @[email protected] the torsion @[email protected] is the @[email protected] (having an absolute value between 0° and 180°) between bonds to two specified (@[email protected]) groups, one from the atom nearer (proximal) to the observer and the other from the further (distal) atom. The torsion @[email protected] between groups A and D is then considered to be positive if the bond A-B is rotated in a clockwise direction through less than 180° in order that it may @[email protected] the bond C-D: a negative torsion @[email protected] requires rotation in the opposite sense. Stereochemical arrangements corresponding to torsion angles between 0° and ±90° are called @[email protected] (s), those corresponding to torsion angles between ±90° and 180° @[email protected] (a). Similarly, arrangements corresponding to torsion angles between 30° and 150° or between -30° and -150° are called clinal (c) and those between 0° and 30° or 150° and 180° are called periplanar (p). The two types of terms can be combined so as to define four ranges of torsion @[email protected]; 0° to 30° synperiplanar (sp); 30° to 90° and -30° to -90° synclinal (sc); 90° to 150°, and -90° to -150° anticlinal (ac); ±150° to 180° antiperiplanar (ap).
The synperiplanar @[email protected] is also known as the @[email protected] or [email protected]@; antiperiplanar as @[email protected] or @[email protected] and synclinal as @[email protected] or @[email protected] For @[email protected] usage the symbols \(\text{T}\), \(\text{C}\), \(\text{G}^{+}\), \(\text{G}^{-}\), \(\text{A}^{+}\) and \(\text{A}^{-}\) are recommended (ap, sp, +sc, -sc, +ac and -ac respectively).
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2220 [Terms] [Paper]