Term: torsion angle
https://doi.org/10.1351/goldbook.T06406

Definition:
In a chain of atoms A-B-C-D, the dihedral angle between the plane containing the atoms A,B,C and that containing B,C,D. In a Newman projection the torsion angle is the angle (having an absolute value between 0° and 180°) between bonds to two specified (fiducial) groups, one from the atom nearer (proximal) to the observer and the other from the further (distal) atom. The torsion angle 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 eclipse the bond C-D: a negative torsion angle requires rotation in the opposite sense. Stereochemical arrangements corresponding to torsion angles between 0° and ±90° are called syn (s), those corresponding to torsion angles between ±90° and 180° anti (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 angle; 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). T06406.png The synperiplanar conformation is also known as the syn- or cis-conformation; antiperiplanar as anti or trans and synclinal as gauche or skew. For macromolecular usage the symbols T, C, G+, G-, A+ and A- are recommended (ap, sp, +sc, -sc, +ac and -ac respectively).

Related Terms:
1) dihedral angle (http://doi.org/10.1351/goldbook.D01730).
2) newman projection (http://doi.org/10.1351/goldbook.N04134).
3) fiducial (http://doi.org/10.1351/goldbook.F02356).
4) eclipse (http://doi.org/10.1351/goldbook.E01884).
5) angle (http://doi.org/10.1351/goldbook.A00346).
6) syn (http://doi.org/10.1351/goldbook.S06216).
7) anti (http://doi.org/10.1351/goldbook.A00381).
8) conformation (http://doi.org/10.1351/goldbook.C01258).
9) trans (http://doi.org/10.1351/goldbook.C01092).
10) gauche (http://doi.org/10.1351/goldbook.G02593).
11) skew (http://doi.org/10.1351/goldbook.S05709).
12) macromolecular (http://doi.org/10.1351/goldbook.M03667).

Image: Not defined (https://goldbook.iupac.org/img/inline/T06406.png) 

Source: PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2220 (https://doi.org/10.1351/pac199668122193)

Citation: 'torsion angle' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.T06406

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