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).
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
,
,
,
,
and
are recommended (
ap,
sp,
+sc,
-sc,
+ac
and
-ac respectively).
Source:
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2220