Dihedral angle

Types of angles |
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2D angles |
Spherical |
2D angle pairs |
Adjacent |
3D angles |
Solid |
A dihedral angle is the angle between two intersecting planes or half-planes. It is a plane angle formed on a third plane, perpendicular to the line of intersection between the two planes or the common edge between the two half-planes. In higher dimensions, a dihedral angle represents the angle between two hyperplanes. In chemistry, it is the clockwise angle between half-planes through two sets of three atoms, having two atoms in common.
Mathematical background
[edit]When the two intersecting planes are described in terms of Cartesian coordinates by the two equations
the dihedral angle, between them is given by:
and satisfies It can easily be observed that the angle is independent of and .
Alternatively, if nA and nB are normal vector to the planes, one has
where nA · nB is the dot product of the vectors and |nA| |nB| is the product of their lengths.[1]
The absolute value is required in above formulas, as the planes are not changed when changing all coefficient signs in one equation, or replacing one normal vector by its opposite.
However the absolute values can be and should be avoided when considering the dihedral angle of two half planes whose boundaries are the same line. In this case, the half planes can be described by a point P of their intersection, and three vectors b0, b1 and b2 such that P + b0, P + b1 and P + b2 belong respectively to the intersection line, the first half plane, and the second half plane. The dihedral angle of these two half planes is defined by
- ,
and satisfies In this case, switching the two half-planes gives the same result, and so does replacing with In chemistry (see below), we define a dihedral angle such that replacing with changes the sign of the angle, which can be between −π and π.
In polymer physics
[edit]In some scientific areas such as polymer physics, one may consider a chain of points and links between consecutive points. If the points are sequentially numbered and located at positions r1, r2, r3, etc. then bond vectors are defined by u1=r2−r1, u2=r3−r2, and ui=ri+1−ri, more generally.[2] This is the case for kinematic chains or amino acids in a protein structure. In these cases, one is often interested in the half-planes defined by three consecutive points, and the dihedral angle between two consecutive such half-planes. If u1, u2 and u3 are three consecutive bond vectors, the intersection of the half-planes is oriented, which allows defining a dihedral angle that belongs to the interval (−π, π]. This dihedral angle is defined by[3]
or, using the function atan2,
This dihedral angle does not depend on the orientation of the chain (order in which the point are considered) — reversing this ordering consists of replacing each vector by its opposite vector, and exchanging the indices 1 and 3. Both operations do not change the cosine, but change the sign of the sine. Thus, together, they do not change the angle.
A simpler formula for the same dihedral angle is the following (the proof is given below)
or equivalently,
This can be deduced from previous formulas by using the vector quadruple product formula, and the fact that a scalar triple product is zero if it contains twice the same vector:
Given the definition of the cross product, this means that is the angle in the clockwise direction of the fourth atom compared to the first atom, while looking down the axis from the second atom to the third. Special cases (one may say the usual cases) are , and , which are called the trans, gauche+, and gauche− conformations.
In stereochemistry
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Configuration names according to dihedral angle |
syn n-Butane in the gauche− conformation (−60°) Newman projection |
syn n-Butane sawhorse projection |

A torsion angle, found in stereochemistry, is a particular example of a dihedral angle describing the geometric relation of two parts of a molecule joined by a chemical bond.[4][5] Every set of three non-colinear atoms of a molecule defines a half-plane. As explained above, when two such half-planes intersect (i.e., a set of four consecutively-bonded atoms), the angle between them is a dihedral angle. Dihedral angles are used to specify the molecular conformation.[6] Stereochemical arrangements corresponding to angles between 0° and ±90° are called syn (s), those corresponding to angles between ±90° and 180° anti (a). Similarly, arrangements corresponding to 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 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 example, with n-butane two planes can be specified in terms of the two central carbon atoms and either of the methyl carbon atoms. The syn-conformation shown above, with a dihedral angle of 60° is less stable than the anti-conformation with a dihedral angle of 180°.
For macromolecular usage the symbols T, C, G+, G−, A+ and A− are recommended (ap, sp, +sc, −sc, +ac and −ac respectively).
Geometry
[edit]Every polyhedron has a dihedral angle at every edge describing the relationship of the two faces that share that edge. This dihedral angle, also called the face angle, is measured as the internal angle with respect to the polyhedron. An angle of 0° means the face normal vectors are antiparallel and the faces overlap each other, which implies that it is part of a degenerate polyhedron. An angle of 180° means the faces are parallel, as in a tiling. An angle greater than 180° exists on concave portions of a polyhedron.
Every dihedral angle in a polyhedron that is isotoxal and/or isohedral has the same value. This includes the 5 Platonic solids, the 13 Catalan solids, the 4 Kepler–Poinsot polyhedra, the 2 convex quasiregular polyhedra, and the 2 infinite families of bipyramids and trapezohedra.
Law of cosines for dihedral angle
[edit]Given 3 faces of a polyhedron which meet at a common vertex P and have edges AP, BP and CP, the cosine of the dihedral angle between the faces containing APC and BPC is:[7]
This can be deduced from the spherical law of cosines, but can also be found by other means.[8]
Higher dimensions
[edit]In m-dimensional Euclidean space, the dihedral angle between the two hyperplanes defined by the equations for vectors nA, nB, x ∈ Rm and constants cA and cB, is given by
See also
[edit]References
[edit]- ^ "Angle Between Two Planes". TutorVista.com. Archived from the original on 2020-10-28. Retrieved 2018-07-06.
- ^ Kröger, Martin (2005). Models for polymeric and anisotropic liquids. Springer. ISBN 3540262105.
- ^ Blondel, Arnaud; Karplus, Martin (7 Dec 1998). "New formulation for derivatives of torsion angles and improper torsion angles in molecular mechanics: Elimination of singularities". Journal of Computational Chemistry. 17 (9): 1132–1141. doi:10.1002/(SICI)1096-987X(19960715)17:9<1132::AID-JCC5>3.0.CO;2-T.
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Torsion angle". doi:10.1351/goldbook.T06406
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Dihedral angle". doi:10.1351/goldbook.D01730
- ^ Anslyn, Eric; Dennis Dougherty (2006). Modern Physical Organic Chemistry. University Science. p. 95. ISBN 978-1891389313.
- ^ "dihedral angle calculator polyhedron". www.had2know.com. Archived from the original on 25 November 2015. Retrieved 25 October 2015.
- ^ "Formula Derivations from Polyhedra". Retrieved 4 December 2024.
External links
[edit]- The Dihedral Angle in Woodworking at Tips.FM
- Analysis of the 5 Regular Polyhedra gives a step-by-step derivation of these exact values.