Dihedral Angles of Four Atoms
The structure of a molecule can be defined with high precision by the dihedral angles between three successive chemical bond vectors (Figure 2). The dihedral angle varies only the distance between the first and fourth atoms; the other interatomic distances are constrained by the chemical bond lengths and bond angles.
To visualize the dihedral angle of four atoms, it's helpful to look down the second bond vector (Figure 3), which is equivalent to the Newman projection in chemistry. The first atom is at 6 o'clock, the fourth atom is at roughly 2 o'clock and the second and third atoms are located in the center. The second bond vector is coming out of the page. The dihedral angle is the counterclockwise angle made by the vectors (red) and (blue). When the fourth atom eclipses the first atom, the dihedral angle is zero; when the atoms are exactly opposite (as in Figure 2), the dihedral angle is 180°.
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