Twist of bond

Figure 2.40. Hypsochromic and bathochromic shift due to steric hindrance the white arrows indicate the steric strain and the black arrows the strain release by molecular deformations (twisting of bonds).

Figure 2.7 (a) Illustration of the twist of (3 sheefs. Befa sfrands are drawn as arrows from the amino end to the carboxy end of the p strand in this schematic drawing of fhe protein thioredoxin from E. coli, fhe sfrucfure of which was defermined in the laboratory of Carl Branden, Uppsala, Sweden, fo 2.8 A resolution. The mixed p sheet is viewed from one of ifs ends, (b) The hydrogen bonds between the P strands in the mixed p sheet of fhe same profein. [(a) Adapfed from B. Furugren.]... 

Nakazaki, M., Yamamoto, K., and Naemura, K. Stereochemistry of Twisted Double Bond Systems, 125, 1-25 (1984). 

The C=C group and all four atoms attached to it lie in the same plane and are locked into that arrangement by the resistance to twisting of the TT-bond (Fig. 18.7). Because alkene molecules cannot roll up into a ball as compactly as alkanes or rotate into favorable positions, they cannot pack together as closely as alkanes so alkenes have lower melting points than alkanes of similar molar mass. 

Figure 1. Hiickel n-MO correlation diagrams for planar- perp twisting of pcntamethine cyanine about the 2-3 and 3-4 bonds (a), and simultaneous twisting about 2-3 and 4-5 bonds (b). For the perp forms the orbitals belonging to the different n fragments are indicated and the CT nature of the HOMO-LUMO excitation is emphasized.

A totally different situation is encountered for dihedral or torsional angles, which describe the twisting of a fragment of four atoms cormected by a sequence of bonds. As the steric energy may have multiple minima around a rotatable bond with similar energy content, this leads to more than one possibility for constructing a 3D model for such molecules, or in other terms, to multiple conformations. 

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