Strain and nonbonded interactions

The interplay between torsional strain and nonbonded interactions can be illustrated by examining conformational isomerism in n-butane. The diagram relating potential energy to torsion angle for rotation about the C(2)-C(3) bond is presented in Fig. 3.3. 

In cyclic systems such as 1, the dominant conformation is the one with the maximum anomeric effect. In the case of 1, only conformation lA provides the preferred antiperiplanar geometry for both oxygens. Antiperiplanar relationships are indicated by including lone pairs in the oxygen orbitals. Other effects, such as torsional strain and nonbonded repulsion, contribute to the conformational equilibrium, of course. Normally, a value of about 1.5 kcal/mol is assigned to the stabilization due to an optimum anomeric interaction in an acetal. 

In the original molecular mechanics work, a steric energy, E, for a molecule was defined as the sum of the potentials for bond stretch, angle bend, Ee, torsional strain, E, nonbonded interactions, Ey f, and other terms, such as Urey-Bradley terms, cross-interaction terms, and electrostatic terms, (8). 

---