Question

Steric strain in a large molecule is often reduced by changes in torsion angles. However, in smaller sets of fused benzene rings, like phenanthrene and 3,4-benzophenanthrene (shown below), the first geometric parameters to vary from reference values are found to be the central bond lengths and bond angles. Why do they expand to relieve steric strain before the molecule undergoes torsion to a non-planar structure?

Answer 1

The molecule expand to relieve steric strain before undergoing torsion to a non-planar structure in order to have several resonating structures.

What is a molecule?

It should be noted that a molecule simply mean the group of atoms that are bonded together that represents the smallest fundamental unit if the chemical compound.

It should be noted that structures containing fused benzene rings have extensive conjugation. Due to this, they can undergo resonance.

This is vital in decreasing the overall energy of the molecule and helps increase its stability. When there's a change in torsion angle to relieve the steric strain, the molecule won't be planar, hence, the energy will increase and the stability will reduce.

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