Introduction

Molecular strain occurs when a molecule deviates from its ideal theoretical geometry. The strain is mainly classified into three types: steric strain (van der Waals strain), torsional strain (Pitzer strain), and bond angle strain (Bayer strain) (Liebman JF, Greenberg in Chem Rev 76:31, 1976). Steric strain is attributed to Pauli repulsion when non-bonding atoms are spatially close together, and the strain generally occurs when the distance between two atoms is less than the sum of their respective van der Waals radii.