Interplay between the crystal stability and the energy of the molecular conformation

A specially designed new compound, 5,5′-bis(4-hydroxyphenyl)-2,2′-dihydroxy-1,1′-biphenyl, can crystallize in different crystallographic systems. The molecule adopts the C-conformation for the torsion angle of around 60° and the T-conformation for the angle of around 130°, which differ in energy by ∼0.8 kJ mol−1. Theoretical studies for the gaseous phase show that the C-conformer has a lower energy. However, crystallization experiments show that the most stable crystal structure consists of only the energetically less stable T-conformer. On the other hand, fast crystal growth at low temperature and crystal growth after milling both lead to the formation of metastable crystals in which the studied molecule adopts the C-conformation. Our study shows that the total crystal net energy is the main factor in determining the molecular conformations even if the molecular conformation has a higher energy in the gaseous phase.