Solvent effects on the infra-red spectra of hindered phenols

Abstract
The effects of variation in the shapes of the solvent and solute molecules have been studied in the hindered phenol series. When a solvent/solute complex is broken by thermal agitation it seems that reassociation of the phenolic OH with another solvent molecule occurs very rapidly. However, the introduction of bulky substituents in the immediate neighbourhood of the OH group lengthens the time between collisions suitably oriented for association. It then becomes possible to detect free OH absorptions corresponding to solute molecules in transit from one molecular association to another. These occur even in such strongly bonding solvents as ethers, and the proportions increase with the complexity of the hindering groups. Similar effects are found when a hindered phenol is examined in a series of solvents of increasing complexity, confirming that there is a dynamic equilibrium between free and associated OH groups. The OH group in the hindered phenols is largely coplanar and only in the 2,6-di-tert.-butyl derivative is there any direct steric hindrance to the approach of the solvent molecule.