Self-association of 1,2-cyclohexanediols: A spectroscopic and computational study

Solutions of the cis and trans isomers of 1,2-cyclohexanediol in carbon tetrachloride, with concentrations varying from 1 to 30 mM and 2 to 20 mM, respectively, are studied by infrared spectroscopy. A peak-fitting analysis of the OH stretching region followed by a statistical treatment of the spectral data reveals that dimers and especially trimers are the most likely oligomers to be formed in the working concentrations range. The equilibrium constants of molecular association and the molar fractions of the different species are estimated and discussed. Despite their chemical similarity, the self-association behavior of the two isomers is found to be different. In order to rationalize the experimental results, DFT calculations are performed on monomers, dimers and trimers using mainly the B3LYP functional in combination with the 6-311++G** basis set. Prior to aggregation, both isomers exist solely as intramolecular H-bonded conformers. These bonds are weakened or broken in the cyclic oligomers but are reinforced in the open ones. Hydrogen bonding cooperativity is found to assume an important role in the oligomers’ structure.