Hydrogen bonding in chloroform solutions of ethylenedioxy ethers. Spectroscopic evidence of bifurcated hydrogen bonds

Chloroform solutions of 1,4-dioxane, 1,2-dimethoxyethane, and diethylene glycol dimethyl ether were studied by means of IR absorption spectroscopy. The intensity of the C-D stretching band of a small amount of deuterated chloroform added to the solutions was used as a probe of the strength of hydrogen bonding between the ethers and chloroform. A two-component model, which assumes that the chloroform molecules can be classified as hydrogen bonded and non-hydrogen bonded, was used to interpret the variations of the intensity of the C-D stretching band on changing the concentration of chloroform. In the frame of this model, the hydrogen bonding was considered as a chemical reaction between the lone electron pairs of the ether oxygens and the chloroform molecules. As a result, the equilibrium constant of hydrogen bonding was estimated and the stoichiometry of the hydrogen bonded complexes of ethers and chloroform was revealed. It was found that the lone electron pairs of the ether oxygens do not participate equally well in hydrogen bonding with chloroform, most probably because of steric hindrance. The analysis of the experimental results also suggested that chloroform can form strong bifurcated hydrogen bonds with the neighboring oxygens of the open-chain ethers. The bifurcated three-centered hydrogen bonds were confirmed by ab initio molecular orbital calculations.