Infrared study and thermodynamics of hydrogen bonding in diethylene glycol monoalkyl ethers

Abstract

Diethylene glycol monoalkyl ethers in dilute CCl₄ solutions exhibit three OH bands, viz. at ∼3635, ∼3600 and ∼3475 cm^–1 attributable to a free monomer (M_F), an intramolecularly hydrogen bonded five-membered ring monomer (M₅) and an intramolecularly hydrogen bonded eight-membered ring monomer (M₈) respectively. In concentrated solutions, the dimer (D) band appears at ∼3440 cm^–1. The relative concentrations of these species involved in the multiple equilibria are observed to be in the order C_D≈C_FC₈≈½C₅ at 298 K. The thermodynamic quantities for the equilibria M₅⇌ M_F, M₈⇌ M_F and D ⇌ 2M_F are ΔH°_5F∼ 12 kJ mol^–1, ΔS°_5F∼ 22 J K^–1 mol^–1; ΔH°_8F∼ 21 kJ mol^–1, ΔS°_8F∼ 56 J K^–1 mol^–1 and ΔH°_DF∼ 58 kJ mol^–1, ΔS°_DF∼ 153 J K^–1 mol^–1 respectively. Finally a ten membered ring, involving the OH group of one molecule bonded to the β-oxygen of the other molecule, similar to that in alkoxy ethanols is found to be the most likely structure for the dimer.