Intramolecular hydrogen bonding and molecular conformation

Part 5 Gas-phase infrared measurements of some hydroxyketones and methoxyalcohols

Abstract

Gas-phase IR OH stretching bands of some hydroxyketones and methoxyalcohols were measured as a function of temperature. The two bands observed were assigned to intramolecularly hydrogen-bonded and hydrogen-bond-free conformers. From comparison of band broadening between the gas-phase and solutions, it is suggested that indirect coupling plays an important role in vibrational dephasing of the hydrogen-bonded OH modes in solutions. Enthalpy change, ΔH_v, for the rupture of an intramolecular hydrogen bond has been obtained by an analysis of the temperature dependence of the OH stretching absorption intensities. The difference between ΔH_v and the enthalpy change in solutions, ΔH_s, obtained previously, provides the contribution of solvation energy, Δ_solvE. The results indicate that solvation, in general, stabilizes the hydrogen-bond-free conformers more than the hydrogen-bonded conformers, but a few exceptions were found. Theoretical calculations of the solvation energy are performed with a simple dielectric model by use of molecular dipole and quadrupole moments estimated by molecular mechanics calculations. The calculated results are not satisfactory for reproducing some of the experimental values, and the problems of the model are discussed.