Proton Magnetic Resonance Study of Intramolecular Hydrogen Bonding in Halophenols

Abstract

The long-range spin–spin coupling constants over five bonds between the hydroxyl proton and the ring protons in a series of trihalophenols imply that the intramolecular hydrogen bond strength (negative enthalpy) to fluorine is greater than that to iodine by 75 ± 20 cal/mol, whereas the strengths to chlorine and bromine are 460 ± 60 cal/mol greater than to iodine. If a distinction can be made between chlorine and bromine, then chlorine–hydrogen bonds more strongly by only a few tens of calories per mol. The measurements were made mainly on dilute solutions in carbon tetrachloride at 32 °C.