Three-centre hydrogen bonding in the complexes of syn-2,4-difluoroadamantane with 4-fluorophenol and hydrogen fluoride

Hydrogen bond enthalpies for the interaction of 4-fluorophenol with syn-2,4-difluoroadamantane (5) and, for comparison, 2-fluoroadamantane (6) and 1,3-difluoroadamantane (7) have been determined by Fourier transform infrared spectrometry. Among a series of seven fluoroalkanes (1-7) it is found that syn-2,4-difluoroadamantane is the best hydrogen bond acceptor. This high hydrogen bond acceptor strength is explained, using electrostatic and 'atoms in molecules' properties and from density functional theory and ab initio calculations on hydrogen fluoride complexes, by the formation of an F... H... F three-centre hydrogen bond. In this system, the three-centre hydrogen bonding is energetically (mainly on the Gibbs energy scale) superior to the two-centre hydrogen bonding. Copyright (C) 2004 John Wiley Sons, Ltd.