The difference in the hydrogen bond acceptor strengths of phenol and a
nisole in their planar and perpendicular conformation has been studied
to assess the effect of conjugation with a pi system on the acceptor
strength of an oxygen atom. Intermolecular perturbation theory calcula
tions were used, together with an analysis of hydrogen bond formation
in molecular crystal structures. Anisoles and phenols form hydrogen bo
nds which are intermediate in strength between those of furan and tetr
ahydrofuran, and are a few kJ/mol stronger for the perpendicular confo
rmations than the planar conformations. This can be partly attributed
to the change in the oxygen charge distribution with conformation. (C)
1997 Elsevier Science B.V.