We have examined a series of cyclic (n,n + 1)-diols in which the hydroxy groups are constrained to be more closely syn-periplanar than in diols with free rotation around the C-C bond, for their ability to form an intramolecular hydrogen bond. Small IR red-shifts, of the order of similar to 50-100 cm(-1), are not necessarily associated with intramolecular hydrogen bond formation but may be generated by sigma double right arrow sigma(*) hyperconjugative effects. Moreover, there exists in (n,n + 1)-diols a clear donor-acceptor cutoff distance below which a (3, -1) bond critical point for an intramolecular hydrogen bond is observed, and above which it is not. (c) 2006 Elsevier B.V. All rights reserved.