Tautomers of the unsubstituted, F-substituted, and BH2-substituted 1,3-diphosphole molecules have been studied at the B3PW91/aug-cc-pVTZ level. The Gibbs free energies calculated for all the molecules demonstrate that, regardless of the character of the substituent, the C2H 1,3-diphosphole tautomer is always the most stable form. In the CH tautomers, the five-membered ring is planar, whereas the pyramidal PH moiety in PH tautomers gives rise to ring non-planarity. For the CH types of the 1,3-phospholes, the substituent effect can well be observed at the nu(CH) modes and for the PH tautomers at the nu(C=C) modes. (c) 2005 Elsevier B.V. All rights reserved.