The reaction energies (RE) for the diagonal, the ultradiagonal, and some homodesmotic processes of oxirane, thiirane, and their fluoro derivatives were computed at the B3LYP/6-311 ++G(3d2f,3p2d) level. The variations displayed by homodesmotic and diagonal REs along the series of compounds are linearly correlated and can be attributed to the combined effect of bond path distortion and sigma delocalization. In contrast, ultradiagonal REs do not correlate with them. According to our analysis, all these reactions include not only the energy due to ring opening but that corresponding to the stabilization gained by the anomeric segments in the antiperiplanar arrangements displayed only by the products. (c) 2005 Elsevier BN. All rights reserved.