ANGULAR GROUP INDUCED BOND ALTERNATION (AGIBA) - THE CASE OF O-DIMETHOXYBENZENE DERIVATIVES

Edyta Pietka, Romana Anulewicz and Tadeusz Marek Krygowski

Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland

Analysis of molecular geometry of 36 o-dimethoxybenzene derivatives revealed a substantial bond length alternation in the ring. A ratio of two Kekule structures weights (I) : (II) calculated from mean geometries is 61.6:38.5, indicating a strong AGIBA effect.1,2 This is also supported by a statistical study of differences between the lengths of the central bond and the two adjacent ones (0.024 and 0.025A) - the quantities which are significantly different from zero. A good correlation between the changes of differences in bond angles (b-a) and bond lengths (R2 - R1) is observed, suggesting an important contribution of the rehybridisation effect in the observed changes in geometry. Ab initio calculations (6-311G**) for o- dimethoxybenzene (optimised) and the constrained structure with both exocyclic bond angles at substituted carbon equal each other lead to the conclusion that the bond alternation is due not only to the rehybridization effect but also due to some kind of through space p-electron effects.3


  1. Howard, S.T., Krygowski, T.M. and G3ówka, M.L., Tetrahedron, 1996, 52, 11379.
  2. Krygowski, T.M., Wisiorowski, M., Howard S.T. and Stolarczyk, L. Tetrahedron, 1997, 53, 13027.
  3. Krygowski, T.M., Anulewicz, R. and Hiberty, Ph. C., J. Org. Chem., 1996, 61, 8533.