EFFECTS OF SUBSTITUTION FOR HYDROXYL IN THE B-RING OF THE FLAVYLIUM CATION

The electronic structure of mono, bis and trihydroxylated derivatives in the B-ring of the flavylium cation was studied. The AM1 semiempiric al method of molecular orbital calculations was used. Data showed that substitution in the B-ring causes resonance increase between this rin g and the C-ring, except for substitution on C(3'). Bond orders were i nterpreted through the variation in the contribution of resonance stru ctures and they helped to explain the relative stability of the severa l dissubstituted isomers. Another factor that contributes to this stab ility is the formation of a hydrogen bond in the B-ring and between th e Band C-rings. The rotation enthalpies in the B-ring were calculated and they confirm that the rotation on the C(2)-C(1') bond is free in t he compounds with substitution on position 3'. The molecule planarity can be attributed to the interaction between the oxygen atom in the C- ring and the hydrogen atoms in the B-ring, or between the hydrogen ato ms in these rings. The charge variations are in agreement with the pro posed resonance structures. Substitution in the B-ring does not affect the sites of basic and nucleophilic attack. (C) 1997 Elsevier Science B.V.