Contrasting roles of bond orbital interactions contributing to conformational stabilities of flavan-3-ol structures

It is not easy to explain the conformational stability of flavan-3-ols. Factors governing it were investigated based on bond orbital interactions. The interactions between the substituent groups on the C2 and C3 positions play a critical role in the conformational stabilities. In the benzoyloxy types, the destabilization effects due to the interaction between the bonding orbital of the aromatic ring on C2 and the lone pair orbital of the oxygen atom on C3 are responsible for the relative stability of the conformations. In contrast, the conformational stabilities of the hydroxy types are determined by the stabilization effects arising from the electron delocalization from the bonding orbital to the vacant orbitals of the atoms. These results show that the stabilization effects due to the electron delocalization between the vicinal substituent groups is an important factor in the conformational stability of molecules in addition to the destabilization effects due to steric repulsion.