Energy Parameters in Polypeptides . 11 . Semiempirical Molecular Orbital Calculations for Model

excited-sta,te charge distribution venting such a conformation. Extended Huckel calculations confirm this assumption. The potential energy curves for the rotation of the aldehyde group away from the plane of the phenyl ring for the ground state and the excited state of benzaldehyde arising from the carbonyl (n,n*) transition are shown in Figure 7. The calculated barrier to rotation is 0.22 eV for the ground The charge redistribution in the excited state is not very state and o.79 ev for the excited state. The theoretical great. This is the consequence of an "n" orbital which analysis is just like that presented for benzophenone, is not localized at oxygen, but significantly delocalized with a predicted higher barrier to internal rotation in the molecule* The absence Of charge rethe (n,7r*) excited state compared to the ground state. distribution of a magnitude consistent with the classical picture of a fully localized oxygen lone pair is supported Aclcnowzedgment* This was supported the