Calculations on the optical activity due to isotopic substitution: [1D]- and [2-18O]-α-fenchocamphoronequinone. Relationship to the optical activity of 1-substituted α-diones

It is shown that an electronic rotational strength R(Q) that depends linearly on the normal vibrational coordinates Q can account for optical activity due to isotopic substitution, provided anharmonicities in the nuclear potentials are taken into account. The expression for this rotational strength is cast into a convenient form by expanding R(Q) around the average geometry of the molecule in its ground state rather than, as usual, around the equilibrium geometry. For [1-D]-α-fenchocamphoronequinone (II) and [2-18O]-α-fenchocamphoronequinone (V) the magnitude of the rotational strength of the first singlet-singlet transition is calculated using a polarizability model. The obtained rotational strength of II and V agree in sign and order of magnitude with the observed values. This result not only supports our description of isotopically induced optical activity but also indicates that the polarizability model is capable of describing adequately the optical activity in the first singlet-singlet transition of α-norbornanediones. This latter conclusion has an interesting implication: when it is assumed that the dicarbonyl moiety in camphorquinone and [1-Br]-α-fenchocamphoronequinone is planar, the calculated rotational strengths agree quite well with the results obtained from experiment. This indicates that in the acquirement of optical activity in the visible absorption band of cisoid α-diketones the effect of vicinal groups may successfully compete with the effect of inherent twist. (Auth.)