Electronic circular dichroism of twisted [4.4]-spirononatetraene

[4.4]-Spirononatetraene belongs to symmetry D_2d in its normal state. The extent of its Jahn-Teller instability with respect to a torsion taking it to D₂ symmetry in an excited degenerate electronic state is calculated by an ab initio method, with STO/3G in SCF/CI. The equilibrium structure is found to be displaced 2.5°from D_2d and is more stable by ≈ 110 cm^-1, though neither result is very precise. The calculated rotatory strength of the lowest frequency optically active electronic transition is found to be negative for the R (or M) enantiomer. The lowest energy singlet transition of the D_2d molecule is found to be ¹B₁ ← ¹A₁, lower by ∼ 10 000 cm^-1 than the Jahn-Teller active ¹E ← ¹A₁, earlier suggested to be the lowest transition.