Knowledge Bank

The results of an $investigation1,2$ of the rotational contribution of a dienic system which is constrained to be non-planar (skewed) have demonstrated that the sign of the rotational strength, and of the resulting Cotton effect, associated with the longest wavelength electronic transition of the system is determined by the skew or helical sense of the four carbon system of which it is composed. Theoretical one-electron HMO calculations are reported of the intensity of the absorption bands, and of the rotational strengths of this transition, as well as that of the transition of next higher energy as a function of the skew angle. In addition, making use of experimentally observed band widths, the contributions of these two transitions to the optical rotatory dispersion has been calculated, also as a function of skew angle, and it is demonstrated that for the one case for which x-ray data are available to determine the skew angle, the shape of the experimentally observed ORD curve is in excellent qualitative agreement with the calculation.