Quantumchemical (MNDO SCF/CI) calculations of the chiroptical properties of twisted conjugated diene and dione chromophores

In this study the utility of wavefunctions, generated with the MNDO method is investigated for the computation of the oscillator strength and the rotational strength of molecules containing twisted conjugated diene and dione chromophores. In order to improve the quality of the excited-state wavefunctions, limited configuration interaction has been applied. For a-dienes (1,3-butadiene, 1,3-cyclohexadiene, 1,4- dimethylcyclohexa-2,4-diene) the method produces results which are comparable with earlier ab initio SCF/CI calculations. The results demonstrate that for these compounds the twisting of the a-diene fragment is the main factor determining the optical activity. On the other hand, for a-diones (glyoxal, bornane-2,3-dione) it is found that significant differences exist between the chiroptical properties of unsubstituted and substituted glyoxal, even if the twist angle of the a-dione fragment is kept unaltered. It appears that for the calculation of the chiroptical properties of such compounds it is essential to be able to treat relatively large molecular systems. It is suggested that the MNDO-based approach used offers a convenient tool for such calculations.