Chiroptical properties of lactones. Part I. Rotatory strengths of electronic transitions in substituted and unsubstituted 1,4-dioxan-2,5-diones (dilactones)

Abstract

The chiroptical properties of dissymmetric conformational isomers and asymmetrically substituted derivatives of 1,4-dioxan-2,5-dione (I) are examined with a theroetical model in which the electronic wave functions are obtained from semi-empirical all-valence-shell molecular orbital calculations. The INDO molecular orbital model is used to perform SCFMO calculations on the ground states of four conformational isomers of (I), seven conformational isomers of the (S)-3-methyl derivative of (I), and five conformational isomers of the (S,S)-3,6-dimethyl derivative of (I). Excited state wave functions are constructed in the virtual orbital-configuration interaction approximation. The rotatory strengths, dipole strengths, oscillator strengths, and transition energies are calculated for the two lowest-lying singlet-singlet transitions (n→π*) in each of the 16 structures. Additionally, ground state dipole moments, net atomic charges, and the first four ionization potentials (calculated according to Koopmans′ theorem) are computed for each structure. The signs and magnitudes of the nπ* rotatory strengths are found to be extremely sensitive to the conformation of the 1,4-dioxan ring as well as to methyl substitution at the α-carbon sites of the ring. Both ring dissymmetry and vicinal effects due to asymmetric rign substitution make substantial contributions to the optical activity of the n→π* transition in the monocyclic dilactone systems examined here.