Conformational dependence of the n→π* rotatory strength in dissymmetric carboxylic acid compounds

The conformational dependence of the chiroptical properties associated with electronic transitions in α-hydroxy, α-fluoro, α-chloro, and α-mercapto carboxylic acids is examined with a theoretical model in which rotatory strengths are calculated directly from molecular orbitals calculated using INDO and CNDO semi-empirical molecular orbital models, and excited state wave functions constructed in the virtual orbital-configuration interaction approximation. Binding energies, ground state dipole moments, and vertical ionization potentials (calculated according to Koopman's theorem) are calculated, as well as transition energies, dipole strengths, oscillator strengths, rotatory strengths and dissymmetry factors. Special emphasis is placed on correlating the signs of the rotatory strengths calculated for the lowest energy singlet-singlet transitions with conformational isomerism about the C(α)-COOH band of acyclic α-substituted carboxylic acids. The calculated results for S-lactic acid are in agreement with and strongly support the empirically based spectra-structure relationships previously proposed for S-lactic acid and its derivatives. The calculated results for 2(S)-chloropropionic acid do not agree with previously proposed spectra-structure relationships and this finding is discussed. The results obtained for 2(S)-mercaptopropionic acid are in partial agreement with experiment and spectra-structure relationships in this case involve some uncertainty.