Conformational dependence of the 1Lb and n → π* rotatory strengths in α-substituted phenylacetic acids

The chiroptical properties of a series of chiral phenylacetic acid derivatives are examined on a theoretical model in which electronic rotatory strengths are calculated directly from molecular wave functions derived from semiempirical molecular orbital calculations. The CNDO/S SCF-MO model is used to calculate ground state wave functions and excited states are constructed in the virtual orbital-configuration interaction approximation. Of special interest are the rotatory strengths associated with the ¹L_b transition of the phenyl chromophore and the n → π^* transition of the carboxyl chromophore. Calculations are carried out on a large number of conformational isomers of the compounds: α-methyl phenylacetie acid (and its methyl ester), α-methylmandelic acid (and its methyl ester), and mandelic acid (and its methyl ester). The dependence of rotatory strength (¹L_b and n → π^*) on conformational variables is examined and discussed, and comparisons between available experimental data and the calculated results are made. Coupling between the phenyl and carboxyl chromophoric moieties is considered and possible spectra-structure relationships are examined.