Conformational dependence of the circular dichroism spectrum of α-hydroxyphenylacetic acid: a ChiraSac study.

The conformational dependence of the circular dichroism (CD) spectrum of a chiral molecule, α-hydroxyphenylacetic acid (HPAA) containing phenyl, COOH, OH and H groups around a chiral carbon atom, has been studied theoretically by using the SAC-CI (symmetry adapted cluster-configuration interaction) theory. The results showed that the CD spectrum of HPAA depends largely on the rotations (conformations) of the phenyl and COOH groups around the single bonds. The first band is due to the excitation of electrons belonging to the phenyl region and therefore sensitive to the phenyl rotation. The second band is due to the excitation of electrons belonging to the COOH region and therefore sensitive to the COOH rotation. From the comparison of the SAC-CI CD spectra calculated at various conformations of phenyl, COOH, and OH groups with the experimental spectrum, we could predict the stable geometry of this molecule, which agreed well with the most stable conformation deduced from the energy criterion. We also calculated the Boltzmann averaged spectrum and obtained better agreement with the experiment. Further, we performed preliminary investigations of the temperature dependence of the CD spectrum of HPAA. In general, the CD spectra of chiral molecules are very sensitive to low-energy processes like the rotations around the single bonds. Therefore, one should be able to study the natures of the weak interactions by comparing the SAC-CI spectra calculated at different geometries and conditions with the experimental spectrum using a new methodology we have termed ChiraSac.