Quantum Chemistry Research Institute, JST, CREST , Kyodai Katsura Venture Plaza, North Building 107, 1-36 Goryo-Oohara, Nishikyo-ku, Kyoto 615-8245, Japan.
J Phys Chem A. 2013 Dec 27;117(51):14065-74. doi: 10.1021/jp407606v. Epub 2013 Dec 10.
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.
手性分子α-羟基苯乙酸（HPAA）中含有苯环、COOH、OH 和 H 基团，围绕手性碳原子，其圆二色性（CD）谱的构象依赖性已通过使用 SAC-CI（对称适应簇-构型相互作用）理论进行了理论研究。结果表明，HPAA 的 CD 光谱在很大程度上取决于苯环和 COOH 基团在单键周围的旋转（构象）。第一谱带是由于苯环区域中电子的激发，因此对苯环的旋转敏感。第二谱带是由于 COOH 区域中电子的激发，因此对 COOH 旋转敏感。通过比较各种苯基、COOH 和 OH 基团的 SAC-CI CD 光谱与实验光谱的计算结果，我们可以预测该分子的稳定几何形状，这与从能量标准推断出的最稳定构象一致。我们还计算了玻尔兹曼平均光谱，并与实验结果吻合得更好。此外，我们对 HPAA 的 CD 光谱的温度依赖性进行了初步研究。一般来说，手性分子的 CD 光谱对单键周围的旋转等低能过程非常敏感。因此，通过使用我们称为 ChiraSac 的新方法，比较不同几何形状和条件下计算的 SAC-CI 光谱与实验光谱，我们应该能够研究弱相互作用的性质。
