Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
J Phys Chem A. 2012 Jul 12;116(27):7372-85. doi: 10.1021/jp304576g. Epub 2012 Jun 28.
The chiroptical properties of a series of carbo[n]helicenes (n = 4-10) were investigated by the state-of-the-art approximate coupled cluster and density functional theory calculations. The theoretical calculation at the RI-CC2/TZVPP//DFT-D2-B97-D/TZVP level nicely reproduced the experimental CD spectra in both excitation energy and rotational strength without any shift or scaling. These calculations afforded the electric and the magnetic transition dipole moment vectors in [n]helicenes, allowing us to discuss the observed rotational strengths as a function of the number of benzene rings. Although the observed CD intensity was not immediately correlated to any of the calculated parameters, the anisotropy (g) factor of the (1)B(b) band and the specific rotation were found inversely proportional to n and nicely correlated with the helical pitch, but discontinuous at n = 6, where the aromatic rings start to overlap. In contrast, the g factor at the (1)B(a) band was rather insensitive to n. It was also revealed that the excitation energies of the (1)B(b) and (1)B(a) bands are inversely proportional to n over the entire range of n examined. The theoretical predictions also enabled us to rectify the erroneous experimental CD spectra of [5]- and [6]helicenes reported earlier, by using the enantiopure samples resolved by chiral HPLC.
一系列碳[n]螺旋烯（n = 4-10）的手性光学性质通过最先进的近似耦合簇和密度泛函理论计算进行了研究。RI-CC2/TZVPP//DFT-D2-B97-D/TZVP 水平的理论计算很好地再现了实验 CD 光谱在激发能量和旋转强度方面，无需任何移位或缩放。这些计算提供了[n]螺旋烯中的电和磁跃迁偶极矩矢量，使我们能够讨论观察到的旋转强度与苯环数量的关系。尽管观察到的 CD 强度与任何计算参数都没有直接相关，但（1）B(b) 带的各向异性 (g) 因子和比旋光度与 n 呈反比关系，与螺旋桨螺距很好地相关，但在 n = 6 时不连续，此时芳环开始重叠。相比之下，（1）B(a) 带的 g 因子对 n 的变化不敏感。还揭示了（1）B(b) 和（1）B(a) 带的激发能在整个 n 范围内与 n 成反比。理论预测还使我们能够通过使用手性 HPLC 分离的对映纯样品来纠正先前报道的[5]-和[6]螺旋烯的错误实验 CD 光谱。
