Quantum Chemical Study on the Circular Dichroism Spectra and Specific Rotation of Donor−Acceptor Cyclophanes

Abstract

The structures of donor,acceptor-substituted cyclophanes were optimized by DFT and MP2 methods and compared with the X-ray crystallographic structures. The electronic circular dichroism (CD) spectra of these chiral cyclophanes were simulated by time dependent density functional theory (TD-DFT) with several functionals including different amounts of "exact" Hartree-Fock exchange. The experimental oscillator and rotatory strengths were best reproduced by the BH-LYP/TZV2P method. The specific rotation and vibrational circular dichroism (VCD) spectra were also calculated at the BH-LYP/aug-cc-pVDZ and B3-LYP/6-31G(d) levels, respectively, and compared with the experimental data. Better performance was obtained with the ECD, rather than the specific rotation or the VCD spectral calculations in view of the computation time and accuracy for the determination of absolute configuration (AC). The exciton coupling model can be applied only for the cyclophanes without CT-character. However, the split pattern found in the experiment does not appear to originate from a simple two-transition coupling, indicating that this method should be applied with caution to the AC determination. This conclusion was supported by the TD-DFT investigations of the transition moments and the roles of excited-state electronic configuration associated with these split bands. Cyclophanes with donor-acceptor interactions showed Cotton effects at the CT band and couplets at the 1La and 1Lb bands. Although the degree of charge transfer between the rings is very small, as revealed by a Mulliken-Hash analysis, the split Cotton effects are due to a large separation in energy of the donor and acceptor orbitals. The effect of the distance and angle between the donor and acceptor moieties in model (intermolecular) CT complexes on the calculated CD spectra was also studied and compared with those obtained for various paracyclophanes.