Conformational Sensitivity of Chiroptical Spectroscopic Methods: 6,6′-Dibromo-1,1′-bi-2-naphthol

Abstract

The experimental vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and optical rotatory dispersion (ORD) spectra for both enantiomers of 6,6'-dibromo-1,1'-bi-2-naphthol have been measured. The corresponding quantum chemical predictions for three different orientations of hydroxyl groups in this molecule were obtained using 6-31G*, 6-311G(2d,2p) and 6-311++G(2d,2p) basis sets and the B3LYP density functional. The absolute configuration inferred by comparing the predicted spectra with experimental data is the same in VCD, ECD, and ORD methods, but spectral sensitivity to conformation varies among these methods. It is found that predicted ECD in the 200-350 nm region does not change significantly for the three conformers. As a result, the sensitivity of ECD to the conformation of hydroxyl groups in 6,6'-dibromo-1,1'-bi-2-naphthol is rather limited. Similarly, a comparison between experimental and predicted ORD spectra indicated that ORD spectral prediction is not the best method of choice for discrimination among different conformations of O-H groups. On the other hand, the predicted VCD, and associated absorption, spectral patterns in the 1600-900 cm(-1) region are found to change significantly for the three conformations, and the experimental spectra correlate well with those predicted for only one conformation. These observations suggest that, among the three chiroptical spectroscopic methods investigated, VCD provides better sensitivity to the conformation of hydroxyl groups. The use of PCM model for exploring the influence of CH(2)Cl(2) solvent on vibrational spectra indicated that this model is not appropriate for modeling the CH(2)Cl(2) solvent influence on the vibrational absorption and VCD spectra of 6,6'-dibromo-1,1'-bi-2-naphthol.