Unusual CD couplet pattern observed for the pi*<--n transition of enantiopure (Z)-8-methoxy-4-cyclooctenone: an experimental and theoretical study by electronic and vibrational circular dichroism spectroscopy and density functional theory calculation.

Ultraviolet absorption (UV) and electronic circular dichroism (ECD) spectra of enantiopure (Z)-8-methoxy-4-cyclooctenone (MCO) were measured in hexane to give a normal single UV absorption band at 298 nm, which is assigned to the carbonyl's pi*<--n transition. Unexpectedly, the ECD spectrum exhibited an apparent couplet pattern with vibrational fine structures. Obviously, the conventional CD exciton coupling mechanism cannot be applied to this bisignate CD signal observed for single-chromophoric MCO. Variable temperature-ECD and vibrational circular dichroism (VCD) spectral measurements, simultaneous UV and ECD spectral band resolution, and density functional theory (DFT) calculations of energy and structure revealed that this apparent CD couplet originates from a rather complicated spectral overlap of more than three conformers of MCO, two of which exhibit mirror-imaged ECD spectra at appreciably deviated wavelengths. In the simultaneous band-resolution analysis, the observed UV and ECD spectra were best fitted to four overlapping bands. Two major conformers were identified by comparing the experimental IR and VCD spectra with the simulated ones, and the other two by comparing the observed UV and ECD spectra with the theoretical ones obtained by time-dependent DFT calculations. It was shown that the combined use of experimental ECD and VCD spectra and theoretical DFT calculations can give a reasonable interpretation for the Cotton effects of the conformationally flexible molecule MCO.