Unusual CD couplet pattern observed for the π*←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

Abstract

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.

Implementation and validation of a multi-purpose virtual spectrometer for large systems in complex environments

“…These results are very different from the previous analysis of the OPA and ECD spectra of MCO, 201 where only the vertical transition between the electronic states has been taken into account, and both absorption and ECD spectra have been simulated by applying a phenomenological broadening to the electronic stick data. Several conformers of MCO have been studied theoretically in order to obtain the best match with the experimental spectra.…”

“…As an example experimental studies of (Z)-8-methoxy-4-cyclooctenone (MCO) considered IR, VDC, OPA and ECD spectra, supported by a standard computational analysis (harmonic approach and vertical transitions). 201 Here, we will focus on the unusual vibronic pattern in the electronic circular dichroism spectrum observed for the p* ' n electronic transition, for which some preliminary results have already been reported by some of us. 53 The simulated absorption and electronic circular dichroism spectra computed within the adiabatic framework (Adiabatic Hessian -AH) with the transition dipole moments described within Franck-Condon (FC|AH) or Franck-Condon Herzberg-Teller (FCHT|AH) models are compared to their experimental counterparts 201 in Fig.…”

“…Nevertheless, even if the main spectral features are related to the most stable conformer, contributions from the second one might slightly modulate the spectrum shape, possibly improving the agreement with experiment. In any case, it should be noted that, within the simplified model where vibrational effects are neglected, 201 as many as four conformers have been necessary for a qualitative reproduction of the experimental spectra. In fact a change of sign can be sometimes observed in the energy window encompassed by a single electronic state, which is often attributed (see e.g.…”

Aiming at an accurate prediction of vibrational and electronic spectra for medium‐to‐large molecules: An overview

“…An important aspect is that, even for fully allowed transitions, the FC contributions can be negligible if the transition moments of the electric and magnetic dipoles moments are near orthogonal ( ). As an example, let us consider the case of (Z)−8‐Methoxy‐4‐Cyclooctenone (MCO) where one broad band is observed in the 340–250 nm range of the experimental OPA spectrum in hexane solution, but a more complex pattern is present in the ECD one . The OPA spectrum is satisfactorily described as the transition of the 1A conformer computed at the TI AH FC level (full convergence), but this is not the case for ECD, as shown in Figure .…”

“… Theoretical ECD spectrum of 1A conformer of (Z)‐8‐Methoxy‐4‐Cyclooctenone in the 340–240 nm range, compared to experiment in hexane solution taken from reference . The theoretical spectra have been simulated with the TI AH FC and AH FCHT models at the TD‐CAM‐B3LYP/aug‐N07D/C‐PCM level .…”