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
Tanaka, Takeyuki, Oelgemöller, Michael, Fukui, Keijiro, Aoki, Fumiko, Mori, Tadashi, Ohno, Takashi, and Inoue, Yoshihisa (2007) 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. Chirality, 19 (5). pp. 415-427.
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[Extract] 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 π*←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.
|Item Type:||Article (Refereed Research - C1)|
|Keywords:||VCD; ECD; TD-DFT; conformation; cotton effect; band resolution|
|Date Deposited:||18 Apr 2010 23:26|
|FoR Codes:||03 CHEMICAL SCIENCES > 0305 Organic Chemistry > 030505 Physical Organic Chemistry @ 40%
03 CHEMICAL SCIENCES > 0306 Physical Chemistry (incl Structural) > 030606 Structural Chemistry and Spectroscopy @ 30%
03 CHEMICAL SCIENCES > 0307 Theoretical and Computational Chemistry > 030799 Theoretical and Computational Chemistry not elsewhere classified @ 30%
|SEO Codes:||97 EXPANDING KNOWLEDGE > 970103 Expanding Knowledge in the Chemical Sciences @ 100%|
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