"Site-specific isotope labeling demonstrates a large mesomeric resonance effect of the methoxy groups on the carbonyl frequency of ubiquinones"Boullais, C., Nabedryk, E., Burie, J.-R.,
Nonella, M., Mioskowski, C., and Breton, J.
Photosynthesis Research 55, 1998, 247-252.
The splitting of the carbonyl infrared bands of
2-methoxy-1,4-benzoquinone in solution can be related to a mesomeric
resonance phenomenon leading to a conformation of the O-CH3 bond coplanar
to the quinone ring. The delocalization of the electron density induces a
frequency downshift of the C4=O carbonyl compared to 1,4-benzoquinone.
This in turns decouples the two carbonyls leading to an upshift of the
C1=O vibration. Using selective 13C-labeling of Q0
(2,3-dimethoxy-5-methyl-1,4-benzoquinone), we show that the effect of
mesomeric resonance is an essential determinant of the carbonyl
frequencies of ubiquinone in solution.
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