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"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. Abstract: 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. BMO authors (in alphabetic order): Marco Nonella |