Site-specific isotope labeling demonstrates a large mesomeric resonance effect of the methoxy groups on the carbonyl frequency in ubiquinones

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 C-4=O carbonyl compared to 1,4-benzoquinone. This in turns decouples the two carbonyls leading to an upshift of the C-1=O vibration. Using selective C-13-labeling of Q(0) (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.