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-benzoqui none in solution can be related to a mesomeric resonance phenomenon le ading to a conformation of the O-CH3 bond coplanar to the quinone ring . The delocalization of the electron density induces a frequency downs hift 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 vibrat ion. 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 e ssential determinant of the carbonyl frequencies of ubiquinone in solu tion.