Methyl beta-substituent effect on NMR O-17 chemical shifts in two-coordinated oxygen atoms: DFT GIAO and NBO and experimental studies

Methyl beta-substituent effects on O-17 chemical shifts in dicoordinated ox ygen atoms in compounds of the type CH3-O-X are found to be insensitive to the nature of the X substrate, even for strongly conjugating groups. Such b ehavior is rationalized in terms of intramolecular electron delocalizing us ing the natural bond orbitals approach to calculate NBO E-(2) charge-transf er energies. The level of theory employed in this analysis was chosen in su ch a way that DFT GIAO at the same level could describe the experimental tr ends adequately. In this way, it was found that charge transfers to (O-C)* antibonds render important deshielding effects on the O-17 chemical shifts. In the particular case of the methyl beta-substituent effect quoted above, such charge transfer originates in the preferential orientation that the C alphaH2-CbetaH3 group adopts in these compounds CH3-O-X. Analogies with F-1 9 chemical shifts are discussed. (C) 1999 John Wiley & Sons, Ltd.