Methyl β-substituent effect on NMR 17O 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 oxygen 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 behavior is rationalized in terms of intramolecular electron delocalizing using the natural bond orbitals approach to calculate NBO E-(2) charge-transfer energies. The level of theory employed in this analysis was chosen in such a way that DFT GIAO at the same level could describe the experimental trends 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 CalphaH2-CbetaH3 group adopts in these compounds CH3-O-X. Analogies with F-19 chemical shifts are discussed. (C) 1999 John Wiley & Sons, Ltd.