Methyl β-substituent effect on NMR17O chemical shifts in two-coordinated oxygen atoms: DFT GIAO and NBO and experimental studies

Abstract

Methyl β-substituent effects on 17 O 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 17O chemical shifts. In the particular case of the methyl β-substituent effect quoted above, such charge transfer originates in the preferential orientation that the CαH2—C βH3 group adopts in these compounds CH 3OX. Analogies with 19F chemical shifts are discussed. © 1999 John Wiley & Sons, Ltd.