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

Methyl β-substituent effects on ¹⁷O 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 - CβH₃ group adopts in these compounds CH₃ - O - X. Analogies with ¹⁹F chemical shifts are discussed.