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

✍ Scribed by J. E. Peralta; R. H. Contreras; O. E. Taurian; F. S. Ortiz; D. G. de Kowalewski; V. J. Kowalewski

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 143 KB
• Volume: 37
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199901)37:1<31::aid-mrc396>3.0.co;2-e

✦ Synopsis

Methyl b-substituent e †ects on 17O chemical shifts in dicoordinated oxygen atoms in compounds of the type are found to be insensitive to the nature of the X substrate, even for strongly conjugating CH 3 ÈOÈX 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 (OÈC)* antibonds render important deshielding e †ects on the 17O chemical shifts. In the particular case of the methyl b-substituent e †ect quoted above, such charge transfer originates in the preferential orientation that the group adopts in these compounds Ana-C a H 2 ÈC b H 3 CH 3 ÈOÈX. logies with 19F chemical shifts are discussed.