DFT–GIAO and DFT–NBO studies of the origin of 19F NMR shielding effects in alkyl fluorides

✍ Scribed by William Adcock; Dieter Lünsmann; Juan E. Peralta; Rubén H. Contreras

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 102 KB
• Volume: 37
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199903)37:3<167::aid-mrc422>3.0.co;2-k

✦ Synopsis

Electron delocalization into the local environment of the Ñuorine nucleus in the anti (1) and syn (2) epimers of 7-norbornenyl Ñuoride, 7-Ñuoronorbornane (3), 7-Ñuoronobornadiene (4) and the equatorial and axial epimers of cyclohexyl Ñuoride (5 and 6, respectively) were studied using the natural bond orbital (NBO) approach. The level of theory, B3LYP/6È311G**, was chosen in such a way that the GIAO-calculated 19F magnetic shielding constants in all these compounds were in very good agreement with experimental values. The large 19F deshielding e †ects known experimentally in 1 and 4 with respect to 2 and 3 are rationalized in terms of di †erences in such electron delocalizations. The main contribution is that corresponding to the n ] (CÈF)* interaction, which is only efficient for an anti orientation of the CÈF bond and the vinyl moiety. 19F and 13C NMR spectra of 5 and 6 were also measured because some ambiguity was detected in the literature. Despite the large calculated charge donation to the Ñuorine environment in 6, its 19F resonance appears upÐeld from that in 5. This trend is rationalized as originating in c e †ects of the bonds of the kind which have been invoked to explain 17O chemical shift CÈH ax trends in di †erent cyclohexanols and their respective ethers. Electron delocalization e †ects on 1J(C,F) and 2J(C,F) couplings are also discussed.