Long-range 37Cl/35Cl and 81Br/79Br isotope shifts in the 19F NMR spectra of some substituted fluorobenzenes

✍ Scribed by Guy M. Bernard; Robert W. Schurko

• Publisher: John Wiley and Sons
• Year: 1995
• Tongue: English
• Weight: 313 KB
• Volume: 33
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260331107

✦ Synopsis

Abstract

Long‐range isotope shifts, Δδ, due to ^37^Cl/^35^Cl and ^81^Br/^79^Br, in the ^19^F NMR spectra of some substituted benzenes are reported. The shifts appear to be invariant to solvent and are additive when the isotopes are in chemically equivalent positions relative to the fluorine site. The three‐bond isotope shift attributed to chlorine is approximately −1.60 ppb and that from bromine varies from −0.25 to −0.28 ppb. These shifts appear to be invariant to the substituent pattern. However, there is a large difference in the four‐bond shift for 2,6‐dichloro‐4‐fluorophenol (−0.16 ppb) and 2‐chloro‐6‐fluorocyanobenzene, (−0.55 ppb), suggesting that this isotope shift is mediated by the intervening group. In all cases, substitution by the heavier chlorine isotope resulted in increased shielding. The nearly equal natural abundance of ^81^Br and ^79^Br resulted in double peaks of approximately equal intensity. However, in almost all cases, the high‐field peak is of slightly lower intensity, suggesting that the sign of this isotope effect is also negative.