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Long‐range 37 Cl/ 35 Cl and 81 Br/ 79 Br isotope shifts in the 19 F NMR spectra of some substituted fluorobenzenes
Author(s) -
Bernard Guy M.,
Schurko Robert W.
Publication year - 1995
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260331107
Subject(s) - chemistry , isotope , kinetic isotope effect , bromine , chlorine , nmr spectra database , chemical shift , isotopic shift , isotopes of chlorine , analytical chemistry (journal) , spectral line , deuterium , atomic physics , organic chemistry , physics , quantum mechanics , astronomy
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.