Fluorine N.S.R. Spectroscopy, II. A “Distant” Carbon-13 Isotope Effect

A nuclear spin resonance isotope upon shielding values for fluorine atoms attached to C/sup 13/was investigated. The small excess shielding produced by distant C/sup 13/ is shown to vary by a statistically significant factor of two among the compounds studied in the F--C/sup 12/--C/sup 13/ system. No is observed between this strongly varying quantity and other closely related measurements, nor to the fluorine shielding values listed for the compounds. The magmtude of the C/sup 13/ effect increases with the number of trigonal carbons in the F--C--C/sup 13/ system; this correlation may perhaps extend to the smaller F--C/sup 13/ system, with the opposite sign. Experiments are underway to determine the relative sign of the two C/sup 13/ coupling constants. (B.O.G.)