Carbon-13 nuclear magnetic resonance spectroscopy. Determination of carbon-fluorine couplings

One-, two-, three-, and four-bond carbon-fluorine coupling constants have been observed for a series of substituted fluorobenzenes. The one-bond coupling constants in the para derivatives are proportional to the fluorine chemical shifts, but a similar correlation shows considerable scatter for the meta derivatives and is invalid for ortho derivatives. The carbon-fluorine coupling constants in fluorobenzene are JCF= -245.3, JCCF= +21.0, JCCGF) = + 7.7, and Jccccr = +3.3 Hz. Consistent explanations of the substituent effects on the long-range coupling constants have not been found. All the carbons in l-fluoronaphthalene are coupled to the fluorine with magnitudes of 0.6 to 250 Hz. The fluorine in 2-fluoronaphthalene is coupled to only six carbons, presumably those in the same ring as the fluorine. The carbon-fluorine coupling constants in n-hexyl fluoride are JCF = - 166.6, JCCF = +19.9, and JcccF= 5.25 Hz. The one-bond carbon-fluorine couplings in cyclic geminal difluorides vary with ring size. The fluorine substituent effect on the chemical shifts of carbons y to the fluorine is discussed in terms of proposed steric perturbation mechanisms.