Internal barriers to rotation in 2,6-difluoroisopropyl and 2,6-difluoroethylbenzenes. Overlap of dynamic nuclear magnetic resonance (DNMR) and the J method

The temperature-dependent 19F nmr spectra of 2,6-difluoroisopropylbenzene yield [Formula: see text], ΔH≠, and ΔS≠ as 6.93 (5) kcal/mol, 6.1(1) kcal/mol, and −5.0(8) cal/mol K, respectively, for the internal rotation of the isopropyl group about the sp2–sp3 carbon–carbon bond. The long-range spin–spin coupling constant over six bonds, 6JpH,CH, combined with the J method gives a twofold internal potential barrier of 5.0 ± 1.6 kcal/mol at 305 K. Although in this barrier range the J method suffers from large errors, the two methods yield comparable values for the barrier height. The lineshape method is inapplicable to 2,6-difluoroefhylbenzene. The J method finds the preferred conformation and a twofold barrier of 6.0 ± 2 kcal/mol, again in a barrier region where this method is inaccurate. Relative to hydrogen, the fluorine substituents cause substantial increases in the barriers to internal rotation. Signs of the stereospecific couplings, 4JoF,CH, are determined.