Limits to barriers to internal rotation in benzal fluoride, chloride, and bromide solution by the J method

On the basis of the long-range couplings over six bonds between the protons on the side chain and the ring protons in benzal fluoride, chloride, and bromide and some of their derivatives, a hindered rotor model (J method) yields limits to the barriers to internal rotation. These are 0.0 to 1.8, 0.7 to 2.4, and 1.7 to 3.8 kcal/mol for the fluoride, chloride, and bromide, respectively. Refinement of the model suggests barriers of 1.1 ± 0.2, 2.2 ± 0.3, and 3.5 ± 0.6 kcal/mol, respectively. The barrier values in the chloride and bromide are considerably less sensitive to the model parameters than is the barrier in the fluoride. It would be of interest to have gas phase values for these barriers. It is also shown that a σ–π mechanism holds for the six-bond proton–proton coupling constant in benzal fluoride but that the analogous coupling between 19F on the side chain and the para ring proton carries a non σ–π contribution of +0.3 Hz.