A proton magnetic resonance determination of the rotational barrier in benzyl chloride and of the low energy conformations in benzyl bromide and iodide derivatives based on motionally averaged spin-spin interactions

The long-range spin-spin coupling constants over six bonds between side-chain and ring protons in benzyl chloride derivatives are combined with solutions to the hindered rotor problem to demonstrate that the twofold barrier to internal rotation in benzyl chloride is 2.1 ± 0.4 kcal/mol. The low energy conformation is that in which the CCl bond lies in a plane perpendicular to the aromatic plane. Molecular orbital calculations at the MINDO/3 and STO-3G levels indicate that the conformation in which the CCl bond lies in a plane having an angle of 60° to the aromatic plane has a higher energy; comparable to kT at ambient temperatures. The geometry-optimized STO-3G results agree quantitatively with the derived barrier for benzyl chloride. A semiempirical relationship between the long-range couplings over four bonds and the internal barrier for a number of benzyl compounds indicates a barrier of 3.1 kcal/mol for benzyl bromide and of 3.6 kcal/mol for benzyl iodide, these values being lower limits. The observed coupling constants are only consistent with low-energy conformations analogous to that for benzyl chloride, so that they contrast with the low-energy conformation for benzyl fluoride in which the CF bond lies in the aromatic plane. The present method for the determination of twofold barriers in benzyl derivatives is useful in the range from about 0.2 to 2 kcal/mol.