Concerning the internal rotational potential in benzal chloride. 37Cl/35Cl isotope effect on the methine 1H NMR

Abstract

1H NMR spectral parameters are reported for benzal chloride in CS2/C6D12, CDCl3, and acetone-d6 solutions. The values of 6J (H,CH), the long-range spin–spin coupling constant between the side-chain and para protons, are consistent with twofold barriers to rotation about the exocyclic C—C bond of 8.6(4), 8.9(4), and 10.2(6) kJ/mol, in the order of the solvents above. These numbers are somewhat smaller than the twofold barrier of 11.6 kJ/mol available from STO-3G MO computations for the free molecule. Alternatively, an internal rotational potential consisting of a twofold term of 17.0 kJ/mol and a fourfold term of −5.5 kJ/mol is also compatible with the 6J (H,CH) measured for the CDCl3 solution and with the 13C spin-lattice relaxation data reported for such a solution. The 6J (H,CH) values demonstrate that the conformer with the a C—H bond situated in the benzene plane is the most stable. A local minimum in the potential for a conformer in which this bond lies in a plane perpendicular to the phenyl group, leading to a population of about 8% for this conformer at 300 K and compatible with the magnitude of 6J (H,CH), is represented by twofold and fourfold terms of 6.0 and 4.5 kJ/mol, respectively. The barrier height is then only 8.0 kJ/mol, however. 37Cl/35Cl isotope effects on the methine proton amount to −0.17, −0.19, and −0.22 parts per billion in CS2, CDCl3, and acetone-d6 solutions, respectively (increased screening). Keywords: benzal chloride, internal potential, 37Cl/35Cl isotope effect on 1H NMR.