Remarks on the barrier to rotation about the Csp2—O bond in anisole

Abstract

Molecular orbital computations with the basis set 6-31G are reported for seven values of θ, the torsion angle about the [Formula: see text] bond in anisole. All bond angles and lengths are optimized but the atoms of the phenyl group are constrained to a plane. The relative energies are fit by V(θ)/kJ mol−1 = 7.78(5) sin2θ + 2.41(5) sin2 2θ − 0.54(5) sin2 3θ, where θ is zero when the heavy-atom skeleton is planar. Computations with the basis set 6-31G*(5D) for three values of θ can be reproduced by V(θ)/kJ mol−1 = 6.07 sin2θ + 2.68 sin2 2θ. These results are compared with experimental gas phase data from the literature. The analysis of the 1H nuclear magnetic resonance spectrum of anisole-α-13C in aCS2/C6D12/TMS solvent mixture yields a value of 6J(1H, 13C), the long-range spin-spin coupling constant between the 13C nucleus in the methyl group and thepara proton. Because this coupling constant is proportional to sin2 θ, it is shown, together with previous dynamic nmr measurements, that the barrier to rotation about the [Formula: see text] bond in solution cannot be purely twofold. The internal potential must also contain a fourfold term of the same sign as that of the twofold component. If the V2/V4 ratio given by the various molecular orbital computations holds in solution, then V2 is 15.0 ± 2.0 kJ/mol and V4 is 5.6 ± 2.2 kj/mol. The apparent doubling of the internal barrier in solution is perhaps unprecedented for such a simple molecule. Keywords: anisole, internal barrier in solution, anisole-α-13C, 1H NMR, conformational behaviour, MO computations.