Resonance Raman studies of benzene derivatives with methoxy substitution: Conformational symmetry-breaking effects

Resonance Raman intensities show that methoxy substitution induces considerable allowed character to the L_a transition of benzene. In symmetric 1,3,5-trisubstituted benzene derivatives the induced allowed character should cancel by symmetry. However, in 1,3,5-trimethoxybenzene the resonance Raman spectrum shows a strong fundamental transition of v₈, the clear signature of an allowed transition. It is shown that this is due to the presence of a C conformer of 1,3,5-trimethoxybenzene in which one of the three methoxy groups is rotated relative to the other two. This is supported by ab initio calculations of conformational energy, induced transition moment and the splitting of the v₈ band.