1H and 13C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution

Abstract

1H NMR and 13C NMR spectral data are presented for 1,2-dimethoxybenzene-α,α′-13C, as well as for a number of other anisole derivatives. For the title molecule, the coupling constants between the 13C nucleus in the side chain and the para ring proton or 13C nucleus in the benzene ring show that the expectation value of sin2 θ is very near 0.2 at 300 K, where θ is the angle by which the methoxy groups twist out of the aromatic plane. This value of [Formula: see text] is much larger than that of 0.05 for anisole in solution, emphasizing the greater conformational mobility of the methoxy groups in 1,2-dimethoxybenzene (DMB). Long-range coupling constants between the methyl and ring protons in DMB are also discussed and compared with those in anisole and some of its derivatives. The preponderance of conformations with large values of θ, thought to occur in the vapor, disappears in solution at ambient temperatures. Under these conditions, the molecule is perhaps best described as preferring a planar conformation in which, however, the methoxy groups undergo excursions in θ whose average value is near 25°. Keywords: 1,2-dimethoxybenzene, conformational behaviour, internal mobility, long-range coupling constants, 1H and 13C NMR.