Proton nuclear magnetic resonance spectra and conformation in 1,4-dihydrobenzenes and 9,10-dihydroanthracenes

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Abstract
Analysis of the 220 MHz 1H n.m.r. spectra of 2,6-dimethoxy- and 2,6-bis(trideuteriomethyl)-1,4-dihydrobenzoic acids leads to the conclusion that the equilibrium geometry of the dihydrobenzene rings in these molecules is essentially planar. Although the equilibrium conformation of the ring in most 1,4-dihydrobenzenes is probably planar, Laane and Lord's far-i.r. data on dihydrobenzene itself suggest that the time-averaged geometry of the thermally populated vibrational states at 300 K is significantly boat-shaped. This allows earlier results, from which a boat conformation had been inferred, to be reconciled with more recent conclusions. The pseudoaxial–pseudoaxial cis homoallylic coupling constant of dihydroanthracene has been estimated to be 4.8 Hz. The values of homoallylic coupling constants in 9,10-dihydroanthracenes are in fair agreement with estimates based on a recent INDO analysis of the angular dependence of homoallylic coupling.

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