Experimental evidence is presented to support the hypothesis that, in triply ortho-substituted diphenyl ethers, conformer interconversion involves both rings. The free energy barrier, measured by p.m.r., is sensitive to substituent effects arising in the ring having only one ortho substituent. The exceptionally high barrier found when an ortho iodine is present is consistent with the existence of a substituent-inside intermediate occurring during conformer conversion.

Using n.m.r. methods developed in earlier papers, the conformations of two series, one doubly ortlro-substituted and one triply ortho-substituted, of six identically substituted bridged diphenyls were compared. The ethers, sulfides, and niethanes have been shown to adopt the same conformation in which the di-ortho-substituted ring is perpendicular to the central bridge plane while the other ring lies in that plane and an ortho hydrogen takes LIP the "inside" o r "proximal" position. The benzophenones and sulfoxides prefer a conforniation in which the dihedral angle between each ring and the central plane is about 40-50" and an orflro hydrogen adopts the "outside" or "distal" position. The sulfones adopt a conformation in which the two rings are more nearly perpendicular to the central plane.

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