Conformations of highly hindered aryl ethers: IX. Dipole moment evidence for an intramolecular donor-acceptor complex

In a search for evidence that a π cloud proximal halogen interaction is of importance to the mode of action of the thyroid hormones, the dipole moments of the 1'-naphthyl (I), (4.29 D), 4'-carboxymethylphenyl (II), (2.90 D), 2'-bromo4'-carboxymethylphenyl (III), (3.56 D), and methyl (IV), (3.24 D) ethers of 2,4dinitro-6-bromophenol were determined in benzene at 25 °C. The moments of (I), (II) and (III) are approximated best by those calculated for the twisted conformation in which the bromine closely approaches the face of the other ring. That of the naphthyl ether (I) can, however, be explained only by inclusion of an intramolecular electron donor-acceptor charge-transfer moment of 1.6 D directed from the π cloud of the naphthyl ring to the proximal nitro group of the other ring, or of 2.8 D to the proximal bromine of that ring. In the preferred conformation of the methyl ether (IV), the methoxy group is coplanar with the ring while the 2-nitro group is twisted 90° from coplanarity to accomodate it. A mesomeric moment of 2.3 D is estimated from this for the 2,4-dinitro-6-bromophenoxy moiety and is used in calculating the expected moments of the other ethers.