Conformations of highly hindered aryl ethers—VIII Interannular HF coupling in diphenyl ethers

A novel type of interannular coupling ( 6 J H,F = 1.0 to 1.3 Hz) has been observed in some fluorodinitrodiphenyl ethers between the fluorine(s) on one ring ortho to the ether linkage and a proton ortho to it on the other ring. 1 H and 19 F chemical shift and coupling constant data support the previous conclusion that these ethers preferentially adopt a twist (propeller) conformation. On account of the geometrical disposition of the six bonds between the interacting nuclei and the latter's spatial proximity in this conformation, direct coupling through‐space is indicated. Concerted libration around the ether bonds brings about magnetic equivalence of the o ‐fluoro substituents on one ring and causes equal coupling to be observed to the o ‐proton on the other ring. The possiblity that the coupling is mediated by an intervening π‐cloud is suggested as an alternate mechanism for indirect coupling. The observed interannular transmission of spin information provides evidence in favor of a recent proposal made concerning the mode of action of the thyroid hormones.