Remarks on the internal motion in diphenyl ether. Fluorophenyl ethers

Abstract

The 13C nuclear magnetic resonance chemical shifts and the 13C,19F spin–spin coupling constants are reported for 4,4′-difluorophenyl ether and 4-fluorophenyl phenyl ether in CS2 and in acetone-d6 solutions. An estimate of 6J90, the extremum in the σ–π coupling constant between the 19F nucleus on one ring and the ipso13C nucleus on the other, is obtained from measurements on 2,6-dibromo-4-fluorophenyl phenyl ether. The ensuing estimates of [Formula: see text], the expectation values of sin2 θ as obtained from 6J(13C,19F), are compared with those obtained from STO-3G MO computations for diphenyl ether and its 4-fluoro derivatives. These computations give conformational energies at 30° intervals of the angles of twist about the two C—O bonds. In rough agreement with C-INDO computations, interconversion of the helical forms is calculated to occur most easily by the so-called one-ring flip mechanism; the barrier to interconversion is less than 1 kJ/mol in the ether and its 4-fluoro derivatives. It appears that the conformational behaviour of these derivatives is unaltered by passage from CS2 to acetone solutions at 300 K. Furthermore, [Formula: see text] values from 6J(13C,I9F) in solution are very similar to those obtained from the computations on the free molecules. If this agreement is not accidental, then it may arise from a high degree of flexibility of the molecules in which, by a disrotatory or one-ring flip mechanism requiring a very low energy of activation, one helical or C2 conformation can be converted to another. The other conformations have considerably higher energies and the solvents do not appear to lower these energies enough to favor their populations significantly at 300 K.