The C-13 nuclear magnetic resonance chemical shifts and the C-13,F-19 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
s-p coupling constant between the F-19 nucleus on one ring and the ipso C-13 nucleus on the other, is obtained from measurements
on 2,6-dibromo-4-fluorophenyl phenyl ether. The ensuing estimates of (sin2t), the expectation values of sin2t as obtained from
6J(C-13,F-19)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 degree intervals of the angles of twist about the two C-0 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, (sin2t) values from 6J(C-13,F-19) 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.
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