Summary: 342 J. Phys. Chem. 1988, 92, 342-346
lation suggesting that the stable conformation of the radical is
planar,gand thus it might be possible to assert that the circum-
stances of the canal complex make the nonplanar structure stable.
If the above assertion is true, it follows that the most stable
conformation ( S ) is pyramidal and the next stable one (U) is
planar as in the MIND0 calculation. These two structures cannot
exist at the same time unless unreasonable structures are assumed.
However, if a pyramidal conformation is the most stable at the
equilibrium state, it is reasonable to suppose that the next stable
(9) Igual, J. THEOCHEM 1985, 121, 221
one is its inverse conformation. This is the present case.
In conclusion, the structure of the methylcyclohexyl radical is
pyramidal at the equilibrium state and the angle of CCC is about
117'. At the low temperature, there are two nonplanar confor-
mations and the radical does the umbrella inversionbetween these
conformations. The existence of the wall of thiourea makes it
possible to observe this inversion. At the higher temperatures,
this umbrella inversion occurs rapidly and the radical can be
thought to be planar.