Cycloalkylmethyl Radicals. 6. The Unexpectedly High Barrier to the Rotation of Axial CH₂· Groups in Cyclohexylmethyl Radicals

The EPR spectra of axial cyclohexylmethyl radicals show temperature-dependent line-broadening effects involving the hydrogen atoms responsible for “long-range” couplings. Since the Hβ hyperfine splittings (hfs) of axial cyclohexylmethyl radicals and of 2-adamantylmethyl (in which the CH2· moiety is necessarily axial to a chair cyclohexane ring) are of similar magnitude and show similar temperature dependencies, it is clear that the conformations of axial cyclohexylmethyl radicals have been correctly assigned. The observed line broadening must therefore be attributed to restricted rotation of the axial CH2·, the barrier to its rotation being ca. 6 kcal/mol. This barrier is enhanced relative to the barrier in equatorial cyclohexylmethyl radicals because of steric interactions with the syn axial hydrogens at positions 3 and 5. Moreover, this barrier is much greater than the ca. 1.5 kcal/mol calculated from the temperature dependence of Hβ hfs of the axial radicals by the “classical limit” procedure. This failure of the “classical limit” procedure is attributed to the fact that the rotational potential function for these radicals is not of the simple 2-fold type. There are no syn axial hydrogens at positions 3 and 5 in 4-tert-butyl-3,5-dioxanylmethyl radicals, and both the axial and equatorial radicals adopt “bisected” conformations, 8 and 9, respectively, in which Hβ lies in the nodal plane of the Cα 2pz orbital, whereas axial and equatorial cyclohexylmethyl radicals adopt a conformation, 3, in which Hβ is eclipsed by this orbital. The cyclohexylmethyl radical was also investigated by semiempirical SCF MO calculations.