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 H_β 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 H_β 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 H_β lies in the nodal plane of the C_α 2p_z orbital, whereas axial and equatorial cyclohexylmethyl radicals adopt a conformation, 3, in which H_β is eclipsed by this orbital. The cyclohexylmethyl radical was also investigated by semiempirical SCF MO calculations.