Conformational analysis and stereodynamics of primary acyclic alkyl radicals by EPR spectroscopy

The EPR spectra of n-alkyl, 2-methylalkyl, 2,2-dimethylalkyl, 2,2,3-trimethylbutyl, and 2,2,3,3-tetramethylbutyl radicals indicate that at 90 K they exist in "rigid" conformations with respect to rotation about the C_β-C_γ bonds. The preferred conformations about the C_α-C_β and C_β-C_γ bonds were deduced by analysis of the β- and γ-H hyperfine splittings (hfs). 2,2,3,3-Tetramethylbutyl radicals, the only radicals with a CH₃ group approximately all-trans with respect to the semioecupied p-orbital, were also the only radicals to show resolved δ-hfs. The barriers to internal rotation of the methyl groups in n-propyl, isobutyl, neopentyl, 2,2-bis(trideuteriomethyl)butyl, and 2,2,3,3-tetramethylbutyl radicals were obtained by line shape analysis; the ethyl rotation barrier in 2,2-bis(trideuteriomethyl)butyl and the tert-butyl rotation barrier in 2,2,3,3-tetramethylbutyl radicals were estimated in a similar way. The experimental hfs of trans γ-hydrogens were shown to fit a relationship of the form a^Hγt = 0.1 + 7.9 cos² Φ, where Φ is the dihedral angle between the SOMO and the plane through C_α, C_β, and C_γ. Trends in the internal rotation barriers of the alkyl groups were adequately accounted for in terms of steric effects.