Alkyl and silyl derivatives of benzene radical-cations formed by radiolysis: an electron spin resonance study

Abstract

Exposure of dilute solutions of various alkyl- and silyl-benzenes in trichlorofluoromethane to ⁶⁰Co γ-rays at 77 K gave species whose e.s.r. spectra are characteristic of substituted benzene cations. For the ethyl derivative large hyperfine coupling to the methyl protons establishes a preferred conformation in which the methyl group lies in the plane of the benzene ring. For the isopropyl derivative, and particularly for p-cymene cations, several conformers were detected, the sterically most favourable being the least stable. This is interpreted in terms of strong electron-donation from the C–H σ-orbitals into the ring π-orbital with is greater than that from C–Me σ-orbitals so that π-overlap with the C-H bonds is maximised. The energy difference is slightly greater than the steric energy differences. The SOMO for the silyl derivatives (SiH₃, SiHMe₂, and SiMe₃) is also the a₁ orbital (ψ_a) which places maximum spin-density on the position of the substituent. However, the degree of hyperconjugation involving Si-H σ-orbitals is reduced by a factor of ca.2, as judged by the ¹H hyperfine coupling constants. Nevertheless, it was not found necessary to invoke a mixture of the a₁ and a₂ orbitals (ψ_S and ψ_A) to explain the results. These results are compared with those for the corresponding radical-anions and for the neutral cyclopentadienyl radicals.