Journal of the Chemical Society. Perkin transactions II p. 1201 - 1206 (1984)
Update date:2022-08-31
Topics:
Ramakrishna Rao, D. N.
Chandra, Harish
Symons, Martyn C. R.
Exposure of dilute solutions of variuos alkyl- and silyl-benzenes in trichlorofluoromethane to 60CoSg-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 conformes were detected, the sterically most favourable being the least stable.This is interpreted in terms of strong electron-donation from the C-H ?-orbitals into the ring ?-orbital with is greater than that from C-Me ?-orbitals so that ?-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 (ΠSa) which places maximum spin-density on the position of the substituent.However, the degree of hyperconjugation involving Si-H ?-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 (ΠSS and ΠSA) to explain the results.These results are compared wiyh those for the corresponding radical-anions and for the neutral cyclopentadienyl radicals.
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