Avila, David Victor; (1990) The molecular and electronic structures of some radical ions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis reports an investigation into the molecular and electronic structures of some aromatic and heteroaromatic radical ions (predominantly cations). The radical ions were identified and characterised by electron spin resonance spectroscopy. Computer-simulated molecular structures of some were optimized by semi-empirical (AM1) and molecular mechanics calculations; these calculations support the experimental observations. The conformational inversion of the cyclohexene ring in two radical cations containing the benzocyclohexene moiety was observed at a range of temperatures, yielding contrasting barriers to inversion. These were found to be in accord with molecular mechanics calculations on the neutral molecules. The permethylbenzocyclohexenes 9,10-dimethyl-1,2,3,4,5,6,7,8-tetrahydroanthracene and 1,2,3,4-tetramethyl-5,6,7,8-tetrahydronaphthalene exhibited unusual molecular orbital perturbation. The radical cations and anions of methylated biphenylenes, tetraphenylene, and octa-β-methyltetraphenylene were investigated. The pairing principle of alternant π-systems was confirmed in all examples where both radical cations and anions could be studied. The tetra-α-methylbiphenylene exhibited mercurideprotonation when its radical cation was generated in trifluoroacetic acid containing mercury(II) trifluoroacetate. When tetra-β-methylbiphenylene in tetrahydrofuran was treated with lithium, reduction and ring opening apparently occurred. The electron distribution observed in the radical cation of octa-β-methyltetraphenylene confirmed the prediction of a perturbation of molecular orbitals approach. The radical cations of a variety of compounds containing the pyrrole moiety are discussed. A series of p-phenylene-bis(dialkylpyrroles) showed no hyperfine coupling to the arene unit, and electron release into the pyrrole ring by 2,5-diethyl groups was found to be greater than by 2,5-dimethyl substituents. X-Ray crystallography showed that the 2,2',5,5'-tetramethylbi-(N-pyrrolyl) has an orthogonal structure and its radical cation shows the unpaired electron to be delocalised over both rings. The radical cation of 8b,8c-diazapyracyclene, which is isoelectronic with the radical anion of pyracyclene, has been observed and undergoes mercurideprotonation. The spectrum of the radical cation of 1-phenyl-2,3,4,5-tetramethylarsole is in accord with the photoelectron spectra and calculations which suggest that the heteroatom is pyramidal and that arsoles may be considered as non-aromatic hetero[5]annulenes.

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