Ng, Kai-Mo; (1993) The E.S.R. spectra of some aromatic radical ions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis reports the results of investigations concerning the generation of radical cations, and the nuclear and electronic structures of some aromatic radical ions. The radical ions were generated in fluid solution, and then identified and characterised by electron spin resonance (e.s.r.) spectroscopy. The interpretation was confirmed by computer simulation. The generation of radical cations by electron-transfer from an electron-rich arene to an electron-acceptor, 2,3-dichloro-5,6-dicyano-l,4-quinone (DDQ) in trifluoroacetic acid (TFAH) in the presence of stronger acids (eg. MeSO3H, CF3SO3H) was examined. This technique can generate the radical cations which we had failed to make previously such as the benzo[l,2:4,5]dicyclobutene radical cation. The most important result with this method is the observation of the e.s.r. spectra of a number of benzo-crown ethers (eg. benzo-18-crown-6, benzo-15-crown-5) and their complexes. The e.s.r. spectra of a series of strained benzocyclobutene and benzocyclopentene radical cations were observed. The McConnell-type Q-values have been derived. The S.O.M.O. of these strained radical cations is the [psi]A M.O., and it is suggested that the ordering of the energy levels ([psi]A above [psi]S) in these ring-strained compound5is a manifestation of the Mills-Nixon effect, and results from rehybridisation of the strained molecular frame work. The e.s.r. spectra of 1,3-benzodioxole radical cations have been observed and characterised. The coupling constants of the aliphatic protons (-OCH3 or -OCH2O-) can be explained in terms of the molecular orbital theory of hyperconjugation, and the Whiffen effect. The e.s.r. spectra of the radical cations of benzo-[l,2-b:4,5-b']-bis-l,4-dioxane, 1,3- dioxolo[4,5-g]benzo-1,4-dioxane, 6,7-dimethyl-1,4-benzodioxane, 6,7-dimethoxy-1,4- benzodioxane and 5,8-di-tert-butyl-l,4-benzodioxane have been observed and analysed. In all five radical cations, the pseudo-axial and pseudo-equatorial protons of the methylene groups in the half-chair dioxene rings are distinguished by different hyperfine coupling constants, and simulation of the spectra over a range of temperatures has given the Arrhenius plots for the ring inversions. When certain substituted benzo-l,4-dioxanes and benzo-l,3-dioxoles are treated with mercury(II) trifluoroacetate in TFAH, the e.s.r. spectra of the mercurated arene radical cations were observed. The aromatic protons with a small hyperfine coupling constant are replaced by mercury and satellites due to 199Hg hyperfine coupling can be observed. Mercuration also increases the g-value of the radical cations, and the ratio a(199Hg)/a(1H) = 13-14 is obtained. Replacement of trifluoroacetate anion by the fluorosulphonate anion, gave an increase of a(199Hg) of about 14[percent] as the electronegativity of the anion increased. The radical anions and cations of benzo[b]biphenylene, binaphthylene and tribenzo[12]annulene were examined. In the case of benzo[b]biphenylene and binaphthylene. The hyperfine coupling constants of the radical cations are less than those of the radical anions, and the pairing principle of alternant hydrocarbons appears to break down.

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