Absorption spectrophotometric and mass spectrometric properties of 1,2-benzoquinone, prepared in aqueous solution by the hexachloroiridate(IV) oxidation of catechol and isolated by HPLC, are reported. Its absorption spectrum has a broad moderately intense band in the near UV with an extinction coefficient of 1370 M(-1)cm(-1) at its 389 nm maximum. The oscillator strength of this band contrasts with those of the order-of-magnitude stronger approximately 250 nm bands of most 1,4-benzoquinones. Gaussian analysis of its absorption spectrum indicates that it also has modestly intense higher energy bands in the 250-320 nm region. In atmospheric pressure mass spectrometric studies 1,2-benzoquinone exhibits very strong positive and negative mass 109 signals that result from the addition of protons and hydride ions in APCI and ESI ion sources. It is suggested that the hydride adduct is formed as the result of the highly polar character of ortho-quinone. On energetic collision the hydride adduct loses an H atom to produce the 1,2-benzosemiquinone radical anion. The present studies also show that atmospheric pressure mass spectral patterns observed for catechol are dominated by signals of 1,2-benzoquinone resulting from oxidation of catechol in the ion sources. Computational studies of the electronic structures of 1,2-benzoquinone, its proton and hydride ion adducts, and 1,2-benzosemiquinone radical anion are reported. These computational studies show that the structures of the proton and hydride adducts are similar and indicate that the hydride adduct is the proton adduct of a doubly negatively charged 1,2-benzoquinone. The contrast between the properties of 1,2- and 1,4-benzoquinone provides the basis for considerations on the effects of conjugation in aromatic systems.

4-Hydroxy-4-methyl-2,5-cyclohexadien-1-one has been conclusively identified by its 1 H and 13 C NMR spectra as a significant initial product in the radiolysis of aqueous solutions of p-cresol. This product is formed as the result of oxidation of the hydroxycyclohexadienyl radicals produced by addition of ∼12% of the • OH radicals to the aromatic ring at the ipso position adjacent to the methyl group. It has a strong absorption band at 228 nm that is similar to the 246 nm band of p-benzoquinone. Its proton NMR spectrum exhibits strong coupling between two pairs of ring protons. A quartet is observed in the spectrum of the methyl-13 C labeled product, confirming that a methyl group is attached to the dienone ring. 13 C chemical shifts and 13 C-H spin-spin splittings of the dienone are reported. Although DFT calculations of the proton NMR parameters are in very good agreement with the experimental values, the calculated chemical shifts of the ring 13 C carbons are 4-8 ppm too high, indicating that the DFT calculations do not properly take into account the dienone π system. This conclusion is confirmed by parallel experimental and theoretical studies of 4H-pyran-4-one that provides a model system closely related to the dienone. The observation of 4-hydroxy-4-methyl-2,5-cyclohexadien-1-one as a product in the radiolysis of aqueous solutions is important in demonstrating that addition of • OH radicals to aromatic rings at positions substituted with alkyl groups can be of appreciable importance in the • OH oxidation of aromatic substrates.

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The concerted effects of hydroxyl and methyl substituents in controlling the site of .OH radical attack on aromatics in aqueous solutions are explored using the cresols as typical examples. The distributions of dihydroxytoluenes produced in the radiolysis of aqueous solutions of the cresols containing ferricyanide as a radical oxidant were examined by capillary electrophoretic and liquid chromatographic methods. Because .OH is a strong electrophile, it adds preferentially at the electron-rich sites of an aromatic ring. As a result, the observed distributions of dihydroxytoluenes reflect the charge distributions in the cresols. It is shown that in the case of m-cresol the hydroxyl substituent has a dominant ortho-para directing effect similar to that observed for phenol. In o- and p-cresol, this effect is modified, indicating that the methyl substituent has a significant effect on the electronic structure of those cresols. Correlation of the charge distribution in the cresols indicated by the observed distribution of dihydroxytoluenes with the unpaired spin distribution in the corresponding methylphenoxyl radicals demonstrates that the electronic structures of o- and p-cresol and their corresponding phenoxyl radicals are similarly affected by hydroxyl and methyl substitution. Addition of .OH at the methyl-substituted positions of o- and p-cresol to produce o- and p-dienone is also reported. The observation of these dienones demonstrates that addition of .OH at the ipso positions of alkylated aromatics can be of considerable importance. Mass spectrometric studies show that these dienones have relatively higher proton affinities than their isomeric analogues.

The relative rates for addition of ‚OH radical to toluene at its ortho, meta, and para sites have been shown to be in the ratios of 0.84:0.41:1. These ratios provide a quantitative basis for considering the effects of alkyl substituents in determining the site of ‚OH attack on other aromatics. Because ‚OH is a strong electrophile, the relative yields of ‚OH adducts provide a measure of the effect of the methyl group on the distribution of negative charge in toluene's aromatic system. Comparison of the partial rate constants observed for reaction at the different sites of toluene, biphenyl, and phenol with theoretical estimates of the distribution of charge in the valence shell indicates that factors other than charge also play some role.

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This paper describes the possible effects of ionizing radiation arising from long-lived soluble radionuclides within clays, in particular 40K, at the epoch of the emergence of life on Earth. The free dispersion of soluble radionuclides constitutes an effective in situ irradiation mechanism that might have acted upon adsorbed nucleic bases and their derivatives on clays, inducing chemical changes on these organic molecules. Several types of well documented reactions for radiolysis of nucleic acid bases and their derivatives are known, even at low doses (i.e., 0.1 Gy). For example, estimates with a dose rate calculated from 40K from deep sea clays at 3.8 Ga ago, indicates that over a period of 1000 years the amount of organic material transformated is 1.8 X 10(-7) moles/kg-clay. Although ionizing radiation may also induce synthetic reactions with prebiological interest, all in all these considerations indicate that nucleic acid bases and their derivatives adsorbed on clays were exposed for long periods to degradation conditions. Such situation promotes decomposition of organic molecules rather than protection of them and enhancement of farther polymerization, as it has been usually taken for granted.

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