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By contrast, the rate of decay of the triplet (<jats:italic>\u03bb<\/jats:italic><jats:sub>max<\/jats:sub>=282 and \u223c410 nm) which is the precursor of these products increases nonlinearly (<jats:italic>k<\/jats:italic>=(2\u21923.8)\u00d710<jats:sup>6<\/jats:sup> s<jats:sup>\u22121<\/jats:sup>) with increasing <jats:bold>Q<\/jats:bold> concentration ((0.2\u219210) m<jats:sc>M<\/jats:sc>). The free\u2010radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing <jats:bold>Q<\/jats:bold> concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet <jats:bold>Q*<\/jats:bold> and an exciplex <jats:bold>Q<\/jats:bold><jats:sub><jats:bold>2<\/jats:bold><\/jats:sub><jats:bold>*<\/jats:bold> (<jats:italic>K<\/jats:italic>=5500\u00b11000\u2009<jats:sc>M<\/jats:sc><jats:sup>\u22121<\/jats:sup>). While <jats:bold>Q*<\/jats:bold> adds water and subsequent enolizes into 1,2,4\u2010trihydroxybenzene <jats:bold>Ph(OH)<\/jats:bold><jats:sub><jats:bold>3<\/jats:bold><\/jats:sub>, <jats:bold>Q<\/jats:bold><jats:sub><jats:bold>2<\/jats:bold><\/jats:sub><jats:bold>*<\/jats:bold> decays by electron transfer and water addition yielding benzosemiquinone <jats:sup>.<\/jats:sup><jats:bold>QH<\/jats:bold> and <jats:sup>.<\/jats:sup>OH adduct radicals <jats:sup>.<\/jats:sup><jats:bold>QOH<\/jats:bold>. The latter enolizes to the 2\u2010hydroxy\u20101,4\u2010semiquinone radical <jats:sup>.<\/jats:sup><jats:bold>Q(OH)H<\/jats:bold> within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by <jats:bold>Q<\/jats:bold> to <jats:bold>HOQ<\/jats:bold> with the concomitant formation of <jats:sup>.<\/jats:sup><jats:bold>QH<\/jats:bold>. On the post\u2010millisecond time scale, that is, when <jats:sup>.<\/jats:sup><jats:bold>QH<\/jats:bold> has decayed, <jats:bold>Ph(OH)<\/jats:bold><jats:sub><jats:bold>3<\/jats:bold><\/jats:sub> is oxidized by <jats:bold>Q<\/jats:bold> yielding <jats:bold>HOQ<\/jats:bold> and <jats:bold>QH<\/jats:bold><jats:sub><jats:bold>2<\/jats:bold><\/jats:sub> as followed by laser flash photolysis with diode array detection. The rate of this pH\u2010 and <jats:bold>Q<\/jats:bold> concentration\u2010dependent reaction was independently determined by stopped\u2010flow. This shows that there are two pathways to photohydroxylation; a free\u2010radical pathway at high and a non\u2010radical one at low <jats:bold>Q<\/jats:bold> concentration. In agreement with this, the yield of <jats:bold>Ph(OH)<\/jats:bold><jats:sub><jats:bold>3<\/jats:bold><\/jats:sub> is most pronounced at low <jats:bold>Q<\/jats:bold> concentration. In the presence of phosphate buffer, <jats:bold>Q*<\/jats:bold> reacts with H<jats:sub>2<\/jats:sub>PO<jats:sub>4<\/jats:sub><jats:sup>\u2212<\/jats:sup> giving rise to an adduct which is subsequently oxidized by <jats:bold>Q<\/jats:bold> to 2\u2010phosphato\u20101,4\u2010benzoquinone <jats:bold>QP<\/jats:bold>. The current view that <jats:sup>.<\/jats:sup>OH is an intermediate in the photohydroxylation of <jats:bold>Q<\/jats:bold> has been overturned. This view had been based on the observation of the <jats:sup>.<\/jats:sup>OH adduct of DMPO when <jats:bold>Q<\/jats:bold> is photolyzed in the presence of this spin trap. It is now shown that <jats:bold>Q*<\/jats:bold>\/<jats:bold>Q<\/jats:bold><jats:sub><jats:bold>2<\/jats:bold><\/jats:sub><jats:bold>*<\/jats:bold> oxidizes DMPO (<jats:italic>k<\/jats:italic> \u22481\u00d710<jats:sup>8<\/jats:sup>\u2009<jats:sc>M<\/jats:sc><jats:sup>\u22121<\/jats:sup>\u2009s<jats:sup>\u22121<\/jats:sup>) to its radical cation which subsequently reacts with water. <jats:bold>Q*<\/jats:bold>\/<jats:bold>Q<\/jats:bold><jats:sub><jats:bold>2<\/jats:bold><\/jats:sub><jats:bold>*<\/jats:bold> react with alcohols by H abstraction (rates in units of <jats:sc>M<\/jats:sc><jats:sup>\u22121<\/jats:sup>\u2009s<jats:sup>\u22121<\/jats:sup>): methanol (4.2\u00d710<jats:sup>7<\/jats:sup>), ethanol (6.7\u00d710<jats:sup>7<\/jats:sup>), 2\u2010propanol (13\u00d710<jats:sup>7<\/jats:sup>) and tertiary butyl alcohol (\u223c0.2\u00d710<jats:sup>7<\/jats:sup>). DMSO (2.7\u00d710<jats:sup>9<\/jats:sup>) and O<jats:sub>2<\/jats:sub> (\u223c2\u00d710<jats:sup>9<\/jats:sup>) act as physical quenchers.<\/jats:p>","DOI":"10.1002\/chem.200305136","type":"journal-article","created":{"date-parts":[[2004,1,16]],"date-time":"2004-01-16T10:48:58Z","timestamp":1074250138000},"page":"440-451","source":"Crossref","is-referenced-by-count":78,"title":["Photohydroxylation of 1,4\u2010Benzoquinone in Aqueous Solution Revisited"],"prefix":"10.1002","volume":"10","author":[{"given":"Justus","family":"von Sonntag","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Eino","family":"Mvula","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Knut","family":"Hildenbrand","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Clemens","family":"von Sonntag","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2004,1,15]]},"reference":[{"key":"e_1_2_6_1_2","doi-asserted-by":"publisher","DOI":"10.1002\/cber.18860190252"},{"key":"e_1_2_6_2_2","doi-asserted-by":"publisher","DOI":"10.1002\/jlac.18882490110"},{"key":"e_1_2_6_3_2","doi-asserted-by":"publisher","DOI":"10.1002\/9780470772126.ch3"},{"key":"e_1_2_6_4_2","doi-asserted-by":"publisher","DOI":"10.1070\/RC1977v046n02ABEH002115"},{"key":"e_1_2_6_5_2","first-page":"465","volume-title":"The Chemistry of the Quinoid Compounds\u2014Part 1","author":"Bruce J. 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