The reaction between p-benzoquinone and H202 in slightly alkaline solutions yields three major quinoid products that accumulate in the reaction mixture: (a) 2.3-epoxy-pbenzoquinone, (b) 2-hydroxy-pbenzoquinone and (c) p-benzohydroquinone. The reaction is accompanied by photoemission, probably originating from excited triplet 2-hydroxy-pbenzoquinone. These products originate from hydrogen peroxide and hydroxide nucleophilic addition to the C2=C3 double bond, as well as secondary redox interactions. The hydroxy substituent and the epoxide ring exert a substantial influence on the electronic distribution in the pbenzoquinone molecule leading t o a decrease in the half-wave potential, as compared to the parent pbenzoquinone.
The generation of electronically excited states is the result of reactions secondary t o the nucleophilic additions involving 2-hydroxy-pbenzosemiquinone, H202 and hydroxyl radical.
The process involves the primary oxidation of 2-hydroxy-pbenzosemiquinone by hydrogen peroxide, followed by oxidation of the semiquinone by hydroxyl radical leading t o the formation of the electronically excited quinone. The decay of the excited triplet t o the ground state is accompanied by photoemission with maximal intensity at 485-530 nm. Thermodynamic calculations along with an observed increase of photoemission intensity in anaerobiosis point t o the triplet (n, n") multiplicity of the excited state. The efficiency of chemiluminescence could be calculated as lo-' photons/2-hydroxy-pbenzoquinone molecule formed.
Photoemission arising from the pbenzoquinone/Hz02 reaction was inhibited efficiently by addition of GSH t o the reaction mixture. This may be due t o deactivation of the triplet quinone by a 2-glutathionyl-pbenzohydroquinone adduct, involving thioether a-hydrogen atom-transfer t o the triplet ketone.