Differential transient behaviour of dihydroxy derivative of thiocoumarin and coumarin towards hydroxyl radical: effect of thiocarbonyl group

Radiation chemical studies of sulfur based compounds are contrasting different from lower chalcogen i.e. oxygen. In most of the cases hydroxyl radical (•OH ) add to coumarin ring or oxidise the phenolic group, but in the case of thiocoumarin majority of •OH radical react with the thiocarbonyl group. For coumarin the carbonyl was inactive towards •OH radical. The thiocarbonyl group in thiocoumarin readily react with •OH radical to form dimer radical of thiocoumarin. The dimer radical was characterised by transient absorption and also by quantum chemical calculation. The bond length between the two sulfur atoms in dimer radical was 2.88 A Åwhich was less than the Van del' Waals distance. Bond order between the two sulfur atoms was 0.55, suggesting that the bond was two center three electron (2c-3e). The fate of dimer radical of thiocoumarin was established experimentally, where it was disproportionate to coumarin and parent thiocoumarin. Detailed reaction of •OH radical with their possible transient behaviour between these two compounds will be discussed. (author)