Formation of two centre three electron bond by hydroxyl radical induced reaction of thiocoumarin: evidence from experimental and theoretical studies

Radiation chemical studies of thioesculetin (1), a thioketone derivative of coumarin, were performed by both pulse radiolysis technique and DFT calculations. Hydroxyl ((OH)-O-center dot) radical reaction with 1 resulted transients absorbing at 320, 360 and 500nm. To identify the nature of the transients, the reaction was studied with specific one-electron oxidant (N-3(center dot)) radical, where 360nm band was absent. The transient absorption at 500nm was concentration-dependent. The overall impression for (OH)-O-center dot radical reaction was that the transient absorbing at 320, 360 and 500nm was due to sulphur centred monomer radical, hydroxysulfuranyl and dimer radical of 1 respectively. The equilibrium constant between the monomer to dimer radical was 3.75x10(4)M(-1). From the transients' redox nature, it was observed that 57 and 24% of (OH)-O-center dot radical yielded to oxidising and reducing products respectively. Further, the product analysis by HPLC suggested that the dimer radical disproportionate to esculetin and thioesculetin. DFT energy calculation for all the possible transients revealed that dimer radical has the lowest energy. The HOMO of 1 and its monomer radical suggested that the electron density was localised on the sulphur atom. The bond length between the two sulphur atoms in dimer radical was 2.88 angstrom which was less than the van der Waals distance. Bond order between the two sulphur atoms was 0.55, suggesting that the bond was two centre three electron (2c-3e). From TD-DFT calculation, the electronic transition of dimer radical was at 479nm which was in close agreement with the experimental value. The nature of the electronic transition was sigma -> sigma* from a 2c-3e bond.