An Initio Theoretical Study for the Electronic Spectra of β-Thioxoketones

Problem statement: β-thioxoketones exist as equilibrium mixtures of the tautomeric enol and enethiol forms which interconvert very rapidly by intramolecular chelate proton transfer. Accordingly their electronic spectra in the u.v.-visible region exhibit absorption bands arising from transitions within the electronic systems involved in these tautomers. Approach: The structures of several β-thioxoketones were geometrically optimized using B3LYP/cc-pVTZ, B3LYP/6-311++G (2d,p), PM2/6-311++G(2d,p) and B3LYPl6-311G(d,p) and their electronic spectra were theoretically calculated. TD-DFT, EOM-CCSD, CIS and semi empirical ZINDO methods were used to calculate the electronic spectra. Results: The spectrum of the enolic form of thiomalonaldehyde is characterized by three absorption bands at 460, 305 and 223 nm due to H→L, H-1→L and both H→L+1 and H→L+2 transitions. The first band is attributed to n→π* transition within the C = S group while the others are due to transitions with the O-C = C-C = S conjugated system. Both TD-DFT and ZINDO methods were able to predict the electronic spectra of the molecules studied that are in very good agreement with the observed spectra. The observed spectra were agreed to large degree with calculated spectra of the enolic tautomer of the studied compounds. Conclusion: The observed spectra were agreed to large degree with calculated spectra of the enolic tautomer of the studied compounds.