Collect. Czech. Chem. Commun. 2003, 68, 2322-2334

A Combined ab initio and Density Functional Study of the Electronic Structure of Thymine and 2-Thiothymine Radicals

Robert Vianelloa and Zvonimir B. Maksića,b,*

a Quantum Organic Chemistry Group, Ruđer Bošković Institute, Bijenička 54, 10002 Zagreb, Croatia
b Faculty of Science and Mathematics, The University of Zagreb, Marulićev trg 19, 10000 Zagreb, Croatia


The electronic and energetic properties of thymine (1) and 2-thiothymine (2) and their neutral and positively charged radicals are considered by a combined ab initio and density functional theory approach. It is conclusively shown that ionization of 1 and 2 greatly facilitates deprotonation of the formed radical cations thus making the proton transfer between charged and neutral precursor species thermodynamically favourable. The adiabatic ionization potential of 1 and 2 are analysed. It appears that ADIP(1) is larger than ADIP(2) by 10 kcal/mol, because of greater stability of the highest occupied molecular orbital (HOMO) of the former. It is also shown beyond any doubt that the spin density in neutral and cationic radical of 2 is almost exclusively placed on the σ-3p AO of sulfur implying that these two systems represent rather rare sigma-radicals. In contrast, the spin density of radicals of 1 is distributed over their π-network.

Keywords: Nucleic acids; DNA; Nucleobases; Pyrimidines; Thymine; 2-Thiothymine; Spin density; Ionization potentials; Ab initio calculations; DFT.

References: 25 live references.