QUANTUM-CHEMICAL INVESTIGATION OF THE STRUCTURE AND STABILITY OF ALL GEOMETRIC ISOMERS AND CONFORMERS OF ALL TAUTOMERIC FORMS OF THYMINE

Optimized structures,and electronic energies are reported for all geometric isomers of all five tautomeric forms of thymine based on quantum chemical HF/6-31G** and (pointwise) MP2/HF/6-31G** computations. Furthermore, electric dipole moments of all isomers and rotational constants and electric field gradients at the ring nitrogen atoms of the four most stable isomers are included. Similarly, electronic energies of all geometric isomers of two more tautomers of both uracil and cytosine are given, thus completing a previously published set of quantum chemical data for these two nucleic acid bases to comprehend conversion energies of geometric isomers of all tautomers. A consistent system of contribution terms for all three bases is determined, from which conversion energies of geometric isomers may be additively expressed within error limits (almost-equal-to 0.5 kcal/mol). The contribution terms represent either repulsive interactions between hydrogen atoms bound to ring atoms and to OH or NH substituents or attractive interactions (''intramolecular H-bonds'') between hydroxy or imino hydrogen atoms and lone-pair electrons localized at adjacent N atoms. Alterations of internal structural parameters accompanying anti-syn conversion of geometric isomers of thymine are described by linear regression expressions and mechanically interpreted in terms of repulsive and attractive interactions. Also, electric field gradients at the N atoms are shown to be correlated closely to these quantities. Predicted electric dipole moments and rotation constants are found to approximate closely empirical data (where available). However, predicted internal structural parameters were found to deviate significantly from X-ray data for thymine.