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 geom etric isomers of all five tautomeric forms of thymine based on quantum chemical HF/6-31G* and (pointwise) MP2/HF/6-31G** computations. Furt hermore, electric dipole moments of all isomers and rotational constan ts and electric field gradients at the ring nitrogen atoms of the four most stable isomers are included. Similarly, electronic energies of a ll geometric isomers of two more tautomers of both uracil and cytosine are given, thus completing a previously published set of quantum chem ical data for these two nucleic acid bases to comprehend conversion en ergies of geometric isomers of all tautomers. A consistent system of c ontribution terms for all three bases is determined, from which conver sion energies of geometric isomers may be additively expressed within error limits (almost-equal-to 0.5 kcal/mol). The contribution terms re present 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 inte rnal structural parameters accompanying anti-syn conversion of geometr ic isomers of thymine are described by linear regression expressions a nd mechanically interpreted in terms of repulsive and attractive inter actions. Also, electric field gradients at the N atoms are shown to be correlated closely to these quantities. Predicted electric dipole mom ents and rotation constants are found to approximate closely empirical data (where available). However, predicted internal structural parame ters were found to deviate significantly from X-ray data for thymine.