QUANTUM-CHEMICAL STUDY OF RELATIVE STABILITY, ELECTRIC-FIELD GRADIENTS AT NITROGEN-ATOMS, ROTATIONAL-CONSTANTS AND ELECTRIC-DIPOLE MOMENTS OF ALL URACIL, THYMINE AND CYTOSINE ISOMERS

For four uracil and five cytosine isomeric forms, fully optimized quan tum chemical structural data computed at the HF/6-31* level of approx imation are reported. Effects of electron correlation (based on pointw ise MP2/HF/6-31G* calculations) on the relative energies of all 13 is omers of uracil and thymine and all 14 isomers of cytosine are investi gated. Principal values of electric field gradients at N atoms in the 1 and 3 positions of the ring and of the amino/imino substituent in th e 6 position of cytosine, rotational constants and electric dipole mom ents were determined for all isomers at the same level of theory. An a ttempt to characterize attractive interactions between hydroxy (imino) substituents and lone pair electrons centered at adjacent N atoms (a hitherto unrecognized ''intramolecular H-bond'') by means of electron density, difference electron density, field lines of the gradient and a contour map of the Laplacian of the electron density is presented. E lectric field gradient tensors and their asymmetry parameters are show n to be correlated in a characteristic manner with repulsive and attra ctive interactions. These interactions were recently found to be conne cted in a simple manner with conversion energies of geometric isomers of tautomeric forms of the three pyrimidine nucleic acid bases.