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

Abstract

Abstract For four uracil and five cytosine isomeric forms, fully optimized quantum chemical structural data computed at the HF/6-31** level of approximation are reported. Effects of electron correlation (based on pointwise MP2/HF/6-31G** calculations) on the relative energies of all 13 isomers of uracil and thymine and all 14 isomers of cytosine are investigated. 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 the 6 position of cytosine, rotational constants and electric dipole moments were determined for all isomers at the same level of theory. An attempt 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. Electric field gradient tensors and their asymmetry parameters are shown to be correlated in a characteristic manner with repulsive and attractive interaction. These interactions were recently found to be connected in a simple manner with conversion energies of geometric isomers of tautomeric forms of the three pyrimidine nucleic acid bases.