Quantum chemical study of structure, energy, rotational constants, electric dipole moments and electric field gradients of all isomeric adenines

Abstract

Results of quantum chemical computations at the HF/6-31G(d, p), pointwise MP2//HF/6-31G(d, p) and MP2/6-31G(d, p) levels of theory for all 12 isomers of adenine are reported, comprising fully optimized structures, total electronic energies, rotation constants, electric dipole moments and electric field gradients at nitrogen sites. For all four purine isomers electronic energies produced by the same quantum chemical approximations are included and used as auxiliary data for analysis of adenines. Relative energies are found to depend on the level of theory. A consistent set of tautomerization energies and attractive and repulsive interactions (increments) involving H substituents and sp 2 lone electron pairs located at ring and substituent N atoms are given, from which conversion energies of tautomers and of geometric isomers may additively be reproduced to within 0.6 kcal mol −1. Increments derived earlier for pyrimidine bases nearly equal corresponding interactions obtained for both 6- and 5-ring N atoms of adenines. Structural parameters predicted by HF and MP2 approximations are found to differ from available empirical data (X-ray) and from each other (statistically) significantly. Structural parameters and non-planarity of amino groups are found to depend systematically on interactions involving amino H atoms. Structural relaxation during anti-syn conversion of imino isomers is related mechanically to interactions involving imino substituents; however there is some indication of structural relaxation not correlated to interactions. Rotation constants predicted by HF and MP2 optimization differ systematically by specific, nearly constant ratios, and predicted electric dipole moments differ slightly but not systematically and seem significantly different from (scarce) empirical data. Electric field gradients are found to follow similar correlations with interactions as observed for cytosines and isocytosines.