AB-INITIO CALCULATIONS WITH ELECTRONIC CORRELATION (MP2) ON THE NUCLEIC-ACID BASES AND THEIR METHYL-DERIVATIVES

High-level ab initio optimizations have been carried out on the nucleic acid bases thymine, cytosine, guanine, and adenine in an attempt to calculate gas-phase structures of these bases and their methyl derivatives. Calculations were carried out at both the Hartree-Fock (HF) and Moller-Plesset (MP2) levels of theory, utilizing the standard 6-31G** basis set. Full optimizations on these structures were undertaken, resulting in planar or near-planar (C-s symmetry) conformers. The results show a large deviation between the HF and MP2 geometries, implying that full optimization is necessary with the inclusion of electron correlation. The calculated rotational constants of the bases are in excellent agreement with those derived from gas-phase microwave results and are in accord with the existence of the planar forms in the gas phase. The MP2 structures of guanine and the methyl derivatives of the nucleic acid bases compare well with structures obtained by room temperature X-ray and neutron diffraction.