Intramolecular Proton Transfer in Mono- and Dihydrated Tautomers of Guanine: An ab Initio Post Hartree−Fock Study

The results of an ab initio post Hartree−Fock study of the molecular structures, relative stabilities, and mechanisms of intermolecular proton transfer in isolated, mono- and dihydrated guanine complexes are reported. The geometries of the local minima and transition states were optimized without symmetry restrictions by the gradient procedure at the HF and the MP2 levels of theory and were verified by energy second derivative calculations. The standard 6-31G(d) basis set was used. The single point calculations have been performed at the MP4(SDQ)/6-31G(d)//MP2/6-31G(d) and the MP2/6-311++G(d,p)//MP2/6-31G(d) approximations. All values of total energies have been corrected for zero point energy contributions scaled by a factor of 0.9. The post Hartree−Fock ab initio theory predicts the height of the proton transfer barrier for monohydrated guanine complexes to be approximately two times lower for the tautomeric oxo−hydroxo reactions and approximately three times lower for the reverse hydroxo−oxo reactions ...