Origin of reverse stability of diphosphouracil tautomers compared to their analogue uracil: DFT and ab initio study

DFT (BMK, MPW1K and B3LYP) and ab initio complete basis set (CBS-QB3) methods are used to study the stability order and tautomerization processes of all possible 13 diphosphouracil tautomers in the gas phase. In contrast to uracil, the dienol tautomers of diphosphouracil are shown to be much more stable than the keto-enol and diketo forms. This reverse stability order has been discussed in the light of electronic and geometrical factors. BMK functional gives better energetics compared to CBS-QB3. Solvation effect has been modeled in water using the polarized continuum model (PCM) at the BMK/6-31+G(d,p) level for the most important tautomers/rotomers and the relevant transition states. Consideration of the solvent causes some reordering of the relative stability of diphosouracil tautomers. The transition states for the diphosphouracil tautomerization processes are investigated for selected systems at five levels in gas phase and at BMK/6-31+G(d,p) level in water. Energy barrier for the proton transfer processes of diphosphouracil, is up to 58 kcal/mol which increased in water (by nearly 10 kcal/mol). This indicates that the tautomerization process is unlikely occurred in polar media under normal experimental conditions.