THEORETICAL EVALUATION OF NEW NUCLEIC-ACID BASES .2. AB-INITIO STUDY ON TAUTOMERISM OF PYRAZOLO [4,3-D]PYRIMIDINE-5,7(4H,6H)-DIONE

The four most important tautomers of the title compound were studied by the ab initio LCAO-MO method at the MP2/6-31G**//HF/3-21G level. All calculated structures are minima at the HF/3-21G potential energy surface with the dioxo tautomer 1 being the global minimum. Also in a polar environment 1 is predicted to be a predominant form. The second most stable species, oxo-hydroxy tautomer 2 has a relative energy of 38.7 kJ mol-1 (MP2/6-31G**+0.9 ZPE). For the 1 --> 2 transition the calculated energy gap corresponds to an equilibrium constant of 10(-7) which is only one order larger than the estimated frequency threshold of spontaneous mutations and it strongly suggests that pi is a new purine base with reduced probability of spontaneous mutations.