THE IMPORTANCE OF KETO-ENOL-TAUTOMERISM IN ALLOXAN - A SEMIEMPIRICAL QUANTUM-MECHANICAL STUDY

The various tautomers and rotamers of alloxan are examined in detail b y the MNDO method. The keto form is predicted to be the most important in the gas phase, while in solution the monohydroxy forms may also co ntribute. Dihydroxy forms are contraindicated. The alloxan ring is fou nd to be most susceptible to nucleophilic attack. In spite of the fact that the carbon C-2 carries a larger positive charge, the site of nuc leophilic attack is C-5. This is explained on the basis of the molecul ar electrostatic potential (MEP) maps in various planes parallel to th e alloxan ring. The larger contribution of the C5O11 antibonding pi or bital in the lowest unoccupied molecular orbital (LUMO) also explains why this carbon is most susceptible to nucleophilic attack. The large negative energies of the LUMOs and highest occupied molecular orbitals (HOMOs) of all the systems indicate that these systems are good elect ron accepters and bad electron donors. The anions are predicted to be highly Stable. Even in the anions, the ring remains electron deficient .