Abstract: The mechanism of photocycloaddition reaction between 6-azauracll and acetone was studied by using semiemptrical SCFMO AMImethod. It was found that this reaction is not a concerted one. The calculated results are as follows:(1) A T₁ state exciplex is on the T₁ state energy surface; (2) Texciplex as a reactant will proceed along the energy surface of T₁ state to form a diradical intermediate. The energy barrier of this reaction step is 63.6 kJ/mol; (3) The T₁ state diradical intermediate happens to be close in energy to the ground state intermediate with a similar geometry. Such a situation turns out to be very favorable for an intersystem crossing (jump from the T, state to the ground state); (4) The final product will be formed from the ground S₀ state intermediate via an energy barrier 88.2 kJ/mol.

Key words: 6-Azauracil, Acetone, Diradical intermediate, the T₁ state, The mechanism of photocycloadditlon reaction, Intersystem crossing.