Abstract: The 1,2-cycloaddition reaction mechanism of singlet molecular oxygen ~1O_2 with imidazole has been investigated by semiempirical molecular orbital AM1 method and energy gradient technique. The calculated results show that the mentioned reaction is a stepwise reaction via a diradical intermediate. The barrier height of the first step reaction is 39.2 kJ·mol~(-1) while the second step reaction is 150.5 kJ·mol~(-1). The second step reaction is the rate determining step.

Key words: Imidazole, Singlet oxygen, Diradical, 1,2-cycloaddition reaction