Cycloaddition reaction of ozone to C_(70) has been studied by using the semi-empirical method AMI. A reasonable mechanism of the reaction has been suggested. All the geometries of the stationary points on the reaction path were optimized by energy gradient technique and the transition states were characterized by vibration frequency analysis. The calculation results show that the cycloaddition of ozone on 6-6 double and single bonds of C_(70) are complicated reactions. Three steps are involved in the reaction mechanism: (l) A molecular ozonide (intermediate (I)) is formed by 1,3-dipolar cycloaddition process. The addition on 6-6 double bond is an exothermal reaction, while the addition of 6-6 single bond is an endothermic reaction. The barriers of 6-6 double and single bonds are 84.7 and 181.2 kJ·mol~(-1) respectively. (2) Cleavage of the C-C bond formed in the addition in intermediate (I) leads to the formation of a zwitterionic intermediate (II), which is an exothermal reaction. The barriers of 6-6 double and single bonds are 61.3 and 13.3 kJ·mol~(-1), respectively. (3) One O2 molecule is eliminated from intermediate (II) resulting in the formation of C_(70)O with an epoxide structure. Both the addition of 6- 6 double and single bonds are exothermic reactions and the barriers are 169.3 and 101.2 kJ·mol~(-1), respectively. The reasons that ozone cycloaddition should take place across 6-6 double bond rather than 6-6 single bond of C_(70) are explained by the reaction mechanism and kinetic factors. The addition of ozone on 6-6 double bond of C_(70) in the first step is a concerted synchronous process, while the addition of 6-6 single bond is a concerted unsynchronous process.

Key words: C70, OZONE, CYCLOADDITION REACTION, TRANSITION STATE THEORY, REACTION MECHANISM