SN R2 R1 R3 Ea     ER
21 -F -CH3 -CH3 48.7 -166.3

0.1

-214.4
22 -Br -CH3 -CH3 55.8 -160.9

2.1

-216.2
23 -NC- -CH3 -CH3 48.0 -167.0

2.8

-214.7
24 -Cl -CH3 -CH3 56.5 -   -213.1
25 -NO2- -CH3 -CH3 50.7 -   -216.2
Table 2: Energetics of the cycloaddition reactions involving electron withdrawing groupsb bEa: activation barrier of first step; : reaction energy of intermediate; : activation energy of second step; ER: reaction energy. All energies are calculated relative to the separated reactants (fullerene+nitrone).