Apparently, the fullerene reaction involves passing through a much flatter barrier than the ethene reaction. Because the latter is more exothermic by 24.9 kcal/mol (Figure ), it leads to the higher energy barrier measured from the products which translates to the stronger thermodynamic driving force and the higher kinetic energy of atoms moving faster away from the transition state. The transition zone (but not the barrier width as defined by IRC!) is also broader for the reaction between fullerene and 2,3-dimethyl-1,3-butadiene as the bonds to be formed range from 1.86 to 2.60 Å compared to the range of 2.02 to 2.52 Å in the reaction between ethene and 1,3-butadiene (see also the histogram of the bond lengths in the initial conditions, Figures ).