product ratio ΔV ‡ (cm The measured activation energies 𝐸 ! for the reactions giving the five products in Figure are within 4 kcal/mol. An even smaller difference in 𝐸 ! is seen between the endo and exo [4+2]-cycloadditions, though the former likely follows a concerted mechanism while the latter a stepwise mechanism, based on the measured difference in ∆𝑉 ‡ . Interestingly, it seems that the more negative the ∆𝑉 ‡ , the lower the 𝐸 ! of the reaction, though the two quantities do not necessarily have a correlation.