Note that 1 cm 3 /mol difference in volume corresponds to 0.24 kcal/mol difference in enthalpy at 1 GPa. The greater overlap of the rings and the highly-ordered staggered conformation in TS-threo-4 is correlated with the small pre-exponential factor (i.e., small entropy of the TS) in the Arrhenius equation for the threo [6+4]-ene reaction (as discussed in the 1 atm PES section), compared with other reactions.