High-Pressure Reaction Profiles and Activation Volumes of 1,3-Cyclohexadiene Dimerizations Computed by the Extreme Pressure-Polarizable Continuum Model (XP-PCM)

Quantum chemical calculations are reported for the thermal dimerizations of 1,3-cyclohexadiene at 1 atm and high pressures up to the GPa range. Computed activation enthalpies of plausible dimerization pathways at 1 atm agree well with the experiment activation energies and the values from previous calculations. High-pressure reaction profiles, computed by the recently developed extreme pressure-polarizable continuum model (XP-PCM), show that the reduction of reaction barrier is more profound in concerted reactions than in stepwise reactions, which is rationalized on the basis of the volume profiles of different mechanisms. A clear shift of the transition state towards the reactant under pressure is revealed for the [6+4]-ene reaction by the calculations. The computed activation volumes by XP-PCM agree excellently with the experimental values, confirming the existence of competing mechanisms in the thermal dimerization of 1,3-cyclohexadiene.

Automated summary

This study reports on the thermal dimerization of 1,3-cyclohexadiene at different pressures, and analyzes the activation enthalpies and activation volumes of different reaction pathways through quantum chemical calculations. The results show that under high pressure, the reduction of reaction barriers is more significant in concerted reactions compared to stepwise reactions, and the transition state of the [6+4]-ene reaction shifts towards the reactant. The computed results agree well with experimental data, confirming the existence of competing mechanisms in the thermal dimerization of 1,3-cyclohexadiene.