High‐Pressure Reaction Profiles and Activation Volumes of 1, 3‐Cyclohexadiene Dimerizations Computed by the Extreme Pressure‐Polarizable Continuum Model (XP‐PCM). Issue 29 (8th April 2022) - Chen, Bo
Abstract: 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. Abstract : XP‐PCM calculations on the thermal dimerizations of 1, 3‐cyclohexadiene show that, going from the stepwise to the concerted [4+2] cycloaddition and finally to the concerted [6+4]‐ene mechanisms, as the facial overlap between the rings increases in the transition state structure, the volume of the transition state decreases, leading to a more negative activation volume and increasing reaction acceleration under pressure.