Force field calculation of equilibrium thermodynamic properties: Diels–Alder reaction of 1,3‐butadiene and ethylene and Diels–Alder dimerization of 1,3‐butadiene

Abstract

AbstractΔH0, ΔS0, and Kp have been calculated by force field methods for two prototype Diels–Alder reactions, the condensation of 1,3‐butadiene and ethylene, and the dimerization of 1,3‐butadiene. Three force field programs were employed, Allinger's MMP2, Warshel and Karplus' CFF/PI, and Boyd's MOLBD3 (with Anet and Yavari diene parameters). MMP2 and CFF/PI were combined to calculate reactant and product ΔH (by MMP2) and thermodynamic functions S0, C, and (H − H) by use of CFF/PI molecular geometry and vibrational frequencies. MOLBD3 was used separately to calculate these properties. The combined MMP2, CFF/PI calculations exhibited excellent agreement with available experimental equilibrium data for both reactions, despite uncertainties in the number and energies of adduct conformers in the dimerization reaction. The MOLBD3 results were less satisfying, particularly for the dimerization reaction, because the entropy of 1,3‐butadiene was overestimated by 2.5 cal mol−1 K−1.