Dual-Level Direct Dynamics Study on the Diels-Alder Reaction of Ethylene and 1,3-Butadiene

Dual-level direct dynamics study was performed on the forward and reverse reaction of the parent DielsAlder reaction of ethylene and 1,3-butadiene. The underlying potential energy surface was mapped with HF/ 6-31G* theory while the correlated methods including MP2/6-31+G* and QCISD(T)/6-31+G* were used to build a more accurate description of the reaction path necessary for the dual-level variational transition state theory calculation including multidimensional tunneling. By modeling the experimental gas-phase rate constants, our calculation estimated a forward classical barrier height of 21.9 kcal/mol, which is significantly lower than that inferred from the experimental activation energy (27.5 kcal/mol). The reverse barrier height is estimated as 69.6 kcal/mol, giving a classical energy of reaction of -47.8 kcal/mol, in good agreement with available experimental values. The variational effects were found to be negligible in this reaction, and the tunneling effects were small except at lower temperature. The deuterium and 13 C kinetic isotope effects (KIEs) of the forward reaction were also calculated and were consistent with available experimental data in related systems. Both the vibrational and rotational motions were found to contribute strongly to the inverse secondary deuterium KIEs, and the tunneling effects make a noticeable contribution to the normal primary 13 C KIEs at