Study of prototypical Diels-Alder reactions by a hybrid density functional/Hartree-Fock approach

Kinetic and thermodynamic parameters for the Diels-Alder reactions of butadiene with ethylene, formaldehyde and thioformaldehyde have been computed by a density functional method (B3LYP) which incorporates gradient corrections and some Hartree-Fock exchange. Post Hartree-Fock computations have also been performed for comparison. The activation energies computed for the addition of C2H4 at the B3LYP and QCISD[T] levels approach the experimental value, whereas HF and MP2 approaches are not sufficiently reliable. The computed activation energy for H2CO is close to that for C2H4 by the B3LYP approach, whereas it significantly decreases at MP2. QCISD[T] computations give an intermediate result. A low activation energy is computed for H2CS, in agreement with observations that thioaldehydes and activated thioketones rapidly undergo hetero-Diels-Alder reactions at or below room temperature. The B3LYP result is closer to the QCISD[T] reference than the MP2 value. These trends have been interpreted by interactions between frontier orbitals or by energy partitioning based on natural bond orbitals.