Diels–Alder reactions: An assessment of quantum chemical procedures

Kinetic and thermodynamic parameters for the Diels–Alder reactions of butadiene with ethylene, formaldehyde, and thioformaldehyde have been computed at a variety of levels of theory with the aim of identifying the most suitable approach for general application. Especially in the presence of heteroatoms, inclusion of electron correlation significantly modifies the geometries of stationary points obtained at the Hartree–Fock level. However, energetic quantities are not very sensitive to the employed geometries provided that the structures of all the species are optimized at the same computational level. On the other hand, both extended basis sets and inclusion of most of the dynamical correlation are needed to obtain reliable energetic quantities. In particular, converged results are obtained only using a triple zeta contraction for the valence space and double sets of polarization functions on nonhydrogen atoms, whereas $f$ functions and hydrogen polarization functions play a minor role. Among cheaper approaches hybrid density functional/Hartree–Fock methods coupled with polarized split valence basis sets provide remarkable geometries and, especially, energetic quantities.