Barrier Heights for Diels-Alder Transition States Leading to Pentacyclic Adducts: A Benchmark Study of Crowded, Strained Transition States of Large Molecules

Theoretical characterization of reactions of complexmoleculesdepends on providing consistent accuracy for the relative energiesof intermediates and transition states. Here we employ the DLPNO-CCSD(T)method with core-valence correlation, large basis sets, andextrapolation to the CBS limit to provide benchmark values for Diels-Aldertransition states leading to competitive strained pentacyclic adducts.We then used those benchmarks to test a diverse set of wave functionand density functional methods for the absolute and relative barrierheights of these transition states. Our results show that only a fewof the tested density functionals can predict the absolute barrierheights satisfactorily, although relative barrier heights are moreaccurate. The most accurate functionals tested are & omega;B97M-V,M11plus, & omega;B97X-V, PBE-D3(0), M11, and MN15 with MUDs from bestestimates less than 3.0 kcal. These findings can guide selection ofdensity functionals for future studies of crowded, strained transitionstates of large molecules.

Automated summary

Theoretical characterization of reactions of complex molecules requires accurate determination of the relative energies of intermediates and transition states. In this study, the DLPNO-CCSD(T) method was employed to provide benchmark values for Diels-Alder transition states leading to strained pentacyclic adducts. Various wave function and density functional methods were compared for their prediction of absolute and relative barrier heights. The results show that only a few density functionals can satisfactorily predict absolute barrier heights, while relative barrier heights are more accurate. The findings of this study can guide the selection of density functionals for future studies on crowded, strained transition states of large molecules.