An assessment of theoretical procedures for π-conjugation stabilisation energies in enones

Abstract

<p>We introduce a representative database of 22α,β-toβ,γ-enecarbonyl isomerisation energies (to be known as the EIE22 data-set). Accurate reaction energies are obtained at the complete basis-set limit CCSD(T) level by means of the high-level W1-F12thermochemical protocol. The isomerisation reactions involve a migration of one double bond that breaks the conjugatedπ-system. The considered enecarbonyls involve a range of common functional groups (e.g., Me, NH2,OMe,F,andCN). Apart from π-conjugation effects, the chemical environments are largely conserved on the two sides of the reactions and therefore the EIE22 data-set allows us to assess the performance of a variety of density functional theory (DFT) procedures for the calculation ofπ-conjugation stabilisation energies in enecarbonyls. We find that, with few exceptions (M05-2X, M06-2X,BMK, and BH&HLYP), all the conventional DFT procedures attain root mean square deviations (RMSDs) between 5.0 and 11.7 kJ mol⁻¹. The range-separated and double-hybrid DFT procedures, on the other hand, show good performance with RMSDs below the 'chemical accuracy' threshold. We also examine the performance of composite and standard <i>ab initio</i> procedures. Of these, SCS-MP2 offers the best performance-to-computational cost ratio with an RMSD of 0.8 kJ mol⁻¹.</p>