Theoretical description of [2+2] photocycloadditions: enone and ethylene as a model of the reactivity of cycloenones

The photoinitiated [2 + 2] cycloaddition of acrolein plus ethylene, as a mo del of cyclobutanes formation, has been studied using semi-empirical, ab in itio and DFT methods. Singlet and triplet potential energy surfaces and the ir relative position were investigated. The intersystem crossing (spin-orbi t coupling) and retrocleavage processes are also reported. The values obtai ned by usual methods (AM1, HF, MP2, B3LYP) are compared to the Coupled Clus ter results. It is shown that the different parts of the reaction pathways have to be described by different levels of approximation. On the triplet s urface, we found a severe spin contamination of the UHF wavefunction, which precludes further use of perturbative methods (MP2). In the intersystem cr ossing, we show how the physically wrong RHF wavefunction (singlet) leads t o erroneous results which are not corrected, even using the CCSD(T) level o f theory. In the ultimate part of the reaction profile (singlet surface), w e report the over-estimation by the UHF method of the energies of the trans ition states, especially for the one leading to the cycloadducts, and the p oor quality of the B3LYP potential energy surface. (C)2001 Elsevier Science B.V. All rights reserved.