Multireference methods Subject

The [4-1-2] cycloaddition of dienes to double bonds, the Diels-Alder reaction (see Pericycttc Reactions The Diels-Alder Reaction), is one of the most useful synthetic reactions in organic chemistry. The mechanism of the Diels-Alder reaction has been the subject of intensive study using a large number of different methods. This is at least in part due to the fact that, based on thermochemical estimates, there is a reasonable possibility for a stepwise mechanism. The energy of concert , i.e., the difference between the concerted and a diradicaloid mechanism, is small enough to be potentially overcome in some cases. The comparison of the results for the two different pathways suffers, however, from the difficulties of comparing closed-shell specie.s and diradicals within a single-determinant MO approach. Only recently, the use of multireference and DFT methods in combination with experimental kinetic isotope effect studies allow an unequivocal assessment of the reaction mechanism of the Diels-Alder reaction. ... 

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