AB-INITIO MC-SCF STUDY OF THERMAL AND PHOTOCHEMICAL [2+2] CYCLOADDITIONS

The results of a study of few prototypical polar and non-polar [2 + 2] cycloadditions are presented at a level of theory where biradical and zwitterionic mechanisms can be treated in a balanced way (CAS-SCF). T he polarity of the reactants is modelled either through the inclusion of donor/acceptor substituents (cycloaddition of dicyanoethene to hydr oxyethene) or through the replacement of.a carbon atom of the ethyleni c C-C double bond with a heteroatom (silaethene and formaldehyde). Two different types of reaction mechanism are documented in each case: (i ) a two-step biradical mechanism and (ii) a concerted mechanism. The s urface topology describing the two-step mechanism turns out to be almo st independent of the polarity introduced by the substituents or by he teroatoms. In contrast, the topology of the surface describing the con certed mechanism is sensitive to substituents and heteroatoms. In non- polar systems (eg. ethene dimerization) a concerted synchronous or asy nchronous supra-supra pathway does not exist; however, in polarized pi -systems we have been able to locate a true supra-supra transition sta te. Since this concerted transition state is at high energy, it will e ventually become important only when solvent effects are considered. A study of the photochemical [2 + 2] dimerization occurring via the low est excited singlet (S1) and triplet states (T1) is also presented. Th e computations demonstrate that the lowest-energy region of the S1 pot ential-energy surface is centred on a singularity (i.e. a cusp) corres ponding to a conical intersection between the S1 and S0 surfaces. The presence of this topological feature seems to be related to the highly stereospecific formation of cyclobutane from simple 2,3-disubstituted alkenes. Both the change in potential surface topology on moving from non-polar to polar reactants and the nature of the excited-state deca y process are rationalized using the same simple VB model.