MCSCF MP2 STUDY OF THE CHELETROPIC ADDITION OF SINGLET AND TRIPLET CF2 AND C(OH)(2) TO THE ETHENE DOUBLE-BOND/

CAS-MCSCF calculations describe the addition of singlet CF2 and C(OH)( 2) to the ethene double bond as a two-step reaction. The energy barrie rs that separate, in the first rate-determining step, loosely bound al pha-complexes from stable CH(2)CH(2)CX(2) diradical intermediates show the expected ordering, smaller for CF2 than for C(OH)(2). Back-dissoc iation of the diradicals into reactants requires the overcoming of non -negligible energy barriers, In both diradicals, the CAS-MCSCF activat ion energy for ring closure is smaller than that required for rotation of their terminal methylenic groups, which models, in these simple sy stems, an isomerization process. However, when the activation free ene rgies are computed, in the case of the difluoro diradical the isomeriz atian process appears to be less disfavored (and possibly competitive to some extent at higher temperatures); in contrast, in the case of th e dioxy diradical, isomerization is never competitive with ring closur e, The small energy barriers for ring closure of the diradicals disapp ear altogether when multireference MP2 energy calculations are carried out on the CAS-MCSCF critical points, casting doubts on the very exis tence of these intermediates, However, in contrast with the ethene rea ction, the addition of singlet CF2 to isobutene involves the formation of a diradical intermediate whose barrier for ring closure persists a lso at the MP2 level. These results suggest that cyclopropanation is L ikely to be a two-step process (with formation of a diradical intermed iate) only with bulky substituted alkenes, while the attack to an unsu bstituted double bond could be an asynchronous but concerted process, The analogous triplet reactions go through transition and stable struc tures of lower symmetry than the singlet and see the intervention of d iradical intermediates. Their formation is easier than that in the sin glet case and their stability with respect to back-dissociation higher . Also the isomerization processes (taking place again through rotatio n of the terminal methylenic group) are easier than those examined on the singlet surfaces.