AB-INITIO MOLECULAR-ORBITAL STUDY OF THE MOLECULAR AND ELECTRONIC-STRUCTURE OF FECH2+ AND OF THE REACTION-MECHANISM OF FECH2++H-2

Both complete active space self-consistent field (CASSCF) and internally contracted single and double configuration interaction from the CASSCF reference functions (MR-SDCI-CASSCF) methods have been used to study the molecular and electronic structures of FeCH2+, as well as the mechanism for the reaction FeCH2++H2. Three quartet electronic states of FeCH2+ are very low-lying; the ground state is a nearly degenerate pair (4B2 and 4B1), with the 4A2 state only 1-2 kcal/mol higher in energy. The lowest doublet states are about 25-35 kcal/mol higher. The binding energy of FeCH2+ (4B 2), relative to the dissociation limit Fe+( 6D,s1d6)+CH2(3B 1), is estimated to be 68 kcal/mol. In the first step of reaction FeCH2++H2, the reactants yield an ion-molecule complex, (H2)FeCH2+, stabilized by about 6 kcal/mol. Subsequently, the H-H bond is activated, leading to a four-center transition state with an energy barrier of about 24 and 14 kcal/mol for the ground 4A″ and excited 2A′ states of the reactants. This step is calculated to be the rate-determining for the entire reaction, and therefore the doublet excited state of FeCH2 + is expected to be more reactive than the ground state because of the lower barrier. The resultant HFeCH3+ complex is predicted to be a minimum at the CASSCF level, but at the highest MR-SDCI-CASSCF level this minimum disappears. Thus, following the H-H bond cleavage, an FeCH4+ ion-molecule complex is directly formed. The ground state of the FeCH4+ complex is the sextet 6A2 state, which is stable relative to the ground state dissociation Fe+(6D,s1d6)+CH 4 by 16 kcal/mol. The entire FeCH2+ hydrogenolysis reaction FeCH2+(4B 2)+H2→Fe+(6D,s 1d6)+CH4 (1) is estimated to be exothermic by 29 kcal/mol. The channels leading to formation of FeH++CH3 (2) and FeCH3++H (3) are thermodynamically almost neutral. The reverse reaction Fe++CH4 for 6D, 4F or 2G states of Fe+ gives only one product, the ion-molecule complex FeCH4+ at moderate temperatures. An increase in the available kinetic energy for 6D and 4F states would make it possible to form dissociation products, FeH ++CH3 and FeCH3++H. Though the channel leading to FeCH2++H2 is thermodynamically as easy as (2) and (3), a large barrier prevents it from taking place. © 1994 American Institute of Physics.