Ab initio molecular orbital study of the molecular and electronic structure of FeCH+2 and of the reaction mechanism of FeCH+2+H2

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 FeCH+2, as well as the mechanism for the reaction FeCH+2+H2. Three quartet electronic states of FeCH+2 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 FeCH+2(4B2), relative to the dissociation limit Fe+(6D,s1d6)+CH2(3B1), is estimated to be 68 kcal/mol. In the first step of reaction FeCH+2+H2, the reactants yield an ion-molecule complex, (H2)FeCH+2, 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 FeCH+2 is expected to be more reactive than the ground state because of the lower barrier. The resultant HFeCH+3 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 FeCH+4 ion-molecule complex is directly formed. The ground state of the FeCH+4 complex is the sextet 6A2 state, which is stable relative to the ground state dissociation Fe+(6D,s1d6)+CH4 by 16 kcal/mol. The entire FeCH+2 hydrogenolysis reaction FeCH+2(4B2)+H2→Fe+(6D,s1d6)+CH4 (1) is estimated to be exothermic by 29 kcal/mol. The channels leading to formation of FeH++CH3 (2) and FeCH+3+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 FeCH+4 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 FeCH+3+H. Though the channel leading to FeCH+2+H2 is thermodynamically as easy as (2) and (3), a large barrier prevents it from taking place.