Ab Initio Study of the Molecular and Electronic Structure of CoCH2+ and of the Reaction Mechanism of CoCH2+ + H2

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Date
1993-11-01
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Musaev, Djamaladdin
Morokuma, Keiji
Gordon, Mark
Nguyen, Kiet
Gordon, Mark
Cundiari, Thomas
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Abstract

Both CASSCF and MR-SDCI-CASSCF methods have been used with two different effective core potentials to investigate the molecular and electronic structures of CoCH2+, as well as the mechanism for the reaction CoCH2+ + H2. Four electronic states of CoCH2+ are very low lying: the ground state is a nearly degenerate pair (3A2 and 3A1), and the 3B1 and 3B2 states are only 4-8 kcal/mol higher in energy. The binding energy of C O C H ~ + ( ~ Are~la)t,iv e to that of C ~ + ( ~ F , s l d+~ C)H 2(3B1), is estimated to be 70-80 kcal/mol. A similar hydrogenolysis reaction mechanism holds for the 3A2 and 3A1 states of the CoCH2+ + H2 reactants: In the first step, the reactants yield an ion-molecule complex, (H2)CoCH2+, stabilized by 8-9 kcal/mol. Subsequently, the H-H bond is activated, leading to a four-center transition state with an energy barrier of about 31-34 kcal/mol. An intermediate complex, HCoCH3+, is predicted to be a minimum at the CASSCF level, but MR-SDCI-CASSCF single-point calculations suggest that this minimum disappears at the higher level of theory. Following H-H bond cleavage, a CoCH4+ ion-molecule complex is formed, with a stabilization energy of 19-22 kcal/mol. The CoCH2+ hydrogenolysis reaction is predicted to be exothermic by 20-30 kcal/mol. The channels leading to formation of CoH+ + CH3 and CoCH3+ + H are endothermic by about 5-1 2 kcal/mol. The reverse reaction Co+ + CH4 may give only one product, the ion-molecule complex CoCH4+ at moderate temperatures. An increase in the available kinetic energy would make it possible to form dissociation products: CoH+ + CH3 and CoCH3+ + H. Although the channel leading to CoCH2+ + H2 is thermodynamically more favorable, a large barrier prevents it from taking place. Hay-Wadt and Stevens-Krauss-Basch-Jasien pseudopotentials give qualitatively the same results.

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Reprinted (adapted) with permission from Journal of Physical Chemistry 97 (1993): 11435, doi:10.1021/j100146a016. Copyright 1993 American Chemical Society.

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