Theoretical Study on the Reaction Mechanism of CO₂ with Mg

Abstract

Ab initio QCISD(T)/6-311+G(3df)//MP2/6-31+G(d) calculations of potential energy surface for the reaction of Mg atoms with CO2 show that re-forming of carbon dioxide to carbon monoxide can be significantly enhanced in the presence of Mg atoms. The overall endothermicity of the Mg + CO2 → MgO + CO reaction is calculated to be about 66 kcal/mol, almost twice lower than the energy needed for spin-forbidden unimolecular decomposition of CO2 to CO + O(3P). The Mg + CO2 reaction is spin-allowed and the barrier, 68.8 kcal/mol, is greatly reduced as compared to the barrier for the unimolecular decomposition, 131 kcal/mol. The reaction proceeds via the MgOCO cyclic intermediate which lies 14.3 kcal/mol higher in energy than the reactants and stabilized with respect to Mg + CO2 by a barrier of 5.9 kcal/mol. The catalytic role of Mg atoms for re-forming of CO2 to CO is discussed. The reverse MgO + CO → Mg + CO2 reaction is highly exothermic and has a barrier of 2.8 kcal/mol indicating that magnesium oxide can be rapidly reduced by carbon monoxide producing Mg atoms and carbon dioxide. Highly accurate full valence active space MRCI calculations with extrapolation to the complete basis set allowed us to propose a new value for the standard heat of formation of MgO in the gas phase, 31.4−31.9 and 33.5−34.0 kcal/mol for ΔH f°(0 K) and ΔH f°(298 K), respectively. The result is 20 kcal/mol higher than the present recommended value and new experimental measurements of thermochemical data for gaseous MgO are suggested.

Ab Initio Study on the Mechanism of the Atmospheric Reaction OH+O3→HO2+O2

“…As a consequence the O(2)ÀC(3) bond length increases from 1.274 (free ozone, C 2v symmetry) to 1.307 ä (TS1). On the whole the structure of TS1 is similar to the nonplanar geometries reported previously for the early stages of the H O 3 , [27] Cl O 3 , [29] NO O 3 , [30] and NH 2 O 3 [31] reactions. The minor differences noted between TS1 of reaction (1) and that previously found for the NO O 3 and NH 2 O 3 reactions [30,31] can be rationalized in terms of the Hammond's principle.…”

“…The corresponding transition vector of TS1 is similar to those of the equivalent transition state structures previously obtained for the NO O 3 , [30] NH 2 O 3 , [31] and Cl O 3 [29] reactions of 353 i, 201 i, and 242 i cm À1 at the UMP2 level. The value of hS 2 i is 0.811 at the UMP2/6-311 G(d,p) level of theory, slightly higher than the 0.75 expected for a pure doublet state.…”

An Ab Initio Study on the Mechanism of the Atmospheric Reaction NH2+O3→H2NO+O2

“…As a consequence, the O(2) À O(3) bond distance increases from 1.274 ä (free ozone, C 2v symmetry) to 1.296 ä (TS1 structure). It is worth mentioning that as a whole, the characterized structure of TS1 is similar to the nonplanar geometries reported previously at the early stages of the reactions between H and O 3 , [36] Cl and O 3 , [38] and NO and O 3 . [39] The minor differences between TS1 of Reaction (3) and a transition state previously found for the reaction between NO and O 3 can be rationalized in terms of the Hammond principle.…”

Ab initio study of the reaction mechanism of CO2 with Ti atom in the ground and excited electronic states

“…Interestingly, the MϩCO 2 →MO ϩCO reaction mechanisms for various alkaline-earth atoms are rather similar to that for MϭNi. Our earlier calculations 29 →ScOϩCO→side-OScCO ͑Ϫ33.8͒. While the insertion mechanism is clearly preferable kinetically, the addition may also contribute because the calculated barriers are rather low.…”

“…At the initial reaction step in the insertion mechanism, the triplet Ti atom attaches to the carbon dioxide with formation of a planar 2 -C,O t-͑TiOC͒O 30 On the other hand, t-OTiCO is predicted to be more stable than OScCO, since the former lies 43.9 and 13.8 kcal/mol lower in energy than Ti( 3 F)ϩCO 2 and TiO( 3 ⌬)ϩCO, respectively, as compared to 38.7 and 6.9 kcal/mol for the corresponding relative energies of OScCO. 30 In the addition reaction mechanism, the triplet Ti atom attaches to CO 2 with formation of a planar cyclic nearly…”