Ab initio G2M(MP2) and CCSD(T)/6-311 + G(3df)//MP2/6-31G(d) calculations show that the Ca +CO2 reaction proceeds by formation of a C2v-symmetric cyclic CaOCO molecule, 1 kcal/mol below the reactants, with the initial reaction step barrier of about 13 kcal/mol. Then, CaOCO dissociates to the CaO +CO products without exit barrier. The total reaction endothermicity is calculated as 35.4 and 40.4 kcal/mol at the G2M(MP2) and CCSD(T)/ 6-311+ G(3df)//QCISD/6-311G(d) levels, respectively. The reverse CaO +CO reaction is predicted to occur without barrier and can rapidly produce Ca +CO2. The reaction mechanisms of dierent alkali earth metals with carbon dioxide are compared.
