Synergism of Catalysis and Reaction Center Rehybridization. A Novel Mode of Catalysis in the Hydrolysis of Carbon Dioxide

Abstract

A quantum mechanical study is presented of the activation barrier of the one-, two-, and three-water hydrolyses of CO2. Geometries were optimized, frequencies were calculated, and NPA charges were determined at MP2(full)/6-31G* and MP2(full)/6-311G**, and energies were also determined at MP4(full,SDTQ)/6-311G**//MP2(full)/6-311G** and QCISD(T)/6-31G**//MP2(full)/6-31G**. The activation barriers are ΔH0 = 223.0 kJ/mol and ΔG298 = 235.6 kJ/mol for the one-water hydrolysis and they are ΔH0 = 149.8 kJ/mol and ΔG298 = 164.4 kJ/mol for the two-water hydrolysis at MP4(full,SDTQ)/6-311G**. The catalytic effect of the second water molecule is due to the alleviation of ring strain in the proton-transfer transition state. The placement of the third water molecule in the proton-transfer ring causes only an insignificant catalytic effect with respect to the two-water hydrolysis. The placement of the third water molecule opposite the site of proton transfer is explored here and it leads to activation barriers of ΔH0 = 122.6 kJ/mol and ΔG298 = 143.1 kJ/mol. The catalytic effect of the third water molecule is approximately 20 kJ/mol with respect to the two-water hydrolysis, and this is attributed to charge relaxation and rehybridization in the transition state. The results are compared to studies on the three-water hydrolysis of carbodiimide to discern the effects of atomic polarizability on the activation barriers of the hydrolyses of heterocumulenes. The conceptional insights predict that the catalytic effect should increase with more polarizable heteroatoms.