CO2 capture in aqueous ammonia solutions: a computational chemistry perspective.

Twenty-five transition structures (TS's) for CO(2) fixation by up to four base molecules (ammonia or ammonia + water) were located using M06-2X/6-311++G(d,p). All lead to either carbamate (NH(2)CO(2)(-)) or bicarbonate (HCO(3)(-)) products. Single-point energies at CCSD(T)/maug-cc-pVTZ//M06-2X/6-311++G(d,p) were added to SM8/M06-2X/6-311++G(d,p) energies to obtain best-estimate aqueous activation energies. All theories agree that: (i) NH(2)CO(2)(-) formation has a lower free energy of activation (best est. 44-45 kJ mol(-1)) than HCO(3)(-) formation (best est. 86 kJ mol(-1)), and (ii) free energies of activation for CO(2) fixation are lowered when an ammonia molecule accepts the proton from the nucleophilic base. The theory also supports a key role for ammonium ions in the observed decomposition of NH(2)CO(2)(-) near pH 9.

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