Twenty-five transition structures (TS's) for CO₂ 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 (NH₂CO₂⁻) or bicarbonate (HCO₃⁻) 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) NH₂CO₂⁻ formation has a lower free energy of activation (best est. 44–45 kJ mol⁻¹) than HCO₃⁻ formation (best est. 86 kJ mol⁻¹), and (ii) free energies of activation for CO₂ 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 NH₂CO₂⁻ near pH 9.