Evaluating the CO2-capturing efficacy of amine and carboxylic acid motifs: ab initio studies on thermodynamic versus kinetic properties

The reaction of alanine with CO2 was investigated to evaluate the CO2-capturing efficacy of its two functional groups, –NH2 and –CO2H, using b3lyp/6-31+G* methods. Optimized structures of initial complexes (IN and IO), transition states (TSN and TSO), and products (PN and PO) from the interactions of CO2 with –NH2 and –CO2H in alanine were calculated. Binding energies to –NH2 and –CO2H in alanine were predicted to be 3.404 and 2.151 kcal/mol, respectively. For the –NH2 and –CO2H reaction pathways, the activation energies were calculated to be 38.149 and 18.882 kcal/mol, and reaction energies were −2.492 and 14.514 kcal/mol, respectively. Kinetically, the route to generate the carbonic anhydride through the –CO2H moiety was more favorable by 20 kcal/mol than reaction at the –NH2 moiety, whereas this route was revealed to be a thermodynamically unfavorable path. These results suggested that the –CO2H functional group reversibly reacted with CO2 more quickly than the –NH2 group, which may afford utility as a carrier in the formation of a CO2-scrubbing membrane. In addition, the effects of a water molecule catalyst in the CO2-capturing reactions of the –NH2 and –CO2H functional groups in alanine were calculated and discussed.