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.

The -CO2H functional group in amino acids reversibly reacts with CO2 more quickly than the -NH2 group, generating the carbonic anhydride, -CO2CO2H.