Nucleophilicity and Accessibility Calculations of Alkanolamines: Applications to Carbon Dioxide Absorption Reactions

Both nucleophilicities and accessibilities of three alkanolamines [monoethanolamine (MEA), (2-(methylamino)ethanol (MAE), and 2-amino-2-methyl-l-propanol (AMP)] were calculated to predict their reactivities with CO2. After DFT geometry-optimization calculations, the global, group, and atomic nucleophilicities of each amine were obtained using MP2 quantum mechanical calculations. Only global nucleophilicity matched an experimental pK(a) order (MAE > AMP > MEA). However, it failed to predict the slow rate of the sterically hindered AMP and the order of rate constants, MAE > MEA > AMP. The accessibilities of amines to CO2 have been calculated by MD simulations by monitoring collisions at the reaction centers: N atoms in amines and C in CO2. The accessibility results indicate that global nucleophilicity needs quantitative correction for steric effects to predict better reactivities of amines with CO2.