Chemical and Molecular Descriptors for the Reactivity of Amines with CO2
Anita S. Lee and John R. Kitchin, “Chemical and Molecular Descriptors for the Reactivity of Amines with CO2”, Industrial & Engineering Chemistry Research 51(42), 13609-13618 (2012). https://doi.org/10.1021/ie301419q
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Note on the source text: the extracted text is a readable author manuscript, but the figures and Tables 1 to 3 (molecular structures and reaction schemes) are not available, so the results below are taken from the prose.
Question
Post-combustion capture with aqueous monoethanolamine (MEA) is the most mature option for coal plants, but regenerating the solvent in the stripper is its largest energy cost, and the amine-CO2 reaction enthalpy sets the heat of desorption and, indirectly, capacity and circulation rate. Steric hindrance and basicity have been the usual descriptors of amine reactivity, yet screening data on many amines did not fit that simple picture. The paper asks which electronic structure properties of an amine control the energies of its two capture products, bicarbonate and carbamate, so that new amines could be designed for a target reaction enthalpy (Lee 2012).
Approach
Three families of amines were studied with B3LYP/6-31+G(d,p) in Gaussian 09: alkylamines, alkanolamines and trifluoroalkylamines, each as primary, secondary and tertiary amines with substituent chains of one to three carbons, alongside NH3 and MEA. Water was represented with the IEFPCM continuum model. Because the CO2 solvation enthalpy was taken as the same for all amines, the liquid-phase reaction energy (with zero point and thermal corrections, using the lowest energy conformer from a stochastic search) served as the reaction enthalpy. Product states included non-interacting ions and interacting ion-pair complexes for both pathways, plus zwitterion and carbamic acid species on the carbamate pathway (primary and secondary amines only). Descriptors came from conceptual density functional theory: electronegativity from a centered finite difference of N-1, N and N+1 electron energies (equivalent to the Mulliken definition), and the local softness of the amine nitrogen from global softness times a condensed Fukui function obtained by Bader analysis (Lee 2012).
Key findings
For every amine the ion-pair complex (amine-bicarbonate or protonated amine-carbamate) was more stable than the separated ions. The bicarbonate complex shows a shared proton between N and O with an N-O distance of 2.66 +/- 0.19 Angstrom, but for NH(CF3)2 and N(CF3)3 no such complex formed (N-O of 3.12 and 4.5 Angstrom), indicating very weak basicity. Zwitterions were always higher in energy than carbamic acid, consistent with a short-lived intermediate, and could not be found for trifluoroalkylamines with CF3 close to nitrogen (Lee 2012).
Adding an alkyl chain to NH3 stabilized both products; further alkylation (secondary, tertiary) made bicarbonate more stable and carbamate less stable, essentially independent of chain length. Trifluoroalkyl groups destabilized both products, most strongly when CF3 sat nearest the nitrogen, with the effect fading as the chain lengthened. Because the effect persists beyond the alpha carbon, the authors argue that it is not only an alpha-carbon orbital interaction. Alkanolamines fell in between, and intramolecular hydrogen bonds (five or six membered rings) stabilized both products further (Lee 2012).
Following Sanderson’s electronegativity equalization idea, the bicarbonate complex formation energy correlated with the electronegativity difference between the amine and carbonic acid: larger differences gave more stable complexes, over a span of roughly 0 to -80 kJ/mol, and the relation held for conformers within 0.1 eV of the lowest energy one. Electron-withdrawing CF3 and OH groups raise amine electronegativity, most strongly when attached directly, which suggests a finite tunable range for amine solvents. On the carbamate pathway, the carbamate complex energy correlated with the carbamic acid formation energy, and the latter followed the local hard and soft acids and bases idea: reaction was more exothermic as the nitrogen softness approached that of the reacting CO2 site. However, there were two separate trends, a strong one for trifluoroalkylamines and a weak one for alkyl and alkanolamines. Longer N-C bonds and larger O-C-O angles in the trifluoroalkylamine carbamic acids suggest a different kind of interaction for that family (Lee 2012).
Significance
The work moves amine design beyond basicity and sterics by tying the bicarbonate pathway to a single computable descriptor (electronegativity) and showing that substituent type, number and distance tune it. It also shows that no single electronic descriptor captures the carbamate pathway across chemically different families, pointing to the need for additional, possibly steric, descriptors (Lee 2012).