TUNING THE IMMISCIBILITY AND DEGREE OF REACTIVITY OF TERTIARY AMINES BY CHANGING THEIR FUNCTIONAL GROUPS: A DFT BASED COMPUTATIONAL STUDY*

The reclamation of water is a very important topic in our current society and critical for human manned space travel such as the International Space Station. Currently, tertiary amines have been used as a switchable polarizable solvent to increase the osmotic potential and reuse carbon dioxide for separation of water by forward osmosis. Both, immiscibility and the degree of reactiviy are identified as requirements to use tertiary amines in the water filtration systems. In this work, we conducted Density Functional Theory (DFT) computations for octanol-water partition coefficient (LogP), HOMO-LUMO gap (ΔE), and study the proton transfer process based on Gibbs free-energy and molecular dynamics simulations on 108 tertiary amines with different functional groups: 36 linear, 36 cyclic and 36 aromatic.

Results indicate that having two groups as increasing carbon chains from two to six and replacing the third group by linear (butyl), cyclic (cyclohexane) and an aromatic (benzene), increase the LogP and reduce ΔE. The HOMO-LUMO gap energy was reduced considerably (≈2.7 eV) with aromatic group in comparison to the tertiary amines with butyl and cyclohexane groups due to π-conjugation in the benzene ring. The immiscibility and degree of reactivity are correlated with the LogP and the ΔE gap and are in the range of 1.28 < LogP < 7.83 and 5.2 < ΔE < 8.6 (in units of eV) for the 108 tertiary amines; 4 of the 108 candidates with values of LogP > 5.0 have high immiscibility. Those with ΔE > 6.0 eV should have low chemical reactivity. The DFT-based molecular dynamics simulations shows how the proton transfer take place in the carbonic acid and tertiary amine acid-base reaction under study in aqueous conditions. In addition, the Gibbs free-energy differences and barriers between reactants (carbonic acid and tertiary amine) and products (bicarbonate and ammonium salt) in water was computed for 18 out of the 108 candidates.