Nuclear Magnetic Resonance Study of CO₂ Capture by Fluoroalkylamines: Ammonium Ion pK_a Depression via Fluorine Modification and Thermochemistry of Carbamylation

Abstract

We are developing energy-efficient and reversible carbon capture and release (CCR) systems that mimic the Lys201 carbamylation reaction in the active site of ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO). The multiequilibria scenario ammonium ion Xa ⇌ amine Xb ⇌ carbamic acid Xc ⇌ carbamate Xd requires the presence of both free amine and CO2 for carbamylation and is affected by the pK a(Xa). Two fluorination strategies aimed at ammonium ion pK a depression and low pH carbamylation were analyzed with (2,2,2-trifluoroethyl)­butylamine 2b and 2,2-difluoropropylamine 3b and compared to butylamine 1b. The determination of K 1 and ΔG 1 of the carbamylation reactions requires the solution of multiequilibria systems of equations based on initial conditions, 1H NMR measurements of carbamylation yields over a wide pH range, and knowledge of K 2–K 5 values. K 2 and K 3 describe carbonic acid acidity, and ammonium ion acidities K 4 were measured experimentally. We calibrated carbamic acid acidities K 5 based on the measured value K 6 of aminocarbamic acid using isodesmic reactions. The proton exchange reactions were evaluated with ab initio computations at the APFD/6-311+G* level in combination with continuum solvation models and explicit solvation. The utilities of 1–3 will be discussed as they pertain to the development of fluorine-modified RuBisCO-mimetic reversible CCR systems.