Effect of protonation on the solvation structure of solute N-butylamine in an aprotic ionic liquid

We report on the acid-base reaction of an amine solute in an aprotic ionic liquid from a structural point of view. Thus, the solvation structures of n-butylamine and n-butylammonium (BuNH₂ and BuNH₃⁺, respectively, acid-base reaction: BuNH₂ + H⁺ ⇄ BuNH₃⁺) in 1-methyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C₂mIm][TFSA]) were investigated by high-energy X-ray total scattering combined with molecular dynamics simulations. We found that the solvation structure drastically changed as a result of the protonation reaction in [C₂mIm][TFSA]. The NH₃⁺ group was preferentially solvated by TFSA^- anions in the protonated BuNH₃⁺ system, whereas the neutral NH₂ group was surrounded by both TFSA^- and C₂mIm⁺ ions in the BuNH₂ system. With regard to the nearest neighbor solute-TFSA interaction, the solvation number for TFSA^- anions (n_TFSA) increased upon protonation (i.e. BuNH₂:n_TFSA = 2 and BuNH₃⁺:n_TFSA = 3). Both BuNH₂ and BuNH₃⁺ interacted with O atoms within TFSA^- through hydrogen bonding (i.e. N-HO) interactions, and the N-HO distance was appreciably shorter for positively charged BuNH₃⁺ as compared to neutral BuNH₂. These results revealed that the solvation is more stable in energy for the protonated BuNH₃⁺ group because of its stronger hydrogen bonding as compared to neutral BuNH₂. This is the origin of the large Gibbs energy for the protonation reaction (ΔG = -94.7 kJ mol^-1) and the high acid dissociation constant (pK_a = 16.6) of BuNH₂ in [C₂mIm][TFSA] solution.