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 (BuNH2and 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 NH2group was surrounded by both TFSA-and C2mIm+ions in the BuNH2system. With regard to the nearest neighbor solute-TFSA interaction, the solvation number for TFSA-anions (nTFSA) increased upon protonation (i.e. BuNH2:nTFSA= 2 and BuNH3+:nTFSA= 3). Both BuNH2and 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 (pKa= 16.6) of BuNH2in [C2mIm][TFSA] solution.
Yokohama National University, 79-5 Tokiwadai Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan.
Kei
K
Hashimoto
Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan. k-fujii@yamaguchi-u.ac.jp.
Kenta
http://orcid.org/0000-0003-0057-1295
K
Fujii
Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Koto, Sayogun Sayocho, Hyogo, 679-5198, Japan.
Koji
K
Ohara
Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan. sibayama@issp.u-tokyo.ac.jp.
Mitsuhiro
M
Shibayama