Hydrogen Bonding Interactions Between Protic Ionic Liquids Consist Of Alkyl Ethylenediaminium With TFSA/TFA And Water Molecules,and Their CO₂ Absorption Study

Ionic liquids?ILs?are molten salts with melting points below 100°C.ILs consist of asymmetric organic cations and inorganic or organic anions.They have unique physicochemical properties such as low melting point,wide liquid range and electrochemical window,non-volatility,good electrical conductivity,and high thermal stability.Since the 21^stt century,ionic liquids have received increasing attention in various fields of research such as catalytic chemistry,materials science,electrochemistry,and separation technology.This is perhaps because ionic liquids exhibit unique properties and can be used as solvents with"designable molecular structure"or as"soft"functional materials.In this study,hydrogen bonding interactions between the cation and anion of protic ionic liquids?PILs?,composed of N-alkylethylenediaminium cations and bis?trifluoromethylsulfonyl?imide?TFSA?anion,as well as those of the cation and anion with water molecules were studied by the density functional theory?DFT?.CO₂ absorption using PILs composed of N-alkylethylenediaminium cations and trifluoroacetic?TFA?anion was studied.This paper mainly discusses the following three aspects:1)the hydrogen bonding study on protic ionic liquids composed of N-alkylethylenediaminium cations with bis?trifluoromethylsulfonyl?imide anionThe intermolecular hydrogen bonding interactions of the protic ionic liquids?PILs?composed of N-alkyl?=hexyl,octyl and 2-ethylhexyl?ethylenediaminium cations with bis?trifluoromethylsulfonyl?imide?TFSA?anion that form[HHex][TFSA],[HOct][TFSA]and[HEtHex][TFSA]were studied by the density functional theory M06-2X/6-311G?d,p?level.The five possible molecular structures S1-S5 for each PIL were optimised and the BSSE-corrected interaction energies(?E_intnt ^BSSE)for all the structures are in the region of?88.8^?108.9 kcal·mol^-1.The length of H?N bond in[HAlkyl]⁺was lengthened and it's IR spectra was redshifted as N?H…N or N?H…O type hydrogen bondings were formed between the[HAlkyl]⁺cation and[TFSA]^-anion.Natural bond orbital?NBO?analysis shows that the stabilization energy mainly caused by the interaction between the lone pair electron LP?N/O?in the[TFSA]and the anti-bonding orbital?*?N?H?in the[HAlkyl]:LP?N/O???*?N?H?.The value of the second order stabilization energy E^?2?,the electron density?_c and Laplacian electron density?²?_c calculated by the AIM show that the strength of hydrogen bonding interaction is related to the alkyl chain length and the introduction of the side chain.The hydrogen bonding interaction is stronger for S1?S3?protonated at the primiray amine?than S4?S5?protonated at the secondary amine?.2)The hydrogen bonding interaction between[HHex][TFSA]and water moleculesIn general,PILs with TFSA anion are hydrophobic.However,the PILs in our study contain two amines in their polar part.Therefore,they are more hydrophilic than mono-amine type PILs.Our previous study shows that at a particular temperature,the viscosity of the PIL decreases and its electrical conductivity increases with increasing water content.Therefore,we studied the hydrogen bonding interaction between a PIL,e.g.,[HHex][TFSA],and the water molecules using the DFT at M06-2X/6-311G?d,p?level.First,the configurations of[HHex][TFSA]-nH₂O?n=1,2,6?were optimised,and the interaction energy,molecular vibration,second-order perturbation,and electron density were calculated and analysed.It was found that N—H···O?O—H···N,and O—H···O type hydrogen bonds are formed between[HHex][TFSA]and the water molecules;this was accompanied by a concomitant decrease in the hydrogen bonding interactions between the cation and anion of[HHex][TFSA]monomer.Higher the?E₀^BSSESSE values,higher is the stability of the[HHex][TFSA]-nH₂O structure.The stability follows the order[HHex][TFSA]-6H₂O>[HHex][TFSA]-2H₂O>[HHex][TFSA]-H₂O.The electron charges increased in the basis set superposition error?BCPs?of the hydrogen bonding;for example,the electron charges increased in the N atoms of[HHex][TFSA]and in the O atoms of H₂O in this study.Also,a higher E^?2?indicates stronger hydrogen bonding interactions.The?_c values in the BCPs of hydrogen bonding for[HHex][TFSA]-nH₂O are in the range 0.0105–0.0468 a.u.,which is typical of a hydrogen bond??_c=0.002–0.040 a.u.?.The values of?_c?sum?and E^?2?increase with increasing water content in[HHex][TFSA].3)The CO₂ absorption using[HHex][TFA],[HOct][TFA],[HEtHex][TFA]was studied according to the theoretical studyThe protic ionic liquids comrising with hexyl?octyl and ethylhexylethylenediaminium caions and trifluoroacetic anion were synthesized using base alkyl?=hexyl?octyland ethylhexyl?ethylenediamine and trifluoroacetic acid?HTFA?.PILs/water systems with different molar ratio?1:10?1:15?1:20 and 1:25?were prepared.pH value,density,dynamic viscosity and electric conductivity of PILs/water systems were studied before and after CO₂ absorption.The results show that density,viscosity decrease and electric conductivity increases with increasing the water molar ratio.The pH value and electrical conductivity decrease,while the density and viscosity increase after CO₂ absorption at 30?,compared with those before CO₂ absorption.