Spectroscopic evidence of ‘jumping and pecking’ of cholinium and H-bond enhanced cation–cation interaction in ionic liquids

Abstract

The subtle energy-balance between Coulomb-interaction, hydrogen bonding and dispersion forces governs the unique properties of ionic liquids. To measure weak interactions is still a challenge. This is in particular true in the condensed phase wherein a melange of different strong and directional types of interactions is present and cannot be detected separately. For the ionic liquids (2-hydroxyethyl)-trimethylammonium (cholinium) bis(trifluoro-methylsulfonyl)amide and N,N,N-trimethyl-N-propylammonium bis(trifluoromethylsulfonyl)amide which differ only in the 2-hydroxyethyl and the propyl groups of the cations, we could directly observe distinct vibrational signatures of hydrogen bonding between the cation and the anion indicated by 'jumping and pecking' motions of cholinium. The assignment could be confirmed by isotopic substitution H/D at the hydroxyl group of cholinium. For the first time we could also find direct spectroscopic evidence for H-bonding between like-charged ions. The repulsive Coulomb interaction between the cations is overcome by cooperative hydrogen bonding between the 2-hydroxyethyl functional groups of cholinium. This H-bond network is reflected in the properties of protic ionic liquids (PILs) such as viscosities and conductivities.

Large Stokes Shift Ionic‐Liquid Dye

“…[27] They are supported by the layered structure (see Figure 2) and the stacking clearly compensates for the anionic repulsive forces.T he situation is similar to that in ILs in which H-bonds can overcome the repulsion between like-charged ions. [28][29][30][31][32][33] TheX -ray structural features are strongly supported by B3LYP/6-311G** and B3LYP-D3/6-311G** calculations on dimers and tetramers of ion-pairs. [34][35][36] Cut-outs of the X-ray structures are used as starting geometry.Only if the dispersion correction is taken into account do the optimized structures resemble the X-ray structures,which are mainly characterized by the stacking between the rings of N-methyl-6-hydroxyquinolinium cations and the hydrogen bond between the hydroxy proton and the sulfonyl oxygen atom (see SI7).…”

When Like Charged Ions Attract in Ionic Liquids: Controlling the Formation of Cationic Clusters by the Interaction Strength of the Counterions

“…[19] This unique behavior is somewhat surprising for ILs wherein all ions are solvated by counterions and the cation-anion interaction strength mainly governs the properties of this unique liquid material. [19] This unique behavior is somewhat surprising for ILs wherein all ions are solvated by counterions and the cation-anion interaction strength mainly governs the properties of this unique liquid material.…”

“…The interpretation of the deconvoluted bands is supported by DFT calculated frequencies of cation-anion and cationcation type clusters including one up to four ion pairs at the B3LYP-D3/6-31 + G* level of theory. [19] The B3LYP-D3/6-31 + G* calculated structures for tetrameric clusters are shown in Figure 4. In particular the red shifts with cationcation cluster size are nicely reflected.…”

Die zweigesichtige Natur der Wasserstoffbrückenbindung in hydroxylfunktionalisierten ionischen Flüssigkeiten, offenbart durch Neutronendiffraktometrie und Molekulardynamik‐Simulation

“…[10][11][12] Jedoch müssen ebenfalls die Wasserstoffbrückenbindungs-und Dispersionskräfte berücksichtigt werden, um zu verstehen, wie sich das empfindliche Gleichgewicht zwischen den verschiedenen Arten von Wechselwirkungen auf die Eigenschaften der ILs auswirkt. [29][30][31][32][33][34][35] Die Schwingungsspektroskopie identifizierte klar zwei unterschiedliche Schwingungsbanden, die (c-a)-und (c-c)-wasserstoffbrückengebundenen Spezies zugeordnet wurden. Es koexistieren zwei verschiedene Arten von Wasserstoffbrückenbindungen:D ie konventionellen Wasserstoffbrückenbindungen zwischen Kationen und Anionen (c-a) sind durch attraktive Coulomb-Wechselwirkungen verstärkt, wohingegen die ungewçhnlichen H-Brücken zwischen gleich geladenen Ionen (c-c) aufgrund der abstoßenden Coulomb-Kraft viel schwächer sein sollten.…”

“…[36][37][38][39][40] Die Streudaten werden mithilfe der "empirical potential structure refinement"(EPSR)-Routine verfeinert, um Strukturinformationen auf atomarer und molekularer Ebene zu extrahieren. [29][30][31][32][33][34][35] Ziel dieser Arbeit ist es,d ie (c-a)-( + OÀH···O À )u nd die (c-c)-Wasserstoffbrücken ( + O À H···O + )i nd ieser hydroxylfunktionalisierten IL mittels ND,M olekulardynamik(MD)-Simulationen und quantenchemischen Berechnungen zu charakterisieren. Zusätzlich ermçglicht die Hydroxybutylgruppe des Kations eine gute Tr ennung zwischen dem positiv geladenen Pyridiniumring und der Hydroxylgruppe.S pektroskopisch konnten das Auftreten der Wasserstoffbrückenbindungen zwischen den Kationen (c-c) fürd iese Kation/Anion-Kombination bereits nachgewiesen werden.…”