Dissecting Noncovalent Interactions in Carboxyl‐Functionalized Ionic Liquids Exhibiting Double and Single Hydrogens Bonds Between Ions of Like Charge

Abstract

We show that the carboxyl‐functionalized ionic liquid 1‐(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC‐CH2‐py][NTf2] exhibits three types of hydrogen bonding: the expected single hydrogen bonds between cation and anion, and, surprisingly, single and double hydrogen bonds between the cations, despite the repulsive Coulomb forces between the ions of like charge. Combining X‐ray crystallography, differential scanning calorimetry, IR spectroscopy, thermodynamic methods and DFT calculations allows the analysis and characterization of all types of hydrogen bonding present in the solid, liquid and gaseous states of the ionic liquid (IL). We find doubly hydrogen bonded cationic dimers (c+=c+) in the crystalline phase. With increasing temperature, this binding motif opens in the liquid and is replaced by (c+−c+−a− species, with a remaining single cationic hydrogen bond and an additional hydrogen bond between cation and anion. We provide clear evidence that the IL evaporates as hydrogen‐bonded ion pairs (c+−a−) into the gas phase. The measured transition enthalpies allow the noncovalent interactions to be dissected and the hydrogen bond strength between ions of like charge to be determined.

Spectroscopic Evidence for Doubly Hydrogen-Bonded Cationic Dimers in the Solid, Liquid, and Gaseous Phases of Carboxyl-Functionalized Ionic Liquids

“…The weaker, lower energy feature near 1635 cm –1 is nearly the same in both sizes, whereas the stronger band is blue-shifted by about 8.8 cm –1 in going from C4 to C2. A feature close to 1635 cm –1 was reported in a previous study of the C1 IL solid and assigned to closely spaced CN stretch (hereafter denoted ν CN ) fundamentals involving the carbon atoms on the chains that bind to the N atoms in the ring . The dominant band in the C1 spectrum, however, was observed at 1750 cm –1 , about 35 cm –1 above the strongest feature displayed by C2 (see Figure ).…”

“…In this paper, we focus on the ILs 1-( n -carboxyalkyl)­pyridinium bis­(trifluoromethylsulfonyl)­imide [HOOC–(CH 2 ) n -py]­[NTf 2 ] with n = 2 and 4. These cations feature the same positive charge motif as that in the earlier study, namely, a pyridinium ring, but the COOH group is displaced away from the charge center by two and four CH 2 links in the alkyl chain. This allows us to explore how the attractive (cc) interactions evolve as the ethyl and butyl chains systematically reduce the repulsive Coulomb interactions between the cations (Scheme ).…”

“…More recently, another variation of the hydrogen bonding motif between cations has been explored in ILs based on carboxy-functionalized cations, as depicted in Scheme . These were chosen because they afford the possibility of forming more strongly bound, doubly H-bonded cationic dimers (hereafter denoted cc), mimicking the structural features of the molecular dimers formed by formic and acetic acid. , Analysis of the diffraction pattern obtained in a recent X-ray crystallographic study of solid 1-(carboxymethyl)­pyridinium bis­(trifluoromethylsulfonyl)­imide [HOOC–CH 2 –py]­[NTf 2 ] revealed the formation of doubly hydrogen-bonded cationic dimers (cc) with a bridge motif similar to that found in the archetypal structure adopted in gaseous phase formic acid dimer. FT-IR spectra in the CO stretching region suggested that the (cc) binding motif opens in the liquid state of the IL when warmed above ∼200 K. The new motif features a single remaining H-bond between cations and an additional H-bond between cation and anion and hence denotes a (c–c–a) interaction.…”

Molecular Liquids versus Ionic Liquids: The Interplay between Inter-Molecular and Intra-Molecular Hydrogen Bonding as Seen by Vaporisation Thermodynamics

“…Admittedly, the reliable measurement of the vapour pressure of extremely low-volatility ionic liquids is an extremely demanding task [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. The reason for this is that the conventional techniques used for ML studies are not applicable as they are not sensitive enough to detect the vanishingly small ILs vapour pressures.…”

“…Nevertheless, the vaporisation enthalpies, , are reliably measured by various experimental techniques [ 1 ], challenging the task of separating and understanding the energetics of molecular interactions using these thermodynamic data. Indeed, in a number of recent publications, we have developed a few thermodynamically based methods to quantify hydrogen bond strengths (inter- and intra-HB) for molecular [ 2 , 3 , 4 ] and ionic liquids [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. In this work, we develop and apply these methods to a series of molecular liquids (MLs) containing a six-membered aliphatic ring (alkyl-piperidines) and a hydroxyl group as a special feature (see Figure 1 left); we also apply these methods to corresponding ionic liquids (ILs) that also contain a six-membered aromatic ring (alkyl-pyridinium) with the hydroxyl attached to the alkyl chain (see Figure 1 right).…”