Abstract
We show that carboxyl-functionalized ionic liquids (ILs) form doubly hydrogen-bonded cationic dimers (c+=c+) despite the repulsive forces between ions of like charge and competing hydrogen bonds between cation and anion (c+–a−). This structural motif as known for formic acid, the archetype of double hydrogen bridges, is present in the solid state of the IL 1−(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC−CH2−py][NTf2]. By means of quantum chemical calculations, we explored different hydrogen-bonded isomers of neutral (HOOC–(CH2)n–py+)2(NTf2−)2, single-charged (HOOC–(CH2)n–py+)2(NTf2−), and double-charged (HOOC– (CH2)n−py+)2 complexes for demonstrating the paradoxical case of “anti-electrostatic” hydrogen bonding (AEHB) between ions of like charge. For the pure doubly hydrogen-bonded cationic dimers (HOOC– (CH2)n−py+)2, we report robust kinetic stability for n = 1–4. At n = 5, hydrogen bonding and dispersion fully compensate for the repulsive Coulomb forces between the cations, allowing for the quantification of the two equivalent hydrogen bonds and dispersion interaction in the order of 58.5 and 11 kJmol−1, respectively. For n = 6–8, we calculated negative free energies for temperatures below 47, 80, and 114 K, respectively. Quantum cluster equilibrium (QCE) theory predicts the equilibria between cationic monomers and dimers by considering the intermolecular interaction between the species, leading to thermodynamic stability at even higher temperatures. We rationalize the H-bond characteristics of the cationic dimers by the natural bond orbital (NBO) approach, emphasizing the strong correlation between NBO-based and spectroscopic descriptors, such as NMR chemical shifts and vibrational frequencies.
Highlights
The common wisdom that “unlike charges attract, like charges repel” has been recently challenged by Weinhold and Klein [1]
For simple hydroxyl-functionalized ionic liquids, we showed earlier that strong cationic cluster formation prevents crystallization, resulting in supercooled and glassy systems, whereas weak cationic cluster formation completely disappears in the solid state and only hydrogen bonding between cation and anion remains [26,31]
We show that carboxyl-functionalized ionic liquids (ILs) form doubly hydrogenbonded cationic dimers (c+=c+) despite the repulsive forces between ions of like charge and competing hydrogen bonds between cation and anion (c+–a−)
Summary
The common wisdom that “unlike charges attract, like charges repel” has been recently challenged by Weinhold and Klein [1]. The stability of the bisulfate and biphosphate anion dimers was discussed before by Mata et al, but attributed to “electrophilic−nucleophilic interactions” rather than charge transfer as an important factor in H-bonding [9,10,11] These anionic dimers show robust binding wells that are stabilized by broad potential barriers opposing “Coulomb explosion” to separated ions, these (a−=a−) complexes are far away from thermodynamic stability and not accessible by experiment at any pressure and temperature. We suggested to consider anionic dimers (HOOC−(CH2)n−COO−) derived from singly deprotonated dicarboxylic acids, such as adipic acid (n = 4), which is a precursor used in the production of nylon [12] These calculated dimers (a−=a−) show thermodynamic stability for n = 6,7, but ionic liquids including carboxyl-functionalized anions showing this structural feature could not be synthesized so far
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