Investigating Local Interactions Between Ions of Like Charge in Carboxyl-Functionalized Ionic Liquids

Carboxyl-functionalized ionic liquids (ILs) exhibit three types of hydrogen bonding: i) single hydrogen bonds between cation and anion, ii) single and iii) double hydrogen bonds between cations. Cation-cation hydrogen bonds must overcome the repulsive Coulomb forces between the ions of like charge. Herein, we investigate the structures adopted by the cryogenically cooled ternary complexes consisting of two cations and one anion (NTf₂^-) by analyzing their size- and isomer-selective vibrational band patterns. These structural motifs are followed as a function of alkyl chain length in the pyridinium carboxylic acids [HOOC-(CH₂)_n-py] with n=1-9. Comparison between the carbonyl stretching bands in the temperature-dependent bulk liquid FTIR and cold ion vibrational spectra confirm that longer chain lengths (n>5) allow the coulombic repulsion between the cationic charge centers to be overcome such that the cations engage in the double hydrogen bond (DHB) motif linking the acid groups. The evolution of the OH bands upon H/D substitution reveal formation of at least two isomers for the n=6 clusters which also display significant differences arising in the aromatic CH stretching bands. Comparison to theoretical calculations indicates chain length dependent structures that account for the observed spectral signatures of the two isomers.