Structural Motifs in Cold Ion Complexes of Carboxy-Functionalized Ionic Liquids Explored by Multi-Nanosecond Extended Tight-Binding Replica Exchange Molecular Dynamics Simulations

Strong, local, and directional hydrogen bonds (HBs) govern the structures and arrangements of carboxy-functionalized ionic liquids. The analysis of infrared spectra in the CO stretching region has revealed doubly hydrogen-bonded cationic dimers (c⁺=c⁺) in the liquid, resembling the archetype HB motif known from carboxylic acids. The like-charge doubly hydrogen-bonded ion pairs are also present in the crystal structure, surviving the phase transition into the liquid state, and are still present in (2,1) complexes in the gas phase. The spectral signatures of cryogenic ion vibrational predissociation spectroscopy showed that no other isomers of (2,1) complexes are present at 40 K. To study the unbiased thermal equilibrium of (2,1) complexes at experimental conditions, we perform extended tight-binding replica exchange molecular dynamics (xTB-REMD) simulations of ([HOOC-(CH₂)-py]⁺)₂([NTf₂]^-)₁, for temperatures ranging from 30 to 545 K employing the GFN1-xTB method. The REMD simulations consisting of 14 replicas allow for frequent conformational transitions at high temperatures and thus ensure proper conformational sampling at the experimental conditions. We demonstrate that the (c⁺=c⁺) HB motif exclusively dominates below about 80 K. In the low-temperature (c⁺=c⁺) configurations, the [NTf₂]^- anion is sandwiched between the two pyridinium rings of the doubly hydrogen-bonded cations in a clamp-like fashion. With increasing temperature, the (c⁺=c⁺) configurations transform into a state mostly devoid of any HBs at a transition temperature of 126 K. The large transition enthalpy of -9.4 kJ mol^-1 underlines the particular strength of the (c⁺=c⁺) HB motif despite the presence of strong repulsive Coulomb forces between the two cations.