The role of isotope effects: From ionic to molecular liquids

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The question sometimes arises whether a hydrogen (H) bond is stronger or weaker than the equivalent deuterium (D) bond. Protein folding dynamics, reactions in atmospheric chemistry or solvation behaviour are strongly affected by the different strength of these interactions. Hydrogen and deuterium are electronically identical and the differences are solely associated with the masses, primarily due to the changing vibrational frequencies, often favoring the D bond. For H and D bonds in neutral and ionic water complexes the effect is, however, the opposite. Zero-point vibrational energies (ZPVE) stabilize the D bonds in the water dimer, whereas they weaken the bonds in the cationic Zundel-Ion. The situation becomes more difficult to analyse in solution, where the “bath” of surrounding solvent molecules might alter the preference for D bonding.The effect of deuteration is almost unknown for ionic liquids (ILs), although these materials can show a broad variety of H/D bonding between opposite- and like-charged ions, and thus might serve as a model system for studying isotope effects depending on the charge of the involved constituents. Here, we chose carboxyl-functionalized ionic liquids, which are capable of forming three different types of H/D bonds: double and single H/D bonds between the cations, and single H/D bonds between cation and anion. The strength of H and D bonds strongly depends on the attractive or repulsive Coulomb forces between the ions. By increasing the alkyl chain length, we can reduce the repulsive Coulomb forces, while strengthening the H and D bonds. The isotope effects in these ionic complexes should then be similar to those in phenyl alkanoic acids, the molecular mimics of the IL cations used here. We address the following questions by means of IR and NMR spectroscopy, neutron diffraction and cryogenic ion vibrational (CIV) spectroscopy in the gas phase, along with supporting DFT calculations: Is the D bond in the carboxyl groups of the ILs and the phenyl alkanoic acids stronger than the corresponding H bonds and why? Does it matter whether the charge of the H or D bonded species is positive or negative? How is the relative stability of the H and D bonds affected by cooperativity? How does entropy and temperature influence H and D bonding? Finally, how strongly are structure and dynamics affected, if the cations are deuterated at different positions that are not involved in hydrogen bonding? Our studies will shed new light on the isotope effects and explain how they change from an ionic to a molecular liquid.