Protic Ionic Liquids DOI: 10.1002/anie.200806224 Hydrogen Bonding in Protic Ionic Liquids: Reminiscent of Water** Koichi Fumino, Alexander Wulf, and Ralf Ludwig* Ionic Liquids (ILs) constitute a promising class of technolog- ically useful and fundamentally interesting materials. Poten- tial applications include novel synthesis, electrolyte devices, photochemical cells, separations, and catalysis. [1–3] Protic ionic liquids (PILs) are a subset of ionic liquids formed by combination of equimolar amounts of a Brønsted acid and a Brønsted base. [4, 5] The key property that distinguishes PILs from other ILs is the proton transfer from the acid to the base, leading to the presence of proton-donor and proton-acceptor sites, which can be used to build a hydrogen-bonded network. PILs have a number of unique properties compared to other ILs, just as water is different from “normal” molecular liquids. Thus it has been suggested that the hydrogen bonds between ammonium cations and nitrate anions, for example, induce a network structure which in some respects mimics the three- dimensional hydrogen-bonded network of water. [6–9] In principle, these H-bond networks can be studied by measuring the low-frequency range below 300 cm À1 (9 THz) using far-IR or Raman spectroscopy. However, only a few infrared studies of ILs have been performed in the far- infrared regime. [10–12] Experimental difficulties arise from the very low intensities of infrared sources. Herein, we demon- strate that these far-IR spectra can be measured for PILs, which show characteristic intramolecular bending modes above 250 cm À1 and intermolecular stretching and bending vibrational bands of hydrogen bonds between 50 and 250 cm À1 . Interestingly, the intermolecular vibrational bands of the PILs show the same structure as the recently measured connectivity bands of water. This finding suggests that PILs form a three-dimensional hydrogen-bonded network that very much resembles that of water. The low-frequency spectra for the neat protic ionic liquids ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and dimethylammonium nitrate (DMAN) in the range between 30 and 600 cm À1 are shown in Figure 1. Overall, the spectra show significant differences but also share some common features. Because we kept the anion (NO 3 À ) constant, the differences can only stem from weak intramolecular vibrations of the various cations and/or from specific cation–anion interactions. Strong support for the interpretation of the low-frequency vibrational bands is provided by DFT calculations of the aggregates ([alkyl ammonium][NO 3 ]) x , where x is the number of ion pairs contributing to the overall cluster. In Figure 2 the measured spectrum of DMAN is shown along with the calculated vibrational frequencies of the clusters with x = 1, 2, 3, 4, 6. In addition, in Figure 3, all low-frequency vibrational spectra are deconvoluted into Voigt functions. The main features of the measured spectra of DMAN (Figure 3 c) are reproduced by the calculated and deconvoluted vibrational bands. The bands above 250 cm À1 can be assigned to the intramolecular bending and torsional modes of the respective Figure 1. Low-frequency vibrational FTIR spectra of the protic ionic liquids ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and dimethylammonium nitrate (DMAN) measured at 353 K. Figure 2. Measured low-frequency vibrational FTIR spectrum of dime- thylammonium nitrate (DMAN) at 353 K compared to the vibrational modes of the corresponding PIL clusters [(DMA)(NO 3 )] x with x = 1, 2, 3, 4, and 6 calculated by DFT at the B3LYP/6-31 + G* level of theory. The major vibrational bands are in agreement with the calculated frequencies, which are corrected for the harmonic approximation. [*] Dr. K. Fumino, Dipl.-Chem. A. Wulf, Prof.Dr. R. Ludwig Institut für Chemie, Abteilung Physikalische Chemie, Universität Rostock, Dr.-Lorenz-Weg 1, 18051 Rostock (Germany) E-mail: ralf.ludwig@uni-rostock.de Prof. Dr. R. Ludwig Leibniz-Institut für Katalyse an der Universität Rostock Albert-Einstein-Strasse 29a, 18059 Rostock (Germany) [**] This work was supported by the German Science Foundation (DFG) priority programme SPP 1191 as well as the Pact for Research and Innovation of the Federal Ministry of Education and Research/ Leibniz Science Association. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200806224. Communications 3184  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2009, 48, 3184 –3186