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How strong is hydrogen bonding in ionic liquids? Combined x-ray crystallographic, infrared/Raman spectroscopic, and density functional theory study

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Publication date 2013
Author Katsyuba Sergey A., Vener Mikhail V., Zvereva Elena E., Fei Zhaofu, Scopelliti Rosario, Laurenczy Gábor, Yan Ning, Pǎunescu Emilia, Dyson Paul J.,
Project Functionalised ionic liquids for catalysis and other applications
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Original article (peer-reviewed)

Journal Journal of Physical Chemistry B
Volume (Issue) 117(30)
Page(s) 9094 - 9105
Title of proceedings Journal of Physical Chemistry B
DOI 10.1021/jp405255w

Abstract

Hydrogen bonding in ionic liquids based on the 1-(2′-hydroxylethyl)- 3-methylimidazolium cation ([C2OHmim]+) and various anions ([A]-) of differing H-bond acceptor strength, viz. hexafluorophosphate [PF6]-, tetrafluoroborate [BF 4]-, bis(trifluoromethanesulfonimide) [Tf 2N]-, trifluoromethylsulfonate [OTf]-, and trifluoroacetate [TFA]-, was studied by a range of spectroscopic and computational techniques and, in the case of [C2OHmim][PF 6], by single crystal X-ray diffraction. The first quantitative estimates of the energy (EHB) and the enthalpy (-ΔH HB) of H-bonds in bulk ILs were obtained from a theoretical analysis of the solid-state electron-density map of crystalline [C2OHmim] [PF6] and an analysis of the IR spectra in crystal and liquid samples. EHB for OH···[PF6] - H-bonds amounts to ∼3.4-3.8 kcal·mol-1, whereas weaker H-bonds (2.8-3.1 kcal·mol-1) are formed between aromatic C2H group of imidazolium ring and the [PF6]- anion. The enthalpy of the OH···[A]- H-bonds follows the order: [PF6] (2.4 kcal·mol-1) < [BF4] (3.3 kcal·mol-1) < [Tf2N] (3.4 kcal·mol-1) < [OTf] (4.7 kcal·mol-1l) < [TFA] (6.2 kcal·mol-1). The formation of aggregates of self-associated [C2OHmim]+ cations is present in liquid [C2OHmim][PF6], [C2OHmim][BF4], and [C2OHmim][Tf2N], with the energy of the OH···OH H-bonds amounting to ∼6 kcal·mol -1. Multiple secondary interactions in the bulk ILs influence their structure, vibrational spectra, and H-bond strength. In particular, these interactions can blue-shift the stretching frequencies of the CH groups of the imidazolium ring in spite of red-shifting CH···[A] - H-bonds. They also weaken the H-bonding in the IL relative to the isolated ion pairs, with these anticooperative effects amounting to ca. 50% of the EHB value. © 2013 American Chemical Society.
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