The chemical nature of the C–H⊕⋯X− (X=Cl or Br) interaction in imidazolium halide ionic liquids

Abstract

The C-H...X (X=Cl or Br) interaction is traditionally characterized as a relatively weak interaction. However, this interaction becomes very strong in the imidazolium-based halide ionic liquids [J. Phys. Chem. 123, 174501 (2005)]. This strong interaction had been attributed to the electrostatic interaction between the imidazolium cation and the halide anion. In this paper, the chemical nature of the (plus sign in circle)C-H...Cl(-) and ( plus sign in circle)C-H...Br(-) interactions is investigated by atoms in molecules (AIM) and natural bond orbital (NBO) analyses. The AIM calculations indicate that in the EmimX complexes, the (plus sign in circle)C-H...Cl(-) and (plus sign in circle)C-H...Br(-) interactions have some covalent character, especially the (plus sign in circle)C-H...Cl(-) interaction. Mulliken, ChelpG charge, and natural bond orbital population analyses for these two kinds of interactions indicate that the charge transfer is important in the interaction of the cation with the anion. In addition, the NBO analysis demonstrated that the stabilization energy is due to an n-->sigma(C-H) (*) orbital interaction. However, in the Emim2X and Emim3X complexes, the calculated results suggested a dominant electrostatic character for the (plus sign in circle)C-H...Cl(-) and (plus sign in circle)C-H...Br(-) interactions.

4,5‐Disubstituted N,N′‐Di‐tert‐alkyl Imidazolium Salts: New Synthesis and Structural Features

“…[23,26] This is probably indicative of very attenuated interactions of the acidic hydrogen of the imidazolium salt with anions and solvents; [27] these interactions are strongly Scheme 1. A new stepwise method for synthesis of the imidazolium salts 1.…”

Preparation, cation-anion interactions and physicochemical properties of ether-functionalized imidazolium ionic liquids

“…A shift to lower frequencies, corresponding to a reduced strength of the aromatic C-H bond, is the consequence of an increased hydrogen bonding participation. 28,29 The value of 3006 cm -1 for [C-H 2 ] corresponds to a very strong hydrogen bonding interaction. Theoretical calculations show that the H 2 proton bears the largest positive charge of the aromatic imidazolium protons and is, therefore, the strongest hydrogen bond donor.…”

“…An infrared spectroscopy study of the ILs 2-4 was performed to obtain information about the inter-ionic hydrogen bonding interactions. [28][29][30] The imidazolium ring proton numbers used in this work are shown in Figure 3.…”

“…An infrared spectroscopy study of the ILs 2-4 was performed to obtain information about the inter-ionic hydrogen bonding interactions. [28][29][30] The imidazolium ring proton numbers used in this work are shown in Figure 3.The IR spectral data of the aromatic C-H stretching region between 3200 and 3000 cm -1 are presented in Table 1. Methanesulfonate RTIL 2a showed three stretching frequencies at 3147, 3096 and 3006 cm -1 , respectively.…”

Notes on the Asymmetric Hydrogenation of Methyl Acetoacetate in Neoteric Solvents

“…The maximum of acetals formation was in all cases higher in the presence of imidazolium cation. It might be attributed to the hydrogen acidic properties of imidazolium [42,43]. In spite of the lower acidity of IL in comparison with methanol the imidazolium based IL are more acidic than ammonium.…”