Ring Hydrogen Atoms Research Articles

Structural analysis of an amino acid ionic liquid: Bulk and electrical double layer

The interfacial structures and macroscopic properties of electrical double layer for 1-ethyl-3-methyl-imidazolium phenyl alanine ([EMIM][PHE]), as an amino acid ionic liquid (AAIL), have been investigated by molecular dynamics simulation and compared with conventional [EMIM][PF6] IL. Results of bulk state showed that hydrogen atoms of ring in [EMIM]+ are capable to form hydrogen bond with the nitrogen and oxygen of [PHE]−. The number of these hydrogen bonds enhances if IL is introduced to graphene sheet. Surprisingly, it was observed that anion-anion hydrogen bond is also probable for [EMIM][PHE]. Introducing the AAIL to graphene does not affect number of this anion-anion hydrogen bond. Being in touch with graphene or not, the main correlation regime between anion and cation is the tail-ring for studied systems. It was observed that phenyl ring of [PHE]− acts as an inhibitor for [EMIM]+ to form π-π stacking with graphene. This behavior leads to higher accumulation of [PHE]− than [EMIM]+ in the first layer. In the current study, a responding electrical potential for an assumed supercapacitor was also introduced. Responding potential of [EMIM][PHE] is lower than [EMIM][PF6].

Can Variations of 1 H NMR Chemical Shifts in Benzene Substituted with an Electron-Accepting (NO2 )/Donating (NH2 ) Group be Explained in Terms of Resonance Effects of Substituents?

The classical textbook explanation of variations of 1 H NMR chemical shifts in benzenes bearing an electron-donating (NH2 ) or an electron-withdrawing (NO2 ) group in terms of substituent resonance effects was examined by analyzing molecular orbital contributions to the total shielding. It was found that the π-electronic system showed a more pronounced shielding effect on all ring hydrogen atoms, relative to benzene, irrespective of substituent +R/-R effects. For the latter, this was in contrast to the traditional explanations of downfield shift of nitrobenzene proton resonances, which were found to be determined by the σ-electronic system and oxygen in-plane lone pairs. In aniline, the +R effect of NH2 group can be used to fully explain the upfield position of meta-H signals and partly the upfield position of para-H signals, the latter also being influenced by the σ-system. The position of the lowest frequency signal of ortho-Hs was fully determined by σ-electrons.

The crystal structures of 1-(4-halo-2,3,5,6-tetrafluorophenyl)-3-benzylimidazolium bromides: The relative importance of anion–π, lone pair–π, π[sbnd]π stacking and halogen bonding interactions

The crystal structures of analogous 1-(4-halo-2,3,5,6-tetrafluorophenyl)-3-benzylimidazolium bromide salts have been determined by single crystal X-ray diffraction. All three structures contain similar charge-assisted hydrogen bonding between the acidic hydrogen atoms of the imidazolium ring and bromide anions, but different structures arise because of the relative importance of other interactions, in particular the halogen bonding between bromide and the covalently bonded halogen atoms. 1-(4-Chloro-2,3,5,6-tetrafluorophenyl)-3-benzylimidazolium bromide (1) comprises columns of π–π stacked phenyl and chlorotetrafluorophenyl rings in three almost orthogonal directions, 1-(4-bromo-2,3,5,6-tetrafluorophenyl)-3-benzylimidazolium bromide (2) contains columns of alternating bromotetrafluorophenyl rings and bromide anions displaying anion–π interactions, and 1-(4-iodo-2,3,5,6-tetrafluorophenyl)-3-benzylimidazolium bromide (3) contains columns of alternating iodotetrafluorophenyl rings and iodine atoms displaying lone pair–π interactions.

MD study of structure and dynamic properties of the 1-n-alkyl-3-methylimidazolium tris(perfluoroalkyl)trifluorophosphate ionic liquids

In this study, we applied classical molecular dynamics (MD) simulations to understand the structure, dynamics and transport properties of the room-temperature 1-alkyl-3-methylimidazolium family with tris(perfluoroalkyl)trifluorophosphate ionic liquids, abbreviated as [Cnmim][eFAP]. Calculated densities of ionic liquids agree well with experimental data. Local structures were characterized by studying the center-of-mass (COM) and partial site-site RDFs, combined distribution functions (CDFs) and dihedral angles distribution of n-alkyl side chains in the imidazolium cations. The hydrogen bonding between Fluor atoms of anions and hydrogen atoms of imidazolium ring of cations were studied by contour maps of CDFs. Also, dynamical properties of these ILs such as mean-square displacement (MSDs) for center-of-mass of ions, ionic diffusion coefficients and the cationic transference numbers are calculated by simulation in the NVT ensemble. Results show that the length of alkyl-side chain of cation is the major factor to affect these properties.

Molecular Dynamics Simulations of Amide Functionalized Imidazolium Bis(trifluoromethanesulfonyl)imide Dicationic Ionic Liquids.

In the present study, the structure and dynamics of three dicationic ionic liquids (DILs) with a functional amide group in the imidazolium ring with bis(trifluoromethanesulfonyl)imide, [TFSI]- anion has been studied by molecular dynamics (MD) simulations. Densities, radial distribution functions (RDFs), combined distribution functions (CDFs), spatial distribution functions, mean-square displacements (MSD), and self-diffusivities for the ions have been calculated from the MD simulations. The calculated densities for [C4(amim)2][TFSI]2 at different temperatures agreed well with the experimental values. The calculated RDFs and CDFs show that the anions are well organized around the amide group and imidazolium rings and the favorite sites of interaction of the [TFSI]- ion are the hydrogen atoms of the amide group and hydrogen atoms of the imidazolium ring of the cation. The calculated MSDs indicated that the diffusion coefficients of the studied DILs are 1 order of magnitude smaller than those of DILs with a comparable molar mass.

Probing the origins of vibrational mode specificity in intramolecular dynamics through picosecond time-resolved photoelectron imaging studies.

We have studied the intramolecular dynamics induced by selective photoexcitation of two near-isoenergetic vibrational states in S1p-fluorotoluene using picosecond time-resolved photoelectron imaging. We find that similar dynamics ensue following the preparation of the 13111 and 7a111 states that lie at 1990 cm-1 and 2026 cm-1, and that these dynamics are mediated by a single strongly coupled doorway state in each case. However, the lifetimes differ by a factor of three, suggesting an influence of the vibrational character of the modes involved. Our results clearly show the contribution of torsion-vibration coupling to the dynamics; this is further corroborated by comparison with the 7a111 state in S1p-difluorobenzene, which lies at 2068 cm-1. We invoke a model in which van der Waals interactions between methyl hydrogen atoms and nearby ring carbon and hydrogen atoms leads to mixing of the vibrational and torsional states. This model predicts that enhanced torsion-vibration coupling occurs when mode 7a is excited, consistent with our observations.

A New Look at the Halogenation of Porphyrins

The present study describes the selective halogenation at the β- or meso-position of the porphyrin macrocycle. The reactions of deuteroporphyrin IX dimethyl ester, mesoporphyrin III dimethyl ester and their Cu(II) and Ni(II) complexes with N-bromosuccinimide or phenylselenyl chloride were investigated. It could be observed that when the bromination of deuteroporphyn IX dimethyl ester using NBS took place, the isolated products were the result of an electrophilic substitution in β-free porphyrin ring positions. Employing the same reagent for halogenating mesoporphyrin III dimethyl ester, which does not possess β-free position, different derivatives were obtained from the allylic bromination of the ethyl side chain. When phenylselenyl chloride was used as halogenating agent with deuteroporphyn IX dimethyl ester or its Cu(II) complex, in both cases the replacement of β-free hydrogen atoms by phenylselenyl group was afforded. When the Ni(II) mesoporphyrin III dimethyl ester was used the desired and selective replacement of meso hydrogen atoms of aromatic ring by chlorine atoms was obtained. The structural assignment of the porphyrin derivatives thus obtained was performed by high-resolution mass spectrometry and detailed analysis of the NMR spectra. Keywords: Halogenation, deuteroporphyrin IX dimethyl ester, mesoporphyrin III dimethyl ester.

Theoretical and infrared investigation of 2-acetylpyridine isolated in solid nitrogen and in neat condensed phases

The geometries of the two conformers of 2-acetylpyridine (2AP) were optimized at the DFT/B3LYP/6–311++G(d,p) level of approximation, and their relative energy and interconversion barrier evaluated. Both conformers were found to belong to the Cs symmetry point group, with conformer cis (with the methyl group and the ring nitrogen atom located in the same side of the molecule) being considerably stabilized over the trans form. The cis conformer exhibits stabilizing interactions between the ortho ring hydrogen atom and the carbonyl oxygen, as well as between the methyl out-of-the-plane hydrogen atoms and the ring nitrogen atom. In the less stable trans conformer (ΔE(trans-cis) = 26.3 kJ mol−1) these stabilizing interactions are replaced by repulsive interactions between the oxygen and nitrogen lone electron pairs and between the ring ortho and methyl out-of-the-plane hydrogen atoms. The energy barrier between the two conformers amounts to 30.7 kJ mol−1 in the cis → trans direction (4.4 kJ mol−1 in the reverse direction). In agreement with the theoretical data, in a cryogenic N2 matrix prepared from the room temperature 2AP gas phase, only the most stable cis conformer was observed. The IR spectra of 2AP isolated in solid N2, and those for the low temperature amorphous and crystalline neat solid states of the compound were recorded, and correlations between the spectroscopic data and the strength and nature of the dominant intermolecular interactions in 2AP neat condensed phases were evaluated. The analysis of the experimental vibrational data was supported by theoretical calculation of harmonic and anharmonic frequencies and IR intensities obtained at the DFT/B3LYP/6–311++G(d,p) level of theory.

Single-Crystal Structures and Typical Hydrogen-Bonding Motifs of Supramolecular Cocrystals Containing 1,4-Di(1H-imidazol-1-yl)benzene

Eight new supramolecular crystals of 1,4-di(1H-imidazol-1-yl)benzene (DIB) have been prepared using water, 4,4′-biphenol, terephthalic acid, isophthalic acid, succinic acid, and 1,2,4,5-tetrafluoro-3,6-diiodobenzene. These materials were subsequently characterized by single crystal X-ray analysis. The results revealed that one-dimensional infinite chains were formed in these crystals through hydrogen bonding or halogen bonding interactions. The hydrogen atoms of the imidazole ring in DIB were found to be involved in three kinds of hydrogen-bonding ring motifs. These motifs could play an important role in the stabilization of the crystals and could therefore be beneficial to DIB in terms of its role as a good building block for supramolecular crystals.

Self-Assembly of Cations in Aqueous Solutions of Hydroxyl-Functionalized Ionic Liquids: Molecular Dynamics Studies.

The effect of presence of a hydroxyl-functionalized alkyl chain of varying carbon number on the self-assembly of cations in aqueous solutions of 1-(n-hydroxyalkyl)-3-decylimidazolium bromide (where the alkyl groups are ethyl, butyl, heptyl, and decyl) has been studied using atomistic molecular dynamics simulations. Spontaneous self-assembly of cations to form aggregates with hydrophobic core and hydrophilic surface is observed. The shape of the aggregates changes from quasispherical in the case of cations with hydroxyheptyl or smaller substituent chain, to a thin film like intercalated aggregate in the case of cations with hydroxydecyl chain. Cations with hydroxydecyl substituent chain exhibit long-range spatial correlations, and the anions are associated with cations to a greater extent due to the higher surface charge density of the aggregate. The ordered film like aggregate is stabilized by the dispersion interactions between the intercalated substituent chains and the intermolecular hydrogen bonds formed between the alkoxy oxygen atoms and the hydrogen atoms of the imidazolium ring. The cations form less compact aggregates with lower aggregation number than their nonhydroxyl analogues in the corresponding aqueous solutions. The intracationic and aggregate structures are governed by the length of the hydroxyalkyl chain.

Theoretical study on the activity of hydrogen atom of imidazolium ring in ionic liquids

The double-bond isomerization of 1-pentene catalyzed by [Emim]BF4 ionic liquid and the interaction relationship of the [Emim]BF4 ion pair have been studied by using density functional theory (DFT) at the B3LYP/6-31G(d,p) level. The obtained results reveal that the [Emim]+ and BF4− ions exist in the form of the ion pairs in the ionic liquid system, and only two stable structures are obtained. The isomerization process of 1-pentene on [Emim]BF4 ionic liquids proceeds via a concerted reaction pathway. The hydrogen atom transfer is accompanied by the migration of the intramolecular double bond. The ionic liquid acts as a proton donor, while it also favors the proton abstraction. The adsorption energies of 1-pentene on the 2-position and 4-position hydrogen atoms of the imidazolium ring are −11.32 and −7.42kJ/mol, respectively. The apparent activation energies of the double-bond isomerization for 2-position hydrogen atom and 4-position hydrogen atom are 196.91 and 214.64kJ/mol, respectively. Those indicate that two kinds of hydrogen atoms of the imidazolium ring have both catalytic activities, but the activity of 2-position hydrogen atom is much higher than the one of 4-position hydrogen atom.

Synthesis, characterization and comparative study of a series of fluorinated Schiff bases containing different orientation [sbnd]CH[dbnd]N[sbnd] spacers

Investigation of the crystal structures of three pairs of multi-fluorinated compounds with bridge-flipped isomeric character, which on the molecular level differ only in the orientation of the bridging bond CHN connecting two larger parts of the molecule, offers a useful context for the examination and evaluation of supramolecular assembly in the case. The results show that the orientation of the bridging bond CHN determines the conformation of six organic molecules 1a–b, 2a–b and 3a–b, thus further influencing intermolecular weak interactions and final supramolecular arrangements. Accordingly, each molecule of six multi-fluorinated compounds as supramolecular building block unit, via strong intermolecular interactions, including interactions between fluorine atoms and aromatic ring hydrogen atoms, between fluorine atoms, and between hydroxyl groups and fluorine atoms, as well as interaromatic π⋯π stacking interactions etc., constructs a series of different and attractive crystal packings. These results demonstrate that by changing CHN orientation, we can obtain different hydrogen-bonded supramolecular structures through different interactions.

Non-covalent interactions in ionic liquid ion pairs and ion pair dimers: a quantum chemical calculation analysis.

Ionic liquids (ILs) being composed of bulky multiatomic ions reveal a plethora of non-covalent interactions which determine their microscopic structure. In order to establish the main peculiarities of these interactions in an IL-environment, we have performed quantum chemical calculations for a set of representative model molecular clusters. These calculations were coupled with advanced methods of analysis of the electron density distribution, namely, the quantum theory of atoms in molecules (QTAIM) and the non-covalent interaction (NCI; J. Am. Chem. Soc., 2010, 132, 6499) approaches. The former allows for profound quantitative characterization of non-covalent interactions between atoms while the latter gives an overview of spatial extent, delocalization, and relative strength of such interactions. The studied systems consist of 1-butyl-3-methylimidazolium (Bmim(+)) cations and different perfluorinated anions: tetrafluoroborate (BF4(-)), hexafluorophosphate (PF6(-)), trifluoromethanesulfonate (TfO(-)), and bis(trifluoromethanesulfonyl)imide (TFSI(-)). IL ion pairs and ion pair dimers were considered as model structures for the neat ILs and large aggregates. Weak electrostatic hydrogen bonding was found between the anions and the imidazolium ring hydrogen atoms of cations. Weaker but still appreciable hydrogen bonding was also noted for hydrogen atoms adjacent to the imidazolium ring alkyl groups of Bmim(+). The relative strength of the hydrogen bonding is higher in BmimTfO and BmimBF4 ILs than in BmimPF6 and BmimTFSI, whereas BmimTfO and BmimTFSI reveal higher sensitivity of hydrogen bonding at the different hydrogen atoms of the imidazolium ring.

Segregation of ions at the interface: molecular dynamics studies of the bulk and liquid-vapor interface structure of equimolar binary mixtures of ionic liquids.

The structures of three different equimolar binary ionic liquid mixtures and their liquid-vapor interface have been studied using atomistic molecular dynamics simulations. Two of these binary mixtures were composed of a common cation 1-n-butyl-3-methylimidazolium and varying anions (chloride and hexafluorophosphate in one of the mixtures and chloride and trifluoromethanesulfonate in the other) and the third binary mixture was composed of a common anion, trifluoromethanesulfonate and two imidazolium cations with ethyl and octyl side chains. Binary mixtures with common cations are found to be homogeneous. The anions are preferentially located near the ring hydrogen atoms due to H-bonding interactions. Segregation of ions is observed at the interface with an enrichment of the liquid-vapor interface layer by longer alkyl chains and bigger anions with a distributed charge. The surface composition is drastically different from that of the bulk composition, with the longer alkyl tail groups and bigger anions populating the outermost layer of the interface. The longer alkyl chains of the cations and trifluoromethanesulfonate anions with a smaller charge density show orientational ordering at the liquid-vapor interface.

Probing structural patterns of ion association and solvation in mixtures of imidazolium ionic liquids with acetonitrile by means of relative (1)H and (13)C NMR chemical shifts.

Mixtures of ionic liquids (ILs) with polar aprotic solvents in different combinations and under different conditions (concentration, temperature etc.) are used widely in electrochemistry. However, little is known about the key intermolecular interactions in such mixtures depending on the nature of the constituents and mixture composition. In order to systematically address the intermolecular interactions, the chemical shift variation of (1)H and (13)C nuclei has been followed in mixtures of imidazolium ILs 1-n-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4), 1-n-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6), 1-n-butyl-3-methylimidazolium trifluoromethanesulfonate (BmimTfO) and 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimTFSI) with molecular solvent acetonitrile (AN) over the entire composition range at 300 K. The concept of relative chemical shift variation is proposed to assess the observed effects on a unified and unbiased scale. We have found that hydrogen bonds between the imidazolium ring hydrogen atoms and electronegative atoms of anions are stronger in BmimBF4 and BmimTfO ILs than those in BmimTFSI and BmimPF6. Hydrogen atom at position 2 of the imidazolium ring is substantially more sensitive to interionic hydrogen bonding than those at positions 4-5 in the case of BmimTfO and BmimTFSI ILs. These hydrogen bonds are disrupted upon dilution in AN due to ion dissociation which is more pronounced at high dilutions. Specific solvation interactions between AN molecules and IL cations are poorly manifested.

Unconventional hydrogen bonding to organic ions in the gas phase: stepwise association of hydrogen cyanide with the pyridine and pyrimidine radical cations and protonated pyridine.

Equilibrium thermochemical measurements using the ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes for the stepwise association of HCN molecules with the pyridine and pyrimidine radical cations forming the C5H5N(+·)(HCN)n and C4H4N2 (+·)(HCN)n clusters, respectively, with n = 1-4. For comparison, the binding of 1-4 HCN molecules to the protonated pyridine C5H5NH(+)(HCN)n has also been investigated. The binding energies of HCN to the pyridine and pyrimidine radical cations are nearly equal (11.4 and 12.0 kcal/mol, respectively) but weaker than the HCN binding to the protonated pyridine (14.0 kcal/mol). The pyridine and pyrimidine radical cations form unconventional carbon-based ionic hydrogen bonds with HCN (CH(δ+)⋯NCH). Protonated pyridine forms a stronger ionic hydrogen bond with HCN (NH(+)⋯NCH) which can be extended to a linear chain with the clustering of additional HCN molecules (NH(+)⋯NCH··NCH⋯NCH) leading to a rapid decrease in the bond strength as the length of the chain increases. The lowest energy structures of the pyridine and pyrimidine radical cation clusters containing 3-4 HCN molecules show a strong tendency for the internal solvation of the radical cation by the HCN molecules where bifurcated structures involving multiple hydrogen bonding sites with the ring hydrogen atoms are formed. The unconventional H-bonds (CH(δ+)⋯NCH) formed between the pyridine or the pyrimidine radical cations and HCN molecules (11-12 kcal/mol) are stronger than the similar (CH(δ+)⋯NCH) bonds formed between the benzene radical cation and HCN molecules (9 kcal/mol) indicating that the CH(δ+) centers in the pyridine and pyrimidine radical cations have more effective charges than in the benzene radical cation.

Effect of metal doping, boron substitution and functional groups on hydrogen adsorption of MOF-5: A DFT-D study

In the present work, adsorption of hydrogen molecules over a metal organic framework (MOF-5) has been investigated by using first principles density functional theory (DFT). Different strategies have been applied for improving hydrogen storage, i.e. metal doping, boron substitution and functionalization. The metal atoms used for enhancing hydrogen adsorption include Li, Ca and Sc. It is found that the binding energy between these metal atoms and MOF is not enough to prevent clustering. Therefore a number of carbon atoms are substituted by boron atoms and it is indicated that boron substitution enhances the binding energies, significantly. Also the results reveal that boron substituted MOF doped by Sc atom has the maximum capacity of hydrogen storage compared to other dopants, with binding energies all in the favorable range of 0.2–0.4eV. On the other hand the effect of functionalization of MOF with halogen atoms is investigated. One and four hydrogen atoms of benzene ring in MOF are replaced by F, Cl and Br atoms. It is found that only MOF functionalized with four Cl and Br atoms improve the hydrogen binding energy. MOF containing four Cl atoms is more promising structure for hydrogen storage in comparison to MOF functionalized with Br atoms because of lower mass of Cl atoms and more favorable hydrogen binding energies.

Competing benzyl cation transfers in the gas-phase fragmentation of the protonated benzyl phenylalaninates

In this study, the competing benzyl cation transfer reactions have been explored by investigating the gas phase chemistry of the protonated benzyl phenylalaninates. Protonation at the carboxylic O atom results in the breakage of the ester CO bond to afford the benzyl cation, which undergoes the competing migration to the amino N atom or the phenyl ring C atom. Both the amino and the phenyl ring hydrogen atoms can be activated to be mobile due to the electrophilic attack of the transferring benzyl cation, and migration of the activated hydrogen atom to the carboxylic hydroxyl leads to (H2O+CO) elimination of the precursor ion. Interestingly, it is much more preferred for the benzyl cation to transfer to the phenyl ring via the amino N, leading to the stepwise benzyl cation transfer, albeit the amino N atom contains more nucleophilic affinity. The mechanistic processes have been confirmed by the MS3 spectra data, along with D-labeling experiments and theoretical calculations.

The pyrolysis of 2-methylfuran: a quantum chemical, statistical rate theory and kinetic modelling study

Due to the rapidly growing interest in the use of biomass derived furanic compounds as potential platform chemicals and fossil fuel replacements, there is a simultaneous need to understand the pyrolysis and combustion properties of such molecules. To this end, the potential energy surfaces for the pyrolysis relevant reactions of the biofuel candidate 2-methylfuran have been characterized using quantum chemical methods (CBS-QB3, CBS-APNO and G3). Canonical transition state theory is employed to determine the high-pressure limiting kinetics, k(T), of elementary reactions. Rice-Ramsperger-Kassel-Marcus theory with an energy grained master equation is used to compute pressure-dependent rate constants, k(T,p), and product branching fractions for the multiple-well, multiple-channel reaction pathways which typify the pyrolysis reactions of the title species. The unimolecular decomposition of 2-methylfuran is shown to proceed via hydrogen atom transfer reactions through singlet carbene intermediates which readily undergo ring opening to form collisionally stabilised acyclic C5H6O isomers before further decomposition to C1-C4 species. Rate constants for abstraction by the hydrogen atom and methyl radical are reported, with abstraction from the alkyl side chain calculated to dominate. The fate of the primary abstraction product, 2-furanylmethyl radical, is shown to be thermal decomposition to the n-butadienyl radical and carbon monoxide through a series of ring opening and hydrogen atom transfer reactions. The dominant bimolecular products of hydrogen atom addition reactions are found to be furan and methyl radical, 1-butene-1-yl radical and carbon monoxide and vinyl ketene and methyl radical. A kinetic mechanism is assembled with computer simulations in good agreement with shock tube speciation profiles taken from the literature. The kinetic mechanism developed herein can be used in future chemical kinetic modelling studies on the pyrolysis and oxidation of 2-methylfuran, or the larger molecular structures for which it is a known pyrolysis/combustion intermediate (e.g. cellulose, coals, 2,5-dimethylfuran).

How Can a Carbene be Active in an Ionic Liquid?

The solvation of the carbene 1-ethyl-3-methylimidazole-2-ylidene in the ionic liquid 1-ethyl-3-methylimidazolium acetate was investigated by ab initio molecular dynamics simulations in order to reveal the interaction between these two highly important classes of materials: N-heterocyclic carbenes with superb catalytic activity and ionic liquids with advantageous properties as solvents and reaction media. In contrast to previously published data on analogous systems, no hydrogen bond is observed between the hypovalent carbon atom and the most acidic ring hydrogen atoms, as these interaction sites of the imidazolium ring are predominantly occupied by the acetate ions. Keeping the carbene away from the ring hydrogen atoms prevents stabilization of this reactive species, and hence any retarding effect on subsequent reactions, which explains the observed high reactivity of the carbene in acetate-based ionic liquids. Instead, the carbene exhibits a weaker interaction with the methyl group of the imidazolium cation by forming a hitherto unprecedented kind of C⋅⋅⋅H-C hydrogen bond. This unexpected finding not only indicates a novel kind of hydrogen bond for carbenes, but also shows that such interaction sites of the imidazolium cation are not limited to the ring hydrogen atoms. Thus, the results give the solute-solvent interactions within ionic liquids a new perspective, and provide a further, albeit weak, site of interaction to tune in order to achieve the desired environment for any dissolved active ingredient.