Unconventional Hydrogen Bonds Research Articles

Synergistic Effects of Unconventional Hydrogen Bonds and π-Stacking Interaction and Their Excited-State Dependence: The Origin of Unusual Photophysical Properties of Aromatic Thioketones in Acetonitrile and Hydrocarbons.

It has been established experimentally that aromatic thioketones possess several inherently unique photophysical properties, some of which are highly sensitive even to common hydrocarbon solvents. However, the deeper reasons and the underlying mechanisms remain unclear up to date. In this study, the multistate complete active space second-order perturbation theory (MS-CASPT2) has been utilized to investigate the five lowest-lying electronic states (S0, T1, S1, T2, and S2) of 4H-1-benzopyran-4-thione (BPT) in acetonitrile and hydrocarbons. The results show that the S1, T1, and T2 states of BPT are close in energy so that the T2-state-mediated S1 → T2 → T1 and T1 → T2 → S1 transitions could occur in tens of picoseconds, which exhibits little dependence on the formation of the BPT-solvent complexes and on the bulk-solvent effect. This explains why thermally activated delayed fluorescence from the S1 state has been observed for many aromatic thioketones in both inert media and hydrocarbons. Meanwhile, our calculations show that the intracomplex noncovalent interactions could be automatically adjusted by the redistribution of π-electrons in the flexible aromatic rings. This allows the S2 → S1 internal conversion to occur efficiently in the vicinity of the two-state conical intersection, which results in the remarkable changes in the S2-state lifetimes and fluorescence quantum yields of many aromatic thioketones from inert media to hydrocarbon solvents. The aforementioned inherent photophysical properties could be qualitatively understood by a simple model of frontier molecular orbitals. This model could be used to understand photophysical properties of other aromatic compounds (such as aldehydes, ketones, amines, and carboxylic acids) in different solvents.

Crystal structure, hydrogen bond patterns, Hirshfeld surface analysis, and topological studies (NCI) of 1,5,5-trimethyl-imidazolidine-2,4-dione; an organic compound with high symmetry crystallizing in the tetragonal space group I4/m

The title compound, 1,5,5-trimethyl-imidazolidine-2,4‑dione was characterized by FT-IR, 1H NMR, and 13C NMR spectroscopy techniques. The crystal structure was established by single-crystal X-ray diffraction. The powder X-ray powder diffraction analysis confirms the phase purity of the crystalline sample. The hydantoin with formula C6H10N2O2 crystallizes in the tetragonal space group I4/m (N°87), Z=8, and unit cell parameters a=15.554(2) Å, c=6.623(6) Å. This space group is very infrequent to find in purely organic molecules and is since, in the asymmetric unit, the hydantoin ring lies on a mirror plane m. The crystalline packing is stabilized by the formation of intermolecular interactions of the strong hydrogen bond type of the N–H···O between the neighboring hydantoin rings. In addition, the crystalline structure presents the formation of weaker unconventional hydrogen bonds of the C–H···O type. These hydrogen bonds give rise to the formation of 8, 24, and 36-membered ring-type supramolecular structures described by the graphs R22(8), R44(24), and R88(36), respectively, which govern the packing of the supramolecular structure of 1,5,5-trimethyl-imidazolidine-2,4‑dione.Hirshfeld surface analysis of the crystal structure shows that the most important contributions for the crystal packing are from H···H (58.3%) and H··O/O··H (34.0%) interactions, which indicate that van der Waals interactions are the dominant forces in the crystal packing. Energy framework calculations suggest that the contacts formed between molecules are slightly electrostatic.These intermolecular interactions were also investigated through topological analysis of the electron density (ρ) employing the NCI method. Experimental and theoretical results obtained for this new hydantoin compound suggest that N–H···O and C–H···O interactions play an important role in the stabilization of its supramolecular structure.

Anticooperative Supramolecular Oligomerization Mediated by V-Shaped Monomer Design and Unconventional Hydrogen Bonds.

After more than three decades of extensive investigations on supramolecular polymers, strategies for self-limiting growth still remain challenging. Herein, we exploit a new V-shaped monomer design to achieve anticooperatively formed oligomers with superior robustness and high luminescence. In toluene, the monomer-oligomer equilibrium is shifted to the monomer side, enabling the elucidation of the molecular packing modes and the resulting (weak) anticooperativity. Steric effects associated with an antiparallel staircase organization of the dyes are proposed to outcompete aromatic and unconventional B-F⋅⋅⋅H-N/C interactions, restricting the growth at the stage of oligomers. In methylcyclohexane (MCH), the packing modes and the anticooperativity are preserved; however, pronounced solvophobic and chain-enwrapping effects lead to thermally ultrastable oligomers. Our results shed light on understanding anticooperative effects and restricted growth in self-assembly.

Anticooperative Supramolecular Oligomerization Mediated by V‐Shaped Monomer Design and Unconventional Hydrogen Bonds

AbstractAfter more than three decades of extensive investigations on supramolecular polymers, strategies for self‐limiting growth still remain challenging. Herein, we exploit a new V‐shaped monomer design to achieve anticooperatively formed oligomers with superior robustness and high luminescence. In toluene, the monomer‐oligomer equilibrium is shifted to the monomer side, enabling the elucidation of the molecular packing modes and the resulting (weak) anticooperativity. Steric effects associated with an antiparallel staircase organization of the dyes are proposed to outcompete aromatic and unconventional B−F⋅⋅⋅H−N/C interactions, restricting the growth at the stage of oligomers. In methylcyclohexane (MCH), the packing modes and the anticooperativity are preserved; however, pronounced solvophobic and chain‐enwrapping effects lead to thermally ultrastable oligomers. Our results shed light on understanding anticooperative effects and restricted growth in self‐assembly.

Regenerable neodymium-doped zirconium-based MOF adsorbents for the effective removal of phosphate from water

UiO-66-NH2 has been widely to remove phosphate from natural water, albeit having poor selectivity and low removal efficiency. Researchers are seeking to enhance the adsorption properties of this Metal-Organic Framework (MOF) and improve its practicability. In this study, the rare earth neodymium (Nd) was doped into UiO-66-NH2 to prepare NdUiO-X (X: mole ratio of Zr/Nd), which was used as an adsorbent to remove phosphate from water. The main adsorption parameters, such as pH value, equilibrium time, and initial concentration were investigated. Furthermore, the adsorption mechanism of NdUiO-0.5 was investigated via spectroscopic analysis (FTIR and XPS) and theoretical calculations (DFT and LOBA). The results showed that when the molar ratio of Nd/Zr was 0.50, the adsorption effect was at its maximum, with the calculated saturation adsorption capacity from the Langmuir model being 106.39 mg P/g. The NdUiO-0.5 adsorbent shows high stability in solutions with pH ranging from 3 to 9. This MOF was applied to the actual water sample to observe its phosphate adsorption capacity. When the dosage of NdUiO-0.5 was 0.15 g/L, the total phosphorus concentration of the actual water sample decreased from 0.57 mg P/L to 0.018 mg P/L. Mechanistic studies have shown that ligand exchange is the key mechanism of action behind phosphate removal via NdUiO-0.5. Besides, unconventional hydrogen bonds also play a very important role. Thus, the prepared NdUiO-0.5 is a high-efficient adsorbent for phosphate removal bearing potential applications in real-life scenarios.

Computational Investigation of Bending Properties of RNA AUUCU, CCUG, CAG, and CUG Repeat Expansions Associated With Neuromuscular Disorders.

Expansions of RNA AUUCU, CCUG, CAG, and CUG repeats cause spinocerebellar ataxia type 10, myotonic dystrophy type 2, Huntington’s disease, and myotonic dystrophy type 1, respectively. By performing extensive molecular dynamic simulations, we investigated the bending propensities and conformational landscapes adopted by 3×3, 2×2, and 1×1 internal loops observed in RNA AUUCU, CCUG, CAG, and CUG repeat expansions using model systems having biologically relevant repeat sizes. We show that the conformational variability experienced by these loops is more complex than previous reports where a variety of unconventional hydrogen bonds are formed. At the global scale, strong bending propensity was observed in r(AUUCU)10, r(CCUG)15, r(CAG)20, and r(CUG)20, and, to a lesser extent, in r(AUUCU)4, r(CCUG)10, r(CAG)10, and r(CUG)10. Furthermore, RNA CAG repeats exhibit a tendency toward bent states with more than 50% of observed conformations having bending angles greater than 50°, while RNA CUG repeats display relatively linear-like conformations with extremely bent conformations accounting for less than 25% of the observed structures. Conformations experienced by RNA AUUCU repeats are a combination of strongly bent and kinked structures. The bent states in RNA CCUG repeats mostly fall into the moderately bent category with a marginal ensemble experiencing extreme bending. The general pattern observed in all the bent structures indicates the collapse of the major groove width as the mechanical trigger for bending, which is caused by alteration of base pair step parameters at multiple locations along the RNA due to local distortions at the loop sites. Overextension is also observed in all the RNA repeats that is attributed to widening of the major groove width as well as undertwisting phenomenon. This information and the rich structural repository could be applied for structure based small molecule design targeting disease-causing RNAs. The bending propensities of these constructs, at the global level, could also have implications on how expanded RNA repeats interact with proteins.

Experimental and Quantum Chemical Studies of Nicotinamide-Oxalic Acid Salt: Hydrogen Bonding, AIM and NBO Analysis.

The computational modeling supported with experimental results can explain the overall structural packing by predicting the hydrogen bond interactions present in any cocrystals (active pharmaceutical ingredients + coformer) as well as salts. In this context, the hydrogen bonding synthons, physiochemical properties (chemical reactivity and stability), and drug-likeliness behavior of proposed nicotinamide–oxalic acid (NIC–OXA) salt have been reported by using vibrational spectroscopic signatures (IR and Raman spectra) and quantum chemical calculations. The NIC–OXA salt was prepared by reactive crystallization method. X-ray powder diffraction (XRPD) and differential scanning calorimetry (DSC) techniques were used for the characterization and validation of NIC–OXA salt. The spectroscopic signatures revealed that (N7–H8)/(N23–H24) of the pyridine ring of NIC, (C═O), and (C–O) groups of OXA were forming the intermolecular hydrogen bonding (N–H⋯O–C), (C–H⋯O═C), and (N–H⋯O═C), respectively, in NIC–OXA salt. Additionally, the quantum theory of atoms in molecules (QTAIM) showed that (C10–H22⋯O1) and (C26–H38⋯O4) are two unconventional hydrogen bonds present in NIC–OXA salt. Also, the natural bond orbital analysis was performed to find the charge transfer interactions and revealed the strongest hydrogen bonds (N7–H8⋯O5)/(N23–H24⋯O2) in NIC–OXA salt. The frontier molecular orbital (FMO) analysis suggested more reactivity and less stability of NIC–OXA salt in comparison to NIC–CA cocrystal and NIC. The global and local reactivity descriptors calculated and predicted that NIC–OXA salt is softer than NIC–CA cocrystal and NIC. From MESP of NIC–OXA salt, it is clear that electrophilic (N7–H8)/(N23–H24), (C6═O4)/(C3═O1) and nucleophilic (C10–H22)/(C26–H38), (C6–O5)/(C3–O2) reactive groups in NIC and OXA, respectively, neutralize after the formation of NIC–OXA salt, confirming the presence of hydrogen bonding interactions (N7–H8⋯O5–C6) and (N23–H24⋯O2–C3). Lipinski’s rule was applied to check the activeness of salt as an orally active form. The results shed light on several features of NIC–OXA salt that can further lead to the improvement in the physicochemical properties of NIC.

Cation-Anion and Cation-Cation Interactions in Mixtures of Hydroxy-functionalized and Aprotic Ionic Liquids

A combination of molecular dynamics simulations, quantum chemical calculations, and vibrational spectroscopy is employed to study ionic interactions in 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C2OHmim]NTf2, 1-methyl-3-propylimidazolium bis(trifluoromethanesulfonyl)imide [C3mim]NTf2, and their double salt mixtures. Ionic liquids containing hydroxy groups are unique in their ability to form unconventional hydrogen bonds between neighboring cations, despite the strong repulsive forces these ions exert upon one another. Our computational and spectroscopic data reveal a rich array of ionic interactions in pure [C2OHmim]NTf2. The hydrogen bonding network inherent to [C2OHmim]NTf2 is partially disrupted when [C3mim]NTf2 is added to the ionic liquid to produce [C2OHmim]0.5[C3mim]0.5NTf2. This leads to increased amounts of uncoordinated hydroxy groups and minor changes in the distribution functions for side chain orientations. Furthermore, OH groups from [C2OHmim]+ are shown to hydrogen bond with the acidic hydrogen atoms attached to the [C3mim]+ or [C2OHmim]+ imidazolium rings. Cation-cation hydrogen bonding is not restricted solely to situations where both cations contain hydroxy groups (i.e., homo-aggregation of cations through OH‧‧‧OH interaction motifs). Rather, hetero-aggregation of chemically distinct cations may also occur through CH‧‧‧OH hydrogen bonds, where the CH hydrogen atom is directly connected to the imidazolium ring. Taken together, our data shows the cationic interactions that occur in the ionic liquid mixtures are controlled by competitive interactions originating from the hydrophobic propyl groups and hydrophilic hydroxyethyl groups.

Chiral Sulfide/Phosphoric Acid Cocatalyzed Enantioselective Intermolecular Oxysulfenylation of Alkenes with Phenol and Alcohol O -Nucleophiles

Chiral Sulfide/Phosphoric Acid Cocatalyzed Enantioselective Intermolecular Oxysulfenylation of Alkenes with Phenol and Alcohol <i>O</i> -Nucleophiles

First-principles study of the adsorption behavior of Octyl-β-D-xyloside surfactant on pristine Al12N12 and B12N12 nanocages

The octyl-β-D-xyloside is a biosurfactant with well-known roles in membrane protein systems. Using an efficient delivery system for these biosurfactants is of primary importance. This paper investigates the potential application of Al12N12 and B12N12 nanocages as an electronic sensor for octyl-β-D-xyloside surfactant detection in the gas phase using density functional theory calculations. Our results show that the electronic properties of Al12N12 and B12N12 nanocages were significantly affected by the adsorption of the octyl-β-D-xyloside molecule. The adsorption energies and enthalpies predicted a thermodynamically favorable chemisorption process. The AIM analysis depicted the formation of normal and bifurcated hydrogen bonds between Al12N12 and B12N12 nanocages and octyl-β-D-xyloside. The existence of the inter/intra-molecular and also conventional and unconventional hydrogen bonds were also detected. Our finding revealed although both Al12N12 and B12N12 nanocages have the ability to detect and adsorb the octyl-β-D-xyloside but, the adsorption over the Al12N12 is not favorable due to the high recovery time. Whilst, the adsorption of the octyl-β-D-xyloside on the B12N12 nanocage through O3 position with less steric factor and the recovery time of 8.14×10−3 S, is the best adsorption site.

Adsorption study of 4-nitrophenol onto kaolinite (001) surface: A van der Waals density functional study

Using clay materials as adsorbents for removing hazardous materials using adsorption has been gaining interest because of the efficiency, ecology, easy access, and low cost of these aluminosilicate materials. Thus, we investigate the possibility of removing 4-nitrophenol, a material present in industrial wastewater that can cause problems to the environment. The adsorption process on the kaolinite (001) surface is investigated via the density functional theory (DFT) using non-local van der Waals functional. We evaluate the structural and electronic properties of several models in terms of adsorption energy. The energy-most stable energy orientation of 4-nitrophenol on the surface under investigation is achieved when the adsorbate forms conventional and unconventional hydrogen bonds with the adsorbent. We show that the investigated clay has the potential to be used as an adsorbent for the removal of environmentally hazardous systems.

Theoretical Study of Intermolecular Interactions between Critical Residues of Membrane Protein MraYAA and Promising Antibiotic Muraymycin D2.

Phospho-N-acetylmuramoyl-pentapeptide translocase (MraYAA) from Aquifex aeolicus is the binding target for the nucleotide antibiotic muraymycin D2 (MD2). MraYAA in the presence of the MD2 ligand has been crystallized and released, while the interactions between the ligand and active-site residues remain less quantitatively and qualitatively defined. We characterized theoretically the key residues involved in noncovalent interactions with MD2 in the MraYAA active site. We applied the quantum theory of atoms in molecules and natural bond orbital analyses based on the density functional theory method on the solved crystal structure of MraY with the MD2 to quantitatively estimate the intermolecular interactions. The obtained results revealed the presence of multiple hydrogen bonds in the investigated active site with strength ranging from van der Waals to covalent limits. Lys70, Asp193, Gly194, Asp196, Gly264, Ala321, Gln305, and His325 are key active-site residues interacting with MD2. Conventional and unconventional hydrogen bonds in addition with charge–dipole and dipole–dipole interactions contribute significantly to stabilize the MD2 binding to the MraYAA active site. It was also found that water molecules inside the active site have substantial effects on its structure stability through hydrogen-bonding interactions with MD2 and the interacting residues.

Molecular Dynamics Insights into the Nanoscale Structural Organization and Local Interaction of Aqueous Solutions of Ionic Liquid 1-Butyl-3-methylimidazolium Nitrate.

Considering the growing number of applications of the aqueous ionic liquids (ILs), atomistic molecular dynamics (MD) simulations were used to probe the effect of water molar fraction, xw, ranging from 0.00 to 0.90, on the nanoscale local structure of 1-butyl-3-methylimidazolium nitrate, [bmim][NO3], IL. The results prove that, with water addition, the cation-anion, cation-cation, and anion-anion structural correlations are weakened, while strong anion-water and unconventional cation-water hydrogen bonds are formed in the solutions. Water molecules were detected as bridges between nitrate anions, and the water cluster size distribution at different xw's was investigated. Simulation shows a similar pattern of probability densities for water and anion around the acidic hydrogen atoms of the reference cation ring, while both species move away from the cation butyl chain. Increasing the water concentration to xw = 0.90 causes decreasing of the local arrangement of the nearest-neighboring cations, because of the weakening of cation-cation π-π stacking. In addition, this dilution reduces the probability of the in-plane cation-anion conformation, disrupts both the polar ionic network and nonpolar domains, and diminishes the nanoaggregation of the cation butyl chains compared to those of the neat IL. These results can rationalize the origins of the fluidity enhancements and transport property trends upon adding water to the imidazolium-based ILs. The current study proposes a deep atomistic-level insight into the complex coupling between water concentration, microscopic structure, and local interactions of aqueous imidazolium-based ILs with hydrophilic anions.

Diffusion of benzene through water film confined in silica mesopores: Effect of competitive adsorption of solvent

To understand the partial hydrogenation of benzene on Ru particles coated with hydrophilic materials, the diffusion of benzene through the water film confined in silica mesopores was studied by molecular dynamics simulation. There is an unconventional hydrogen bond between benzene and the surface hydroxyl group, which was previously supposed to retard benzene diffusion in hydrophilic mesopores. Driven by stronger hydrogen bonding, solvent water, however, is preferentially adsorbed onto the hydroxylated silica surface. Benzene dissolved in the water film has to diffuse away from the interfacial region, thereby weakening its interaction with the surface. Benzene may diffuse faster in hydroxylated mesopores than in bare mesopores, and the difference depends on the pore diameter. With a pore diameter of 4 to 6 nm, the diffusion coefficient of benzene along the pore axis, varying from 0.04 × 10−8 to 0.12 × 10−8 m2/s, could be adjusted by surface hydroxylation.

Reductively Stable Hydrogen-Bonding Ligands Featuring Appended CF2-H Units.

We present the development of ligands featuring the unconventional hydrogen bond donor, -CF2H, within a metal's secondary coordination sphere. When metalated with palladium, o-CF2H-functionalized 1,10-phenanthroline provides highly directed H-bonding interactions with Pd-coordinated substrates. Spectroscopic and computational analyses with a series of X-type ligand acceptors (-F, -Cl, -Br, -OR) establish the H-bonding interaction strength for the -CF2H group (∼3 kcal/mol). The synthesis of Pd0/Ni0 complexes and subsequent coupling (Ni) highlight the unique reductive and base compatibility of the -CF2H hydrogen bond donor group.

C–H···Y (Y=N, O, π) Hydrogen Bond: A Unique Unconventional Hydrogen Bond

We present a spectroscopic overview of the C–H···Y (Y = hydrogen bond acceptors) hydrogen bonded (HB or H-bond) complexes in this article. Although C–H···Y interactions have been recognized as H-bonding interactions for quite some time, they have not been investigated spectroscopically until recently. Recent results indicated that unlike the conventional hydrogen bond, C–H···Y H-bond has interesting spectroscopic characteristics, i.e. it shows both red as well as blue shift in C–H stretching frequency upon H-bond formation. This review presents examples of red, blue, and zero shifted C–H···Y H-bonds investigated in our laboratory that were characterized using laser-based IR and UV spectroscopic techniques applied to the cold isolated molecular complexes formed under supersonic expansion conditions. Along with spectroscopic information, ab initio/DFT-predicted geometry optimized structures of various conformers, harmonic frequency calculations of the optimized structures, and a number of properties such as electron densities at the bond critical points, orbital interaction energies, binding energies of the C–H···Y bound complexes are also summarized for better understanding of this type of H-bond. Not only the spectroscopic shift in C–H stretching frequency, but also the role of C–H···O H-bonds in microsolvation of several organic molecules has been highlighted. It has been found that depending upon activation of C–H moiety, C–H···Y H-bonds can provide primary or secondary stabilization for the growth of the primary solvation shell around organic molecules.

Dihydrogen Bonds in Salts of Boron Cluster Anions [BnHn]2− with Protonated Heterocyclic Organic Bases

Dihydrogen bonds attract much attention as unconventional hydrogen bonds between strong donors of H-bonding and polyhedral (car)borane cages with delocalized charge density. Salts of closo-borate anions [B10H10]2− and [B12H12]2− with protonated organic ligands 2,2’-dipyridylamine (BPA), 1,10-phenanthroline (Phen), and rhodamine 6G (Rh6G) were selectively synthesized to investigate N−H...H−B intermolecular bonding. It was found that the salts contain monoprotonated and/or diprotonated N-containing cations at different ratios. Protonation of the ligands can be implemented in an acidic medium or in water because of hydrolysis of metal cations resulting in the release of H3O+ cations into the reaction solution. Six novel compounds were characterized by X-ray diffraction and FT-IR spectroscopy. It was found that strong dihydrogen bonds manifest themselves in FT-IR spectra that allows one to use this technique even in the absence of crystallographic data.

Side chain to main chain hydrogen bonds stabilize a polyglutamine helix in a transcription factor

Polyglutamine (polyQ) tracts are regions of low sequence complexity frequently found in transcription factors. Tract length often correlates with transcriptional activity and expansion beyond specific thresholds in certain human proteins is the cause of polyQ disorders. To study the structural basis of the association between tract length, transcriptional activity and disease, we addressed how the conformation of the polyQ tract of the androgen receptor, associated with spinobulbar muscular atrophy (SBMA), depends on its length. Here we report that this sequence folds into a helical structure stabilized by unconventional hydrogen bonds between glutamine side chains and main chain carbonyl groups, and that its helicity directly correlates with tract length. These unusual hydrogen bonds are bifurcate with the conventional hydrogen bonds stabilizing α-helices. Our findings suggest a plausible rationale for the association between polyQ tract length and androgen receptor transcriptional activity and have implications for establishing the mechanistic basis of SBMA.

Ionic Hydrogen and Halogen Bonding in the Gas Phase Association of Acetonitrile and Acetone with Halogenated Benzene Cations.

We report on the gas phase association of the small polar and aprotic solvent molecules acetonitrile (CH3CN) and acetone (CH3COCH3) with the halogenated benzene radical cations (C6H5X•+, X = F, Cl, Br, and I) using the mass-selected ion mobility technique and density functional theory calculations. The association energies (-Δ H°) of CH3CN (CH3COCH3) with C6H5F•+ and C6H5I•+ are similar [13.0 (13.3) and 13.2 (14.1) kcal/mol, respectively] but higher than those of CH3CN (CH3COCH3) with C6H5Cl•+ and C6H5Br•+ [10.5 (11.5) and 10.9 (10.6) kcal/mol, respectively]. However, the electrostatic potentials of the lowest energy structures of C6H5Br•+(CH3CN) and C6H5Br•+(CH3COCH3) or C6H5I•+(CH3CN) and C6H5I•+(CH3COCH3) complexes clearly show the formation of the ionic halogen bonds (IXBs) C-Brδ+- -NCCH3 and C-Brδ+- -OC(CH3)2 or C-Iδ+- -NCCH3 and C-Iδ+- -OC(CH3)2 driven by positively charged σ-holes on the external sides of the C-Br and C-I bond axes of the bromobenzene and iodobenzene radical cations, respectively. For the C6H5F•+(CH3CN) complex, the dominant interaction involves a T-shaped structure between the N atom of CH3CN and the C atom of the C-F bond of C6H5F•+. The structure of the C6H5Cl•+(CH3CN) complex shows the formation of unconventional ionic hydrogen bonds (uIHBs) between the N atom of CH3CN and the C-H bonds of the C6H5Cl•+ cation. Similar results are obtained for the association of acetone with the halogenated benzene radical cations. The formation of IXBs of the iodobenzene cation with acetonitrile or acetone involves a significant entropy loss (-Δ S° =25-27 cal /(mol K)) resulting from the formation of more ordered and highly directional structures between the nitrogen or oxygen lone pair of electrons of acetonitrile or acetone, respectively, and the electropositive region around the iodine atom of the iodobenzene cation. In comparison, for the association of acetonitrile or acetone with the fluorobenzene, chlorobenzene, and bromobenzene cations, -Δ S° = 16-23 cal/(mol K), consistent with the formation of less ordered structures and loose interactions. The lowest energy structures of the C6H5Br•+(CH3COCH3)2 and C6H5I•+(CH3COCH3)2 clusters show a novel combination of ionic halogen bonding and hydrogen bonding where the oxygen atom of one acetone molecule forms the halogen bond while the oxygen atom of the second acetone molecule becomes the hydrogen acceptor from the methyl group of the first acetone molecule.

N–H···S Interaction Continues To Be an Enigma: Experimental and Computational Investigations of Hydrogen-Bonded Complexes of Benzimidazole with Thioethers

The N-H···S hydrogen bond, even though classified as an unconventional hydrogen bond, is found to bear important structural implications on protein structure and folding. In this article, we report a gas-phase study of the N-H···S hydrogen bond between the model compounds of histidine (benzimidazole, denoted BIM) and methionine (dimethyl sulfide, diethyl sulfide, and tetrahydrothiophene, denoted Me2S, Et2S, and THT, respectively). A combination of laser spectroscopic methods such as laser-induced fluorescence (LIF), two-color resonant two-photon ionization (2cR2PI), and fluorescence depletion by infrared spectroscopy (FDIR) is used in conjunction with DFT and ab initio calculations to characterize the nature of this prevalent H-bonding interaction in simple bimolecular complexes. A single conformer was found to exist for the BIM-Me2S complex, whereas the BIM-Et2S and BIM-THT complexes showed the presence of three and two conformers, respectively. These conformers were characterized on the basis of IR spectroscopic results and electronic structure calculations. Quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO), and energy decomposition (NEDA) analyses were performed to investigate the nature of the N-H···S H-bond. Comparison of the results with the N-H···O type of interactions in BIM and indole revealed that the strength of the N-H···S H-bond is similar to N-H···O in these binary gas-phase complexes.