Hydrogen-bonded Complexes Research Articles

Constrained nuclear-electronic orbital QM/MM approach for simulating complex systems with quantum nuclear delocalization effects incorporated

The hybrid quantum mechanics/molecular mechanics (QM/MM) approach, which combines the accuracy of QM methods with the efficiency of MM methods, is widely used in the study of complex systems. However, past QM/MM implementations often neglect or face challenges in addressing nuclear quantum effects, despite their crucial role in many key chemical and biological processes. Recently, our group developed the constrained nuclear-electronic orbital (CNEO) theory, a cost-efficient approach that accurately addresses nuclear quantum effects, especially quantum nuclear delocalization effects. In this work, we integrate CNEO with the QM/MM approach through the electrostatic embedding scheme and apply the resulting CNEO QM/MM to two hydrogen-bonded complexes. We find that both solvation effects and nuclear quantum effects significantly impact hydrogen bond structures and dynamics. Notably, in the glutamic acid–glutamate complex, which mimics a common low barrier hydrogen bond in biological systems, CNEO QM/MM accurately predicts nearly equal proton sharing between the two residues. With an accurate description of both quantum nuclear delocalization effects and environmental effects, CNEO QM/MM is a promising new approach for simulating complex chemical and biological systems.

Charge-Assisted Anion-π Interaction and Hydrogen Bonding Involving Alkylpyridinium Cations.

Competition and cooperation of charge-assisted anion-π interactions and hydrogen bonding were explored in the solid state and in solutions of 1-ethyl-4-carbomethoxypyridinium iodide, the compound utilized by Kosower to calculate solvent polarity Z-indices. X-ray structural analysis of this salt revealed multiple short contacts of iodide anions with hydrogen atoms and aromatic rings of pyridinium cations. Geometric characteristics, quantum theory of atoms in molecules (QTAIM), and noncovalent interaction (NCI) analysis of these contacts indicated comparable interaction energies of the anion-π and hydrogen bonding between iodide and pyridinium cation. 1H NMR (indicating the presence of the hydrogen-bonded complexes) and UV-vis measurements (which were consistent with the formation of anion-π associations) pointed out that both these supramolecular interactions also coexist in solutions. The comparable interaction energies (ΔE) of these modes were confirmed by the DFT computations. Also, while the variations of ΔE with the dielectric constant of the solvents for the complexes of iodide with the neutral π-acceptors were related to the increase of the effective radii of hydrogen- or anion-π bonded iodides, the changes in ΔE for the complexes with pyridinium followed interaction energies between two unit charges. However, the distinction of the bonding in hydrogen-bonded and anion-π complexes of iodide with pyridinium led to a switch of their relative energies with an increase of the polarity of the medium.

Hydrogen-Bonded Complexes of HPN• and HNP• Radicals with Carbon Monoxide.

Phosphorus mononitride (PN) is a carrier of phosphorus in the interstellar medium. As the simplest derivatives of PN, the radical species HPN• and HNP• have remained elusive. Herein, we report the generation, characterization, and photochemistry of HPN• and HNP• in N2-matrix at 3 K. Specifically, HPN• was formed as a weakly bonded complex with CO in the matrix by 254 nm photolysis of the novel phosphinyl radical HPNCO•. The •NPH-CO complex is extremely unstable, as it undergoes spontaneous isomerization to the lower-energy isomer •PNH-CO through fast quantum mechanical tunneling (QMT) with a half-life of 6.1 min at 3 K. Upon further irradiation at 254 nm, the reverse conversion of •PNH-CO to •NPH-CO along with dehydrogenation to yield PN was observed. The characterization •NPH-CO and •PNH-CO with matrix-isolation IR spectroscopy is supported by D, 15N, and 13C isotope labeling and quantum chemical calculations at the XYGJ-OS/AVTZ level of theory, and the mechanism by hydrogen atom tunneling is consistent with multidimensional instanton theory calculations.

Effects of Cosolvent on the Intermolecular Interactions between an Analyte and a Gold Nanostar Surface Studied Using SERS.

For surface-enhanced Raman scattering (SERS), reproducible solution-phase results are typically obtained using nanoparticles functionalized with surface-stabilizing molecules that can prevent the adsorption of analyte molecules with surface affinities lower than those of the stabilizing agent. Herein, we investigate the effects of intermolecular interactions between a nonthiolated analyte and cosolvent to facilitate and modulate analyte adsorption to gold nanostars. SERS, extinction spectroscopy, transmission electron microscopy (TEM), and density functional theory (DFT) calculations are employed in this regard. Tetrahydrofuran (THF) is utilized as a cosolvent in water to facilitate the detection of acetylsalicylic acid (aspirin) on anisotropic gold nanoparticles. Intermolecular interactions between the analyte, solvent, and surface are modulated by changing the solution composition to understand how THF facilitates the SERS detection of aspirin in THF-water cosolvents. SERS signals for 5 mM aspirin arise only in the presence of THF at and above 60 mM, while no signal with or without THF below 60 mM is observed. SERS detection of aspirin is hypothesized to depend on THF forming a hydrogen-bonded complex with aspirin that reduces aspirin hydrophobicity, thus stabilizing the acid form of the molecule and allowing it to weakly interact with the gold nanoparticles. The aspirin-THF complex adsorbs to the gold surface through π-orbital overlap between the aromatic ring and gold, where additional THF weakens orbital overlap. Understanding the mechanism by which organic cosolvents facilitate the SERS detection of nonthiolated analytes such as aspirin, in aqueous media, allows other cosolvents, nonthiolated analytes, and other surfaces to obtain a SERS signal in a variety of systems.

Highly enhanced fluoride removal from aqueous systems via solvent extraction utilizing trialkyl phosphine oxide as an efficient extractant

Solvent extraction (SX) has been recognized as an efficient approach for the large-scale removal of impurities, demonstrating considerable potential for fluoride extraction in industrial applications. However, existing fluoride extraction methods typically require the incorporation of additives to form complexes with fluoride for its removal. Due to the excessive additive requirements for efficient fluoride removal, these methods inevitably result in secondary contamination of feed solution, necessitating additional purification efforts to remove them. In this study, four extractants (trioctyl amine, tributyl phosphate, trioctyl phosphate, and trialkyl phosphine oxide (TRPO)) were investigated for fluoride removal via hydrogen bonding with hydrofluoric acid (HF). Various parameters, including the concentrations of extractants and stripping agents, pH values, extraction time, and temperature were initially optimized. It was found that TRPO exhibits the highest extraction efficiency for HF, with a single-stage extraction rate exceeding 70 %. Complete fluoride removal (> 99.9 %) was achieved through a consecutive four-stage extraction process. Further experiments, including maximum loading and slope analysis, combined with FT-IR and 19F NMR characterization, and DFT calculations elucidate the mechanism of the extraction. The transfer of HF into the organic phase is accomplished by the formation of a 1:1 hydrogen-bonded complex with TRPO. The loaded fluoride in the organic phase can be easily stripped by a base to get a fluoride salt. This study paves the way for the construction of novel SX systems to remove fluoride as HF.

Comments on the thermodynamic and dielectric studies on hydrogen-bonded liquid crystal complexes

A research article titled as “Hydrogen-bonded liquid crystal complexes: A comprehensive study of structure, behaviour and potential applications” has been recently published by Meddeb et al., J. Mol. Liq. 395 (2024) 123838. Authors have reported and analysed thermodynamic and dielectric studies on highly ordered SmB and SmG mesophases in 14OBAF/DABCO complexes. But explanations of the dielectric data reported on the material are flawed based on various facts. Unintended wrong interpretations by the authors in reporting and explaining the thermodynamic and dielectric results are discussed here. Influence of various artefacts on acquired dielectric data has also been discussed in order to avoid such problems in further reports.

Blue Phase III from Bent-Core Liquid Crystals: Unveiling Thermal Stability & Unique Traits - A Comprehensive Review.

This review article mainly delves into the comprehensive development, thermal stabilization, characteristics, and applications of Blue Phase III (BPIII) derived from non-calamitic, mainly T-shaped and bent-core liquid crystals (BCLC). The discussion begins with discovering and characterizing various liquid crystal (LC) phases of BCLCs, emphasizing the significance of the nematic (N) phase in three and four-ring BCLCs. Following this, the focus shifts to the stabilization, properties, and potential applications of BPIII, particularly those derived from non-conventionalliquid crystals. The review highlights the exceptional electro-optical (E-O) properties of BPIII, including high Kerr constants and distinct phase transitions. Studies reveal the impact of chirality on thermal behavior, microscopic observations, and the influence of molecular structures on mesophase formation. Investigations into asymmetrical chiral liquid crystal diads and hydrogen-bonded complexes underscore the importance of molecular design in expanding BPIII ranges. Furthermore, achiral unsymmetrical BCLC designs reveal significant insights into the interplay between molecular structure, phase transitions, and E-O behavior.Additionally, the structural transformation and E-O properties of highly polar BCLCs are examined to stabilize BPIII at room temperature, achieving notable Kerr constants and low voltage requirements. These collective studies provide a thorough understanding of BPIII and its promising applications in materials science and technology.

Regularities and Anomalies in Neon Matrix Shifts of Hydrogen-Bonded O-H Stretching Fundamentals.

O-H bond stretching vibrations in hydrogen-bonded complexes embedded into cryogenic neon matrices are subtly downshifted from cold gas phase reference wavenumbers. To the extent that this shift is systematic, it enables neon matrices as more universally applicable spectroscopic benchmark environments for quantum chemical predictions. Outliers are indicative of either an assignment problem in one of the two cryogenic experiments or they reveal interesting dynamics or structural effects on the complexes as a function of the environment. We compile 6 literature-known pairs of experimental data in jet and neon matrix expansions and realize a 6-fold expansion of that number through targeted matrix isolation and/or slit jet expansion spectroscopy presented in this work. In many cases, the neon matrix shift is less than the uncertainty of the currently best-performing blind quantum chemical predictions for the gas phase, but in specific cases, it may exceed the currently achievable theoretical accuracy. Some evidence for a positive correlation of the matrix shift with the hydrogen bond shift is found, similar to observations for helium nanodroplets. Outliers in particular for water acting as a donor are discussed, and in a few cases they call for a future reinvestigation. Substantial improvement in the correlation of the matrix shift with the hydrogen bond shift is achieved for ketone monohydrates by removing a vibrational resonance. New insights into nitrile hydration isomerism are obtained, and the linear OH stretching spectrum of the jet-cooled ammonia-water complex is presented for the first time. Vibrational spectroscopy in weakly perturbing solid rare gas quantum matrices for the benchmarking of gas phase theory and future explicit theoretical treatments of the quantum matrix environment to better understand the outliers are both encouraged.

One-pot construction of gradient colloidal gel coating for stable and efficient emulsion separation

Gradient gel coatings possess superior interfacial bonding strength and surface wettability, providing new ideas for the structure design of separation materials. However, simply and effectively combining colloidal gels that have excellent separation potential with gradient structure design remains a great challenge. Herein, a polymerization-induced microphase separation strategy based on metal ions is developed for constructing colloidal gel coatings with gradient structure on the stainless steel mesh. The colloidal gel is formed by hydrogen-bond complexation and microphase separation of acrylamide and methacrylic acid during polymerization. The metal ions generated by the hydrogen evolution corrosion of stainless steel mesh can not only be used to construct a continuous gradient structure by influencing the microphase separation process but also enhance the mechanical properties and stability of the gel coating. The preparation strategies of traditional gradient materials are limited in the manufacturing and application processes due to their complicated preparation procedures and potential requirement for special equipment assistance. In contrast, this strategy can simply and efficiently prepare colloidal gel coatings with continuous gradient structure by one-pot method under strict monomer ratio. Its unique colloid network demonstrates better wettability and permeability than conventional polymer gel. The prepared gradient colloidal gel coating exhibits excellent separation efficiency (above 99.9 %) and water flux recovery rate (≈98.75 %) in separating surfactant-stabilized emulsion. Therefore, this work provides new insights into the design and preparation of gradient colloidal gels, promoting their development in the field of water treatment.

Ultra-stretchable and adhesive hydrogel based on double network structure as flexible strain sensor for human motion detection

In recent years, conductive hydrogels were widely applied as flexible strain sensor due to its outstanding stretchability, high flexibility and conductivity. However, most conductive hydrogels would reduce the sensitivity and accuracy of hydrogel-based strain sensor for the lack of self-adhesion. Herein, a highly stretchable and excellent adhesive hydrogel based on chitosan network and ionic liquid crosslinking network was prepared. Chitosan (CS) worked as network skeleton, which could enhance the mechanical properties of hydrogel by participating in the construction of double network structure. Furthermore, dimethylaminoethyl methacrylate maleate (DM-M) as cross-linker, the hydrogel was imparted with high ionic conductivity (0.29 S/m) while it also enhanced the mechanical properties of the hydrogel. The ionic bonding conferred high toughness to the hydrogel for avoiding stress concentration during tensile process. Additionally, the hydrogel could form strong adhesion to a variety of substrate surfaces through electrostatic interaction, hydrogen bond and metal complexation. Meanwhile, the hydrogel remained strong adhesion under different pH, solvent and temperature condition. Therefore, the hydrogel with excellent adhesive property, mechanical properties and high electrical conductivity have been designed as flexible strain sensor, which could detect limb movement and physiological signals with real-time feedback electrical signals. Thus, this highly stretchable, tough and adhesive hydrogel based on double network structure would promote the development of flexible electronic device.

Biological and computational exploration of a hydrogen-bonded charge transfer complex between 8-Hydroxyquinoline and Benzene-1,4-diol in different polar solvents: Synthesis and spectrophotometric analysis

The formation and characterization of a charge transfer complex (CTC) between 8-Hydroxyquinoline and Benzene-1,4-diol (Quinol) in various polar solvents. CTCs, marked by weak yet significant interactions, are pivotal in understanding electron transfer processes and molecular recognition phenomena. The choice of 8-Hydroxyquinoline (8HQ) as the donor and Quinol (Q) as the acceptor facilitates the investigation of their interaction, given their respective electron-rich and electron-poor properties. The spectroscopic analysis, including Fourier-transform infrared (FTIR) and UV-visible spectroscopy, reveals shifts in molecular vibrations and electronic transitions, indicating the formation of the CTC. Additionally, thermal analysis techniques, such as thermogravimetric analysis (TGA) and differential thermal analysis (DTA), provide insights into the thermal stability and decomposition behavior of the CTC. Powder X-ray diffraction (PXRD) analysis offers data on the crystalline structure and morphology of the complex. Furthermore, the manuscript explores the biological significance of the synthesized CTC, including its antioxidant, antibacterial, and antifungal activities. Conductivity measurements and computational studies, such as Density Functional Theory (DFT) and molecular docking, provide further insights into the electronic properties and structural characteristics of the CTC. Overall, the study contributes to advancing the understanding of CTCs and their applications, particularly in the context of novel donor-acceptor systems. The insights gained have implications in diverse fields, including materials science, biomedical engineering, and fundamental chemistry, paving the way for tailored molecular design and functional applications.

Defluoridation by positive single-pulse current electrocoagulation from photovoltaic wastewater: Energy consumption assessment and mechanism analysis

The presence of fluoride ions (F−) in photovoltaic (PV) wastewater significantly affects the integrity of the ecological environment. In contrast to direct current electrocoagulation (DC-EC), positive single-pulse electrocoagulation (PSPC-EC) shows a significant reduction in both the formation of passivation films on electrodes and the consumption of electrical energy. Under the experimental conditions of an Al–Al–Al–Al electrode combination, an electrode spacing of 1.0 cm, a NaCl concentration of 0.05 mol L−1, an initial pH of 5.6, an initial F− concentration of 5 mg L−1, a current density of 5 A m−2, a pulse frequency of 500 Hz, and a 40 % duty cycle, the achieved equilibrium F− removal efficiencies were 84.0 % for DC-EC and 88.0 % for PSPC-EC, respectively, accompanied by power consumption of 0.0198 kWh·mg−1 and 0.0073 kWh·mg−1. The flocs produced in the PSPC-EC process were characterized using scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy and it is revealed that the F− removal mechanisms in the PSPC-EC process include co-precipitation, hydrogen bond complexation, and ion exchange. When the actual PV wastewater was finally subjected to treatment under the optimal PSPC-EC conditions, the F− concentration in the wastewater was reduced from 4.6 mg L−1 to 1.4 mg L−1. This paper provides both a theoretical framework and a technological basis for the application of PSPC-EC in the advanced treatment of PV wastewater.

Monohydrides of the Group 13 Elements M=B, Al and Ga: Axial Bi-Nucleophilicity and the Propensity to Form Both H-M⋅⋅⋅HX and M-H⋅⋅⋅HX Hydrogen Bonds (X=F, Cl, Br, I, CN, CCH, CP).

Equilibrium dissociation energies De of the hydrogen-bonded complexes HAl⋅⋅⋅HX and HGa⋅⋅⋅HX (X=F, Cl, Br, I, CN, CCH, and CP) were calculated ab initio at the CCSD(T)-(F12c)/cc-pVDZ-F12 level of theory. The gradients of graphs of De versus the electrophilicity EHX of the Lewis acids HX yielded the nucleophilicities NM-X of the Group 13 atoms M in these diatomic molecules. Molecular electrostatic surfaces potentials reveal that H-Al and H-Ga are bi-nucleophilic and that the H ends of these H-M molecules are more nucleophilic than the M ends for M=Al and Ga, but not when M=boron. Therefore, the complexes M-H⋅⋅⋅HX were investigated using the same approach. It was concluded for M=Al and Ga that, for a given X, the M-H⋅⋅⋅HX complexes were more strongly bound than the corresponding H-M⋅⋅⋅HX complexes for both M=Al and Ga but the reverse order applies for M = boron. The effects of substituting the H atoms in the M-H molecules by F atoms and by methyl groups were investigated to measure the -I and +I inductive effects relative to H, respectively, on the nucleophilicities of the molecules M-H when M is acting as hydrogen-bond acceptor in complexes H-M⋅⋅⋅H-X.

Carbamoylphosphinidene: A Phosphorus Analogue of Carbonyl Nitrene.

Carbonyl nitrenes are versatile intermediates that have been extensively characterized; however, their phosphorus analogues remain largely unknown. Herein, we report the observation of a rare example of carbonyl phosphinidene NH2C(O)P, which was generated through the photolytic (193 nm) dehydrogenation of phosphinecarboxamide (NH2C(O)PH2) in a solid N2-matrix at 12 K. The characterization of NH2C(O)P in the triplet ground state with matrix-isolation IR and ultraviolet-visible (UV-vis) spectroscopy is supported by comprehensive isotope labeling experiments (D and 15N) and quantum chemical calculations. Upon visible-light irradiation at 680 nm, NH2C(O)P inserts into dihydrogen by the reformation of NH2C(O)PH2 with concomitant isomerization to the more stable aminophosphaketene (NH2PCO). Additionally, the photoisomerization of NH2C(O)PH2 to NH2C(OH) = PH along with decomposition by yielding hydrogen-bonded complexes HNCO···PH3 and HPCO···NH3 has been observed in the matrix.

Influence of the structure and gas-phase acidity of aromatic disulfonic acids on proton transfer and the formation of hydrogen-bonded complexes with pyridine derivatives

The structure and conformational properties of a number of aromatic disulfonic acids were studied,using quantum chemistry methods (DFT(B3LYP)/cc-pVTZ). Their gas-phase acidity was assessed, and their hydrogen-bonded complexes with 4-pyridyl-4′-propyloxybenzoate were modeled in the variant with proton transfer from the sulfo group to pyridyl and without proton transfer. It has been shown that the gas-phase acidity of disulfonic acids is more pronounced than in the corresponding sulfonic acids due to competition between electron-withdrawing electron substituents. Modeling of the complexes was carried out in the gas phase and in solvents of different polarities. The potential proton transfer functions have two minima, corresponding to structures with and without proton transfer. The magnitude of the barrier and the relative energies of the complexes depend on the gas-phase acidity of benzene disulfonic acids: components with lower gas-phase acidity give a larger transition barrier and a larger difference in the energies of complexes with and without proton transfer. However, proton transfer complexes for such acids are characterized by the strongest hydrogen bonds.

Superelectrophilic Activation of Phosphacoumarins towards Weak Nucleophiles via Brønsted Acid Assisted Brønsted Acid Catalysis.

The electrophilic activation of various substrates via double or even triple protonation in superacidic media enables reactions with extremely weak nucleophiles. Despite the significant progress in this area, the utility of organophosphorus compounds as superelectrophiles still remains limited. Additionally, the most common superacids require a special care due to their high toxicity, exceptional corrosiveness and moisture sensitivity. Herein, we report the first successful application of the "Brønsted acid assisted Brønsted acid" concept for the superelectrophilic activation of 2-hydroxybenzo[e][1,2]oxaphosphinine 2-oxides (phosphacoumarins). The pivotal role is attributed to the tendency of the phosphoryl moiety to form hydrogen-bonded complexes, which enables the formation of dicationic species and increases the electrophilicity of the phosphacoumarin. This unmasks the reactivity of phosphacoumarins towards non-activated aromatics, while requiring only relatively non-benign trifluoroacetic acid as the reaction medium.

Transient spectroscopic insights into nitroindole’s T1 state: Elucidating its intermediates and unique photochemical properties

Indoles are notable for their distinct photophysical and photochemical properties, making them useful indicators in biological systems and promising candidates for a variety of pharmaceutical applications. While some indoles exhibit room temperature phosphorescence, such a phenomenon has not been observed in nitroindoles. Typically, adding of a nitro group into aromatic compounds promotes ultrafast intersystem crossing and increases the formation quantum yield of the lowest excited triplet (T1). Therefore, understanding the reactivity of nitroindoles′ T1 states is imperative. This study investigated the physical properties and chemical reactivities of the T1 state of 6-nitroindole (3HN-6NO2) in both polar aprotic and protic solvents, using transient absorption spectroscopy. Our results demonstrate the basicity and acidity of 3HN-6NO2, emphasizing its potential for protonation and dissociation in mildly acidic and basic conditions, respectively. Furthermore, 3HN-6NO2 has a high oxidizing capacity, participating in electron transfer reactions and proton-coupled electron transfer to produce radicals. Interestingly, in protic solvents like alcohols, 3HN-6NO2 dissociates at the –NH group and forms N-H…O hydrogen-bonded complexes with the nitro group. By identifying transient absorption spectra of intermediates and quantifying kinetic reaction rate constants, we illuminate the unique properties of the T1 state nitroindoles, enriching our understanding of their photophysical and photochemical behaviors. The results of this study have significant implications for their potential application in both biological systems and materials science.

Hydrogen bond complexation of ethanol with chlorobenzene: FTIR studies

ABSTRACT FTIR spectral studies has been carried out on ethanol (EtOH), chlorobenzene (PhCl) and their binary solutions at various EtOH/PhCl mole fractions. The bonded O − H stretching region has been deconvoluted into four Gaussian peaks centring at 3543.6, 3489.4, 3406.3 and 3254.3 cm−1. The first and the last peaks have been assigned to the hydroxyl stretching bands of dimer and tetramer, while the remaining two to the trimer bands. The presence of dimer in liquid PhCl has been confirmed by the experimental C − Cl stretching bands. Experimentally, two C − Cl stretching bands have been observed, which fall outside the 1096–1089 cm−1 range reported for chlorobenzenes in literature. The positional changes in O − H , C H 3 , C H 2 , C − H and C − Cl stretching bands in the spectrum of the solutions have been analysed in terms of heteromolecular H-bond interactions. This analysis shows that the classical O − H ⋯ Cl hydrogen bonds are the dominant interaction forces.

Shedding light on the strength, nature, and cooperativity/anti-cooperativity between intermolecular H…N hydrogen and Cl…C halogen bonds in the competitive binary and ternary complexes

In equimolar mixture of monomers N≡C-Cl (M−1) and C=N-H (M−2), formation of N…H hydrogen-bonded complex HB-Dim and C…Cl halogen-bonded complex XB-Dim compete with each other. Values of counterpoise-corrected interaction energy indicate that HB-Dim is absolute winner in such a competition. Existence of N…H HB- and C…Cl XB-interactions is characterized through geometrical parameters analysis, Quantum theory of atoms-in-molecules descriptors, and Independent gradient model based on Hirshfeld partition of molecular density. Interaction energy decomposition reveals that electrostatic with orbital interactions and, electrostatic with dispersion interactions are main driving forces for the formation of, respectively, HB- and XB-Dim. Moreover, substantially stronger electrostatic and orbital interactions characterized with Molecular electrostatic potential and Extended Transition State-Natural Orbitals for Chemical Valence analyses explain why formation of HB-Dim is quite preferred over that of XB-Dim. Two N…H HB- and C…Cl XB-interactions, present at the same time, make each other slightly strength in ternary complex Trim.

The Stability of Hydrogen-Bonded Ion-Pair Complex Unexpectedly Increases with Increasing Solvent Polarity.

The generally observed decrease of the electrostatic energy in the complex with increasing solvent polarity has led to the assumption that the stability of the complexes with ion-pair hydrogen bonds decreases with increasing solvent polarity. Besides, the smaller solvent-accessible surface area (SASA) of the complex in comparison with the isolated subsystems results in a smaller solvation energy of the latter, leading to a destabilization of the complex in the solvent compared to the gas phase. In our study, which combines Nuclear Magnetic Resonance, Infrared Spectroscopy experiments, quantum chemical calculations, and molecular dynamics (MD) simulations, we question the general validity of this statement. We demonstrate that the binding free energy of the ion-pair hydrogen-bonded complex between 2-fluoropropionic acid and n-butylamine (CH3CHFCOO-…NH3But+) increases with increased solvent polarity. This phenomenon is rationalized by a substantial charge transfer between the subsystems that constitute the ion-pair hydrogen-bonded complex. This unexpected finding introduces a new perspective to our understanding of solvation dynamics, emphasizing the interplay between solvent polarity and molecular stability within hydrogen-bonded systems.