Counterions Research Articles

Anion Modulation of Ag‐Imidazole Cuboctahedral Cage Microenvironments for Efficient Electrocatalytic CO2 Reduction

AbstractHow to achieve CO2 electroreduction in high efficiency is a current challenge with the mechanism not well understood yet. The metal‐organic cages with multiple metal sites, tunable active centers, and well‐defined microenvironments may provide a promising catalyst model. Here, we report self‐assembly of Ag4L4 type cuboctahedral cages from coordination dynamic Ag+ ion and triangular imidazolyl ligand 1,3,5‐tris(1‐benzylbenzimidazol‐2‐yl) benzene (Ag‐MOC‐X, X=NO3, ClO4, BF4) via anion template effect. Notably, Ag‐MOC‐NO3 achieves the highest CO faradaic efficiency in pH‐universal electrolytes of 86.1 % (acidic), 94.1 % (neutral) and 95.3 % (alkaline), much higher than those of Ag‐MOC‐ClO4 and Ag‐MOC‐BF4 with just different counter anions. In situ attenuated total reflection Fourier transform infrared spectroscopy observes formation of vital intermediate *COOH for CO2‐to‐CO conversion. The density functional theory calculations suggest that the adsorption of CO2 on unsaturated Ag‐site is stabilized by C−H⋅⋅⋅O hydrogen‐bonding of CO2 in a microenvironment surrounded by three benzimidazole rings, and the activation of CO2 is dependent on the coordination dynamics of Ag‐centers modulated by the hosted anions through Ag⋅⋅⋅X interactions. This work offers a supramolecular electrocatalytic strategy based on Ag‐coordination geometry and host–guest interaction regulation of MOCs as high‐efficient electrocatalysts for CO2 reduction to CO which is a key intermediate in chemical industry process.

Transition metal salt catalysed green synthesis of mesoporous silica nanoparticles

Conventionally, mesoporous silica nanoparticles are prepared by catalysing silicon alkoxides using acids or bases and are highly important in storage, delivery, and catalysis. Here, for the first time, we demonstrate that a transition metal ion (such as Ni(II), Co(II), and Mn(II)) also catalyses the hydrolysis and condensation reactions of silicon alkoxides in aqueous media without any additional acid or base to synthesize mesostructured and micro/mesostructured silica nanoparticles. An aqueous solution of a transition metal salt (specifically, nitrate salts of Ni(II), Co(II), or Mn(II), or chloride and sulphate salts of Ni(II)), 10-Lauryl ether (C12H25(OCH2CH2)10OH, C12E10) and cetyltrimethylammonium bromide (C16H33N(CH3)3Br, CTAB), and tetramethyl orthosilicate (TMOS) undergoes a precipitation reaction at room temperature, yielding ultra-small ordered mesostructured silica nanoparticles. These nanoparticles are subsequently calcined to produce mesoporous silica (meso-SiO2) with a high surface area (680–871 m2/g), large pore-volume (2.2–3.71 cm3/g), and small pore-size (1.2–3.0 nm). Moreover, the counter anions of the salts play an important role in the assembly process to obtain nanoparticles with an additional well-defined secondary pore (7.5–33.4 nm or larger). Coordinated water of the metal ion and methoxy group of the silica source react to produce a complex in which two hydroxy sides are in close vicinity to speed up the condensation reaction. We propose a hydrolysis and condensation reaction mechanism for TMOS to highlight the role of the metal ion as a catalyst.

Precise counter anion modulation of the self-assembly behavior-endowed ultrasensitive and specific dual-mode visualization of nitrate

Pt(II) polypyridine complex-based probe exhibits promising performance in anion detection by the change of the absorption and emission properties based on supramolecular self-assembly. However, whether one can develop a modulation strategy of the counter anion to boost the detection sensitivity and anti-interference capability of the Pt(II) complex-based probe remains a big challenge. Here, an effective modulation strategy was proposed by precisely regulating the interaction energy through adjusting the type of the counter anions, and a series of probes have been synthesized by counter anion (X = Cl-, ClO4-, PF6-) exchange in [Pt(tpy)Cl]·X (tpy=2,2′:6′,2′’-terpyridine), and thus the colorimetric-luminescence dual-mode detection toward nitrate was achieved. The optimal [Pt(tpy)Cl]·Cl probe shows superior nitrate detection performance including a limit of detection (LOD) (8.68 nM), rapid response (<0.5 s), an excellent selectivity and anti-interference capability even facing 14 common anions. Moreover, a polyvinyl alcohol (PVA) sponge-based sensing chip loaded with the probe enables the ultra-sensitive detection of nitrate particles with an ultralow detection limit of 7.6 pg, and it was further integrated into a detection pen for the accurate recognition of nitrate particles in real scenarios. The proposed counter-anion modulation strategy is expected to start a new frontier for the exploration of novel Pt(II) complex-based probes.

Ion-Specific Effects on Ion and Polyelectrolyte Solvation.

Ion-specific effects on aqueous solvation of monovalent counter ions, Na , K , Cl , and Br , and two model polyelectrolytes (PEs), poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA) were here studied with ab initio molecular dynamics (AIMD) and classical molecular dynamics (MD) simulations based on the OPLS-aa force-field which is an empirical fixed point-charge force-field. Ion-specific binding to the PE charge groups was also characterized. Both computational methods predict similar response for the solvation of the PEs but differ notably in description of ion solvation. Notably, AIMD captures the experimentally observed differences in Cl and Br anion solvation and binding with the PEs, while the classical MD simulations fail to differentiate the ion species response. Furthermore, the findings show that combining AIMD with the computationally less costly classical MD simulations allows benefiting from both the increased accuracy and statistics reach.

Regulating room temperature phosphorescence of carbazole quaternization pyridine in polymer through Hofmeister effect

AbstractHofmeister effect is a famous physical chemistry phenomenon that was reported a hundred years ago, which firstly refers to the action of certain salts to decrease the solubility of proteins while others increase. The Hofmeister effect on the luminescent properties of cationic organic fluorophore is still obscure, especially for their room temperature phosphorescence (RTP). Herein, hydrophilic groups (quaternization pyridine) were introduced into carbazole molecules to obtain a series of carbazole derivatives (named CZ‐Py+) with different counter anions in the Hofmeister series. These carbazole derivatives displayed tunable fluorescent color from cyan to yellow in the solid state following the Hofmeister sequence and anti‐Hofmeister behavior in an aqueous solution. Moreover, RTP material with tunable emission color and lifetime was achieved by doping CZ‐Py+ with Hofmeister series anion in polymethyl methacrylate and polyvinyl alcohol, which displayed good performance in time getting information encryption and anti‐counterfeiting.

Counter anions modulate selectivity in the divergent copper-catalyzed dehydrogenation of cyclohexanones

Counter anions modulate selectivity in the divergent copper-catalyzed dehydrogenation of cyclohexanones

Rational design of bilayer heterogeneous poly(ionic liquid)s electrolytes for high-performance flexible supercapacitors with ultraslow self-discharge rate

Supercapacitors have high power density and long cyclic life, but always face a great challenge of rapid self-discharge. Herein, we design a series of bilayer heterogeneous poly(ionic liquid)s electrolytes (BHPE) with anionic and cationic poly(ionic liquid)s (PolyILs), to construct high-performance supercapacitors with ultraslow self-discharge rate. Zeta potential and ionic conductivity of cationic PolyILs are adjusted by changing the volume of counter anions (BF4-, PF6-, TFSI-) and the length of alkyl side chain (ethyl, butyl and hexyl). Both experimental and simulated results reveal that the interaction energy between the cationic PolyILs and counter anion greatly depends on the sizes of anions and the lengths of alkyl side chains of cationic PolyILs, which has intrinsic effect on the zeta potential difference between anionic and cationic PolyILs of the BHPEs, and further can efficiently regulate the self-discharge rate of supercapacitors. The developed supercapacitor based on BHPE with using cationic PolyILs with small anion of BF4- and long alkyl side chain of hexyl exhibits an ultraslow self-discharge rate of −0.04 V h−1 and a high energy density of 34.8 Wh kg−1, which shows great promise for further practical applications in the future.

N-type small molecule electrolyte cathode interface layer with thickness insensitivity for organic solar cells

Interface engineering has a critical impact on the performances of organic solar cells (OSCs). And cathode interface layer (CIL) with thickness insensitivity is urgently pursued to improve the possibility of industrialization of OSCs. N-type self-doping has been proven effective in increasing electron mobility. Here, four novel n-type small molecule electrolytes (SMEs) with diverse counter anions (CAs), PDIN-BF4, PDIN-BPh4, PDIN-BPhF4, and PDIN-BIm4 were synthesized and employed as cathode interface layers (CILs). Among them, PDIN-BIm4-based OSCs with PM6:Y6 active layer achieved the most glorious electron mobility and thickness insensitivity with a power conversion efficiency (PCE) of 16.98 % due to outstanding self-doping effect and interfacial regulation ability. However, the multi-F atoms on PDIN-BPhF4 may prevent self-doping progress and impede electron transport, thus leading to a low PCE of 11.53 %. Meanwhile, the PDIN-BIm4-based device can maintain over 80 % of the optimal PCE with a thickness of 43 nm or storing in a glove box for 600 h. In addition, PM6: BTP-eC9-based device with PDIN-BIm4 CIL acquired a PCE of 17.82 %, highlighting the broad applicability of PDIN-BIm4. Our work demonstrates that the introduction of CAs into n-type organic materials helps promote the progress of efficient and stable OSCs.

Carbon dots as a sustainable electrolyte enhancer in aqueous alkaline electrochemical capacitors

In the endeavour to increase the energy density and to widen the potential window of aqueous alkaline electrochemical capacitors (EC), this study explores the role of carbon dots (CDs) as an additive in potassium hydroxide electrolyte. The CDs with an average size of ∼2.2 nm and negative surface potential are synthesized from a dispersion of palm kernel shell powder in water using a low-temperature hydrothermal process. Electrochemical measurements show that the CD–electrolyte ion (K+) interaction has improved counter ion adsorption in porous carbon electrodes via lowering the characteristic resistances and time constants, which significantly improved the fraction of adsorbed charges than diffusively stored. The improved ionic conductivity is attributed to the improved wettability introduced by the hydrophilic functional groups in the CDs. These parametric changes widened the potential window and marked an 80 % increase in the specific energy and over a 10 % increase in specific power in a practical EC with similar mass-loading as commercial devices. The device with CDs demonstrated superior cycling stability and Coulombic efficiency than the control device without them. These findings underscore the potential of CDs as a promising avenue for advancing the performance parameters of aqueous electrolyte EC, aligning with the overarching goal of realising economical, environmentally friendly, and sustainable energy storage solutions.

Mesomorphism, high-contrast negative fluorosolvatochromism and photoinduced fluorescence conversion of thienylcyanostyrene-pyridinium salts

Novel thienylcyanostyrene and pyridinium-based luminescent ionic liquid crystals (ILCs) characterized with different length of N-alkyl chains (N–C3H7 and N–C8H17) and different size of counter anions (Br−, BF4−, PF6− and Tf2N−) on pyridinium moiety have been synthesized. The chemical modifications of N-alkyl chains and counter anions are easily achieved via pyridine alkylation followed with further ion exchanges, which played a key role in tuning supramolecular self-assembly of these ILCs. The compounds with shorter N–C3 alkyl chain (E-CTPC3X, X = Br−, BF4−, PF6− and Tf2N−) can self-assemble into 3D cubic phase or 2D hexagonal columnar phase depending on the size of counter anion, while the analogues with longer N–C8 alkyl chain (E-CTPC8X, X = BF4− and Tf2N−) only result smectic A phases. Moreover, the prolongation of the N-alkyl chain or enlargement of counter anion could significantly decrease the clear points of LC phases avoiding the thermal decomposition at high temperature, whereas introducing counter anions such as BF4−, PF6− that can form hydrogen bonds have stabilized the LC phases. The notable negative fluorosolvatochromism and the photoinduced fluorescence conversion with high-contrast of these ILCs were observed using UV–vis absorption and photoluminescence (PL) spectra, and was further analyzed by dynamic 1H NMR. These ILCs had been used as dopant to modify the n-Si/PEDOT:PSS heterojunction solar cells (HSCs) with 2 fold hole mobility (μhole) enhancement of the PEDOT:PSS layer.

Ways to improve understanding of PFAS contamination - A case study within a waste management facility

Waste management facilities are a known source for per- and polyfluoroalkyl substances (PFAS) to the environment. In this study, water samples from seven subsections within a waste management facility in Sweden were analyzed for PFAS and extractable organofluorine (EOF). Oxidative conversion was used to investigate how much PFAS precursors could contribute to the EOF. Out of the 23 analyzed PFAS, ten compounds accounted for a major proportion of the concentrations. Before oxidative conversion the ∑10PFAS were between 0.44 μg/L and 17 μg/L. The EOF ranged from 2 μg/L F up to 79 μg/L F. There was a greater difference in concentrations and profiles between the subsections in comparison to the four sampling dates at respective sampling point, suggesting different sources of PFAS from the waste. Oxidative conversion revealed presence of precursors by elevated concentrations of perfluoroalkyl acids after oxidation, which increased the explained EOF up to 25%. Seven samples from one sampling date were selected to investigate if other fluorinated compounds (inorganic anions, ultra-short-chain PFAS, and zwitterions) could be a part of the unexplained EOF fraction. The contribution of fluorine from tetrafluoroborate and hexafluorophosphate were equal or higher proportions than the ∑10PFAS. The presence of the ionic liquids tetrafluoroborate and hexafluorophosphate could originate from battery waste, due to their use as counter ions in batteries. Ultra-short-chain PFAS increased the explained EOF by an average of 8%, with trifluoroacetic acid and trifluoromethane sulfonic acid being the main contributors. However, the reported concentrations of ultra-short-chain PFAS, were underestimated due to low recovery by the additional washing step to remove inorganic fluoride for EOF analysis. The concentrations of zwitterions were low and increased the explained EOF by < 1%. Our results suggest that EOF, selected PFAS, oxidative conversion and anionic fluorinated substances give a better picture of PFAS contamination.

Counterion selection for ion controls of CuO-nanoparticles is important for assessing soil biological responses

Environmental evaluations of metal nanoparticles (NP) rely on seperating the effects of the metal NP from its dissolution products. However, the coordinating or counter anion used in experimental controls may potentially influence biotic indicators used in ecotoxicology and soil health monitoring, thereby compromising the ability to detect real nanoparticle effects and potentially confounding interpretation of metal-NP impacts. Using the example of copper oxide (CuO) NP, we demonstrate for the first time that depending on the anion used in the metal ion control (CuCl2 versus CuSO4), different and even opposite conclusions may be drawn for CuO-NP effects. This include a key biological indicator such as enzyme activity in soil samples. Moreover, this effect was specific to environmental conditions and indicator type, raising important methodological and interpretive implications to assess the CuO-NP impacts on soils. Our findings imply that assessments of soil health impacts of metal-NP should consider multiple coordinating anion controls for a given metal, especially when the counterion is known to impact the biological indicator including nutrient ions.

Counter ions in tuning the extended arrangements of pyrazolone-thiosemicarbazone-based Fe(III) complexes

Counter ions in tuning the extended arrangements of pyrazolone-thiosemicarbazone-based Fe(III) complexes

Structure and properties of electrochemical interfaces with grafting polyelectrolyte: A fluid density functional theory study

The structure of the electrode/electrolyte interface directly determines electrochemical performance. One effective method for controlling this interface structure is by grafting polyelectrolyte onto the electrode surface, which can significantly alter the double layer structure and enhance electrochemical performance. This study focuses on the graft modification of polyelectrolytes at the electrochemical interface and investigates the impact of the length and density of the grafted polyelectrolyte chain on the structure and capacitance of the electrode/ionic liquid interface using fluid density functional theory (FDFT). The findings indicate that the grafted polymerbrush disrupts the alternating layer of anions and cations at the conventional interface and exerts an additional adsorption effect on counter ions, thereby reducing the density of freely movable co-ions. Consequently, this process increases the effective charge and capacitance of the interface. The results of this investigation contribute to a deeper comprehension of polyelectrolyte solutions at solid–liquid interfaces and guide the regulation of electrochemical interface properties.

A Systematic Study of Nonionic Di- and Multiborane Catalysts for the Oligomerization and Polymerization of Epoxides.

Borane catalysis has emerged as a powerful technology in epoxide polymerization. Still, the structure-activity correlations for these catalysts are not fully understood to date, especially regarding compounds with nonionic backbones. Thus, in this work, 13 different borane catalysts of this respective type are described and investigated for their epoxide oligomerization and polymerization performance, using propylene oxide (PO), 1-butylene oxide (BO) and allyl glycidyl ether (AGE) as monomers. Structurally, special emphasis is put on catalysts with different linker lengths and linker flexibilities as well as the introduction of more than two borane functionalities. Importantly, this screening is conducted both under typical polymerization conditions as well as under the chain transfer agent (CTA)-rich conditions relevant for large-scale production. It is found that suitable preorganization of the borane groups, such as present in biphenyl derivatives, offers a simple route to high-performing catalysts and quantitative monomer conversion of the investigated epoxides. Furthermore, it is demonstrated that a diborane-catalyzed oligomerization can be kept active over weeks, whereby repeated addition of monomer batches (14 steps) constantly results in full conversion and well-defined oligoethers, underlining the practical potential of this method. The absence of co-initiating counter ions is suggested as an inherent advantage of nonionic catalysts.

Preparation of a class of cucurbit[6]uril-light rare-earth-based supramolecular assemblies and its efficient detection of chloramphenicol

By introducing 4,4′-naphthalenedisulfonic acid (4,4′-H2BPs) as a structure-induced agent, four novel cucurbit[6]uril-based supramolecular assemblies have been successfully prepared in the cucurbit[6]uril (Q[6]) system with light rare-earth elements (La3+, Ce3+, Pr3+, Nd3+) under hydrothermal conditions. Crystal structure analysis shows that the assemblies 1 (La3+), 2 (Ce3+), 3 (Pr3+) are isomorphism 3D frameworks, where Q[6] can directly coordinate with light rare-earth metal ions to form 1D −Ln(III)- Q[6]-Ln(III)- chains, while the 4 (Nd3+) presents 3D structures stacking by the unit of Q[6]@Nd3+@Q[6]@Nd3+@Q[6], respectively. In these structures, 4,4′-H2BPs ligands all can act as an organic counter anion and bridging via the outer-surface interaction of cucurbit[n]uril (OSIQ) in the system, which is the main driving force from low-dimensional to high-dimensional. Meanwhile, the fluorescence sensing performance of the two assemblies for antibiotics was investigated, and the results showed that assembly 1 could be used as a promising fluorescent sensor material to detect chloramphenicol (CAP).

Closing the Gap Between Gas Phase and Condensed Phase: Fragmentation Studies of the Thiomethoxymethyl Complex [(bipy)Pt(CH3)2(CH2SCH3)]+

AbstractCollision‐induced dissociation (CID) of the mass‐selected cation [(bipy)Pt(CH3)2(CH2SCH3)]+ in an ion‐trap mass‐spectrometer results in the losses of the neutrals ethane, ethene, dimethyl sulfide, methane, ethyl methyl sulfide, and propane. The generation of the first four neutrals is also observed, when the isolated, dimeric complex [(bipy)Pt(CH3)2(CH2SCH3)]2[SbF6]2 is heated in the condensed phase. While the product distribution is very similar regardless of whether the complex is heated in the solid state or in solution, ethane loss, i. e. the quite rare event of an sp3‐sp3 C−C reductive elimination from a platinum(IV) complex, dominates in the gas‐phase experiment. However, when the counter anion [SbF6]− is exchanged by the more weakly coordinating anion [BAr4F]−, the amounts of ethane and ethene are increased also in the condensed phase. These sum of observations indicates the similarities of the fragmentation of [(bipy)Pt(CH3)2(CH2SCH3)]+ in both phases. Thus, a correlation of processes in both environments is possible for selected systems (and particularly for unimolecular processes), but the choice of the proper conditions is crucial for a meaningful comparison.

Synthesis, Characterisation, and In Vitro Evaluation of Biocompatibility, Antibacterial and Antitumor Activity of Imidazolium Ionic Liquids.

In recent decades, ionic liquids (ILs) have garnered research interest for their noteworthy properties, such as thermal stability, low or no flammability, and negligible vapour pressure. Moreover, their tunability offers limitless opportunities to design ILs with properties suitable for applications in many industrial fields. This study aims to synthetise two series of methylimidazolium ILs bearing long alkyl chain in their cations (C9, C10, C12, C14, C16, C18, C20) and with tetrafluoroborate (BF4) and the 1,3-dimethyl-5-sulfoisophthalate (DMSIP) as counter ions. The ILs were characterised using 1H-NMR and MALDI-TOF, and their thermal behaviour was investigated through DSC and TGA. Additionally, the antimicrobial, anticancer, and cytotoxic activities of the ILs were analysed. Moreover, the most promising ILs were incorporated at different concentrations (0.5, 1, 5 wt%) into polyvinyl chloride (PVC) by solvent casting to obtain antimicrobial blend films. The thermal properties and stability of the resulting PVC/IL films, along with their hydrophobicity/hydrophilicity, IL surface distribution, and release, were studied using DSC and TGA, contact angle (CA), SEM, and UV-vis spectrometry, respectively. Furthermore, the antimicrobial and cytotoxic properties of blends were analysed. The in vitro results demonstrated that the antimicrobial and antitumor activities of pure ILs against t Listeria monocytogenes, Escherichia coli, Pseudomonas fluorescens strains, and the breast cancer cell line (MCF7), respectively, were mainly dependent on their structure. These activities were higher in the series containing the BF4 anion and increased with the increase in the methylimidazolium cation alkyl chain length. However, the elongation of the alkyl chain beyond C16 induced a decrease in antimicrobial activity, indicating a cut-off effect. A similar trend was also observed in terms of in vitro biocompatibility. The loading of both the series of ILs into the PVC matrix did not affect the thermal stability of PVC blend films. However, their Tonset decreased with increased IL concentration and alkyl chain length. Similarly, both the series of PVC/IL films became more hydrophilic with increasing IL concentration and alkyl chain. The loading of ILs at 5% concentration led to considerable IL accumulation on the blend film surfaces (as observed in SEM images) and, subsequently, their higher release. The biocompatibility assessment with healthy human dermal fibroblast (HDF) cells and the investigation of antitumoral properties unveiled promising pharmacological characteristics. These findings provide strong support for the potential utilisation of ILs in biomedical applications, especially in the context of cancer therapy and as antibacterial agents to address the challenge of antibiotic resistance. Furthermore, the unique properties of the PVC/IL films make them versatile materials for advancing healthcare technologies, from drug delivery to tissue engineering and antimicrobial coatings to diagnostic devices.

The Effect of Weakly Coordinating Solvent and Counter Anion on Diphosphine‐Palladium(II) Catalytic Reactivity for Copolymerization of 1‐Butene with Carbon Monoxide in Aqueous Media

AbstractWater soluble bis(acetonitrile) diphospalladium (II) complexes cis‐[Pd(L1–L5)(NCCH3)2][BF4]2 (C1–C5) were obtained by the reaction [Pd(NCCH3)4][BF4]2 with diphosphines ligands L1–L5 (CH2(CH2PR2)2: where R=(CH2)nP(O)(OEt)2, n=2–4, n=6 and n=8)) in dichloromethane–acetonitrile. The isolated complexes were characterized by elemental analysis, FTIR, 1H‐ and 31P‐NMR spectroscopy. The complexes were applied as catalysts for the copolymerization of 1‐butene with carbon monoxide in aqueous media. The present work emphasized on the effect of weakly coordinated solvent and counterion on the catalytic behavior of cis‐[Pd(L1–L5)(NCCH3)2][BF4]2 complexes compared to previously prepared cis‐[ Pd(L1–L5)(OAc)2] (A1–A5). The exchange of π‐bonding NCCH3 ligand with π‐bonding CO and alkene was more favored than the exchange of σ‐bonding OAc− as reflected by the much higher catalytic activity (TON) of the former compared to the latter. In order to compare the catalytic efficiency of water soluble complexes with the performance of a standard, the water insoluble complex, cis‐[Pd(CH2(CH2P(CH2CH3)2)2(NCCH3)2][BF4]2, C6, was prepared and incorporated in the investigations. The isolated alternating 1‐butene/CO copolymer exhibits glass transition temperature of Tg=−3–−15 °C, which is in the range desirable for latex emulsion application. The solid content is up to 21 % with high molecular weight (Mw=4×104 g mol−1) and narrow polydispersities (Mw/Mn=2–3). The C5 complex was found to be fourteen times more active in aqueous polymerization medium (6369 mol substr. mol Pd−1 h−1) than C6 complex in nonaqueous medium (441 mol substr. mol Pd−1 h−1).

Disentangling Acid-Base Chemistry through Blue Shifting Hydrogen Bond Contributions.

The blue shifting of vibrational frequencies in hydrogen bonded molecules, as observed in aqueous environments, has been attributed to local partial charge transfer from solvation. Here, we extrapolate the blue shift model to the stronger ionic interactions between hydrogen bond acceptors associated with protonation through augmented pH levels and competitive interactions with counter ion pairing. The chemical model we utilize in this work is the aqueous pyridine-pyridinium equilibrium to characterize the blue shifts observed in the pyridinium chloride ionic system. The observed agreement between observed experimental and calculated spectral shifts shows that the blue shifting model can be extrapolated to stronger interactions and accurately describe the nature of the hydrogen bond.