Ion-pair Formation Research Articles

A Conserved Lysine in an Ion-Pair with a Catalytic Glutamate Is Critical for U-to-C RNA Editing but Restricts C-to-U RNA Editing.

Plants make pyrimidine base substitutions in organellar mRNAs through the action of sequence-specific nuclear-encoded enzymes. Pentatricopeptide repeat (PPR) proteins are essential for ensuring specificity, while the enzymatic DYW domain is often present at the C-terminus of a PPR protein and dependent on the variant possessing C-to-U and/or U-to-C RNA editing activities. Expression of exogenous DYW-KP variant enzymes in bacteria leads to the modification of RNAs suggestive of U-to-C base changes. The modified RNAs could only be purified from the interphase of an acidic guanidinium thiocyanate-phenol-chloroform experiment. It was projected that in bacteria stable RNA-enzyme cross-links form from a lysyl attack. In this study, RNA editing was examined for dual U-to-C/C-to-U editing enzyme KP6 with conserved lysine residues substituted by alanine. A single lysine was found to be essential for U-to-C editing and, based on the crystal structures of DYW domains, would likely be present in the active site. Crystal structures also suggest that the lysine can potentially form an ion pair with the catalytic glutamate critical for C-to-U RNA editing. Mutation of lysine to alanine greatly stimulated the C-to-U RNA editing by KP6. A ∼319 Da adduct observed on DYW-KP proteins could not be detected on the U-to-C-deficient lysine to alanine point mutant enzymes. This work establishes the critical role for a single lysine in the DYW-KP domain specifically for U-to-C editing activity but also highlights a secondary role for the lysine in modulating C-to-U editing through the formation of an inhibitory ion pair with the catalytic glutamate.

Correlating Ionic Conductivity to Structure and Dynamics of Li-Ion Battery Electrolyte Systems: Raman Spectroscopy, Dielectric Relaxation Measurements, and Streak Camera Solvation Data Analysis.

A correlation between ionic conductivity and electrolyte solution structure and dynamics was explored by performing electrolyte concentration- and temperature-dependent measurements of conductivity, viscosity, and dielectric relaxation (DR) in solutions of lithium bis(trifluoromethane)sulfonimide (LiTFSI) in triethylene glycol dimethyl ether (also known as triglyme, G3). In addition, an electrolyte concentration-dependent Raman spectroscopic study and ultrafast dynamic fluorescence Stokes shift measurements of solvation of a dissolved solute by employing a streak camera detection technique were carried out. Measured conductivities (σ) and the average DR times (⟨τDR⟩) were found to be partially decoupled from the solution viscosities (η) and obeyed the relation, σ or ⟨τDR⟩-1 ∝(η/T)-p, with p = 0.6-0.75 for σ and p = 0.2-0.45 for ⟨τDR⟩-1. Raman data indicated the formation of ion pairs and ionic aggregates in these solutions, while the measured glass transition temperature increased with LiTFSI concentration. Conductivities (σ) showed a nonlinear concentration dependence but increased linearly with the solution static dielectric constants (εs). The latter may be explained by considering the temperature effects on complex electrolyte species and the subsequent solution dielectric behavior. Interestingly, an inverse power-law dependence of σ on the measured DR or solvation time scales, σ ∝ (τx)-m, with m = 1.2-1.9, was observed. This dependence may be explained by considering that the same environmental friction regulates both the ion diffusion and the medium polarization relaxation. The control of a slow process (ion translation) by relatively faster medium dynamics (dipolar rotation), although quite fascinating for the present system, warrants further experimental scrutiny for other electrolyte systems relevant to battery applications.

DEVELOPMENT AND VALIDATION OF NEW SPECTROPHOTOMETRIC PROCEDURES FOR THE ANALYSIS OF ROXITHROMYCIN IN BULK AND DOSAGE FORMS

Novel sensitive spectrophotometric techniques were developed and validated for a quantitative estimation of roxithromycin (ROX) in bulk and pharmaceuticals. In alkaline medium, procedure (A) was performed by ion-pairs formation between the studied macrolide and 1,2-naphthoquinone-4-sulphonate to formed orange - products with maxima absorption at 454 nm. While the second method (B) was carried out for coupling the amino group of the cited macrolide with phylloquinone using the charge transfer reaction to form coloured products with λmax at 457 nm. The methods presented good linearity in the range of concentrations between 1.0 - 28.8 μg/mL. Regression coefficients were 0.9997 and 0.9998 for methods (A) and (B), respectively, as well the detection limits were cited in 0.26-0.23 μg/mL. Validation measurements were rebuts, accurate and precise within %RSD values lower than 3.5%. These methods were successfully used to investigate roxithromycin quantitatively in tablets. Recoveries were 98.94–101.01% and no interferences were detected from excipients. Both procedures are rapid, simple and cost-effective and it can adopted quality control application of this antibiotic.

Probing the potential of type V Deep eutectic solvents as sustainable electrolytes

The increasing interest within the scientific community in environmentally friendly solvents has led to a focus on Deep Eutectic Solvents (DES), which have natural components. DES are viewed as alternatives to traditional organic solvents and have the potential to be used as electrolytes. For the first time, transport properties of four Type V Deep Eutectic Salt Solutions (DESS) were accessed to investigate the potential of this technology, selecting precursors ranked as excellent in Eco-Scale metrics. The DESS were composed of terpene and trioctylphosphine oxide (TOPO), and varying concentrations of lithium bis(trifluoromethane)sulfonimide (LiTFSI), and their properties were assessed through self-diffusion, viscosity, density, and conductivity measurements. While Type V DESS are capable of dissolving significant amounts of LiTFSI (up to 30 % molar), their ionic conductivity is low, with values ranging from 3.6·10−3 to 9.3·10−2 mS·cm−1 at 25 °C, thus limiting their suitability as electrolytes, for instance, for lithium-ions batteries applications. Similar diffusion coefficients for Li+ and TFSI− ions suggest the formation of long-lived ion pairs moving as a neutral species. Therefore, future research aims to introduce additives to disrupt contact ion pairs and enhance transport properties, leveraging the sustainable appeal of DES and their use in advanced energy storage technologies.

Why is binding of a divalent metal cation to a structural motif containing four carboxylate residues not accompanied by a conformational change?

We earlier showed that Torpedo californica acetylcholinesterase (AChE) contains a cluster of four conserved aspartates that can strongly bind divalent cations, which we named the 4D motif. Binding of the divalent metal cations greatly increases its thermal stability. Here we systematically examined all available crystallographic structures of T. californica AChE. Two additional metal-binding sites were identified, both composed of acidic and histidine residues. Relative binding to the 4D and additional sites was studied using metadynamics simulations. It was observed that in crystal structures devoid of metal ions in the 4D site, the conformation of T. californica AChE is almost identical to that in structures in which it is occupied by a divalent metal ion. Closer examination of the 4D motif reveals that three of the four acidic residues form ion pairs with conserved basic residues surrounding them. We named this new motif the 4A/3B motif. Molecular dynamics with quantum potential simulations was used to quantify the 4D motif's binding strength compared with that of the metal-binding site in the protein fXIIIa, which consists of four aspartates, but is devoid of adjacent cationic residues. Whereas fXIIIa's 4D site, in the absence of a metal cation, expanded significantly in the simulation, that of Torpedo AChE displayed only minor periodic changes in size. Furthermore, the energy of metal ion unbinding from the two sites differs by ca. 10 kcal/mol. We identified several other proteins in the PDB that contain the 4A/3B motif, whose conformations are identical in the presence or absence of a metal ion. An animated Interactive 3D Complement (I3DC) is available in Proteopedia at https://proteopedia.org/w/Journal:Protein_Science:4.

Reviving Multivalent-Metal Anodes in Simple Salt Electrolytes via Component Modifier Design.

Using combinatory electrolyte blends represents an imperative avenue to achieve good magnesium (Mg)-metal anode compatibility and commercial feasibility in fields of promising rechargeable Mg batteries. However, fundamental challenges of how to manipulate component modifier reactivity on molecule level still remain to be solved. Here, molecular structure design concepts towards seeking bromophenyl complex-based component modifiers has been proposed according to implications of electron-donating and/or electron-withdrawing substituents on Br-C bond dissociation reactivity. Exceptional Mg electro-plating/stripping properties (a stable cycle life of 250 days in Mg//Cu asymmetric cells) have been firstly achieved in a simple salt electrolyte with 1-(3-bromophenyl)-N,N-dimethylmethanamine (BPDMA) as optimal component modifier. Comprehensive analyses disclose the unique electrochemically-active Br-containing ion-pairs formation, such as [(Mg2+)2(TFSI-)Br-]2+ and [(Mg2+)2(TFSI-)(Br-)(G2)2]2+, which results in the much thinner Br- containing and organic-inorganic mixed interphases on Mg-metal anodes. Furthermore, conventional MgSO4-based electrolytes and even calcium (Ca)-ion electrolytes can also be revived by similar strategy, demonstrating its generality and superiority.

Noncovalent Inhibition and Covalent Inactivation of Proline Dehydrogenase by Analogs of N-Propargylglycine.

The flavoenzyme proline dehydrogenase (PRODH) catalyzes the first step of proline catabolism, the oxidation of l-proline to Δ1-pyrroline-5-carboxylate. The enzyme is a target for chemical probe discovery because of its role in the metabolism of certain cancer cells. N-propargylglycine is the first and best characterized mechanism-based covalent inactivator of PRODH. This compound consists of a recognition module (glycine) that directs the inactivator to the active site and an alkyne warhead that reacts with the FAD after oxidative activation, leading to covalent modification of the FAD N5 atom. Here we report structural and kinetic data on analogs of N-propargylglycine with the goals of understanding the initial docking step of the inactivation mechanism and to test the allyl group as a warhead. The crystal structures of PRODH complexed with unreactive analogs in which N is replaced by S show how the recognition module mimics the substrate proline by forming ion pairs with conserved arginine and lysine residues. Further, the C atom adjacent to the alkyne warhead is optimally positioned for hydride transfer to the FAD, providing the structural basis for the first bond-breaking step of the inactivation mechanism. The structures also suggest new strategies for designing improved N-propargylglycine analogs. N-allylglycine, which consists of a glycine recognition module and allyl warhead, is shown to be a covalent inactivator; however, it is less efficient than N-propargylglycine in both enzyme inactivation and cellular assays. Crystal structures of the N-allylglycine-inactivated enzyme are consistent with covalent modification of the N5 by propanal.

Direct observation of the complex S(IV) equilibria at the liquid-vapor interface

The multi-phase oxidation of S(IV) plays a crucial role in the atmosphere, leading to the formation of haze and severe pollution episodes. We here contribute to its understanding on a molecular level by reporting experimentally determined pKa values of the various S(IV) tautomers and reaction barriers for SO2 formation pathways. Complementary state-of-the-art molecular-dynamics simulations reveal a depletion of bisulfite at low pH at the liquid-vapor interface, resulting in a different tautomer ratio at the interface compared to the bulk. On a molecular-scale level, we explain this with the formation of a stable contact ion pair between sulfonate and hydronium ions, and with the higher energetic barrier for the dehydration of sulfonic acid at the liquid-vapor interface. Our findings highlight the contrasting physicochemical behavior of interfacial versus bulk environments, where the pH dependence of the tautomer ratio reported here has a significant impact on both SO2 uptake kinetics and reactions involving NOx and H2O2 at aqueous aerosol interfaces.

Effect of Methyl Red on the surface properties of DTAB in CH3OH–H2O

The purpose of this study is to investigate the effect of Methyl Red (MR) on the micellization and surface properties of a cationic surfactant, dodecyl trimethylammonium bromide (DTAB), in CH3OH–H2O mixed solvent systems at varying CH3OH parts by volumes (0.1, 0.2, 0.3, and 0.4) and temperatures (298.15 K, 308.15 K and 318.15 K). Surface tension and conductivity measurements were conducted to obtain the critical micelle concentration (CMC) and various surface properties such as premicellar slope (dγdlogC), maximum surface concentration (Γmax), minimum surface area occupied (πCMC), free energy of adsorption (ΔGadso), adsorption efficiency (pC20), packing parameter (P), and aggregation number (Nagg). The results indicate that the presence of MR significantly influences the behavior of DTAB in the mixed solvent system. In the absence of MR, the CMC increased with higher CH3OH content, while MR reduced the CMC, promoting micelle formation through dye-surfactant-ion-pair (DSIP) formation. Surface properties were enhanced in the presence of MR leading to higher surface pressure. Additionally, the free energy of adsorption (ΔGadso) became more negative with MR, indicating a more favorable adsorption process. Correlations of (dγdlogC), Γmax,γ0/γcmc, pC20, Nagg, CMC/C20 with the parts by volume of CH3OH at 298.15 K, 308.15 K, and 318.15 K are discussed with and without MR. The obtained results were used to examine the effect of MR on the surface properties of DTAB in the CH3OH–H2O medium. The change in properties can be explained in terms of the change in polarity of the medium and dye-surfactant ion pair (DSIP) formation.

Quantification of Extractable Total Per- and Polyfluoroalkyl Substances in Water Using Persulfate Preoxidation, Solid Phase Extraction, and Chemical Defluorination with Sodium Biphenyl or Ion-Pair Formation with Methylene Blue

Quantification of Extractable Total Per- and Polyfluoroalkyl Substances in Water Using Persulfate Preoxidation, Solid Phase Extraction, and Chemical Defluorination with Sodium Biphenyl or Ion-Pair Formation with Methylene Blue

Solvent effects on the prediction of redox potentials: Application to nitroxides

This paper investigates the impact of solute-solvent interactions on the redox potentials of nitroxides, with a focus on ionic interactions caused by the presence of electrolytes found in different environment such as batteries. The analysis of various nitroxide families shows that ion-substituent interactions, especially in aromatic systems, significantly influence complex stability. Indeed, the counterion typically prefers to position near the substituent. However, in acetonitrile, the hydroxylamine anion and its cation show strong interactions when the cation is near the nitroxyl moiety. The study also confirm that an electrostatic interaction model can predict the effects of substituents, aromaticity, and ring size on redox potentials of nitroxides. Concerning the impact of the environment, solute-ion interactions and ion-pairs formation play a crucial role in modulating the redox potential.

Ion-Pairing Propensity in Guanidinium Salts Dictates Their Protein (De)stabilization Behavior.

Since the proposition of the Hofmeister series, guanidinium (Gdm) salts hold a special mention in protein science owing to their contrasting effect on protein(s) depending on the counteranion(s). For example, while GdmCl is known to act as a potential protein denaturant, Gdm2SO4 offers minimal effect on protein structure. Despite the fact that theoretical studies reckon the formation of ion-pairing to be responsible for such behavior, experimental validation of this hypothesis is still in sparse. In this study, we combine electrochemical impedance spectroscopy (EIS) and THz spectroscopy to underline the effect of GdmCl and Gdm2SO4 on a model amide molecule N-methylacetamide (NMA). Molecular dynamics (MD) simulation studies predict that Gdm2SO4 forms heteroion pairing in water, which inhibits Gdm+ ions to approach NMA molecules, while in case of GdmCl, Gdm+ ions directly interact with NMA. The experimental findings on ion hydration, specifically the detailed analysis of the ion-water rattling mode, which appears in the THz frequency domain, unambiguously endorse this hypothesis. Our study establishes the fact that the propensity of ion-pairing in Gdm salts dictates their (de)stabilization effect on proteins.

Ion Association and Hydration of Some Heavy-Metal Nitrate Salts in Aqueous Solution.

Aqueous solutions of four heavy-metal nitrate salts (AgNO3, TlNO3, Cd(NO3)2 and Pb(NO3)2) have been studied at 25 °C using broadband dielectric relaxation spectroscopy (DRS) at frequencies 0.27 ≤ ν/GHz ≤ 115 over the approximate concentration range 0.2 ≲ c/mol L-1 ≲ 2.0 (0.08 ≲ c/mol L-1 ≲ 0.4 for the less-soluble TlNO3). The spectra for AgNO3, TlNO3, and Pb(NO3)2 were best described by assuming the presence of three relaxation processes. These consisted of one solute-related Debye mode centered at ∼2 GHz and two higher-frequency solvent-related modes: one an intense Cole-Cole mode centered at ∼18 GHz and the other a small-amplitude Debye mode at ∼500 GHz. These modes can be assigned, respectively, to the rotational diffusion of contact ion pairs (CIPs), the cooperative relaxation of solvent water molecules, and its preceding fast H-bond flip. For Cd(NO3)2 solutions an additional solute-related Debye mode of small-amplitude, centered at ∼0.5 GHz, was required to adequately fit the spectra. This mode was consistent with the presence of small amounts of solvent-shared ion pairs. Detailed analysis of the solvent modes indicated that all the cations are strongly solvated with, at infinite dilution, effective total hydration numbers (Zt0 values) of irrotationally bound water molecules of ∼5 for both Ag+ and Tl+, ∼10 for Pb2+, and ∼20 for Cd2+. These results clearly indicate the presence of a partial second hydration shell for Pb2+(aq) and an almost complete second shell for Cd2+(aq). However, the hydration numbers decline considerably with increasing solute concentration due to ion-ion interactions. Association constants for the formation of contact ion pairs indicated weak complexation that varies in the order: Tl+ < Ag+ < Pb2+ < Cd2+, consistent with the charge/radius ratios of the cations and their Gibbs energies of hydration. Where comparisons were possible the present constants mostly agreed well with the rather uncertain literature values.

Development of levamlodipine long-acting patches based on an ion-pair strategy: Investigation of the mechanism for reducing skin irritation

The aim of this study was to develop a long-acting transdermal patch of levamlodipine (LAM) using an ion-pair strategy to reduce the skin irritation induced by topical application of LAM and explore the mechanism underlying the improvement of skin irritation. The formulation was optimized through porcine in vitro transdermal experiments and rabbit in vivo skin irritation tests. The obtained formulation consisted of poly (2-Ethylhexyl acrylate-co-N-Vinyl-2-pyrrolidone-co-N-(2-Hydroxyethyl) acrylamide) (PENH) as the adhesive matrix, 13.00 % levamlodipine-sorbic acid ion-pair complex (LAM-SA) (w/w), and 10 % isopropyl myristate (IPM) (w/w), with a patch thickness of 70 μm, achieving an erythema index of 188 for rabbit skin and 117–187 for human skin (264 for rabbit skin and 110–260 for human skin in the absence of sorbic acid (SA)). In vivo rabbit and human skin erythema analysis and H&E staining verified that the optimized ion-pair patch effectively reduced skin irritation. Drug distribution experiments in the skin, ATR-FTIR, and molecular simulation were used to characterize the mechanism by which the ion-pair reduced skin irritation. Excessive accumulation of LAM in the epidermis induced secondary structural changes in keratin, resulting in skin barrier damage and inflammatory response. The formation of the LAM-SA ion pair altered physicochemical properties of LAM, reducing drug retention in the epidermis and, thereby, reducing skin irritation. This study demonstrated the potential of the ion-pair strategy to improve the safety of transdermal drug delivery system (TDDS) and provided a means for reducing skin irritation caused by the active pharmaceutical ingredient (API) itself.

SF6 Negative Ion Formation in Charge Transfer Experiments.

In the present work, we report an update and extension of the previous ion-pair formation study of Hubers, M.M.; Los, J. Chem. Phys.1975, 10, 235-259, noting new fragment anions from time-of-flight mass spectrometry. The branching ratios obtained from the negative ions formed in K + SF6 collisions, in a wide energy range from 10.7 up to 213.1 eV in the centre-of-mass frame, show that the main anion is assigned to SF5- and contributing to more than 70% of the total ion yield, followed by the non-dissociated parent anion SF6- and F-. Other less intense anions amounting to <20% are assigned to SF3- and F2-, while a trace contribution at 32u is tentatively assigned to S- formation, although the rather complex intramolecular energy redistribution within the temporary negative ion is formed during the collision. An energy loss spectrum of potassium cation post-collision is recorded showing features that have been assigned with the help of theoretical calculations. Quantum chemical calculations for the lowest-lying unoccupied molecular orbitals in the presence of a potassium atom are performed to support the experimental findings. Apart from the role of the different resonances participating in the formation of different anions, the role of higher-lying electronic-excited states of Rydberg character are noted.

Selective colorimetric sensors and diagnostic microfiber yarn for carbonyl groups via ion-pairing dyes with plural chromatic behaviors

An ideal chemo-responsive dye requires several essential features, including a facile preparation method, a broad range of dynamic color changes, and colorimetric responses to multiple stimuli. Herein, we present a simple and reliable procedure for synthesis of chemo-responsive dye from ion-pair formation of oppositely charged dyes. The ion-pairing dye exhibits various colorimetric responses to pH levels and amine concentrations, indicating that the dye has plural chromatic behaviors and wide visible spectrum: ‘halochromism’ and ‘solvatochromism’. Due to this, our colorimetric sensors can selectively identify various carbonyl groups such as ketone, aldehyde, and carboxylic acid, in which the detection limits range from a minimum of 0.019 to maximum of 2.506 ppm. Furthermore, the sensing array composed of a single ion-pairing dye enables the generation of easily identifiable color-patterns as unique fingerprints for each carbonyl groups across a range of gas levels. Based on these colorimetric responses to carbonyl groups, we develop dye-loaded microfiber yarn for biomarker diagnosis, which can check for the presence of acetone gas in the exhaled breaths and confirm whether an individual is in the optimal ketosis state or not.

Carbon black dispersing agents based on fully hydrophilic poly(N‐vinylamide): Surface adsorption via selective interactions

AbstractInterface‐stabilizing polymers are frequently used in commercial applications to provide long‐lasting dispersions of colloids in liquid media. Dispersing high concentrations of pigments in aqueous solution can be particularly challenging. Here, we present a novel, fully hydrophilic dispersing agent system for carbon black (CB) pigments based on partially hydrolyzed poly(N‐vinylamides). These highly water‐soluble polymers adsorb onto CB through specific interactions between the polymer and surface‐bound moieties, rather than the hydrophobic effect. They participate in hydrogen bonding, ion–dipole interactions and can be partially positively charged to engage in ion pair formation, providing strong surface affinity. Dispersants are synthesized using free radical copolymerization of N‐vinylform‐ and ‐acetamide, acidic hydrolysis, and subsequent PEGylation. Molecular water solubility of the synthesized polymers is confirmed by using fluorescence spectroscopy. Highly concentrated CB dispersions are prepared in acidic, neutral, and basic media, which we identify to strongly affect the interactions between polymer and CB. The stability of CB dispersions is analyzed using ultracentrifugation and thermogravimetric analysis. The presented polymers are able to provide significantly more stable dispersions than a commercial reference product, showcasing the suitability of poly(N‐vinylamines) as potent CB dispersing agents.

Structure and intermolecular interactions in ionic liquid 1-ethyl-3-methylimidazolium bromide and its aqueous solutions investigated by vibrational spectroscopy and quantum chemical computations.

The recently developed efficient protocol to explicit quantum mechanical modeling of structure and IR spectra of liquids and solutions (S. A. Katsyuba, S. Spicher, T. P. Gerasimova, S. Grimme, J. Phys. Chem. B 2020, 124, 6664) is applied to ionic liquid (IL) 1-ethyl-3-methylimidazolium bromide (EmimBr), its C2-deuterated analog [Emim-d]Br and its aqueous solutions. It is shown that the solvation strongly modifies frequencies and IR intensities of the CH/CD stretching vibrations (νCH/νCD) of the imidazolium ring. The main vibrational spectroscopic features of the neat IL are reproduced by the simulations for a cluster (EmimBr)9, in which all three imidazolium CH moieties of the solvated cation form short contacts with three Br- anions, and another two Br- anions are located on top and bottom of imidazolium ring. Cluster models of aqueous solutions reproduce the experimental vibrational frequencies of actual solutions, provided that the Br- anion of solvated contact ion pair (CIP) is situated on top of imidazolium ring, and CH/CD moieties of the latter participate in short contacts with surrounding water molecules. Both structural and spectroscopic analysis allow to interpret the short contacts CH/CD⋯Br- and CH/CD⋯OH2 as hydrogen bonds of approximately equal strength. Enthalpies of bonding of these liquid-state H-bonds, estimated with the use of empirical correlations, amount to ca. 1.4 kcal⋅mol-1, while the analogous estimates obtained for the gas-phase charged species [Emim]2Br+ increase to 5.6 kcal⋅mol-1. It is shown that formation of solvent-shared ion pair (SIP) in aqueous solution, where the counterions of IL are separated by two water molecules H-bonded to a Br- anion, produces frequency shifts ΔνCH/CD, strongly different from the case of CIP formation. This difference can be used for IR/Raman spectroscopic differentiation of the type of solvated ion pairs of EmimBr or other related ILs.

The solvation shell probed by resonant intermolecular Coulombic decay

Molecules involved in solvation shells have properties differing from those of the bulk solvent, which can in turn affect reactivity. Among key properties of these molecules are their nature and electronic structure. Widely used tools to characterize this type of property are X-ray-based spectroscopies, which, however, usually lack the capability to selectively probe the solvation-shell molecules. A class of X-ray triggered “non-local” processes has the recognized potential to provide this selectivity. Intermolecular Coulombic decay (ICD) and related processes involve neighbouring molecules in the decay of the X-ray-excited target, and are thus naturally sensitive to its immediate environment. Applying electron spectroscopy to aqueous solutions, we explore the resonant flavours of ICD and demonstrate how it can inform on the first solvation shell of excited solvated cations. One particular ICD process turns out to be a potent marker of the formation of ion pairs. Another gives a direct access to the electron binding energies of the water molecules in the first solvation shell, a quantity previously elusive to direct measurements. The resonant nature of the processes makes them readily measurable, providing powerful new spectroscopic tools.

Theoretical Study of an Authentic Hydrocarbon Ion Pair.

For many years researchers believed that hydrocarbons only contain covalent bonds. However, since 1985 Okamoto et al. demonstrated the formation of hydrocarbon salts in several systems, demolishing the structural principle that hydrocarbons only contain covalent bonds. Despite the great importance of this outcome to the study of chemical bonds, quantum chemical calculations on these systems are essentially nonexistent. The stability of the hydrocarbon ions along with the steric hindrance associated with the formation of the covalent bond contribute to their occurrence either in solution (dissociated) or in the solid state. These facts along with the common formation of ion pairs in solvents of low polarity motivated us to search for hydrocarbon ion pairs in the gas phase. Its energetics has also been studied in four nonprotic solvents, through a continuum solvation model (CPCM). DFT and CASSCF calculations indicate a metastable and highly polar ion pair between the tricyclopropylcyclopropenylium cation and a simplified Kuhn's anion. The barrier to the covalent structure varies from ∼4.8 to 14.4 kcal/mol, while the energy difference between the ion pair and the covalent form varies from ∼4.3 to 25.4 kcal/mol. The obtained theoretical results along with previous experimental results suggest the following strategy to obtain kinetically and thermodynamically stable hydrocarbon ion pairs: choose very stable hydrocarbon ions and systematically increase the steric hindrance between them.