Anchor Points for the Unified Brønsted Acidity Scale: The rCCC Model for the Calculation of Standard Gibbs Energies of Proton Solvation in Eleven Representative Liquid Media

Hybrid quantum-classical polarizability model for single molecule biosensing

Optical whispering gallery mode biosensors are able to detect single molecules through effects of their polarizability. We address the factors that affect the polarizability of amino acids, which are the building blocks of life, via electronic structure theory. Amino acids are detected in aqueous environments, where their polarizability is different compared to the gasphase due to solvent effects. Solvent effects include structural changes, protonation and the local field enhancement through the solvent (water). We analyse the impact of these effects and find that all contribute to an increased effective polarizability in the solvent. We also address the excess polarizability relative to the displaced water cavity and develop a hybrid quantum-classical model that is in good agreement with self-consistent calculations. We apply our model to calculate the excess polarizability of 20 proteinogenic amino acids and determine the minimum resolution required to distinguish the different molecules and their ionised conformers based on their polarizability.

Tunable Metal-Free Imidazole-Benzimidazole Electrocatalysts for Oxygen Reduction in Aqueous Solutions

A series of metal-free imidazole-benzimidazole catalysts (ImBenz-H, ImBenz-NO2 , ImBenz-OCH3 ) for oxygen reduction reaction (ORR) were prepared. We demonstrate that the electrocatalytic O2 reduction by ImBenz-NO2 with the electron-withdrawing group showed high selectivity toward H2 O with the number of electrons transferred (n=3.7) in a neutral aqueous solution. The highest ORR selectivity toward H2 O2 was achieved using ImBenz-H (n=2.4) in an alkaline solution. Electrochemical studies of reaction kinetics disclosed that the highest turnover frequencies were obtained from ImBenz-H in both neutral and alkaline aqueous solutions. The results prove that the ORR selectivity is tunable by modulating the substituent of the ImBenz catalysts. Furthermore, DFT calculations suggested that the ORR mechanism of ImBenz-H involves the electron transfer from imidazole-benzimidazole to O2 resulting in the formation of H2 O2 which supports the redox active properties of the catalysts ImBenz.

The Unified Redox Scale for All Solvents: Consistency and Gibbs Transfer Energies of Electrolytes from their Constituent Single Ions

We have devised the unified redox scale Eabs H2O , which is valid for all solvents. The necessary single ion Gibbs transfer energy between two different solvents, which only can be determined with extra-thermodynamic assumptions so far, must clearly satisfy two essential conditions: First, the sum of the independent cation and anion values must give the Gibbs transfer energy of the salt they form. The latter is an observable and measurable without extra-thermodynamic assumptions. Second, the values must be consistent for different solvent combinations. With this work, potentiometric measurements on silver ions and on chloride ions show that both conditions are fulfilled using a salt bridge filled with the ionic liquid [N2225 ][NTf2 ]: if compared to the values resulting from known pKL values, the silver and chloride single ion magnitudes combine within a uncertainty of 1.5 kJ mol-1 to the directly measurable transfer magnitudes of the salt AgCl from water to the solvents acetonitrile, propylene carbonate, dimethylformamide, ethanol, and methanol. The resulting values are used to further develop the consistent unified redox potential scale Eabs H2O that now allows to assess and compare redox potentials in and over six different solvents. We elaborate on its implications.

Universal Trends between Acid Dissociation Constants in Protic and Aprotic Solvents

pKa values in non-aqueous solvents are of critical importance in many areas of chemistry. Our knowledge is, despite their relevance, still limited to the most fundamental properties and few pKa values in the most common solvents. Taking advantage of a recently introduced computationally efficient procedure we computed the pKa values of 182 compounds in 21 solvents. This data set is used to establish for the first time universal trends between all solvents. Our computations indicate, that the total charge of the molecule and the charge of the acidic group combined with the Kamlet-Taft solvatochromic parameters are sufficient to predict pKa values with at least semi- quantitative accuracy. We find, that neutral acids such as alcohols are strongly affected by the solvent properties. This is contrasted by cationic acids like ammonium ions whose pKa is often almost completely independent from the choice of solvent.

Modeling pKa of the Brønsted Bases as an Approach to the Gibbs Energy of the Proton in Acetonitrile

A simple but efficient computational approach to calculate pK_a in acetonitrile for a set of phosphorus, nitrogen, and carbon bases was established. A linear function that describes relations between the calculated ΔG’_a.sol(BH⁺) and pK_a values was determined for each group of bases. The best model was obtained through the variations in the basis set, in the level of theory (density functionals or MP2), and in the continuum solvation model (IPCM, CPCM, or SMD). The combination of the IPCM/B3LYP/6-311+G(d,p) solvation approach with MP2/6-311+G(2df,p)//B3LYP/6-31G(d) gas-phase energies provided very good results for all three groups of bases with R² values close to or above 0.99. Interestingly, the slopes and the intercepts of the obtained linear functions showed significant deviations from the theoretical values. We made a linear plot utilizing all the conducted calculations and all the structural variations and employed methods to prove the systematic nature of the intercept/slope dependence. The interpolation of the intercept to the ideal slope value enabled us to determine the Gibbs energy of the proton in acetonitrile, which amounted to −258.8 kcal mol⁻¹. The obtained value was in excellent agreement with previously published results.

Measurements and Utilization of Consistent Gibbs Energies of Transfer of Single Ions: Towards a Unified Redox Potential Scale for All Solvents

Utilizing the “ideal” ionic liquid salt bridge to measure Gibbs energies of transfer of silver ions between the solvents water, acetonitrile, propylene carbonate and dimethylformamide results in a consistent data set with a precision of 0.6 kJ mol⁻¹ over 87 measurements in 10 half‐cells. This forms the basis for a coherent experimental thermodynamic framework of ion solvation chemistry. In addition, we define the solvent independent peabsH2O ‐ and the EabsH2O values that account for the electronating potential of any redox system similar to the pHabsH2O value of a medium that accounts for its protonating potential. This EabsH2O scale is thermodynamically well‐defined enabling a straightforward comparison of the redox potentials (reducities) of all media with respect to the aqueous redox potential scale, hence unifying all conventional solvents′ redox potential scales. Thus, using the Gibbs energy of transfer of the silver ion published herein, one can convert and unify all hitherto published redox potentials measured, for example, against ferrocene, to the EabsH2O scale.

How to Predict the pK a of Any Compound in Any Solvent

Acid-base properties of molecules in nonaqueous solvents are of critical importance for almost all areas of chemistry. Despite this very high relevance, our knowledge is still mostly limited to the pK _a of rather few compounds in the most common solvents, and a simple yet truly general computational procedure to predict pK _a's of any compound in any solvent is still missing. In this contribution, we describe such a procedure. Our method requires only the experimental pK _a of a reference compound in water and a few standard quantum-chemical calculations. This method is tested through computing the proton solvation energy in 39 solvents and by comparing the pK _a of 142 simple compounds in 12 solvents. Our computations indicate that the method to compute the proton solvation energy is robust with respect to the detailed computational setup and the construction of the solvation model. The unscaled pK _a's computed using an implicit solvation model on the other hand differ significantly from the experimental data. These differences are partly associated with the poor quality of the experimental data and the well-known shortcomings of implicit solvation models. General linear scaling relationships to correct this error are suggested for protic and aprotic media. Using these relationships, the deviations between experiment and computations drop to a level comparable to that observed in water, which highlights the efficiency of our method.

A theoretical study on the pKa values of selenium compounds in aqueous solution

The pK_a values of different kinds of selenium compounds (R-SeH) were investigated by using the ωB97XD method with a SMD model.

Global and local minima of protonated acetonitrile clusters

Potential energy surfaces of protonated acetonitrile clusters have been explored to locate global and local minima energy structures. The structures are stabilized by strong hydrogen bonds, anti-parallel dimers, dipole–dipole and CH⋯N interactions.

Proton Thermodynamics in a Protic Ionic Liquid, Ethylammonium Nitrate

In order to investigate the proton solvation state in protic ionic liquids (PILs), ten acid dissociation enthalpies and entropies of eight compounds were determined in ethylammonium nitrate (EAN). Regardless of the nature of the compound, 24 kJ mol^-1 larger enthalpy and 65 J mol^-1  K^-1 larger entropy than those in water, respectively, were observed. These values were reasonably explained by the differences in the proton solvation structure in EAN and water. Namely, protons in EAN exist as HNO₃ , having a higher reaction energy than that of H₃ O⁺ in water, undergo entropic stabilization as a result of the less-structured solvation. As such, the entropic effect of the proton solvation structure on the acid-base property is possibly applicable to all PILs. In addition, based on these proton thermodynamics, enthalpy and entropy windows were proposed as a novel perspective for the characterization of solvents. Use of this concept enabled the visualization of similarities and differences between EAN and water.

Theoretical modeling of pKa's of thiol compounds in aqueous solution

The pK_a's of different kinds of thiols (R-SH) were investigated by using the M06-2X method with a SMD_sSAS model.

Validation of pH Standards and Estimation of the Activity Coefficients of Hydrogen and Chloride Ions in an Ionic Liquid, Ethylammonium Nitrate

We selected and validated the pH values of three standard materials that function in the protic ionic liquid, ethylammonium nitrate (EAN). The pH values of 0.05 mol kg^-1 phthalate, oxalate, and phosphate buffers were 4.93 (0.04), 2.12 (0.04), and 7.13 (0.06), respectively (the values in the parentheses denote the standard deviation). Because the pH of EAN ranges from 0 to 10, with a neutral pH of 5, these materials are usable as acidic, basic, or neutral standards. The standard electrode potential of silver-silver chloride in EAN was 127.2 (1.7) mV. The activity coefficients of hydrogen and chloride ions remain equal to unity in EAN of a wide concentration range, which indicates that the effective ionic strength is independent of the solute ion concentration. In addition, the estimated value of the transfer activity coefficient of chloride ion suggests a weaker solvation in EAN compared with water in spite of a ubiquitous cation (C₂H₅NH₃⁺). These behaviors of ions in EAN can be explained by the unique solvation in the ionic liquid through direct ion-ion electrostatic interactions.

Basic Remarks on Acidity

This Review provides a unified view on Brønsted acidity. For this purpose, a brief overview of the concepts acidity, acid strengths, and pH value is given, including problems, proposed solutions, and the use of the pH_abs /pHabsH2O scale as a unifying concept. Thereafter, some examples of the accessibility and application of unified pH_abs values are given. The Review is rounded off with the analogy of acid-base chemistry to redox chemistry with the introduction of the unified redox scale pe_abs . The combination of pH_abs and pe_abs values in the protoelectric potential map (PPM), as elaborated in ongoing studies on the thermochemistry of single ions, provides a means to classify and to compare all possible acid-base/redox reactions in a medium-independent and, thus, unified fashion.

A unified view to Brønsted acidity scales: do we need solvated protons?

The most comprehensive solvent acidity scale spanning 28 orders of magnitude of acidity was measured in the low-polarity solvent 1,2-dichloroethane (DCE). Its experimental core is linked to the unified acidity scale (pH_abs) in an unprecedented and generalized approach only based on experimental values. This enables future measurements of acid strengths and acidity adjustments in low polarity solvents. The scale was cross-validated computationally. The purely experimental and computational data agree very well. The DCE scale includes 87 buffer systems with values between -13.0 and +15.4, i.e. similar to water at hypothetical and extreme pH values of -13.0 to +15.4. Unusually, such high acidities in DCE are not realized via solvated protons, but rather through strongly acidic molecules able to directly donate their proton, even to weak bases dissolved in the solution. Thus, in all examined cases, not a single solvated proton is present in one liter of DCE.

The Evolution of Electrospray Generated Droplets is Not Affected by Ionization Mode

Ionization efficiency and mechanism in ESI is strongly affected by the properties of mobile phase. The use of mobile-phase properties to accurately describe droplets in ESI source is convenient but may be inadequate as the composition of the droplets is changing in the plume due to electrochemical reactions occurring in the needle tip as well as continuous drying and fission of droplets. Presently, there is paucity of research on the effect of the polarity of the ESI mode on mobile phase composition in the droplets. In this paper, the change in the organic solvent content, pH, and droplet size are studied in the ESI plume in both ESI+ and ESI- ionization mode. We introduce a rigorous way - the absolute pH (pH_abs^H₂^O) - to describe pH change in the plume that takes into account organic solvent content in the mobile phase. pH_abs^H₂^O enables comparing acidities of ESI droplets with different organic solvent contents. The results are surprisingly similar for both ionization modes, indicating that the dynamics of the change of mobile-phase properties is independent from the ESI mode used. This allows us to conclude that the evolution of ESI droplets first of all proceeds via the evaporation of the organic modifier and to a lesser extent via fission of smaller droplets from parent droplets. Secondly, our study shows that qualitative findings related to the ESI process obtained on the ESI+ mode can almost directly be applied also in the ESI- mode. Graphical Abstract ᅟ.

From an Easily Accessible Pentacarbonylcobalt(I) Salt to Piano-Stool Cations [(arene)Co(CO)2 ]. Chemistry 2017;23:14658-14664. [PMID: 28796933 DOI: 10.1002/chem.201703589]

The facile synthesis of a pentacarbonyl cobalt(I) salt without the need for a superacid as solvent is presented. This salt, [Co(CO)₅ ]⁺ [Al(OR^F )₄ ]^- {R^F =C(CF₃ )₃ }, readily accessible on a multigram scale, undergoes substitution reactions with arenes yielding the hitherto unknown class of two-legged cobalt piano-stool complexes [(arene)Co(CO)₂ ]⁺ with four different arene ligands. Such a substitution chemistry would have been impossible in superacid solution, as the arenes used would have been oxidized and/or protonated. Thus, the general approach described herein may have a wide synthetic use. Additionally, the thermochemistry of the piano-stool complexes is shown to be not easy to describe computationally and most of the established DFT methods overestimate the reaction energies. Only CCSD(T) calculations close to the basis set limit gave energies fully agreeing with the experiment.

Reaction Mechanism of Covalent Modification of Phosphatidylethanolamine Lipids by Reactive Aldehydes 4-Hydroxy-2-nonenal and 4-Oxo-2-nonenal

4-Hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE) are biologically important reactive aldehydes formed during oxidative stress in phospholipid bilayers. They are highly reactive species due to presence of several reaction centers and can react with amino acids in peptides and proteins, as well as phosphoethanolamine (PE) lipids, thus modifying their biological activity. The aim of this work is to study in a molecular detail the reactivity of HNE and ONE toward PE lipids in a simplified system containing only lipids and reactive aldehydes in dichloromethane as an inert solvent. We use a combination of quantum chemical calculations, ¹H NMR measurements, FT-IR spectroscopy, and mass spectrometry experiments and show that for both reactive aldehydes two types of chemical reactions are possible: formation of Michael adducts and Schiff bases. In the case of HNE, an initially formed Michael adduct can also undergo an additional cyclization step to a hemiacetal derivative, whereas no cyclization occurs in the case of ONE and a Michael adduct is identified. A Schiff base product initially formed when HNE is added to PE lipid can also further cyclize to a pyrrole derivative in contrast to ONE, where only a Schiff base product is isolated. The suggested reaction mechanism by quantum-chemical calculations is in a qualitative agreement with experimental yields of isolated products and is also additionally investigated by ¹H NMR measurements, FT-IR spectroscopy, and mass spectrometry experiments.

Experimental Insight into the Thermodynamics of the Dissolution of Electrolytes in Room-Temperature Ionic Liquids: From the Mass Action Law to the Absolute Standard Chemical Potential of a Proton

Room-temperature ionic liquids (ILs) are a class of nonaqueous solvents that have expanded the realm of modern chemistry, drawing increasing interest over the last few decades, not only in terms of their own unique physical chemistry but also in many applications including organic synthesis, electrochemistry, and biological systems, wherein charged solutes (i.e., electrolytes) often play vital roles. However, our fundamental understanding of the dissolution of an electrolyte in an IL is still rather limited. For example, the activity of a charged species has frequently been assumed to be unity without a clear experimental basis. In this study, we have discussed a standard component-based scheme for the dissolution of an electrolyte in an IL, supported by our observation of ideal Nernstian responses for the reduction of silver and ferrocenium salts in a representative IL, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim⁺][NTf₂ ^-] or [emim⁺][TFSI^-]). Using this scheme, which was also supported by temperature-dependent measurements with ILs having longer alkyl chains in the imidazolium ring, and the solubility of the IL in water, we established the concept of Gibbs transfer energies of "pseudo-single ions" from the IL to conventional neutral molecular solvents (water, acetonitrile, and methanol). This concept, which bridges component- and constituent-based energetics, utilizes an extrathermodynamic assumption, which itself was justified by experimental observations. These energies enable us to eliminate inner potential differences between the IL and molecular solvents (solvent-solvent interactions), that is, on a practical level, conditional liquid junction potential differences, so that we can discuss ion-solvent interactions independently. Specifically, we have examined the standard electrode potential of the ferrocenium/ferrocene redox couple, Fc⁺/Fc, and the absolute intrinsic standard chemical potential of a proton in [emim⁺][NTf₂ ^-], finding that the proton is more acidic in the IL than in water by 6.5 ± 0.6 units on the unified pH scale. These results strengthen the progress on the physical chemistry of ions in IL solvent systems on the basis of their activities, providing a rigorous thermodynamic framework.

Reactive p-block cations stabilized by weakly coordinating anions

The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the - naturally subjective -"reactive"-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

Complexes of carborane acids linked by strong hydrogen bonds: acidity scales

Scales based on DFT results of calculations and on the topological QTAIM parameters are introduced and discussed to order the species analyzed here by acidity; in particular, carborane acids are analyzed and the theoretical results are compared with experimental results.

Conformational Switching of a Foldamer in a Multicomponent System by pH-Filtered Selection between Competing Noncovalent Interactions

Biomolecular systems are able to respond to their chemical environment through reversible, selective, noncovalent intermolecular interactions. Typically, these interactions induce conformational changes that initiate a signaling cascade, allowing the regulation of biochemical pathways. In this work, we describe an artificial molecular system that mimics this ability to translate selective noncovalent interactions into reversible conformational changes. An achiral but helical foldamer carrying a basic binding site interacts selectively with the most acidic member of a suite of chiral ligands. As a consequence of this noncovalent interaction, a global absolute screw sense preference, detectable by (13)C NMR, is induced in the foldamer. Addition of base, or acid, to the mixture of ligands competitively modulates their interaction with the binding site, and reversibly switches the foldamer chain between its left and right-handed conformations. As a result, the foldamer-ligand mixture behaves as a biomimetic chemical system with emergent properties, functioning as a "proton-counting" molecular device capable of providing a tunable, pH-dependent conformational response to its environment.

Superacidity of closo-dodecaborate-based Brønsted acids: a DFT study

The structures and intrinsic gas-phase acidities (GA) of some dodecaborane acids, the derivatives of YB12H11H (Y = PF3, NH3, NF3, NMe3), B12H12H2, and B12H12H(-) (HA, H2A, and HA(-), respectively) have been computationally explored with DFT B3LYP method at the 6-311+G** level of theory as new possible directions of creating superstrong Brønsted acids. Depending on the nature and number of the substituents different protonation geometries were investigated. In general, the GA values of the neutral systems varied according to the substituents in the following order: CF3 < F < Cl and in case of anionic acids: CF3 < Cl < F. The dodecatrifluoromethyl derivative of H2A, B12(CF3)12H1H2, emerges as the strongest among the considered acids and is expected to be in the gas phase at least as strong as the undecatrifluoromethyl carborane, CB11(CF3)11H1H. The GA values of the respective monoanionic forms of the considered acids all, but the (CF3)11 derivative, remained higher than the widely used threshold of superacidity. The HA derivatives' (Y = PF3, NF3) GA's were approximately in the same range as the H2A acids'. In the case Y = NH3 or NMe3 the GA values were significantly higher. Also, the pKa values of B12H12H2, CB11H12H, and their perfluorinated derivatives in 1,2-dichloroethane (DCE) were estimated with SMD and cluster-continuum model calculations. The obtained estimates of pKa values of the perfluorinated derivatives are by around 30 units lower than that of trifluoromethylsulfonylimide, making these acids the strongest ever predicted in solution. The derivatives of B12H12H2 are as a rule not significantly weaker acids than the respective derivatives of CB11H12H. This is important for expanding practical applicability of this type of acids and their anions, as they are synthetically much more accessible than the corresponding CB11H12(-) derivatives.

From unsuccessful H2-activation with FLPs containing B(Ohfip)3 to a systematic evaluation of the Lewis acidity of 33 Lewis acids based on fluoride, chloride, hydride and methyl ion affinities

From hard to soft: The ion affinities of a large set of 33 Lewis acids towards hard and soft bases were examined with a unified isodesmic approach.

Are thermodynamic cycles necessary for continuum solvent calculation of pKas and reduction potentials?

Continuum solvent calculations of pKas and reduction potentials usually entail the use of a thermodynamic cycle to express the reaction free energy in terms of gas phase energies and free energies of solvation. In this work, we present a systematic study comparing the solution phase free energy changes obtained in this manner with those directly computed within the SMD solvation model against a large test set of 117 pKas and 42 reduction potentials in water and DMSO. The inclusion of vibrational contributions in the free energy of solvation has a negligible impact on the accuracy of thermodynamic cycle predictions of pKas and reduction potentials. Additionally, when gas phase energies in the thermodynamic cycle are computed at more accurate levels of theory, very similar results (mean unsigned difference of 0.5 kcal mol(-1)) can be achieved when the high-level computations (MP2/GTMP2Large and G3(MP2)-RAD(+)) are directly carried out within the continuum model. Increasing the accuracy of the electronic structure theory may or may not improve the agreement with experiment suggesting that the error is largely in the solvation model. For amino acids where their gas and solution phase species exist as different tautomers, the direct approach provided a significant improvement in calculated pKas. These results demonstrate that direct calculation of solution phase pKas and reduction potentials within the SMD model provides a general and reliable approximation to corresponding thermodynamic cycle based protocols, and is recommended for systems where solvation induced changes in geometry are significant. Further studies are necessary to ascertain whether the results are generalisable to other continuum solvation models.

Predicting pKa in Implicit Solvents: Current Status and Future Directions

Computational prediction of condensed phase acidity is a topic of much interest in the field today. We introduce the methods available for predicting gas phase acidity and pKas in aqueous and non-aqueous solvents including high-level electronic structure methods, empirical linear free energy relationships (LFERs), implicit solvent methods, explicit solvent statistical free energy methods, and hybrid implicit–explicit approaches. The focus of this paper is on implicit solvent methods, and we review recent developments including new electronic structure methods, cluster-continuum schemes for calculating ionic solvation free energies, as well as address issues relating to the choice of proton solvation free energy to use with implicit solvation models, and whether thermodynamic cycles are necessary for the computation of pKas. A comparison of the scope and accuracy of implicit solvent methods with ab initio molecular dynamics free energy methods is also presented. The present status of the theory and future directions are outlined.