A thermodynamic study of the protonation of some sulphur-containing α,ω-diamines in aqueous solution

The protonation equilibria in aqueous solution of α,ω-thiadiamines of general formula (R)(R′)N(CH₂)_n,S(CH₂)_mNH₂ (R,R′= CH₃ or H) have been investigated potentiometrically and calorimetrically at 25° C in 0.5 mole dm^?3 KNO₃ solution. The enthalpy and entropy changes are discussed in terms of intrinsic proton affinities and solvation effects.     

Mechanism of atmospheric pressure chemical ionization of morphine,codeine, and thebaine in corona discharge–ion mobility spectrometry: Protonation,ammonium attachment,and carbocation formation

Atmospheric pressure chemical ionizations (APCIs) of morphine, codeine, and thebaine were studied in a corona discharge ion source using ion mobility spectrometry (IMS) at temperature range of 100°C–200°C. Density functional theory (DFT) at the B3LYP/6‐311++G(d,p) and M062X/6‐311++G(d,p) levels of theory were used to interpret the experimental data. It was found that in the presence of H₃O⁺ as reactant ion (RI), ionization of morphine and codeine proceeds via both the protonation and carbocation formation, whereas thebaine participates only in protonation. Carbocation formation (fragmentation) was diminished with decrease in the temperature. At lower temperatures, proton‐bound dimers of the compounds were also formed. Ammonia was used as a dopant to produce NH₄⁺ as an alternative RI. In the presence of NH₄⁺, proton transfer from ammonium ion to morphine, codeine, and thebaine was the dominant mechanism of ionization. However, small amount of ammonium attachment was also observed. The theoretical calculations showed that nitrogen atom of the molecules is the most favorable proton acceptor site while the oxygen atoms participate in ammonium attachment. Furthermore, formation of the carbocations is because of the water elimination from the protonated forms of morphine and codeine.     

Molecular Engineering of Cation Solvation Structure for Highly Selective Carbon Dioxide Electroreduction

Balancing the activation of H₂O is crucial for highly selective CO₂ electroreduction (CO₂RR), as the protonation steps of CO₂RR require fast H₂O dissociation kinetics, while suppressing hydrogen evolution (HER) demands slow H₂O reduction. We herein proposed one molecular engineering strategy to regulate the H₂O activation using aprotic organic small molecules with high Gutmann donor number as a solvation shell regulator. These organic molecules occupy the first solvation shell of K⁺ and accumulate in the electrical double layer, decreasing the H₂O density at the interface and the relative content of proton suppliers (free and coordinated H₂O), suppressing the HER. The adsorbed H₂O was stabilized via the second sphere effect and its dissociation was promoted by weakening the O−H bond, which accelerates the subsequent *CO₂ protonation kinetics and reduces the energy barrier. In the model electrolyte containing 5 M dimethyl sulfoxide (DMSO) as an additive (KCl-DMSO-5), the highest CO selectivity over Ag foil increased to 99.2 %, with FE_CO higher than 90.0 % within −0.75 to −1.15 V (vs. RHE). This molecular engineering strategy for cation solvation shell can be extended to other metal electrodes, such as Zn and Sn, and organic molecules like N,N-dimethylformamide.     

Isobutane chemical ionization mass spectra of unsaturated alcohols

Analysis of the isobutane chemical ionization mass spectra of hexenols, cyclohexenols and various syn/anti pairs of bicyclic and tricyclic homoallylic alcohols shows that: (i) the spectra of the allylic alcohols are dominated by [M + H – H₂O]⁺ and [M + C₄H₉–H₂O]⁺ ions and contain traces of [M + H]⁺ ions; (ii) [M + H]⁺ ions are prominent in the spectra of acyclic and certain cyclic homoallylic alcohols; and (iii) [M + H]⁺ ions dominate the spectra of other acyclic unsaturated alcohols. The [M + H]⁺ ions may result from either: (a) protonation of the hydroxyl group, followed by a very rapid intramolecular proton transfer from the protonated hydroxyl group to the carbon–carbon double bond or internal solvation of the protonated hydroxyl group by the carbon–carbon double bond; and/or (b) direct protonation of the carbon–carbon double bond with significant internal solvation of the resulting carbocation by the hydroxyl group, which may lead to carbon–oxygen bond formation to give a protonated cyclic ether. The consequences of placing various geometric constraints on the possible intramolecular interactions between the hydroxyl group and the carbon–carbon double bond in unsaturated alcohols are explored.     

Oxygen reduction on a platinum cluster

The reaction Pt₅O₂(ads)+H⁺(aq)+e⁻→Pt₅–OOH is analyzed on the basis of density functional theory calculations. It is found that the electron transfer process takes place gradually as the hydronium ion gets close to the adsorbed oxygen. At a certain small O_ads–H⋯O_water distance, the barrier for proton and electron transfer becomes negligible. The effect of an electric field on the reaction is studied by charging the metal/adsorbate complex, and that of a solvation shell on the proton transfer process is explored by using the H₃O⁺(H₂O)₂ ion cluster to model the hydrated proton.     

Heat Effect of the Protonation of Glycine and the Enthalpies of Resolvation of Participating Chemical Species in Water–Dimethylsulfoxide Solvent Mixtures

Enthalpies of the protonation of glycine in water?dimethylsulfoxide (DMSO) mixed solvents are determined calorimetrically in the range of DMSO mole fractions of 0.0 to 0.9, at T = 298.15 K and an ionic strength μ = 0.3 (NaClO₄). It is established that the protonation of glycine becomes more exothermic with an increasing mole fraction of DMSO, and the enthalpies of resolvation of glycine and glycinium ions in water?DMSO solvent mixtures are calculated. It is shown that the small changes in the enthalpy of protonation observed at low mole fractions of DMSO are caused by the contributions from the solvation of proton and protonated glycine cancelling each other out. The enthalpy term of the Gibbs energy of the reaction leading to the formation of glycinium ion is estimated along with the enthalpy of resolvation of the reacting species in the water?DMSO mixed solvent.     

Complexation of Ni(II) with Benzoic,p-Methoxybenzoic,and Isonicotinic Acids Hydrazides in Aqueous Acetonitrile

Solvation and complexation of Ni(II) with benzoic (L¹), p-methoxybenzoic (L³), and isonicotinic (L) acids hydrazides in water and aqueous acetonitrile were studied. The coordination of acetonitrile with Ni(II) was qualitatively estimated, and the formation constant were determined for the complexes Ni(L¹)^{2 +}, Ni(L¹)2^{2 +}, Ni(L³)^{2 +}, Ni(L³)2^{2 +}, Ni(HL)^{3 +}, NiL^{2 +}, NiL(HL)^{3 +}, and NiL₂ ^{2 +}. The effects of dilution, ligand basicity, and ligand solvation on the stability of Ni(II) compounds with hydrazides of benzoic acid and its derivatives were demonstrated. The stability of the Ni(II) complexes with isonicotinic acid hydrazide is governed by dehydration of the metal ion, decrease in the donor power of the coordinating hydrazide fragment on protonation of the pyridine substituent L, formation of the intracomplex hydrogen bond between the protonated and deprotonated pyridine nitrogen atoms in NiL(HL)^{3 +}, and stacking interaction between the heterocycles in NiL₂ ^{2 +}.     

The site of gas phase cation attachment. The protonation,methylation and ethylation of morpholine,thiomorpholine and 1,4-thioxane

The attachment of gaseous positive ions ([H]⁺, [CH₃]⁺ and [C₂H₅]⁺) to morpholine, thiomorpholine and 1,4-thioxane, through chemical ionization, has been studied by collision spectroscopy. The daughter ion spectra of the ion/molecule reaction products were compared to those of model ions, generated by fast-atom bombardment of corresponding quaternary ammonium salts, in order to determine the preferred site of reaction for the protonation and alkylation of these multifunctional nucleophilic compounds. For novel entities with no model precursors, the site of cation attachment was postulated on the basis of characteristic fragmentations and trends established by the study of other bifunctional heterocycles. The site of protonation followed predicted trends in proton affinity differences for the various heteroatoms (N>S>O), and the alkyl ion reactivities followed differences in electronegativity or nucleophilicity (S>N>O).     

Three‐Way Cooperativity in d8 Metal Complexes with Ligands Displaying Chemical and Redox Non‐Innocence

Reversible proton‐ and electron‐transfer steps are crucial for various chemical transformations. The electron‐reservoir behavior of redox non‐innocent ligands and the proton‐reservoir behavior of chemically non‐innocent ligands can be cooperatively utilized for substrate bond activation. Although site‐decoupled proton‐ and electron‐transfer steps are often found in enzymatic systems, generating model metal complexes with these properties remains challenging. To tackle this issue, we present herein complexes [(cod_?H)M(μ‐L^2?) M (cod_?H)] (M=Pt^II, [ 1 ] or Pd^II, [ 2 ], cod=1,5‐cyclooctadiene, H₂L=2,5‐di‐[2,6‐(diisopropyl)anilino]‐1,4‐benzoquinone), in which cod acts as a proton reservoir, and L^2? as an electron reservoir. Protonation of [ 2 ] leads to an unusual tetranuclear complex. However, [ 1 ] can be stepwise reversibly protonated with up to two protons on the cod_?H ligands, and the protonated forms can be stepwise reversibly reduced with up to two electrons on the L^2? ligand. The doubly protonated form of [ 1 ] is also shown to react with OMe^? leading to an activation of the cod ligands. The site‐decoupled proton and electron reservoir sources work in tandem in a three‐way cooperative process that results in the transfer of two electrons and two protons to a substrate leading to its double reduction and protonation. These results will possibly provide new insights into developing catalysts for multiple proton‐ and electron‐transfer reactions by using metal complexes of non‐innocent ligands.     

Quantum chemical calculations on the interaction of diazomethane with proton acids

Diazoalkanes may form H-bonded associates with weak proton acids, whereas with strong acids proton transfer leads to the diazonium ion. In order to get information about protonation and association at and N, the corresponding energy balances, electron charge distributions and bond strengths have been calculated by means of quantum chemical ab initio methods with diazomethane as substrate and HF,NH₄⁺, OH₃⁺ as acids.     

Effect of water—ethanol solvent on the stability of copper(<Emphasis Type="SmallCaps">ii</Emphasis>) coordination compound with the nicotinate ion

The stability constants of copper(II) complexes with nicotinate ion in water—ethanol solvent were determined by the potentiometric method at 25.0±0.1 °C and ionic strength of 0.25 (NaClO₄) in the range X_EtOH = 0—0.7 mole fractions. The stability constant of copper(II) nicotinate complex considerably increases with increasing ethanol concentration in the solvent. The contributions of reactants to the Gibbs energy of the complex formation reaction on going from water to aqueous ethanol were analyzed. The results of thermodynamic analysis of solvation effects were used to evaluate the ratio of the ion and ligand transfer Gibbs energy contributions to the change in the reaction Gibbs energy in the water—ethanol system.     

Proton solvation in protic and aprotic solvents

Protonation pattern strongly affects the properties of molecular systems. To determine protonation equilibria, proton solvation free energy, which is a central quantity in solution chemistry, needs to be known. In this study, proton affinities (PAs), electrostatic energies of solvation, and pK_A values were computed in protic and aprotic solvents. The proton solvation energy in acetonitrile (MeCN), methanol (MeOH), water, and dimethyl sulfoxide (DMSO) was determined from computed and measured pK_A values for a specially selected set of organic compounds. pK_A values were computed with high accuracy using a combination of quantum chemical and electrostatic approaches. Quantum chemical density functional theory computations were performed evaluating PA in the gas‐phase. The electrostatic contributions of solvation were computed solving the Poisson equation. The computations yield proton solvation free energies with high accuracy, which are in MeCN, MeOH, water, and DMSO ?255.1, ?265.9, ?266.3, and ?266.4 kcal/mol, respectively, where the value for water is close to the consensus value of ?265.9 kcal/mol. The pK_A values of MeCN, MeOH, and DMSO in water correlates well with the corresponding proton solvation energies in these liquids, indicating that the solvated proton was attached to a single solvent molecule. © 2016 Wiley Periodicals, Inc.     

Effect of protonation exothermicity on the reaction of gaseous Brønsted acids with halobenzene derivatives

The protonation of haloaromatics by [N₂H]⁺ and [CO₂H]⁺ has been studied by chemical ionization mass spectrometry. In general, the fragmentation reactions following protonation by [CO₂H]⁺ are similar to those observed following protonation by [CH₅]⁺, while the fragmentation reactions induced by protonation by [N₂H]⁺ are intermediate between those observed on reaction with [CH₅]⁺ and with [H₃]⁺. These results are consistent with the conclusion that the fragmentation mode is determined by the protonation exothermicity since the proton affinity of CO₂ is the same as that of CH₄ while the proton affinity of N₂ is intermediate between that of CH₄ and H₂.     

Theoretical studies on model reaction pathways of prostaglandin H<Subscript>2</Subscript> isomerization to prostaglandin D<Subscript>2</Subscript>/E<Subscript>2</Subscript>

Model reaction mechanisms in the biosynthesis of prostaglandin D₂ (PGD₂) and prostaglandin E₂ (PGE₂) from prostaglandin H₂ with PGD₂/E₂ synthase were examined using the ab initio second-order Møller–Plesset perturbation method and density functional theory. The reaction was modeled similar to the isomerization of 2,3-dioxabicyclo[2.2.1]heptane to 3-hydroxycyclopentanone in the presence of MeS^?. An explicit solvation of two H₂O molecules was also considered, and two probable types of reaction mechanisms were demonstrated. One mechanism starts with proton abstraction from an oxygen-bound carbon at the endoperoxide by a thiolate ion and the other is stepwise and involves attack of a thiolate anion on an oxygen of the endoperoxide group in the first step with protonation of the other oxygen, followed by deprotonation from a carbon-attached oxygen to break an O–S bond to yield PGD₂ or PGE₂. We also found that the mPW1LYP hybrid method was superior to the B3LYP functional for systems with respect to the state-of-the-art CCSD(T) energetics.     

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 H₃O⁺ 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.     

Equilibrium and kinetics of proton transfer from thiocarboxylic acids to γ-collidine

NMR and IR spectroscopy have been used in studying the equilibrium in the reaction of proton transfer from thiocarboxylic acids RCOSH [R=CH₃ (a), C₆H₅ (b) or CH₂Cl (c)] to -collidine (d), and also the kinetics of CH/NH proton exchange between protonated -collidine and excess RCOSH. For compoundsa and b, partial protonation of the -collidine is observed; and for compound c, complete protonation. The heat of reaction of proton transfer with the participation of binary acidamine associates is 30 (a) and 45 (b) kJ/mole. The rate of proton exchange decreases and the activation energy E increases with increasing acidity of the RCOSH [E=44 (b) and 88 (c) kJ/mole] and with increasing basicity of the amine (E_d < E_TEA), which, in accordance with the orders of reaction that were found for the exchanged components, is due to a mechanism in which the slow stage is proton transfer in the ion pair NH⁺...^–SOCR. The thiocarboxylate ion of c is unstable; and after splitting out Cl^–, it forms the compounds Cl(CH₂COS)₂ ^– and (CH₂COS)₂.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 187–194, March–April, 1985.     

Complexation of divalent metal ions with diols in the presence of anion auxiliary ligands: zinc‐induced oxidation of ethylene glycol to glycolaldehyde by consecutive hydride ion and proton shifts

Ternary complexes of the type AH???M²⁺???L^– (AH = diol, including diethylene and triethylene glycol, M = Ca, Mn, Fe, Co, Ni, Cu and Zn and auxiliary anion ligand L^– = CH₃COO^–, HCOO^– and Cl^–) have been generated in the gas phase by MALDI and ESI, and their dissociation characteristics have been obtained. Use of the auxiliary ligands enables the complexation of AH with the divalent metal ion without AH becoming deprotonated, although A^–???M²⁺ is often also generated in the ion source or after MS/MS. For M = Ca, dissociation occurs to AH + M²⁺???L^– and/or to A^–???M²⁺ + LH, the latter being produced from the H‐shifted isomer A^–???M²⁺???LH. For a given ligand L^–, the intensity ratio of these processes can be interpreted (barring reverse energy barriers) in terms of the quantity PA(A^–) – Ca_aff(A^–), where PA is the proton affinity and Ca_aff is the calcium ion affinity. Deuterium labeling shows that the complex ion HOCH₂CH₂OH???Zn²⁺???^–OOCCH₃, in addition to losing acetic acid (60 Da), also eliminates glycolaldehyde (HOCH₂CH=O, also 60 Da); it is proposed that these reactions commence with a hydride ion shift to produce the ion–dipole complex HOCH₂CHOH⁺??? HZnOOCCH₃, which then undergoes proton transfer and dissociation to HOCH₂CH=O + HZn⁺???O = C(OH)CH₃. In this reaction, ethylene glycol is oxidized by consecutive hydride ion and proton shifts. A minor process leads to loss of the isomeric species HOCH=CHOH. Copyright © 2012 John Wiley & Sons, Ltd.     

Large-Sized Ammonia Clusters and Solvation Energies of the Proton in Ammonia

The absolute solvation energies (free energies and enthalpies) of the proton in ammonia are used to compute the pK_a of species embedded in ammonia. They are also used to compute the solvation energies of other ions in ammonia. Despite their importance, it is not possible to determine experimentally the solvation energies of the proton in a given solvent. We propose in this work a direct approach to compute the solvation energies of the proton in ammonia from large-sized neutral and protonated ammonia clusters. To undertake this investigation, we performed a geometry optimization of neutral and protonated ammonia 30-mer, 40-mer, and 50 mer to locate stable structures. These structures have been fully optimized at both APFD/6-31++g(d,p) and M06-2X/6-31++g(d,p) levels of theory. An infrared spectroscopic study of these structures has been provided to assess the reliability of our investigation. Using these structures, we have computed the absolute solvation free energy and the absolute solvation enthalpy of the proton in ammonia. It comes out that the absolute solvation free energy of the proton in ammonia is calculated to be −1192 kJ mol^–1, whereas the absolute solvation enthalpy is evaluated to be −1214 kJ mol^–1. © 2019 Wiley Periodicals, Inc.     

Ground‐State Structure of the Proton‐Bound Formate Dimer by Cold‐Ion Infrared Action Spectroscopy

The proton‐bound dicarboxylate motif, RCOO^??H⁺?^?OOCR, is a prevalent chemical configuration found in many condensed‐phase systems. The proton‐bound formate dimer HCOO^??H⁺?^?OOCH was studied utilizing cold‐ion IR action spectroscopy in the range 400–1800 cm^?1. The spectrum obtained at ca. 0.4 K of ions captured in He nanodroplets was compared to that measured at ca. 10 K by photodissociation of Ar‐ion complexes. Similar band patterns are obtained by the two techniques that are consistent with calculations for a C₂ symmetry structure with a proton shared equally between the two formate moieties. Isotopic substitution experiments point to the nominal parallel stretch of the bridging proton appearing as a sharp, dominant feature near 600 cm^?1. Multidimensional anharmonic calculations reveal that the bridging proton motion is strongly coupled to the flanking ?COO^? framework, an effect that is in line with the expected change in ?C=O bond rehybridization upon protonation.     

Dependence of the enthalpies of formation of Ag<Superscript>+</Superscript> complexes with glycinate ion and the protonation of glycinate ion on the content of aqueous ethanol solvent

Enthalpy changes are determined by calorimetry for the reactions of glycinate ion (Gly⁻) proto- nation and its complexation with Ag⁺ ion at a temperature of 298 K and ionic strength 0.1 (NaClO₄) in an aqueous ethanol solvent containing 0.0–0.4 and 0.0–0.3 mole fraction of alcohol, respectively. An abnormal relationship of enthalpy changes is found for the processes of stepwise formation of mono- and bis-glycinates of silver(I) in water. It is shown that varying the ethanol content has virtually no effect on the exothermicity of Ag⁺ complexation reactions with glycinate ions at either coordination step and does not change the relationship of the step enthalpies. An analogy is observed in the relationship of solvation contributions from the reagents to the value of Δ_tr H° for the reactions of glycinate ion protonation and its complexation with silver(I) in aqueous ethanol solvents.