Selective Encapsulation of Chloride Ions within Novel Cage Host Complexes in the Presence of Equimolar Amounts of Chloride and Bromide Ions

Four macrotricyclic cage hosts which feature four positive binding sites oriented toward the center of the intramolecular cavity are presented as promising candidates for anion receptors and they have been expected to play a important role in the selective encapsulation of the halide ion Cl⁻ or Br⁻. The complementarity between a macrotricyclic quaternary ammonium ion and Cl⁻ was achieved by fine-tuning of the four ammonium nitrogen atoms and the endocyclic methylene groups. The cage hosts [R₄N₄(C₅H₁₀)₄(C₆H₁₂)₂]⁴⁺ (abbreviated as [556]) showed perfect encapsulation of all chloride ions in acetonitrile at 0<r=([Cl⁻]_o/[[556]]_o)≤1 within the sensitivity of the ¹H NMR spectra in combination with a rather slow chemical exchange of the Cl⁻ ion in an encapsulation/decapsulation equilibrium with [556]. Further, the selective encapsulation of all the chloride ions into [556] cage occurs unambiguously at r=1 in the presence of equimolar amounts of Br⁻. The structural complementarity of the newly designed [556] host prevails over the Hofmeister-series restraints determined by differences in Gibbs free energy of halide anion solvation.     

Neighbouring group participation in reactions of sulphides with chloramine-t

Neighbouring group participation in reactions of chloramine-T with ortho-substituted aryl methyl sulphides and diaryl sulphides has been studied. The reaction is markedly hindered by the steric effect of the ortho substituent of the phenyl ring, but groups having a CO moiety show an anchimeric effect in the following order: o-CH₂CO₂Me ~ o-CH₂CO₂H < o-CH₂CO₂^- < o-CO₂Me ~ o-CO₂H < o-CO₂^- ? 2o-CO₂ in the rate-determining step may be ruled out on the basis of salt and isotope effect. Substituents with neighbouring group participation diminish the yield of sulphilimine in solvents containing water. The electrophilic chlorination of sulphides by TsNHCl may be assumed to be the rate-determining step with the positively charged sulphonium centre stabilized by the negatively polarized or charged carbonyl-oxygen in the transition state. This type of interaction hinders the nucleophilic attack of sulphonamidate ion at the sulphonium centre in the fast product-controlling steps, decreasing the yield of sulphilimine.     

Benzene-based tripodal isothiouronium compounds as sulfate ion receptors

Novel benzene-based tripodal isothiouronium receptors are synthesized for the selective recognition of tetrahedral oxoanions. The binding study by isothermal titration calorimetry indicates that the cationic receptors bind sulfate ions preferably in a tripodal mode, while they show a mixed binding mode toward phosphate ion. The tripodal isothiouronium receptors show large ΔG⁰ values toward sulfate ions in methanol, which are entropy driven. The results demonstrate that a subtle structural constraint can lead to different binding modes toward structurally similar anions.     

Electrochemical determination of lead and lead ion transfer at liquid-liquid interfaces

The transfer behavior of lead (II) ion at a liquid-liquid interface, facilitated by neutral carriers (polyethylene glycols), is studied by an electrochemical method. The transfer process is discussed in terms of the formation of complexes in two phases and attributed to the transfer of a complex ion across the interface. The apparent standard transfer potential, ΔwoφPbL2+o, apparent standard Gibbs energy of transfer, ΔwoGPbL2+o, and the dissociation constant of the complex, KPbL2+, in the aqueous phase are obtained from the experimental data. The results suggest a new electrochemical method for the determination of lead.     

The association of hexacyanoferrate(II) ions and group IIA cations

A thermodynamic study has been made of the equilibrium $$M^{2 + } {\text{ + }}Fe(CN)_6^{4 - } {\text{ }} \rightleftharpoons {\text{ }}MFe(CN)_6^{2 - } $$ in dilute aqueous solution by a potentiometric method using a divalent-cation electrode where, M²⁺ is a group IIA cation. Ion pairing was found to be quite extensive: Thus, at 25°C, the association constants extrapolated to zero ionic strength were 5840, 4730, 4560, and 6080M ^?1 for M²⁺=Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, respectively. In each case, ion pairing is slightly endothermic, the corresponding ΔH ^o values being 18.0, 13.0, 7.8, and 17.5 kJ-mole^?1. However, the values of ΔS ^o are larger and positive, thus favoring ion pairing: the values are 133, 115, 96, and 131 J^o-K^?1-mole^?1 for the same series of cations. There was also some indication of a small amount of ion triplet formation, M₂Fe(CN)₆. Comparisons were made with the predictions of the Fuoss theory, and these support the view that the bonding in the ion is largely electrostatic and not covalent.     

The standard enthalpies of formation of alkoxy-NNO-azoxy compounds

The standard enthalpies of combustion Δ_c H ^o and formation Δ_f H ^o of seven alkoxy-NNO-azoxy compounds containing the-N⁺(O^?)=NO-characteristic group were determined by combustion in a calorimetric bomb in the atmosphere of oxygen. The contribution of this group to the Δ_f H ^o enthalpies of the substances studied was calculated. The Δ_f H ^o enthalpies found by the method of group contributions were in satisfactory agreement with experimental data.     

Fragmentation of protonated amides through intermediate ion-neutral complexes: Neighboring group participation

It has been shown that neighboring group participation plays an important role in the fragmentation of protonated amides; the attachment of an adjacent functional group capable of accepting a proton provides alternative pathways of low energy for the formation of the inevitable N-protonated species in the fragmentation of the amide bond. Under methane chemical ionization (CI) conditions, protonated aniline (m/z 94) is only 1. 6% of the base peak MH⁺ ion for acetanilide; the abundance of the m/z 94 ion is increased to 15% for acetoacetanilide and protonated o-methoxyaniline reaches a relative intensity of 49% for N-acetyl-o-methoxyaniline. A more striking difference in ease of the formation of protonated anilines is found for acetanilides bearing a nitro group at different positions. Protonated nitroaniline (m/z 139) is the base peak in the methane CI spectrum of N-acetyl-o-nitroaniline; the m/z 139 ion drops to only 0.7% for the para isomer, and this ion is increased to 31.5% in the spectrum of N-acetoaceto-p-nitroaniline. By employing low energy collision-induced dissociation, it has been found that the fragmentation of protonated amides proceeds by way of ion-neutral complexes. In the case of acetanilide, for example, the cleavage of the amide bond gives rise to an acetylium ion and neutral aniline, which are bound together as a complex. An α-hydrogen of the acetylium ion, which is activated by the positive charge, is captured by aniline due to its higher proton affinity as compared with ketene. For those compounds having mobile protons other than the amidic hydrogen, it is indicated that such proton has the priority to be transferred in the reaction. Thus, the proton on the free carboxyl group of N-phenyl succinic and maleic monoamides is transferred in the fragmentation, leading to anhydrides as the neutral species in the formation of protonated aniline.     

Ionization of aqueous benzoic acid: Conductance and thermodynamics

Molar conductances of dilute aqueous benzoic acid solutions are presented for temperatures from 5 to 80°C. The data have been analyzed to give acid dissociation constants as well as ΔH ^o, ΔS ^o, and ΔC _p ^o for the ionization process and the limiting conductance of the benzoate ion. The conductance-viscosity product changes less than 4% over the temperature range, indicating that the interaction of the benzoate ion with the solvent changes little if at all with increasing temperature. The pK _a(m) vs.T data show that ΔH ^o decreases quadratically while ΔC _p ^o increases linearly withT although, over the 75°C range, ΔC _p ^o increases only about 6 cal-mole^?1 deg^?1 around an average of ?37 cal-mole^?1deg^?1. The acid dissociation constants as derived from the conductance-molal concentration analysis show an average uncertainty of about 0.1% and are fitted to within about 0.01% by the equation $$p{\text{K}}_{\text{a}} (m) = - 75.5422 + 3136.34/T + 28.7965 log T - 6.8139 {\text{x}} 10^{ - 3{\text{T}}} $$ whereT is the absolute temperature.     

Thermodynamic properties of molybdenum pentafluoride vapor

There are calculated for 298 K and higher temperatures the standard entropies of the liquid and the important gas species (monomer, dimer, and trimer) of MoF₅, and their differences in enthalpy and Gibbs energy. Besides estimated mean heat capacities, the input data are: the vapor pressure at one temperature and the saturated-vapor density at two temperatures (based on measurements in this laboratory); the standard entropies of the three gas species (estimated from published spectroscopic data on the crystal and monomer); and an ion-abundance proportion (from a published mass-spectral study). This ion proportion is corrected for fragmentation after demonstrating that changes in the ion proportions for a similar fluoride, RhF₅, can be explained by assuming that the rates of fragmentation of the dimer and trimer are equal. For those input data considered most uncertain (the vapor pressure, the entropies of dimer and trimer, and the ratios of mass-spectral ionization efficiencies) several sets of values covering their respective estimated ranges of uncertainty are used alternatively in the calculations. Results at 298.15 K—expressed in the form “preferred value (probable range due to uncertainty)”, with H^o and G^o relative to the liquid, and S^o—are as follows: liquid, S^o = 46 (44.5 to 49) cal_th K^?1 mol^?1; monomer, H^o = 19 (17 to 22) kcal_th mol^?1, G^o = 8.5 (7.5 to 11.5) kcal_th mol^?1, S^o = 80.6 cal_thK^?1; dimer, H^o = 15.7 (15 to 16) kcal_th mol^?1, G^o = 6.48 (6.45 to 6.51) kcal_th mol^?1, S^o = 123 (121 to 127) cal_th K^?1 mol^?1; trimer, H^o = 17.7 (16 to 21) kcal_th mol^?1, G^o = 8.4 (8.1 to 8.7) kcal_th mol^?1, S^o = 169 (161 to 185) cal_th K^?1 mol^?1. The calculated mole fractions of monomer, dimer, and trimer in the saturated vapor at 298.15 K average 0.027, 0.94, and 0.035, respectively.     

The effects of biologically important divalent and trivalent metal cations on the cyclization step of dopamine autoxidation reaction: A quantum chemical study

Dopamine is the most essential monoaminergic neurotransmitter involved in the pathophysiology of neurodegenerative disorders, and its autoxidation has been recognized as one of the potential trigger factors for dopaminergic neuron loss. The cyclization of dopamine o-quinone was shown to be the irreversible and rate-limiting step of the autoxidation reaction at physiologic pH values. Furthermore, various metal ions such as Al³⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, and Mn²⁺ have been clinically associated with neurodegeneration, especially Parkinsonism and dementia. It has been proposed that these metal ions could increase the rate of the dopamine autoxidation reaction; however, the exact mechanism has not yet been fully understood. Using advanced quantum chemical calculations with the inclusion of solvent effects we showed that except for Mn²⁺, the studied metal cations could form complexes with dopamine o-quinone and significantly increase the dopamine o-quinone cyclization rate in aqueous solution; first, by enabling the cyclization to proceed spontaneously without the attack of the unprotonated amino group by hydroxide ion; second, by decreasing the intrinsic activation energy; and third, by decreasing the free energy of protonated amino group deprotonation. The latter also decreases the protective effect of acidic pH on dopamine autoxidation found in synaptic vesicles. The results are fully consistent with experimental data and provide deeper understanding of the effects of metal cations on the dopamine autoxidation reaction at physiologic pH values.     

The Heats of Mixing of Aqueous Solutions of Dipeptides with Solutions of Nitric Acid and Potassium Hydroxide over the Temperature Range 288.15–308.15 K

The heats of interaction of D,L-α-alanyl-D,L-valine, β-alanyl-β-alanine, and α-alanyl-β-alanine with solutions of nitric acid and potassium hydroxide were determined calorimetrically at 288.15, 298.15, and 308.15 K and solution ion strengths of 0.5, 1.0, and 1.5 in the presence of KNO₃ and LiNO₃. The heat effects of step dissociation of the dipeptides were calculated using the RRSU universal program. The standard thermodynamic characteristics (Δ_r H ^o, Δ_r G ^o, Δ_r S ^o, and ΔC _p ^o) of proton interaction with the ligands specified were determined. The data obtained were analyzed in terms of Herny concepts. The thermodynamic characteristics of ionization correlated with the structural features of the dipeptides.     

Determination of enthalpies of formation of organic free radicals from bond dissociation energies 1. Hydrocarbon radicals

The enthalpies of formation (ΔH _f ^o) for 24 hydrocarbon radicals (R^?), mainly polycyclic aromatic radicals with the complex structure, were determined from the published data on bond dissociation energies. The ΔH _f ^o values of the corresponding molecules were calculated, in the majority of cases, by the macroincrement method. Calculations by the group contribution method were performed. Some ΔH _f ^o(R^?) values were compared to those calculated by the additive-group method. Calculations were performed, and the conjugation energies of the radicals were discussed. The errors of determination of the ΔH _f ^o(R^?) values found were estimated. Due to this work, the database for ΔH _f ^o values of hydrocarbon radicals was increased more than by 25%.     

Acid-catalysed hydrolysis of N-sulphonyl sulphilimines—II

The basicity and the acid-catalysed hydrolysis of ph(R)SNTs and o-HC₆H₄(Me)SNTs sulphilimines have been studied by UV spectrophotometric and kinetic methods, respectively, in aqueous HClO₄ (1–10 M) and 1:1 (v/v) EtOH/H₂O-HClO₄ (0.5–6 M). Depending on the constitution of the substrates, sulphilimine hydrolysis can follow three different courses, according to rate-acidity profiles, Bunnett-Olsen's treatment, activation parameters and product analysis. Most typical for sulphilimines is S_N2 hydrolysis with S^IV-N bond cleavage. In this case the reaction starts with the nucleophilic addition of water and is promoted by acid-base catalysis. If a relatively stable carbenium ion can be formed from R group, an S_N1 reaction with S^IVC bond cleavage takes place. Sulphilimine with X = o-CO₂H due to neighbouring-group participation hydrolyses very rapidly via acyloxy-sulphurane and acyloxy-sulphonium ion intermediates with five-memembered ring (S_Ni reaction involving S^IVN bond cleavage).     

Proportionality of intrinsic heat of transport to standard entropy of hydration for aqueous ions

Intrinsic ionic heats of transport q _o ^* (ion) and ionic heats of transport Q _o ^* (ion) have been evaluated for 53 aqueous ions at infinite dilution at 25°C using the reduction rule proposed by the authors and the limiting laws of Agar, and of Helfand and Kirkwood without electrophoretic terms. q _o ^* (ion) have been found to correlate linearly with the standard ionic entropies of hydration for the 38 ions investigated. The correlation yields three distinctive proportionality constants indicating that the ions may be divided into three distinctive groups. Although the sign of Q _o ^* (ion) is not definite, all values of q _o ^* (ion) are positive. For 17 ions Q _o ^* (ion) are in good agreement with TS _o ^* (ion). Here, S _o ^* (ion) is the absolute standard ionic entropy of transport which can be obtained from potentiometric measurements on cells. The values of S _o ^* (ion) were determined by Agar, and recently by Lin and coworkers.     

Negative ion photoelectron spectroscopy of teo−

We have recorded the photoelectron spectrum of Te0⁻ using a hot-cathode discharge ion source and a negative ion photoelectron spectrometer. The adiabatic electron affinity of TeO is determined to be 1.697±0.022 eV. The negative ion parameters determined in this work are: (w_e″(TeO⁻) = 690 ± 80 cm⁻¹, r_e″(TeO⁻) = 1.884 ± 0.028 Å. and D_o     

Universality of the Sodium Ion Binding Mechanism in Class A G‐Protein‐Coupled Receptors

The allosteric modulation of G‐protein‐coupled receptors (GPCRs) by sodium ions has received significant attention as crystal structures of several receptors show Na⁺ ions bound to the inactive conformations at the conserved Asp^2.50. To date, structures from 24 families of GPCRs have been determined, though mechanistic insights into Na⁺ binding to the allosteric site are limited. We performed hundreds‐of‐microsecond long simulations of 18 GPCRs and elucidated their Na⁺ binding mechanism. In class A GPCRs, the Na⁺ ion binds to the conserved residue 2.50 whereas in class B receptors, it binds at 3.43b, 6.53b, and 7.49b. Using Markov state models, we obtained the free energy profiles and kinetics of Na⁺ binding to the allosteric site, which reveal a conserved mechanism of Na⁺ binding for GPCRs and show the residues that act as major barriers for ion diffusion. Furthermore, we also show that the Na⁺ ion can bind to GPCRs from the intracellular side when the allosteric site is inaccessible from the extracellular side.     

Ortho effects in organic molecules on electron impact: XI—the interesting ortho effect of the methoxy group in o-methoxy aromatic thioamides

It has been noticed that the major part of the loss of ?H from the molecular ion of most of the o-methoxythioamides results from an ortho effect of the methoxy group. Comparison of the MIKE spectra of the [M? SH]⁺ of 1-(2-methoxyphenylthioxomethyl)piperidine and 1-(2-methoxyphenylthioxomethyl)pyrrolidine with the MIKE spectra of [M? SH]⁺ of the corresponding unsubstituted compounds, reported earlier, indicated two parallel pathways for the formation of [M? SH]⁺ in the o-methoxy compounds. In the first pathway, as has been noticed in thioamides in general, the loss of ?H involves the migration of either the α-hydrogen in the amine moiety or the hydrogen attached to nitrogen. In the second pathway, the migration of a hydrogen from the o-methoxy group to the sulphur atom followed by ejection of SH from the molecular ion leads to a stable cyclized ion. Interesting secondary fragmentations as a consequence of this ortho effect have also been noticed.     

Simple imine linked colorimetric and fluorescent receptor for sensing Zn ions in aqueous medium based on inhibition of ESIPT mechanism

Herein, we report the highly selective binding of Zn²⁺ ion by the salicylaldimine based Schiff base chromogenic receptor 1 [(N,N′-bis (salicylidine)-o-phenylenediamine]. Receptor 1 senses Zn²⁺ ion in aqueous medium by colorimetric and fluorescent response in the presence of other metal ions like Pb²⁺, Hg²⁺, Sn²⁺, Cd²⁺. Receptor 1 on binding with Zn²⁺ ion exhibits fluorescence enhancement which is due to the inhibition of the (ESIPT) mechanism.     

Strontium(II)-selective electrode based on a macrocyclic tetraamide

A new PVC membrane electrode based on 5,7,12,14-dibenzo-2,3,9,10-tetraoxa-1,4,8,11-tetraazacyclotetradecane (I) as an ion carrier, o-nitrophenyloctyl ether (o-NPOE) as solvent mediator and sodium tetraphenylborate (NaTPB) as lipophilic additive was fabricated and investigated as Sr²⁺-selective electrode. The best performance was exhibited by the membrane having composition 8:200:4:120 (I:o-NPOE:NaTPB:PVC). The electrode exhibited a Nernstian response for strontium ion over a wide concentration range 3.98 × 10⁻⁶ to 1.0 × 10⁻¹ M with a slope of 29.0 ± 0.1 mV/decade of concentration and a detection limit of 2.82 × 10⁻⁶ M. It showed a response time of less than 10 s and could be used for at least 3 months without any divergence in potential. The proposed electrode showed a good discriminating ability towards strontium(II) ion over a wide variety of other metal ions including alkali, alkaline earth, transition, and heavy metal ions. The electrode can be used in the pH range of 2.5-10.5 and in mixtures containing up to 35% (v/v) non-aqueous content. It was used as an indicator electrode in potentiometric titration of strontium ion against EDTA.     

Hydrogen-bonded clusters and solvate structures in the supercritical CO2-water-o-hydroxybenzoic acid system: the car-parrinello molecular dynamics

Hydrogen-bonded clusters and solvate structures formed by o-hydroxybenzoic acid (o-HBA) and water in supercritical CO₂ were studied (T = 318 K, 348 K, ρ = 0.7 g/cm³). The atom-atom radial distribution functions, coordination numbers, average numbers of hydrogen bonds for individual atomic groups, and power spectrum were calculated by the Car-Parrinello molecular dynamics. Despite the high polarity of the cosolvent, the hydroxyl group of o-HBA predominantly forms intramolecular hydrogen bond, while hydrogen bonds with water involve only the atoms of carboxyl groups. The temperature effect on the stability of these bonds showed itself in different ways. The intermolecular interactions of o-HBA with carbon dioxide were found to be weaker than those with water. It was established that the Lewis acid-Lewis base interactions between CO₂ and the hydroxyl group of the solute increase with increasing temperature. Instantaneous configurations illustrating the temperature effects on the molecular structures were obtained.