Protic Solutions Research Articles

ChemInform Abstract: Protic Ionic Liquids: Evolving Structure‐Property Relationships and Expanding Applications

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Protic Ionic Liquids: Evolving Structure-Property Relationships and Expanding Applications.

The thermal and physicochemical properties of protic ionic liquids (PILs) are reported. It is highly evident that there has been an extensive range of alkylammonium, imidazolium, and heterocyclic cations paired with many organic and inorganic anions that have been employed to prepare PILs. There has been strong interest in modifying the properties of PILs through the addition of water or other molecular solvents. For many applications, the presence of some water in the PILs is not detrimental, and instead leads to enhanced solvent properties such as lower viscosity, higher conductivities, and lower melting points. It remains an issue of definition though of how to refer to these resulting protic solutions. There is also an ongoing difficulty surrounding how to describe the proton activity in the PILs, analogous to pH in aqueous systems. For a broad range of applications, it has been reported that the acidity/basicity of the PIL or PIL-solvent system is crucial for their beneficial properties. It is expected that the fundamental properties of PILs will continue to be explored, along with continued interest in many existing and new applications, such as in electrochemistry, organic and inorganic synthesis, and biological applications. In particular, there has been a significant interest in a broad- range of PILs for use as electrolytes and incorporation in polymer electrolytes for fuel cells, and other energy storage devices.

Hydrogen-bonding dynamics of photoexcited coumarin 138 and 339 in protic methanol solution: Time-dependent density functional theory study

The ground and electronically excited-state intermolecular hydrogen bonds between coumarin 138 (C138)/coumarin 339 (C339) and the protic methanol solvent are investigated by using time-dependent density functional theory method. The methanol solvent can act as a hydrogen-donating or hydrogen-accepting moiety at the proper site of organic chromophore. Our theoretical investigation explores the formation of one and multiple hydrogen bonds and demonstrates the strengthening of the intermolecular hydrogen bonds in the excited state. According to our calculation, upon photoexcitation the isolated coumarins and hydrogen-bonded clusters are initially excited to the first excited state and the intermolecular hydrogen bonds are strengthened in the excited state. The obvious red shifts, as large as 21nm, of the steady-state absorption spectra are observed. In addition, the infrared spectra in the ground and excited states are calculated to explore the hydrogen bonding dynamics. The calculated CO, NH, OH stretching modes are red shifted induced by the electronic excitation and intermolecular hydrogen bond interaction. The strengthening of the intermolecular hydrogen bonds is also confirmed by the geometric parameters in the ground and excited states with TDDFT method.

A preliminary study of sodium octahydrotriborate NaB3H8 as potential anodic fuel of direct liquid fuel cell

Theoretically, sodium octahydrotriborate NaB3H8, solved in alkaline aqueous solution, is a potential boron-based liquid anodic fuel of direct liquid fuel cell, since the B3H8− anion could be oxidized (B3H8− + 20OH− → 3BO2− + 14H2O + 18e−) in a reaction ideally involving 18 electrons. Hence, we focus on this material. We firstly synthesize NaB3H8 and fully characterize it (NMR, FTIR, Raman, XRD, TGA, DSC), then study its stability in protic solutions by NMR, and lastly investigate its oxidation with the help of electrochemical techniques. The diffusion coefficient for the B3H8− anion is calculated (3.7·10−5 cm2 s−1) in order to assess the coulombic efficiency for the oxidation on platinum and gold electrodes. It is found effective numbers of electrons of 5.2 and 10.1 (out of a theoretical total of 18 electrons) respectively. Neither platinum nor gold smooth surfaces enable the complete oxidation of B3H8−, which is explained mainly by the occurrence of heterogeneous hydrolysis (i.e. hydrogen evolution), but also by the difficulty to break the B–B bonds (difficult to oxidize) present in the B3H8− anion. Yet, these first results confirm that the B3H8− anion has a potential as anodic fuel, provided the proper electrocatalyst is isolated: this is discussed herein.

Conformational preferences of Ac-Gly-NHMe in solution

The conformational behaviour of Ac-Gly-NHMe and its fluorinated [CF3-C(O)-Gly-NHMe] and N-methyl[Ac-Gly-N(Me)2] derivatives is investigated in nonpolar, polar and polar protic solutions by NMR and IR spectroscopies and theoretical calculations.

Design of chemically stable, potent, and efficacious MDM2 inhibitors that exploit the retro-mannich ring-opening-cyclization reaction mechanism in spiro-oxindoles.

Inhibition of the MDM2–p53 protein–protein interaction is being actively pursued as a new anticancer therapeutic strategy, and spiro-oxindoles have been designed as a class of potent and efficacious small-molecule inhibitors of this interaction (MDM2 inhibitors). Our previous study showed that some of our first-generation spiro-oxindoles undergo a reversible ring-opening-cyclization reaction that, from a single compound in protic solution, results in an equilibrium mixture of four diastereoisomers. By exploiting the ring-opening-cyclization reaction mechanism, we have designed and synthesized a series of second-generation spiro-oxindoles with symmetrical pyrrolidine C2 substitution. These compounds undergo a rapid and irreversible conversion to a single, stable diastereoisomer. Our study has yielded compound 31 (MI-1061), which binds to MDM2 with Ki = 0.16 nM, shows excellent chemical stability, and achieves tumor regression in the SJSA-1 xenograft tumor model in mice.

Competing intramolecular vs. intermolecular hydrogen bonds in solution.

A hydrogen bond for a local-minimum-energy structure can be identified according to the definition of the International Union of Pure and Applied Chemistry (IUPAC recommendation 2011) or by finding a special bond critical point on the density map of the structure in the framework of the atoms-in-molecules theory. Nonetheless, a given structural conformation may be simply favored by electrostatic interactions. The present review surveys the in-solution competition of the conformations with intramolecular vs. intermolecular hydrogen bonds for different types of small organic molecules. In their most stable gas-phase structure, an intramolecular hydrogen bond is possible. In a protic solution, the intramolecular hydrogen bond may disrupt in favor of two solute-solvent intermolecular hydrogen bonds. The balance of the increased internal energy and the stabilizing effect of the solute-solvent interactions regulates the new conformer composition in the liquid phase. The review additionally considers the solvent effects on the stability of simple dimeric systems as revealed from molecular dynamics simulations or on the basis of the calculated potential of mean force curves. Finally, studies of the solvent effects on the type of the intermolecular hydrogen bond (neutral or ionic) in acid-base complexes have been surveyed.

Evaluation of homogeneous electrocatalysts by cyclic voltammetry.

The pursuit of solar fuels has motivated extensive research on molecular electrocatalysts capable of evolving hydrogen from protic solutions, reducing CO2, and oxidizing water. Determining accurate figures of merit for these catalysts requires the careful and appropriate application of electroanalytical techniques. This Viewpoint first briefly presents the fundamentals of cyclic voltammetry and highlights practical experimental considerations before focusing on the application of cyclic voltammetry for the characterization of electrocatalysts. Key metrics for comparing catalysts, including the overpotential (η), potential for catalysis (E(cat)), observed rate constant (k(obs)), and potential-dependent turnover frequency, are discussed. The cyclic voltammetric responses for a general electrocatalytic one-electron reduction of a substrate are presented along with methods to extract figures of merit from these data. The extension of this analysis to more complex electrocatalytic schemes, such as those responsible for H2 evolution and CO2 reduction, is then discussed.

Silver(I) and zinc(II) organometallic intermediates, catalysing coupling reactions of polysubstituted benzoic acids – Experimental and theoretical study

The paper highlighted AgI- and ZnII-organometallic catalysing CC coupling processes of polysubstituted benzoic acids in polar protic solutions. Six novel interaction products of substituted biphenyl, 6H-benzo-[c]-chromene[6]-one, chromeno[5,4,3-cde]chromene-5,10-dione, and 4,10-dioxa-pyrene-5,9-dione are isolated. The study, therefore impact the fields of inorganic, metal–organic and organometallic chemistry, catalysis, semi-synthetic natural products and materials research, polymer science, and more. The mechanistic aspects of the reactions with participation of nine MC bonded organometallic precursors (M=ZnII, AgI/AgII) and four metal-organic coordination compounds are comprehensively discussed. The factors stabilizing the metal ion – containing systems in polar environment and gas–phase are elucidated, employing the method of quantum chemistry and variety of mass spectrometric approaches, utilizing the ESI-, APCI- and MALDI-methods. These latters provided experimental physical evidences for formation of ZnII and AgI-containing organometallics species with closed d10 shell electronic configuration of the metal ions. The data reported are of methodological importance for the field of the mass spectrometry as well. Due to the matrix-analyte sample preparation techniques in UV–MALDI–MS method widely utilize polysubstituted benzoic acids as organic matrixes. The study contributed as well to the fundamentals of CT- and MLCT-effects in coordination compounds of metal ions with closed d10-electronic configuration. Electronic absorption spectroscopy, single crystal X-ray diffraction and nuclear magnetic resonance methods are also utilized.

Phase Transition of Poly(N-isopropylacrylamide) in Aqueous Protic Ionic Liquids: Kosmotropic versus Chaotropic Anions and Their Interaction with Water

We have investigated the influence of a series of triethylammonium-based protic ionic liquid-water solutions on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM). We find that kosmotropic anions lower the LCST of PNIPAM more dramatically when compared with chaotropic anions. In addition, we have probed the solvent properties of the hydrated protic ionic liquid solutions using (1)H NMR, polarity measurements, and solvatochromic analysis of the Kamlet-Taft parameters, β and π*. We find that the hydrogen bond character--more specifically, the interactions between water and pIL--is the dominant parameter responsible for lowering the LCST of PNIPAM. We have added choline dihydrogen phosphate (choline dhp) into this study on the basis of positive results from previously reported protein folding studies using this ionic liquid.

Specific interactions in N-ylid solutions, studied by nuclear magnetic resonance and electronic absorption spectroscopy

Compounds with basic character, N-ylids can be involved in specific interactions with protic solvents. Consequently, “long range” (universal) and “short range” (specific) forces act simultaneously in the protic solutions of N-ylids. The contributions of these interactions to the total spectral shift measured in the Electronic Absorption Spectra (EAS) were separated on the basis of solvatochromic effects.Supplemental spectral shifts measured in protic solvents compared with aprotic ones increases with the chemical shift of –OH signal from the 1H Nuclear Magnetic Resonance (NMR) spectra of the protic solvents.Important information about the strength of the specific interactions was obtained from the spectral study of N-ylids ternary solutions in binary solvents with protic and non-protic molecules. The model of ternary solutions was used to estimate the potential energy in the molecular pairs of the type N-ylid-protic solvent.

Syntheses and highly enantioselective fluorescent recognition of α-hydroxyl/amino carboxylic acid anions in protic solutions

Abstract The multibinding-site macrocycle R-4 was synthesized and the interactions of R-4 with various α-hydroxyl/amino carboxylate were studied in DMSO/H2O system (1:1, 0.01 M Tris–HCl buffer, pH 7.4) by fluorescence titration experiments. The sensor R-4 was found to present high enantioselective fluorescent sensing ability to α-hydroxyl/amino carboxylate. It was observed that within a certain concentration range, the senor R-4 enantioselective responded to d -α-hydroxyl/amino carboxylates rather l -α-hydroxyl/amino carboxylates according to the fluorescence intensity. The results indicated that the sensor R-4 was very promising to be used as a fluorescent sensor in determining the enantiomeric composition of α-hydroxyl/amino carboxylates in DMSO/H2O system.

Luminescence, absorption, and Stern–Volmer studies of cerium chloride and nitrate compounds in acidic and neutral aqueous, and non-aqueous solutions

Complexation of cerium chloride and nitrate in neutral and acidic aqueous solutions as well as in anhydrous alcohol solutions were investigated using emission, excitation, and absorption spectroscopic techniques. In aqueous solution cerium chloride shows a strong, and broad emission centering at 365nm. The excitation spectra are observed at 266 and 296nm with the shorter wavelength showing the highest intensity. Cerium chloride compound also strongly emits in methanol (MeOH), where the broad emission spectrum is red shifted by ∼10–375nm. The excitation spectrum in MeOH shows bands at 255 and 309nm, respectively with the longer wavelength band (at 309nm) dominating. The relative intensities of these two excitation bands are reversed in protic aqueous solution. In contrast, solutions of cerium nitrate are only weakly luminescent in aqueous media, while the emission is totally quenched in MeOH solution. These observations indicate that the spectral profiles are largely influenced by the extent of inner-sphere coordination and the type of the dominant species in solutions. Both nitrate and chloride anions show enhanced inner-sphere coordination in MeOH when compared with that of the aqueous media. However, enhanced inner-sphere complexation of the NO3- ion quenches the emission, while the reverse effect is observed upon Cl− coordination. Stern–Volmer studies provide quenching constant, Ksv, value of 577M−1. The calculated rate constant kr is 1.3×1010M−1s−1 indicating diffusion controlled bimolecular process as the major mode of interaction.

A ruthenium(ii) complex with environment-responsive dual emission and its application in the detection of cysteine/homocysteine

A ruthenium polypyridine complex [Ru(phen)(2)(IPBA)](PF(6))(2) (complex 1) (IPBA = 4-(1H-imidazo[4,5-f][1,9]phenanthroline-2-yl)benzaldehyde), which displays environment-responsive dual emissive properties, was designed and synthesized. In aprotic solvent, such as DMSO, DMF or CH(3)CN, the complex emits strong cyan light. When in protic solvent, it emits orange light. Similarly, the response of the complex to homocysteine and cysteine (Hcy/Cys) also shows obvious solvent dependence. TDDFT calculations reveal that the protonation of imidazole nitrogen in protic solution is responsible for the luminescent red shift. Hence, the complex could be used to detect Hcy/Cys in aprotic solvent, as well as in protic solvent.

A Chiral Perazamacrocyclic Fluorescent Sensor for Cascade Recognition of Cu(II) and the Unmodified α-Amino Acids in Protic Solutions

A novel chiral Perazamacrocyclic fluorescent sensor (1) was designed and synthesized. It can serve as a fluorescent turn-off sensor with high selectivity toward Cu(II) among 14 metal ions. Furthermore, though 1 exhibits no enantioselectivity, after adding Cu(II), the in situ generated Cu(II)-containing complex of 1 (Cu(II)-1) can exhibit remarkable fluorescent enhancement responses and considerable enantioselectivities toward unmodified α-amino acids in protic solutions via a ligand displacement mechanism; i.e. a cascade recognition of Cu(II) and unmodified α-amino acids has been achieved.

How Linear Is “Linear” Polyaniline?

The structure of emeraldine salt and emeraldine bases with different molar weight and their behavior in electrochemical doping was studied by different spectroscopic and spectroelectrochemical techniques. By Fourier transform infrared (FTIR) spectroscopy, the branching of the polymer chain at tri- and tetrasubstituted benzene rings as well as the presence of small amounts of phenazine units are shown. The branching of the polymer chains increases with the increasing of the molar weight of emeraldines. The optical transitions in protonated and unprotonated emeraldine were studied by ultraviolet-visible near-infrared (UV-vis NIR) spectroscopy. By comparison of the electron spin resonance (ESR) spectra of emeraldine in protic solvents and acidic solutions, the emeraldine bases are shown to be to some extent protonated. Applying in situ ESR-UV-vis NIR spectroelectrochemistry, the charged states in emeraldines upon p-doping were investigated considering the influence of the nonideal "linear" polymer structures. The initial stage of oxidation of the emeraldine base and salt consists of the different charged states. The phenazine units in the polymer chains stabilize the charged states in the emeraldines upon p-doping.

Electrochemical quartz crystal microbalance studies of polymerization and redox process of poly(1,8-diaminocarbazole) in protic and aprotic solutions

Electropolymerization and doping/undoping process of poly(1,8-diaminocarbazole) (PDACz) were investigated by cyclic voltammetry combined with electrochemical quartz microbalance (EQCM), in aqueous solution of 0.1 M HClO 4 and in 0.1 M LiClO 4 in acetonitrile. The results demonstrated that protonation of the polymer in HClO 4 strongly influences the gravimetric response obtained during polymerization as well as the mechanism of redox process of the polymer in the monomer-free solution. The data are discussed in terms of the presence of protons and ClO 4 − counter ions in the reduced (neutral) polymer matrix. In aprotic acetonitrile containing LiClO 4 as the supporting electrolyte, the polymer doping/undoping occurs according to the simple mechanism with exchange of anions. The gravimetric results obtained in acidic aqueous solution allowed for some revision of the scheme of the redox process proposed by the authors in the previous paper.

Quick and simple modification of a poly(dimethylsiloxane) surface by optimized molecular design of the anti-biofouling phospholipid copolymer

The optimal molecular design of an amphiphilic copolymer composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) and dimethylsiloxane (DMS) units for modifying a poly(dimethylsiloxane) (PDMS) surface in a quick and simple manner was developed. Block- and random-type copolymers with three different compositions were each coated on a PDMS surface in a protic solution. The resulting surfaces were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, contact angle measurement. From the results, the random-type copolymer containing 86% hydrophobic DMS unit was the most suitable molecular design to be stably coated on the PDMS surface. From view point of bioengineering application, it was confirmed that for optimal suppression of protein adsorption and cell adhesion on a PDMS surface, the surface should be coated by immersing it in the polymer solutions with a concentration of 30 mg mL−1 for more than 30 s.

Kinetics of Electron Transfer Reactions of H2-Evolving Cobalt Diglyoxime Catalysts

Co-diglyoxime complexes catalyze H(2) evolution from protic solutions at modest overpotentials. Upon reduction to Co(I), a Co(III)-hydride is formed by reaction with a proton donor. Two pathways for H(2) production are analyzed: one is a heterolytic route involving protonation of the hydride to release H(2) and generate Co(III); the other is a homoytic pathway requiring association of two Co(III)-hydrides. Rate constants and reorganization parameters were estimated from analyses of laser flash-quench kinetics experiments (Co(III)-Co(II) self-exchange k = 9.5 x 10(-8) - 2.6 x 10(-5) M(-1) s(-1); lambda = 3.9 (+/-0.3) eV: Co(II)-Co(I) self-exchange k = 1.2 (+/-0.5) x 10(5) M(-1) s(-1); lambda = 1.4 (+/-0.05) eV). Examination of both the barriers and driving forces associated with the two pathways indicates that the homolytic reaction (Co(III)H + Co(III)H --> 2 Co(II) + H(2)) is favored over the route that goes through a Co(III) intermediate (Co(III)H + H(+) --> Co(III) + H(2)).

One-Size Adjustable-Gap Cluster Anions: Systematic Approach to Multinary, Water-Soluble Chalcogenidometalate Compounds

Reactions of defined mixtures of ternary salts [K 4(H 2O) 4][SnSe 4] and [K 4(H 2O) 0.5][SnTe 4] with transition-metal ions M = Zn (2+) or Mn (2+) provide an easy approach to penternary salts of quaternary cluster anions [M 4Sn 4Se 17- x Te x ] (10-), with x corresponding to the Se/Te ratio of the reactant mixture. In this way, it is possible to generate both protic solutions and single crystals with finely adjustable electronic excitation energies E g; the fact that the E g values cover the whole visible part of the electromagnetic spectrum as a function of the parameter x spotlights these cluster compounds with respect to diverse photocatalytic applications.