Insights into Tris‐(2‐Hydroxylethyl)methylammonium Methylsulfate Aqueous Solutions

We report herein a combined experimental–computational study on tris‐(2‐hydroxylethyl)methylammonium methylsulfate in water solutions, as a representative ionic liquid of the aqueous‐solution behavior of hydroxylammonium‐based ionic liquids. Relevant thermophysical properties were measured as a function of mixture composition and temperature. Classical molecular dynamics simulations were performed to infer microscopic structural features. The reported results for ionic liquid in water‐rich solutions show that it behaves as isolated non‐interacting ions solvated by water molecules, through well‐defined solvation shells, exerting a disrupting effect on the water hydrogen bonding network. Nevertheless, as ionic liquid concentration increase, interionic association increases, even for diluted water solutions, evolving from the typical behavior of strong electrolytes in solution toward large interacting structures. For ionic‐liquid‐rich mixtures, water exerts a minor disrupting effect on the fluid’s structuring because it occupies regions around each ion (developing water–ion hydrogen bonds) but without significantly weakening anion–cation interactions.     

The Electronic Nature of the 1,4‐β‐Glycosidic Bond and Its Chemical Environment: DFT Insights into Cellulose Chemistry

The molecular understanding of the chemistry of 1,4‐β‐glucans is essential for designing new approaches to the conversion of cellulose into platform chemicals and biofuels. In this endeavor, much attention has been paid to the role of hydrogen bonding occurring in the cellulose structure. So far, however, there has been little discussion about the implications of the electronic nature of the 1,4‐β‐glycosidic bond and its chemical environment for the activation of 1,4‐β‐glucans toward acid‐catalyzed hydrolysis. This report sheds light on these central issues and addresses their influence on the acid hydrolysis of cellobiose and, by analogy, cellulose. The electronic structure of cellobiose was explored by DFT at the BB1 K/6‐31++G(d,p) level. Natural bond orbital (NBO) analysis was performed to grasp the key bonding concepts. Conformations, protonation sites, and hydrolysis mechanisms were examined. The results for cellobiose indicate that cellulose is protected against hydrolysis not only by its supramolecular structure, as currently accepted, but also by its electronic structure, in which the anomeric effect plays a key role.     

The Mechanism of Biochemical NO-Sensing: Insights from Computational Chemistry

The binding of small gas molecules such as NO and CO plays a major role in the signaling routes of the human body. The sole NO-receptor in humans is soluble guanylyl cyclase (sGC) – a histidine-ligated heme protein, which, upon NO binding, activates a downstream signaling cascade. Impairment of NO-signaling is linked, among others, to cardiovascular and inflammatory diseases. In the present work, we use a combination of theoretical tools such as MD simulations, high-level quantum chemical calculations and hybrid QM/MM methods to address various aspects of NO binding and to elucidate the most likely reaction paths and the potential intermediates of the reaction. As a model system, the H-NOX protein from Shewanella oneidensis (So H-NOX) homologous to the NO-binding domain of sGC is used. The signaling route is predicted to involve NO binding to form a six-coordinate intermediate heme-NO complex, followed by relatively facile His decoordination yielding a five-coordinate adduct with NO on the distal side with possible isomerization to the proximal side through binding of a second NO and release of the first one. MD simulations show that the His sidechain can quite easily rotate outward into solvent, with this motion being accompanied in our simulations by shifts in helix positions that are consistent with this decoordination leading to significant conformational change in the protein.     

Volumetric Properties of Aqueous 2-Chloroethanol Solutions and Volumes of Transfer of Some Amino Acids and Peptides from Water to Aqueous 2-Chloroethanol Solutions

Densities and apparent molar volumes of aqueous 2-chloroethanol were determined at temperatures from 15.0 to 34.4°C using digital densimetry. The results of the volumetric measurements have been used to calculate the following thermodynamic quantites at 25°C: V ₂ ^o = 55.05 ± 0.02 cm³-mol^–1, (V ₂ ^o/T)_p = 0.01486 ± 0.00318 cm³-K^–1-mol^–1, and (² V ₂ ^o/T ²)_p = 0.02972 ± 0.00318 cm³-K^–2-mol^–1. Partial molar volumes of transfer from water to 1 mol-dm^–3 2-chloroethanol have also been determined for L-glycine, L-alanine, L-serine, L-glutamic acid, and L-aspartic acid at 35.0°C. The transfer results have been explained in terms of the nature of the interactions of the groups in the solute and solvent. Hydration numbers of L-glycine and L-alanine have also been calculated in aqueous 2-chloroethanol.     

Dynamical Dimension to the Hofmeister Series: Insights from First‐Principles Simulations

A systematic characterization of the competing kosmotropic and chaotropic effects of a series of divalent salts on the aqueous H‐bonding structure by means of first‐principles molecular dynamics simulations is presented. The structural properties are quantified by means of experimental and computed ¹H NMR chemical shifts, whereby the local environments of cations and anions can be discriminated. Complementary to the well‐established structural features, a dynamical aspect is added to the concept of kosmotropes and chaotropes. The H‐bond dynamics, quantified in terms of the H‐bonding autocorrelation functions, shows a good correlation with the structural kosmotropic and chaotropic modifications, which are commonly referred to as the Hofmeister series. The considerably enhanced (reduced) fluctuations of the H‐bonding network in the hydration shells around the anions (cations) are a complementary dynamical dimension to the concept of kosmotropic/chaotropic behavior of solvated ions.     

Self‐Assembly of Calix[6]arene–Diazapyrenium Pseudorotaxanes: Interplay of Molecular Recognition and Ion‐Pairing Effects

The calix[6]arene wheel CX forms pseudorotaxane species with the diazapyrenium‐based axle 1? 2PF₆ in CH₂Cl₂ solution. The macrocyclic component is a heteroditopic receptor, which can complex the electron‐acceptor moiety of the axle inside its cavity and the counterions with the ureidic groups on the upper rim. The self‐assembled supramolecular species is a complex structure, which involves three components—the wheel, the axle and its counterions—that can mutually interact and affect. The stoichiometry of the resulting supramolecular complex depends on the nature and concentration of the counterions. Namely, it is observed that in dilute solution and with low‐coordinating anions the axle takes two wheels, whereas with highly coordinating anions or in concentrated solutions the complex has a 1:1 stoichiometry.     

Ion Pairing and Salt Structure in Organic Salts through Diffusion,Overhauser, DFT and X‐ray Methods

Pulsed gradient spin‐echo (PGSE) diffusion characteristics for a) the new [brucinium][X] salts 6 a – f [ a : X=BF₄^?; b : X=PF₆^?; c : X=MeSO₃^?, d : X=CF₃SO₃^?; e : X=BArF^?; f : X=PtCl₃(C₂H₄)^?], b) 4‐tert‐butyl‐N‐benzyl analogue, 7 and c) the aryl carbocations (p‐R‐C₆H₄)₂CH 9 a (R=CH₃O) and 9 b (R=(CH₃)₂N), (p‐CH₃O‐C₆H₄)_xCPh_3?x⁺ 10 a – c (x=1–3, respectively) and (p‐R‐C₆H₄)₃C⁺ 11 (R=(CH₃)₂N) and 12 (R=H) all in several different solvents, are reported. The solvent dependence suggests strong ion pairing in CDCl₃, intermediate ion pairing in CD₂Cl₂ and little ion pairing in [D₆]acetone. ¹H, ¹⁹F HOESY NMR spectra (HOESY: heteronuclear Overhauser effect spectroscopy) for 6 and 7 reveal a specific approach of the anion with respect to the brucinium cation plus subtle changes, which are related to the anion itself. Further, for carbocations 9 – 12 , (all as BF₄^? salts) based on the NOE results, one finds marked changes in the relative positions of the BF₄^? anion. In these aryl cationic species the anion can be located either a) very close to the carbonium ion carbon b) in an intermediate position or c) proximate to the N or O atom of the p‐substituent and remote from the formally positive C atom. This represents the first example of such a positional dependence of an anion on the structure of the carbocation. DFT calculations support the experimental HOESY results. The solid‐state structures for 6 c and the novel Zeise's salt derivative, [brucinium][PtCl₃(C₂H₄)], 6 f , are reported. Analysis of ¹⁹⁵Pt NMR and other NMR measurements suggest that the η²‐C₂H₄ bonding to the platinum centre in 6 f is very similar to that found in K[PtCl₃(C₂H₄)]. Field dependent T₁ measurements on [brucinium][PtCl₃(C₂H₄)] and K[PtCl₃(C₂H₄)], are reported and suggested to be useful in recognizing aggregation effects.     

Impedance Analysis of Inherently Redox‐Active Ionic‐Liquid‐Based Photoelectrochemical Cells: Charge‐Transfer Mechanism in the Presence of an Additional Redox Couple

An intensive electrochemical impedance study was carried out to understand the charge‐transfer processes in photoelectrochemical (PEC) cells based on ionic liquid (IL) electrolytes. Three different electrolytes were utilized to understand the role of redox species as well as the medium on the charge‐transfer mechanism. The negligible diffusion resistance, despite the presence of two different redox species in the case of Fe(CN)₆^?4/?3 in IL, was explained on the basis of charge transfer between species of two different redox couples. Accordingly, the redox species are not required to travel through the bulk of the electrolyte for the removal of accumulated charges, as short‐range charge transfer between the IL and the Fe(CN)₆^?4/?3 species facilitates the removal of accumulated charges. It is also shown that PEC cells utilizing dual redox couples are highly stable with larger photoelectrochmeical windows, >3 V.     

Ion Association and Hydration in Aqueous Solutions of Nickel(II) and Cobalt(II) Sulfate

Aqueous solutions of nickel(II) and cobalt(II) sulfate have been investigated at 25 ^∘C by dielectric relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 ≤ ν (GHz) ≤ 89) and salt concentrations (0.025 ≤ c(mol-L⁻¹) ≤ 1.4). The spectra indicate, as for MgSO₄(aq) studied previously, the simultaneous presence of double solvent-separated, solvent-shared and contact ion pairs in both NiSO₄(aq) and CoSO₄(aq). The stepwise formation constants for each ion-pair type and the overall association constant, obtained from the data are in good agreement with ultrasonic relaxation and other estimates. The DR spectra at higher concentrations (c ≥ 0.5 mol-L⁻¹) suggest the existence of a nonlinear triple ion M₂SO₄²⁺(aq). Consistent with the very strong hydration of the salts, which have ‘effective’ hydration numbers approaching 27 at infinite dilution, there are no significant differences in any of the relaxation or thermodynamic parameters for NiSO₄(aq) and CoSO₄(aq), except that the triple ion appears to be somewhat more stable for the latter.     

Spectroscopic and Electrochemical Evaluation of Salt Effects on Electron‐Transfer Equilibria between Donor/Acceptor and Ion‐Radical Pairs in Organic Solvents

Addition of “inert” tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) to a solution of TMDO/DDQ in dichloromethane (where TMDO= 2,2,6,6‐tetramethylbenzo[1,2‐d;4,5‐d]bis[1,3]‐dioxole, donor, and DDQ= diclorodicyano‐p‐benzoquinone, acceptor) is accompanied by drastic changes in the electronic spectrum, which are related to the appearance of the DDQ ^{? .} and TMDO ^+. ion radicals and a decrease in the concentration of the neutral molecules and the charge‐transfer complex [ TMDO,DDQ ]. These changes point to a considerable rise (of about three orders of magnitude) in the apparent electron‐transfer equilibrium constant (K_ET) for this donor/acceptor pair upon increasing the electrolyte concentration from 0 to 0.5 M . Accordingly, the ion‐radical fractions and K_ET values are higher in dichloromethane, at high electrolyte concentrations, than in acetonitrile (where the effect of Bu₄NPF₆ is less pronounced). Similar trends of the apparent equilibrium constants are observed for the tetramethyl‐p‐phenylenediamine/tetracyanoethylene pair. Electron‐transfer equilibrium constants for both donor/acceptor dyads obtained from spectral measurements are related to those derived from the redox potentials of the reactants. The effects of media variations on the electron‐transfer equilibria are discussed within the ion‐pairing and ionic‐activity frameworks.     

Defective Continuous Hydrogen‐Bond Networks: An Alternative Interpretation of IR Spectroscopy

We apply our previously developed deconvolution method and interpretation to analyze changes in the OH stretching band [ν(OH) band] of low‐concentration (≤0.2 m) aqueous solutions of NaCl and KCl. We treat these simple, monovalent ions as defects in the hydrogen‐bond network of pure water and quantify the changes in the spectra at low defect concentration with an “order parameter”. Order‐parameter analysis of difference spectra of the two solutions leads to hydration numbers of 7.0±1.0 and 5.9±0.3 for K⁺ and Na⁺, respectively. Additionally, we find that changes in the ν(OH) band due to low concentrations of ions result from changes in the topology of the hydrogen‐bond network.     

Base‐Pairing Directed Folding of a Bimolecular G‐Quadruplex: New Insights into G‐Quadruplex‐Based DNAzymes

Base pairs, magic hands : An additional base‐pairing duplex is utilized to control the folding topologies of a bimolecular G‐quadruplex formed by two G‐rich single‐stranded DNAs (see picture), which is dependent on the position of base pairs. This study clearly reveals an important intrinsic role of additional base pairs in the G‐quadruplex structure, and also provides a clue to the formation mechanism of the G‐quadruplex‐based DNAzyme.

     

Janus‐Like Squaramide‐Based Hosts: Dual Mode of Binding and Conformational Transitions Driven by Ion‐Pair Recognition

New tripodal squaramide‐based hosts have been synthesised and structurally characterised by spectroscopic methods. In 2.5 % (v/v) [D₆]DMSO in CDCl₃, compound 4 formed dimeric assemblies [log K_dim=3.68(8)] as demonstrated by ¹H NMR spectroscopy and UV dilution experiments. AFM and SEM analyses revealed the formation of a network of bundled fibres, which indicates a preferential mechanism for aggregation. These C₃‐symmetric tripodal hosts exhibited two different and mutually exclusive modes of binding, each one easily accessible by simultaneous reorientation of the squaramide groups. In the first, a convergent disposition of the NH squaramide protons allowed the formation of an array of N? H???X^? hydrogen bonds with anions. In the second mode, reorientation of carbonyl squaramide groups allowed multiple C?O???H interactions with ammonium cations. The titration of 4 with different tetraalkylammonium iodides persistently showed the formation of 1:1 complexes, as well as 1:2 and 1:3 complexes. The corresponding stoichiometries and binding affinities of the complexes were evaluated by multi‐regression analysis. The formation of high‐order complexes, supported by ROESY, NOESY and mass spectrometry experiments, has been attributed to the insertion of NR₄I ion pairs between the carbonyl and NH protons of the squaramide groups located in adjacent arms of 4 . The observed effects reflect the induction of significant conformational changes in the hosts, mainly in relation to the relative orientation of the squaramide groups adapting their geometries to incoming ion‐pair complementary substrates. The results presented herein identify and fully describe two different modes of ion‐pair recognition aimed at directing conformational transitions in the host, therefore establishing a base for controlling more elaborate movements of molecular devices through ion‐pair recognition.     

Unprecedented Oxidative Addition and Metal‐Only Lewis Pair Chemistry of Antimony Trihalides

Reaction of the zero‐valent platinum complex [Pt(PCy₃)₂] with SbF₃ generates the cationic diplatinum stibenium complex [{(Cy₃P)₂Pt}₂(μ‐SbF₂)]⁺, the first unsupported metal‐only Lewis pair containing an antimony‐centered Lewis acid. In contrast, SbCl₃ undergoes oxidative addition to [Pt(PCy₃)₂], resulting in the dihalostibanyl complex trans‐[PtCl(SbCl₂)(PCy₃)₂], the first example of oxidative addition of an antimony–halide bond to a transition metal.     

Insights into the dynamics of evaporation and proton migration in protonated water clusters from Large‐scale Born–Oppenheimer direct dynamics

Large‐scale on‐the‐fly Born–Oppenheimer molecular dynamics simulations using recent advances in linear scaling electronic structure theory and trajectory integration techniques have been performed for protonated water clusters around the magic number (H₂O)_nH⁺, for n = 20 and 21. Besides demonstrating the feasibility and efficiency of the computational approach, the calculations reveal interesting dynamical details. Elimination of water molecules is found to be fast for both cluster sizes but rather insensitive to the initial geometry. The water molecules released acquire velocities compatible with thermal energies. The proton solvation shell changes between the well‐known Eigen and Zundel motifs and is characterized by specific low‐frequency vibrational modes, which have been quantified. The proton transfer mechanism largely resembles that of bulk water but one interesting variation was observed. © 2012 Wiley Periodicals, Inc.     

The Ambivalent Chemistry of a Free Anionic N‐Heterocyclic Carbene Decorated with a Malonate Backbone: The Plus of a Negative Charge

The anionic heterocycle “[maloNHC]^?”, ([ 1 ]^?), is the archetype of a growing family of N‐heterocyclic carbenes incorporating an anionic backbone; here, a malonate group. A comprehensive experimental exploration of its chemistry as a free entity (in the form of its lithium salt [ 1 ] ? Li) is presented, and rationalized using DFT calculations at the B3LYP/6‐31+G** level of theory. For the sake of comparison, similar computations were performed on other representative carbene types. Reactions of [ 1 ]?Li with a broad series of electrophilic reagents were used to ascertain its intrinsic nature as a nucleophilic carbene. Unexpectedly, [ 1 ]?Li was also seen to react with the nucleophilic tert‐butylisocyanide, to give an anionic ketenimine, which could be subsequently derivatized, either into an imine by protonation of the ketenimine moiety, or into a neutral ketenimine by alkylation of the intracyclic malonate moiety. Further experiments on the electrophilic behavior of [ 1 ]?Li revealed its unexpected reactivity toward p‐chlorobenzaldehyde, resulting in a formal C? H activation and the first structurally characterized keto‐tautomer of the Breslow intermediate. Finally, [ 1 ]?Li remarkably activates polar E? H bonds, including N? H bonds from ammonia and amines, Si? H bonds, and B? H bonds. Importantly, DFT calculations indicate the importance of counterion effects. In particular, the key to the observed reactivity appears to be a modulation of energy levels associated with a dynamic variability of the Li? O distance between the remote malonate group and the counterion.     

Ion‐Specific Oil Repellency of Polyelectrolyte Multilayers in Water: Molecular Insights into the Hydrophilicity of Charged Surfaces

Surface wetting on polyelectrolyte multilayers (PEMs), prepared by alternating deposition of polydiallyldimethylammonium chloride (PDDA) and poly(styrene sulfonate) (PSS), was investigated mainly in water‐solid‐oil systems. The surface‐wetting behavior of as‐prepared PEMs was well correlated to the molecular structures of the uncompensated ionic groups on the PEMs as revealed by sum frequency generation vibrational and X‐ray photoelectron spectroscopies. The orientation change of the benzenesulfonate groups on the PSS‐capped surfaces causes poor water wetting in oil or air and negligible oil wetting in water, while the orientation change of the quaternized pyrrolidine rings on the PDDA‐capped surfaces hardly affects their wetting behavior. The underwater oil repellency of PSS‐capped PEMs was successfully harnessed to manufacture highly efficient filters for oil‐water separation at high flux.