Internal Solvation Research Articles

Nanoporous Carbon Supercapacitors in an Ionic Liquid: A Computer Simulation Study

Supercapacitors composed of carbon nanotube (CNT) micropores in the room-temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4(-)) are studied via molecular dynamics (MD)computer simulations. It is found that the distribution of RTIL ions inside the micropore varies significantly with the pore size. Internal solvation of small (6,6) and (7,7) CNTs with an electrified interior wall is effected almost exclusively via counterions. Surprisingly, these counterions, even though they all have the same charge, lead to a charge density characterized by multiple layers with alternating signs. This intriguing feature is attributed to the extended nature of RTIL ion charge distributions, which result in charge separation through preferential orientation inside the electrified nanotubes. In the case of larger (10,10) and (15,15) CNTs, counterions and coions develop multilayer solvation structures. The specific capacitance normalized to the pore surface area is found to increase as the CNT diameter decreases from (15,15) to (7,7). As the pore size further reduces from (6,6) to(5,5), however, the specific capacitance diminishes rapidly. These findings are in excellent agreement with recent experiments with carbon-based materials. A theoretical model based on multiple charge layers is proposed to understand both the MD and experimental results.

Guided ion beam and theoretical studies of sequential bond energies of water to sodium cysteine cation

Absolute bond dissociation energies of water to sodium cysteine (Cys) cations and cysteine to hydrated sodium cations are determined experimentally by collision-induced dissociation of Na(+)Cys(H(2)O)(x), where x = 1-4, complexes with xenon in a guided ion beam mass spectrometer. Experimental results show that the binding energies of water and cysteine to the complexes decrease monotonically with increasing number of water molecules. Quantum chemical calculations at three different levels show reasonable agreement with the experimental bond energies. The calculations indicate that the primary binding site for Na(+) changes from charge-solvated tridentate chelation at the amino nitrogen, carbonyl oxygen, and sulfur side-chain for x = 0 and 1 to the C terminus of zwitterionic cysteine for x = 4, whereas different levels of theory provide conflicting predictions for x = 2 and 3. The first solvent shell of Na(+)Cys is found to be complete at four waters. This is fewer than needed for the aliphatic amino acid glycine, because the functionalized side-chain of Cys provides an internal solvation site, a binding motif that probably applies for most other functionalized amino acids.

Solvation of Carbon Nanotubes in a Room-Temperature Ionic Liquid

Single- and double-walled carbon nanotubes in the armchair configuration solvated in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4−) are studied via molecular dynamics (MD) computer simulations. Cations and anions show smeared-out, cylindrical shell-like distributions outside of the nanotubes irrespective of the nanotube diameter. The ion distributions inside the nanotubes vary markedly with their diameter. For example, in the case of (n,n) single-walled nanotubes, EMI+ and BF4− ions separately form single-shell zigzag and chiral distributions for (8,8) and (10,10), respectively, while (12,12) develops a second internal solvation structure. The first internal solvation shell of (15,15) nanotubes consists of alternating layers of cations and anions along the nanotube axis. In the azimuthal direction, these cations and anions, respectively, form a pentagonal structure, whereas the corresponding ions for (20,20) show disordered octagonal structures. The smallest nanotube that allows solvent ions inside the tunnel is (7,7) with a diameter of 0.95 nm, which shows a single file distribution of internal ions. Imidazole rings of cations in the first internal and external solvation shells are mainly parallel to the nanotube surface, indicating π-stacking between the nanotubes and EMI+ ions there.

Interpretation of the emission fluorescence spectra of two fluoroionophores: DMABN‐Crown4 and DMABN‐Crown5

AbstractWe discuss an interpretation of the emission signal of a class of dimethylaminobenzonitrile (DMABN) derivatives, based on a stochastic model approach. This study is prompted by the interest in building relatively simple theoretical tools for understanding the static and dynamic properties of metal chelation‐controlled fluorescent molecular systems with internal flexibility and solvation effects, which can be used as fluorescent sensors of metal ions. We proceed by extending a stochastic modeling approach, previously used for DMABN, in which internal degrees of freedom are coupled to an effective solvent relaxation variable. The model is based on an integrated approach that combines stochastic methods and QM calculations. The static and dynamic properties of the molecule are obtained by a combination of a continuum treatment and a detailed solute dynamics. Simple computational approximations lead to a semianalytical treatment of the resulting equations, which reproduce correctly the static spectra at room temperature of 4‐(1‐aza‐4,7,10‐trioxacyclododecyl) benzonitrile (DMABN‐Crown4) and 4‐(1‐aza‐4,7,10,13‐tetraoxacyclododecyl) benzonitrile (DMABN‐Crown5) in solvents with different polarity. A qualitative analysis of the dynamic spectra of the DMABN‐Crown4 is also presented. A discussion is finally provided on how combined quantum and classical methods can be used to define a general robust and reliable computational approach for predicting fluorescence properties of organic molecules in solution. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Electronic Properties of Charge-Tagged Peptides upon Electron Capture

We report a computational study of Ala-Lys (AK) and Lys-Ala (KA) dipeptide ions furnished with fixed-charged pyridinium groups that were attached by amide linkers to the N-terminal amino groups. Cation-radicals from one-electron reduction of the doubly charged AK and KA peptide conjugates showed various extents of unpaired electron density being delocalized between the pyridine and peptide moieties. The delocalization depended on the local recombination energies (RE(loc)) of the charged groups. The RE(loc) of the pyridine moieties were modified by introducing electron-donating substituents (CH(3), OCH(3), and N(CH(3))(2)). The RE(loc) of the peptide moieties were found to depend on the peptide conformation and internal solvation of the Lys ammonium groups. Substantial electron delocalization was found for combinations of pyridine substituents and peptide conformers with closely matched RE(loc), such as 4-dimethylamino-pyridine and internally solvated Lys ammonium or unsubstituted pyridine and free (unsolvated) Lys ammonium. The dissociation (DeltaH(diss)) and transition state energies (E(TS)) for the loss of the pyridine ring from the conjugates were found to be DeltaH(diss) = 34-36 kJ mol(-1) and E(TS) = 67-69 kJ mol(-1) for the unsubstituted pyridine moieties, but did not depend much on the peptide sequence.

Revisiting the internal conformational dynamics and solvation properties of cyclodextrins

Molecular dynamics simulations were used to investigate the internal conformational dynamics and solvation properties of the three natural cyclodextrins, a-, b- and g-cyclodextrin in aqueous solution at room temperature. These glucose-derived oligosacharides present a molecular structure that confers them the ability to complex guest molecules and change their physicochemical properties. The structural behavior of cyclodextrins in solution is crucial for their complexation abilities. Analyses of the obtained trajectories show that inter-glucose hydrogen bonds between the secondary hydroxyl groups are present in solution, but show a very dynamical character where alternative hydrogen bonds to water molecules can be formed. Despite the lower hydrophilicity of the cyclodextrins inner-cavities, they were found to be solvated and the number of water molecules inside the cavity roughly doubles per glucose unit added to the ring. The residence times for water molecules inside the cavities are inversely proportional to the cavity size.

Determination of the Gas-Phase Acidities of Cysteine-Polyalanine Peptides Using the Extended Kinetic Method

We determined the gas-phase acidities of two cysteine-polyalanine peptides, HSCA3 and HSCA4, using a triple-quadrupole mass spectrometer through application of the extended kinetic method with full entropy analysis. Five halogenated carboxylic acids were used as the reference acids. The negatively charged proton-bound dimers of the deprotonated peptides with the conjugate bases of the reference acids were generated by electrospray ionization. Collision-induced dissociation (CID) experiments were carried out at three collision energies. The enthalpies of deprotonation (Delta(acid)H) of the peptides were derived according to the linear relationship between the logarithms of the CID product ion branching ratios and the differences of the gas-phase acidities. The values were determined to be Delta(acid)H(HSCA3) = 317.3 +/- 2.4 kcal/mol and Delta(acid)H (HSCA4) = 316.2 +/- 3.9 kcal/mol. Large entropy effects (Delta(DeltaS) = 13-16 cal/mol K) were observed for these systems. Combining the enthalpies of deprotonation with the entropy term yielded the apparent gas-phase acidities (Delta(acid)G(app)) of 322.1 +/- 2.4 kcal/mol (HSCA3) and 320.1 +/- 3.9 kcal/mol (HSCA4), in agreement with the results obtained from the CID-bracketing experiments. Compared with that in the isolated cysteine residue, the thiol group in HSCA3,4 has a stronger gas-phase acidity by about 20 kcal/mol. This increased acidity is likely due to the stabilization of the negatively charged thiolate group through internal solvation.

Thermal Decomposition of Multiply Charged T-Rich Oligonucleotide Anions in the Gas Phase. Influence of Internal Solvation on the Arrhenius Parameters for Neutral Base Loss

Arrhenius activation parameters (E(a), A) for the loss of neutral nucleobases from a series of T-rich, doubly and triply deprotonated 15- and 20-mer oligodeoxynucleotides (ODN) containing a single reactive base (X = A or C) with the sequence, XT14, XT19 and T19X, have been determined using the blackbody infrared radiative dissociation technique. The A-containing anions are significantly more reactive (> or =3000 times) than the C-containing ions over the temperature range investigated. Importantly, the Arrhenius parameters for the loss of AH exhibit a strong dependence on size of the ODN and, to some extent, the charge state; the Arrhenius parameters increase with size and charge (Ea = 29-39 kcal mol(-1), A = 10(15)-10(20) s(-1)). In contrast, the parameters for the loss of CH are much less sensitive to size (Ea = 35-39 kcal mol(-1), A = 10(14)-10(17) s(-1)). The results are consistent with a greater contribution from the internal solvation of the reactive base to the Arrhenius parameters for the loss of A, compared with C, from the 15- and 20-mers. To further probe differences in internal solvation of A and C, hydrogen/deuterium exchange was carried out on AT19(-3), T19A(-3), CT19(-3) and T19C(-3) using D2O as the exchange reagent. However, the H/D exchange results did not reveal any differences in internal solvation within the ODN anions. Arrhenius parameters for the dissociation of noncovalent complexes of T20(-3) and the neutral nucleobase AH or CH have also been determined. Differences in the parameters indicate differences in the nature of the intermolecular interactions. It is proposed that neutral A-T interactions (i.e., base-base), which originate in solution, dominate in the case of (T20 + AH)(-3), while charge solvation, involving CH and a deprotonated phosphate group, is present for (T20 + CH)(-3).

Gaseous Bradykinin and Its Singly, Doubly, and Triply Protonated Forms: A First-Principles Study

The conformers of gaseous bradykinin, BK, (Arg(1)-Pro(2)-Pro(3)-Gly(4)-Phe(5)-Ser(6)-Pro(7)-Phe(8)-Arg(9)) and its protonated forms, [BK + H](+), [BK + 2H](2+), and [BK + 3H](3+), were examined theoretically using a combination of the Merck molecular force field, Hartree-Fock, and density functional theory. Neutral BK, [BK + H](+), and [BK + 2H](2+) exist in zwitterionic forms that are stabilized by internal solvation and have compact structures; [BK + 3H](3+) differs by the absence of a salt bridge and adopts an elongated form. The common structural feature in all four BK species is a beta-turn in the Ser(6)-Pro(7)-Phe(8)-Arg(9) sequence. The gas-phase basicity of [BK + H](+) estimated from the calculated protonation energy is in accord with published experimental basicity; population-weighted collision cross-sections of the three ionic forms are in agreement with experimental cross-sections in the literature.

Chiral Diamine−Silver(I)−Alkene Complexes: A Quantum Chemical and NMR Study

The ability of chiral diamine silver complexes to bind chiral and prochiral alkenes has been analyzed in detail. The stereoselectivity in binding of alkenes to a chiral ethanediamine silver complex has been investigated by NMR. The low-energy conformations of several small model complexes have been explored by DFT methods. By successive substitution of the computational model complexes, it has been possible to elucidate the role of each amine substituent in achieving successful discrimination of alkenes. The conformational space has been fully explored using small model systems, allowing an unbiased calculation of stereoselectivities that match well the experimental results. For a chiral allylic alcohol substrate, the correct stereoselectivity was obtained only when the structures were optimized with a continuum representation of the solvent. The discrepancy between gas phase and solution data is found to result from a competition between internal stabilization and solvation of the OH group of the substrate.

Dynamics and fluidity of amyloid fibrils: a model of fibrous protein aggregates.

A previous experimentally defined model for the fibril formed from the core residues of the beta-amyloid (Abeta) peptides of Alzheimer's disease, 10YEVHHQKLVFFAEDVGSNKGAIIGLM, Abeta(10-35) using spectroscopic and scattering analyses reports on the average structure, benefiting immensely from the homogeneous assembly of Abeta(10-35). However, the energetic constraints that contribute to fibril dynamics and stability remain poorly understood. Here we perform molecular dynamics simulations to extend the structural assignment by providing evidence for a dynamic average ensemble with transient backbone H-bonds and internal solvation contributing to the inherent stability of amyloid fibrils.

UV−vis Subpicosecond Spectroscopy of 4-(9-Anthryl)-N,N‘-dimethylaniline in Polar and Nonpolar Solvents: A Two-Dimensional View of the Photodynamics

Transient absorption and gain spectra are reported for 4-(9-anthryl)-N,N'-dimethylaniline (ADMA) in polar and nonpolar solvents in the 330-780 nm wavelength range after excitation with a 500-fs laser at 370 nm. In acetonitrile, the initial transient spectra can be interpreted by the superposition of the dimethylaniline cation radical and the anthracene anion radical absorption bands, resulting from the well-known ultrafast photoinduced charge separation. In benzonitrile, similar transient spectra are reached 20-30 ps after excitation. In this solvent, as well as in alcohols, tetrahydrofuran and cyclohexane, the initial UV and visible absorption bands exhibit the spectral characteristics of the locally excited (LE) state. With increasing time, while these bands decay, the signature of the charge-transfer (CT) state appears and one observes a red shift of the region of minimum differential absorption, where gain is expected. These changes occur at a solvent-dependent rate, except in acetonitrile in which the spectra show little evolution because of our limited time resolution. Discrepancies with previously published models for ADMA photodynamics are discussed. In polar solvents except acetonitrile, a quasi-barrierless or slightly activated electron transfer is proposed to explain the long-time decay of the UV absorption band attributed to the LE state. On the other hand, inertial internal torsion around the bond linking the aniline and anthracene moieties in the LE state followed by solvation-induced charge separation similar to that in bianthryl is proposed to explain differences between acetonitrile and benzonitrile or other solvents in the UV range. A two-dimensional picture in which both internal torsion and solvation dynamics determine the excited-state reaction path is discussed, taking into account the initial proposal made by Mataga's group. In cyclohexane, in which solvation effects are not expected, the spectral changes in the picosecond time range are attributed to geometrical relaxation within the S, state.

Probing hydrogen bond network formation in anion–water clusters through high energy collision experiments

In high-energy collisions (50 keV in the laboratory system) between anion–water clusters [X−⋅(H2O)n, X−=Cl−, CN−, O2−, NO2−, NO3−; n=1–6] and argon, H3O+ is formed with an abundance that is determined by the cluster size—the larger the cluster, the more H3O+. The mechanism for H3O+ formation is believed to be a nearly vertical ionization process (two-electron stripping) followed by an exothermic intracluster hydrogen transfer reaction between an ionized water and a neutral water. The abundance of H3O+ can be used as a probe to determine how extended the water hydrogen bond network is in the initial anion–water cluster and to distinguish between surface solvation (water network preserved) and internal solvation (water network broken). In this work, it is demonstrated that in the hydration of hexacyanoplatinate(IV) dianion complexes, surface solvation is important despite the large number of available water binding sites; however, the competition between a cyanide-bound water and a “naked” cyanide ligand for a water molecule favors the cyanide-bound water because of the splitting of the excess charges between six ligands (between −1/3 and −1 charge at each ligand on average). We also investigate anion–methanol clusters in which the hydrogen bond network is less extended with the result of a less abundant oxonium ion compared to the hydronium ion from similar size water clusters.

ChemInform Abstract: Consequences of Internal Solvation in the Gas Phase

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Gas-phase basicity of polyfunctional amidinazines: experimental evidence of preferred site(s) of protonation

Gas-phase basicities of four polyfunctional N1,N1-dimethyl-N2-azinylformamidines (1-4) are obtained from proton-transfer equilibrium constant determinations, using Fourier transform ion-cyclotron resonance mass spectrometry. Comparison with model amidines and azines (GB revised according to the recent compilation of Hunter and Lias) indicates the aza group as the favored site of protonation. The strong basicity of ortho derivatives is explained in term of intramolecular stabilization (the so-called "internal solvation").

Ab initio investigations on the photophysics of indole

Reaction paths and potential-energy profiles for detachment of the hydrogen atom of the NH group in excited singlet states of indole have been investigated using the CIS, CASSCF and CASPT2 ab initio methods. The potential-energy profile of the lowest π σ ∗ excited singlet state is found to be essentially repulsive. It crosses the potential-energy functions of the 1 L b and 1 L a excited states of ππ ∗ character as well as those of the ground state. The resulting multiple conical intersections can provide the mechanism for efficient internal conversion to the ground state. The polarities of the excited states are remarkably different, indicating a complex interplay of internal conversion and solvation dynamics of photoexcited indole in polar solvents.

ChemInform Abstract: Gaseous Arenium Ions at Atmospheric Pressure: Elementary Reactions and Internal Solvation Effects

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Size Dependence of Blackbody Radiation Induced Hydrogen Formation in Al+(H2O)n Hydrated Aluminum Cations and Their Reactivity with Hydrogen Chloride

Trapped hydrated aluminum cluster ions are studied by FT-ICR mass spectrometry on a time scale of several seconds. Blackbody radiation, besides causing the fragmentation of the cluster by stepwise loss of individual water molecules, induces in hydrated aluminum clusters Al+(H2O)n an intracluster reaction yielding hydrated hydroxide and releasing molecular hydrogen. Local deviations of the individual rate constants for the water loss process from a linear size dependence are due to increased rigidity of certain sizes due to the formation of stabilizing hydrogen-bonded bridges. In comparison with previously studied hydrated ions, surface versus internal solvation is discussed. The preferential occurrence of the intracluster reaction in the size region of n = 11−24 is attributed to a concerted proton-transfer mechanism, in which a chain of at least two water molecules is needed to transfer a proton between two first solvation shell water molecules, leading to formation of an Al(OH)2+(H2O)n hydrated aluminum dihydroxide cation and molecular hydrogen. The Al+(H2O)n species with n ≥ 13 and all investigated Al(OH)2+(H2O)m species are able to react with and “dissolve” HCl. The maximum number of HCl molecules in the cluster strongly depends on the number of water molecules available for solvation. The presence of HCl in the cluster removes the upper limit for the intracluster reaction, which leads to the formation of molecular hydrogen, driven by blackbody radiation. This is taken as further evidence for the validity of the proton-transfer mechanism.

Mobile protons in large gaseous alkylbenzenium ions. The 21-proton equilibration in protonated tetrabenzylmethane and related “proton dances”

The proton transfer between the protonated arene (arenium) and the neutral arene rings of protonated tribenzylmethane ( 8), tetrabenzylmethane ( 9), several higher triphenylisoalkylalkanes ( 10– 12) as well as of protonated 2-benzylindane ( 13) and 2,2-dibenzylindane ( 14) has been studied in detail. In all regimes of the ions’ lifetimes (ion source, first and second field-free region), the interannular proton exchange leads to complete equilibration. As an extreme, 21 protons are randomized in protonated tetrabenzylmethane 9 prior to fragmentation within ∼1 μs. The hydrogens of the aliphatic or alicyclic spacer do not participate in the exchange process. The interannular proton exchange is independent of the fragmentation path; both single and twofold loss of benzene from 8 and protonated tris(β-phenylethyl)methane 12 are preceded by complete proton equilibration between all of the rings. The individual proton transfer rate for 9 is estimated to be >10 8 s −1, and the readiness of proton equilibration is assigned to the efficient internal solvation, or spectator ring effects, in these large alkylbenzenium ions, possibly giving rise to a merge between the numerous inter- and intra-annular proton transfer steps during the ions’ lifetime.

Gaseous Arenium Ions at Atmospheric Pressure: Elementary Reactions and Internal Solvation Effects

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTGaseous Arenium Ions at Atmospheric Pressure: Elementary Reactions and Internal Solvation EffectsSimonetta Fornarini and Maria Elisa CrestoniView Author Information Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Università di Roma “La Sapienza”, P. le A. Moro 5, I-00185 Roma, Italy Cite this: Acc. Chem. Res. 1998, 31, 12, 827–834Publication Date (Web):October 24, 1998Publication History Received21 January 1998Published online24 October 1998Published inissue 1 December 1998https://doi.org/10.1021/ar960247uCopyright © 1998 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views166Altmetric-Citations48LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (246 KB) Get e-AlertsSUBJECTS:Aromatic compounds,Hydrocarbons,Ions,Reaction mechanisms,Reactivity Get e-Alerts