Directly relating reduction energies of gaseous Eu(H2O)n(3+), n = 55-140, to aqueous solution: the absolute SHE potential and real proton solvation energy

First and Second Reductions in an Aprotic Solvent: Comparing Computational and Experimental One-Electron Reduction Potentials for 345 Quinones

Using reference reduction potentials of quinones recently measured relative to the saturated calomel electrode (SCE) in N,N-dimethylformamide (DMF), we benchmark absolute one-electron reduction potentials computed for 345 Q/Q•- and 265 Q•-/Q2- half-reactions using adiabatic electron affinities computed with density functional theory and solvation energies computed with four continuum solvation models: IEF-PCM, C-PCM, COSMO, and SM12. Regression analyses indicate a strong linear correlation between experimental and absolute computed Q/Q•- reduction potentials with Pearson's correlation coefficient (r) between 0.95 and 0.96 and the mean absolute error (MAE) relative to the linear fit between 83.29 and 89.51 mV for different solvation methods when the slope of the regression is constrained to 1. The same analysis for Q•-/Q2- gave a linear regression with r between 0.74 and 0.90 and MAE between 95.87 and 144.53 mV, respectively. The y-intercept values obtained from the linear regressions are in good agreement with the range of absolute reduction potentials reported in the literature for the SCE but reveal several sources of systematic error. The y-intercepts from Q•-/Q2- calculations are lower than those from Q/Q•- by around 320-410 mV for IEF-PCM, C-PCM, and SM12 compared to 210 mV for COSMO. Systematic errors also arise between molecules having different ring sizes (benzoquinones, naphthoquinones, and anthraquinones) and different substituents (titratable vs nontitratable). SCF convergence issues were found to be a source of random error that was slightly reduced by directly optimizing the solute structure in the continuum solvent reaction field. While SM12 MAEs were lower than those of the other solvation models for Q/Q•-, SM12 had larger MAEs for Q•-/Q2- pointing to a larger error when describing multiply charged anions in DMF. Altogether, the results highlight the advantages of, and further need for, testing computational methods using a large experimental data set that is not skewed (e.g., having more titratable than nontitratable substituents on different parent groups or vice versa) to help further distinguish between sources of random and systematic errors in the calculations.

An Emerging Chemistry Revives Proton Batteries

Developing new energy techniques that simultaneously integrate the fast rate capabilities of supercapacitors and high capacities of batteries represents an ultimate goal in the field of electrochemical energy storage. A new possibility arises with an emerging battery chemistry that relies on proton-ions as the ion-charge-carrier and benefits from the fast transportation kinetics. Proton-based battery chemistry starts with the recent discoveries of materials for proton redox reactions and leads to a renaissance of proton batteries. In this article, the historical developments of proton batteries are outlined and key aspects of battery chemistry are reviewed. First, the fundamental knowledge of proton-ions and their transportation characteristics is introduced; second, Faradaic electrodes for proton storage are categorized and highlighted in detail; then, reported electrolytes and different designs of proton batteries are summarized; last, perspectives of developments for proton batteries are proposed. It is hoped that this review will provide guidance on the rational designs of proton batteries and benefit future developments.

Development of a multi-step screening procedure for redox active molecules in organic radical polymer anodes and as redox flow anolytes

Benzo[d]-X-zolyl-pyridinyl (XO, S, NH) radicals represent a promising class of redox-active molecules for organic batteries. We present a multistep screening procedure to identify the most promising radical candidates. Experimental investigations and highly correlated wave function-based calculations are performed to determine benchmark redox potentials. Based on these, the accuracies of different methods (semi-empirical, density functional theory, wave function-based), solvent models, dispersion corrections, and basis sets are evaluated. The developed screening procedure consists of three steps: First, a conformer search is performed with CREST. The molecules are selected based on the redox potentials calculated using GFN2-xTB. Second, HOMO energies calculated with reparametrized B3LYP-D3(BJ) and the def2-SVP basis set are used as selection criteria. The final molecules are selected based on the redox potentials calculated from Gibbs energies using BP86-D3(BJ)/def2-TZVP. With this multistep screening approach, promising molecules can be suggested for synthesis, and structure-property relationships can be derived.

Synchronous Redox Reactions in Copper Oxalate Enable High-Capacity Anode for Proton Battery

Proton batteries are competitive due to their merits such as high safety, low cost, and fast kinetics. However, it is generally difficult for current studies of proton batteries to combine high capacity and high stability, while the research on proton storage mechanism and redox behavior is still in its infancy. Herein, the polyanionic layered copper oxalate is proposed as the anode for a high-capacity proton battery for the first time. The copper oxalate allows for reversible proton insertion/extraction through the layered space but also achieves high capacity through synchronous redox reactions of Cu2+ and C2O42-. During the discharge process, the bivalent Cu-ion is reduced, whereas the C═O of the oxalate group is partially converted to C-O. This synchronous behavior presents two units of charge transfer, enabling the embedding of two units of protons in the (110) crystal face. As a result, the copper oxalate anode demonstrates a high specific capacity of 226 mAh g-1 and maintains stable operation over 1000 cycles with a retention of 98%. This work offers new insights into the development of dual-redox electrode materials for high-capacity proton batteries.

Hydronium Intercalation Enables High Rate in Hexagonal Molybdate Single Crystals

Rapid proton transport in solid-hosts promotes a new chemistry in achieving high-rate Faradaic electrodes. Exploring the possibility of hydronium intercalation is essential for advancing proton-based charge storage. Nevertheless, this is yet to be revealed. Herein, a new host is reported of hexagonal molybdates, (A2 O)x ·MoO3 ·(H2 O)y (A = Na+ , NH4 + ), and hydronium (de)intercalation is demonstrated with experiments. Hexagonal molybdates show a battery-type initial reduction followed by intercalation pseudocapacitance. Fast rate of 200 C (40 A g-1 ) and long lifespan of 30 000 cycles are achieved in electrodes of monocrystals even over 200 µm. Solid-state nuclear magnetic resonance confirms hydronium intercalations, and operando measurements using electrochemical quartz crystal microbalance and synchrotron X-ray diffraction disclose distinct intercalation behaviours in different electrolyte concentrations. Remarkably, characterizations of the cycled electrodes show nearly identical structures and suggest equilibrium products are minimally influenced by the extent of proton solvation. These results offer new insights into proton electrochemistry and will advance correlated high-power batteries and beyond.

Precise Fingerprint Determination of Vibrational Infrared Spectra in a Series of Co(II) Clathrochelates through Experimental and Theoretical Analyses

The energetic demands of modern society for clean energy vectors, such as H2, have caused a surge in research associated with homogeneous and immobilized electrocatalysts that may replace Pt. In particular, clathrochelates have shown excellent electrocatalytic properties for the hydrogen evolution reaction (HER). However, the actual mechanism for the HER catalyzed by these d-metal complexes remains an open debate, which may be addressed via Operando spectroelectrochemistry. The prediction of electrochemical properties via density functional theory (DFT) needs access to thermodynamic functions, which are only available after Hessian calculations. Unfortunately, there is a notable lack in the current literature regarding the precise evaluation of vibrational spectra of such complexes, given their structural complexity and the associated tangled IR spectra. In this work, we have performed a detailed theoretical and experimental analysis in a family of Co(II) clathrochelates, in order to establish univocally their IR pattern, and also the calculation methodology that is adequate for such predictions. In summary, we have observed the presence of multiple common bands shared by this clathrochelate family, using the B3LYP functional, the LANL2DZ basis, and effective core potentials (ECP) for heavy atoms. The most important issue addressed in this article was therefore related to the detailed assignment of the fingerprint associated with cobalt(II) clathrochelates, which is a challenging endeavor due to the crowded nature of their spectra.

Structure Dependence of CO2 Reduction Electrocatalyzed by Metal-Nanographene Complexes: A Computational Study

Improving the energy efficiency of electrocatalytic reduction of CO2 requires tuning of redox properties of electrocatalysts to match redox potentials of the substrate. Recently, we introduced nanographenes as ligands for metal complexes for such purposes by taking advantage of size-dependent properties of the conjugated systems. Here, we use computations to investigate the structure dependence of the electrocatalysis at Re(diimine)(CO)3Cl complexes with nanographene ligands that contain a polycyclic aromatic hydrocarbon moiety through a pyrazinyl linkage. We show that the reduction potentials of the complexes depend not only on conjugation size but also on shape and geometry of the ligands, revealing another parameter in tuning the redox properties of the electrocatalysts. In addition, our work reveals a compromise between reduction potentials and activation of this class of electrocatalysts.

Nanostructured TiO2 Arrays for Energy Storage

Because of their extensive specific surface area, excellent charge transfer rate, superior chemical stability, low cost, and Earth abundance, nanostructured titanium dioxide (TiO₂) arrays have been thoroughly explored during the past few decades. The synthesis methods for TiO₂ nanoarrays, which mainly include hydrothermal/solvothermal processes, vapor-based approaches, templated growth, and top-down fabrication techniques, are summarized, and the mechanisms are also discussed. In order to improve their electrochemical performance, several attempts have been conducted to produce TiO₂ nanoarrays with morphologies and sizes that show tremendous promise for energy storage. This paper provides an overview of current developments in the research of TiO₂ nanostructured arrays. Initially, the morphological engineering of TiO₂ materials is discussed, with an emphasis on the various synthetic techniques and associated chemical and physical characteristics. We then give a brief overview of the most recent uses of TiO₂ nanoarrays in the manufacture of batteries and supercapacitors. This paper also highlights the emerging tendencies and difficulties of TiO₂ nanoarrays in different applications.

Rational Design of Electrode-Electrolyte Interphase and Electrolytes for Rechargeable Proton Batteries

Rechargeable proton batteries have been regarded as a promising technology for next-generation energy storage devices, due to the smallest size, lightest weight, ultrafast diffusion kinetics and negligible cost of proton as charge carriers. Nevertheless, a proton battery possessing both high energy and power density is yet achieved. In addition, poor cycling stability is another major challenge making the lifespan of proton batteries unsatisfactory. These issues have motivated extensive research into electrode materials. Nonetheless, the design of electrode-electrolyte interphase and electrolytes is underdeveloped for solving the challenges. In this review, we summarize the development of interphase and electrolytes for proton batteries and elaborate on their importance in enhancing the energy density, power density and battery lifespan. The fundamental understanding of interphase is reviewed with respect to the desolvation process, interfacial reaction kinetics, solvent-electrode interactions, and analysis techniques. We categorize the currently used electrolytes according to their physicochemical properties and analyze their electrochemical potential window, solvent (e.g., water) activities, ionic conductivity, thermal stability, and safety. Finally, we offer our views on the challenges and opportunities toward the future research for both interphase and electrolytes for achieving high-performance proton batteries for energy storage.

Surface-Induced Desolvation of Hydronium Ion Enables Anatase TiO2 as an Efficient Anode for Proton Batteries

Hydrogen ion is an attractive charge carrier for energy storage due to its smallest radius. However, hydrogen ions usually exist in the form of hydronium ion (H₃O⁺) because of its high dehydration energy; the choice of electrode materials is thus greatly limited to open frameworks and layered structures with large ionic channels. Here, the desolvation of H₃O⁺ is achieved by using anatase TiO₂ as anodes, enabling the H⁺ intercalation with a strain-free characteristic. Density functional theory calculations show that the desolvation effects are dependent on the facets of anatase TiO₂. Anatase TiO₂ (001) surface, a highly reactive surface, impels the desolvation of H₃O⁺ into H⁺. When coupled with a MnO₂ cathode, the proton battery delivers a high specific energy of 143.2 Wh/kg at an ultrahigh specific power of 47.9 kW/kg. The modulation of the interactions between ions and electrodes opens new perspectives for battery optimizations.

Ab Initio Evaluation of the Redox Potential of Cytochrome c

Various biochemical activities of metabolism and biosynthesis are fulfilled by redox processes with explicit electron exchange, which furnish redox enzymes with high chemical reactivity. However, theoretical investigation of a redox process, which simultaneously involves a complex electronic change at a redox metal center and conformational reorganization of the surrounding protein environment coupled to the electronic change, requires computationally conflicting approaches, highly accurate quantum chemical calculations, and long-time molecular dynamics (MD) simulations, limiting the physicochemical understanding of biological redox processes. Here, we theoretically examined a redox process of cytochrome c by means of a hybrid molecular simulation technique, which enables one to consistently treat the redox center at the ab initio quantum chemistry level of theory and the protein reorganization with long-time MD simulations on the microsecond timescale. The calculations successfully evaluated a large absolute redox potential, 4.34 eV, with errors of only 0.03 to 0.34 eV to the experimental ones without any problem-specific empirical parameters. Through the long-time MD sampling, large and nonlinear reorganization of the protein environment was unveiled and the molecular determinants for the redox potential were identified. The present ab initio approach significantly expands the applicability of theoretical investigation to biological redox systems with more electronically complicated redox centers such as polynuclear transition metal complexes.

Method for the accurate prediction of electron transfer potentials using an effective absolute potential

A protocol for the accurate computation of electron transfer (ET) potentials from ab initio and density functional theory (DFT) calculations is described. The method relies on experimental pKa values, which can be measured accurately, to compute a computational setup dependent effective absolute potential. The effective absolute potentials calculated using this protocol display strong variations between the different computational setups and deviate in several cases significantly from the "generally accepted" value of 4.28 V. The most accurate estimate, obtained from CCSD(T)/aug-ccpvqz, indicates an absolute potential of 4.14 V for the normal hydrogen electrode (nhe) in water. Using the effective absolute potential in combination with CCSD(T) and a moderately sized basis, we are able to predict ET potentials accurately for a test set of small organic molecules (σ = 0.13 V). Similarly we find the effective absolute potential method to perform equally good or better for all considered DFT functionals compared to using one of the literature values for the absolute potential. For, M06-2X, which comprises the most accurate DFT method, standard deviation of 0.18 V is obtained. This improved performance is a result of using the most appropriate effective absolute potential for a given method.

Development of a photoinduced fragmentation ion trap for infrared multiple photon dissociation spectroscopy

RATIONALE

Methods for isomer discrimination by mass spectroscopy are of increasing interest. Here we describe the development of a three-dimensional ion trap for infrared multiple photon dissociation (IRMPD) spectroscopy that enables the acquisition of the infrared spectrum of selected ions in the gas phase. This system is suitable for the study of a myriad of chemical systems, including isomer mixtures.

METHODS

A modified three-dimensional ion trap was coupled to a CO₂ laser and an optical parametric oscillator/optical parametric amplifier (OPO/OPA) system operating in the range 2300 to 4000 cm^-1 . Density functional theory vibrational frequency calculations were carried out to support spectral assignments.

RESULTS

Detailed descriptions of the interface between the laser and the mass spectrometer, the hardware to control the laser systems, the automated system for IRMPD spectrum acquisition and data management are presented. The optimization of the crystal position of the OPO/OPA system to maximize the spectroscopic response under low-power laser radiation is also discussed.

CONCLUSIONS

OPO/OPA and CO₂ laser-assisted dissociation of gas-phase ions was successfully achieved. The system was validated by acquiring the IRMPD spectra of model species and comparing with literature data. Two isomeric alkaloids of high economic importance were characterized to demonstrate the potential of this technique, which is now available as an open IRMPD spectroscopy facility in Brazil.

A P/N type silicon semiconductor loaded with silver nanoparticles used as a SERS substrate to selectively drive the coupling reaction induced by surface plasmons

Semiconductor materials are favoured in the field of photocatalysis due to their unique optoelectronic properties. When a semiconductor is excited by external energy, electrons will transition through the band gap, providing electrons or holes for the reaction. This is similar to the chemical enhancement mode of a catalytic reaction initiated by the rough noble metal on the surface excited by plasmon resonance. In this study, different types of semiconductor silicon loaded with silver nanoparticles were used as SERS substrates. SERS detection of p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) probe molecules was performed using typical surface plasmon-driven coupling reactions, and the mechanism of optical drive charge transfer in semiconductor-metal-molecular systems was investigated. Scanning electron microscopy and plasmon luminescence spectroscopy were used to characterize the silver deposited on the substrate surface. Mapping technology and electrochemistry were used to characterize the photocatalytic reaction of the probe molecules. This study proposed a mechanism for the coupling reaction of "hot electrons" and "hot holes" on the surface of plasmon-driven molecules and provides a method for preparing a stable SERS substrate.

On the mechanism of protein supercharging in electrospray ionisation mass spectrometry: Effects on charging of additives with short- and long-chain alkyl constituents with carbonate and sulphite terminal groups

Small organic molecules are used as solution additives in electrospray ionisation mass spectrometry (ESI-MS) to increase the charge states of protein ions and improve the performance of intact protein analysis by tandem mass spectrometry. The properties of the additives that are responsible for their charge-enhancing effects (e.g. dipole moment, gas-phase basicity, Brønsted basicity, and surface tension) have been debated in the literature. We report a series of solution additives for ESI-MS based on cyclic alkyl carbonates and sulphites that have alkyl chains that are from two to ten methylene units long. The extent of charging of [Val [5]]-angiotensin II, cytochrome c, carbonic anhydrase II, and bovine serum albumin in ESI-MS using the additives was measured. For both the alkyl carbonate and sulphite additives with up to four methylene units, ion charging increased as the side chain lengths of the additives increased. At a critical alkyl chain length of four methylene units, protein ion charge states decreased as the chain length increased. The dipole moments, gas-phase basicity values, and Brønsted basicities (i.e. the pK _a of the conjugate acids) of the additives were obtained using electronic structure calculations, and the surface tensions were measured by pendant drop tensiometry. Because the dipole moments, gas-phase basicities, and pK _a values of the additives did not depend significantly on the alkyl chain lengths of the additives and the extent of charging depended strongly on the chain lengths, these data indicate that these three additive properties do not correlate with protein charging under these conditions. For the additives with alkyl chains at or above the critical length, the surface tension of the additives decreased as the length of the side chain decreased, which correlated well with the decrease in protein charging. These data are consistent with protein charging being limited by droplet surface tension below a threshold surface tension for these additives. For additives with relatively high surface tensions, protein ion charging increased as the amphiphilicity of the additives increased (and surface tension decreased) which is consistent with protein charging being limited by the emission of charge carriers from highly charged ESI generated droplets.

Redox "Innocence" of Re(I) in Electrochemical CO2 Reduction Catalyzed by Nanographene-Re Complexes

Improving energy efficiency of electrocatalytic CO₂ conversion to useful chemicals poses a significant scientific challenge. Recently we reported on using a colloidal nanographene as the diimine ligand to form a molecular complex Re(diimine)(CO)₃Cl to tackle this challenge, leading to significantly improved CO₂ reduction potential. In this work, we use theoretical computations to investigate the roles of the nanographene ligand in the reduction and the reaction pathways. Remarkably, our results show that the metal center merely provides a binding site for CO₂ and a conduit for electron transfer between the nanographene ligand and the substrate instead of changing its own oxidation state in the processes. Thus, despite its multiple oxidation states, the Re is redox "innocent" in the CO₂ reduction catalyzed by the nanographene complex.

Solvation of the Guanidinium Ion in Pure Aqueous Environments: A Theoretical Study from an "Ab Initio"-Based Polarizable Force Field

We report simulation results regarding the hydration process of the guanidinium cation in water droplets and in bulk liquid water, at a low concentration of 0.03 M, performed using a polarizable approach to model both water/water and ion/water interactions. In line with earlier theoretical studies, our simulations show a preferential orientation of guanidinium at water-vacuum interfaces, i.e., a parallel orientation of the guanidinium plane to the aqueous surface. In an apparent contradiction with earlier simulation studies, we show also that guanidinium has a stronger propensity for the cores of aqueous systems than the ammonium cation. However, our bulk simulation conditions correspond to weaker cation concentrations than in earlier studies, by 2 orders of magnitude, and that the same simulations performed using a standard nonpolarizable force field leads to the same conclusion. From droplet data, we extrapolate the guanidinium single hydration enthalpy value to be -82.9 ± 2.2 kcal mol^-1. That is about half as large as the sole experimental estimate reported to date, about -144 kcal mol^-1. Our result yields a guanidinium absolute bulk hydration free energy at ambiant conditions to be -78.4 ± 2.6 kcal mol^-1, a value smaller by 3 kcal mol^-1 compared to ammonium. The relatively large magnitude of our guanidinium hydration free energy estimate suggests the Gdm⁺ protein denaturing properties to result from a competition between the cation hydration effects and the cation/protein interactions, a competition that can be modulated by weak differences in the protein or in the cation chemical environment.

Noncovalent Halogen Bonding as a Mechanism for Gas-Phase Clustering

Gas-phase clustering of nonionizable iodylbenzene (PhIO₂) is attributed to supramolecular halogen bonding. Electrospray ionization results in the formation of ions of proton-charged and preferably sodium-charged clusters assignable to [H(PhIO₂) _n ]⁺, n = 1-7; [Na(PhIO₂) _n ]⁺, n = 1-6; [Na₂(PhIO₂) _n ]²⁺, n = 7-20; [HNa(PhIO₂) _n ]²⁺, n = 6-19; [HNa₂(PhIO₂) _n ]³⁺, n = 15-30; and [Na₃(PhIO₂) _n ]³⁺, n = 14-30. The largest cluster detected has a supramolecular mass of 7147 Da. Electronic structure calculations using the M06-2X functional with the 6-311++G(d,p) basis set for C, H, and O, and LANL2DZ basis set for I and Na predict 298 K binding enthalpies for the protonated and sodiated iodylbenzene dimers and trimers are greater than 180 kJ/mol. This is exceptionally high in comparison with other protonated and sodiated clusters with well-established binding enthalpies. Strongly halogen-bonded motifs found in the crystalline phases of PhIO₂ and its derivatives serve as models for the structures of larger gas-phase clusters, and calculations on simple model gas-phase dimer and trimer clusters result in similar motifs. This is the first account of halogen bonding playing an extensive role in gas-phase associations. Graphical Abstract ᅟ.

Sequential water molecule binding enthalpies for aqueous nanodrops containing a mono-, di- or trivalent ion and between 20 and 500 water molecules

Sequential water molecule binding enthalpies, ΔH_n,n-1, are important for a detailed understanding of competitive interactions between ions, water and solute molecules, and how these interactions affect physical properties of ion-containing nanodrops that are important in aerosol chemistry. Water molecule binding enthalpies have been measured for small clusters of many different ions, but these values for ion-containing nanodrops containing more than 20 water molecules are scarce. Here, ΔH_n,n-1 values are deduced from high-precision ultraviolet photodissociation (UVPD) measurements as a function of ion identity, charge state and cluster size between 20-500 water molecules and for ions with +1, +2 and +3 charges. The ΔH_n,n-1 values are obtained from the number of water molecules lost upon photoexcitation at a known wavelength, and modeling of the release of energy into the translational, rotational and vibrational motions of the products. The ΔH_n,n-1 values range from 36.82 to 50.21 kJ mol^-1. For clusters containing more than ∼250 water molecules, the binding enthalpies are between the bulk heat of vaporization (44.8 kJ mol^-1) and the sublimation enthalpy of bulk ice (51.0 kJ mol^-1). These values depend on ion charge state for clusters with fewer than 150 water molecules, but there is a negligible dependence at larger size. There is a minimum in the ΔH_n,n-1 values that depends on the cluster size and ion charge state, which can be attributed to the competing effects of ion solvation and surface energy. The experimental ΔH_n,n-1 values can be fit to the Thomson liquid drop model (TLDM) using bulk ice parameters. By optimizing the surface tension and temperature change of the logarithmic partial pressure for the TLDM, the experimental sequential water molecule binding enthalpies can be fit with an accuracy of ±3.3 kJ mol^-1 over the entire range of cluster sizes.

Extrapolating Single Organic Ion Solvation Thermochemistry from Simulated Water Nanodroplets

We compute the ion/water interaction energies of methylated ammonium cations and alkylated carboxylate anions solvated in large nanodroplets of 10 000 water molecules using 10 ns molecular dynamics simulations and an all-atom polarizable force-field approach. Together with our earlier results concerning the solvation of these organic ions in nanodroplets whose molecular sizes range from 50 to 1000, these new data allow us to discuss the reliability of extrapolating absolute single-ion bulk solvation energies from small ion/water droplets using common power-law functions of cluster size. We show that reliable estimates of these energies can be extrapolated from a small data set comprising the results of three droplets whose sizes are between 100 and 1000 using a basic power-law function of droplet size. This agrees with an earlier conclusion drawn from a model built within the mean spherical framework and paves the road toward a theoretical protocol to systematically compute the solvation energies of complex organic ions.

Thermochemistry of the Reaction of SF6 with Gas-Phase Hydrated Electrons: A Benchmark for Nanocalorimetry

The reaction of sulfur hexafluoride with gas-phase hydrated electrons (H2O)n(-), n ≈ 60-130, is investigated at temperatures T = 140-300 K by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. SF6 reacts with a temperature-independent rate of 3.0 ± 1.0 × 10(-10) cm(3) s(-1) via exclusive formation of the hydrated F(-) anion and the SF5(•) radical, which evaporates from the cluster. Nanocalorimetry yields a reaction enthalpy of ΔHR,298K = 234 ± 24 kJ mol(-1). Combined with literature thermochemical data from bulk aqueous solution, these result in an F5S-F bond dissociation enthalpy of ΔH298K = 455 ± 24 kJ mol(-1), in excellent agreement with all high-level quantum chemical calculations in the literature. A combination with gas-phase literature thermochemistry also yields an experimental value for the electron affinity of SF5(•), EA(SF5(•)) = 4.27 ± 0.25 eV.

Hydration of guanidinium depends on its local environment

Hydration of gaseous guanidinium (Gdm+) with up to 100 water molecules attached was investigated using infrared photodissociation spectroscopy in the hydrogen stretch region between 2900 and 3800 cm-1. Comparisons to IR spectra of low-energy computed structures indicate that at small cluster size, water interacts strongly with Gdm+ with three inner shell water molecules each accepting two hydrogen bonds from adjacent NH2 groups in Gdm+. Comparisons to results for tetramethylammonium (TMA+) and Na+ enable structural information for larger clusters to be obtained. The similarity in the bonded OH region for Gdm(H2O)20+vs. Gdm(H2O)100+ and the similarity in the bonded OH regions between Gdm+ and TMA+ but not Na+ for clusters with <50 water molecules indicate that Gdm+ does not significantly affect the hydrogen-bonding network of water molecules at large size. These results indicate that the hydration around Gdm+ changes for clusters with more than about eight water molecules to one in which inner shell water molecules only accept a single H-bond from Gdm+. More effective H-bonding drives this change in inner-shell water molecule binding to other water molecules. These results show that hydration of Gdm+ depends on its local environment, and that Gdm+ will interact with water even more strongly in an environment where water is partially excluded, such as the surface of a protein. This enhanced hydration in a limited solvation environment may provide new insights into the effectiveness of Gdm+ as a protein denaturant.

Structural, energetic, and electronic properties of La(III)-dimethyl sulfoxide clusters

By using accurate density functional theory calculations, we have studied the cluster complexes of a La(3+) ion interacting with a small number of dimethyl sulfoxide (DMSO) molecules of growing size (from 1 to 12). Extended structural, energetic, and electronic structure analyses have been performed to provide a complete picture of the physical properties that are the basis of the interaction of La(III) with DMSO. Recent experimental data in the solid and liquid phase have suggested a coordination number of 8 DMSO molecules with a square antiprism geometry arranged similarly in the liquid and crystalline phases. By using a cluster approach on the La(3+)(DMSO)n gas phase isolated structures, we have found that the 8-fold geometry, albeit less regular than in the crystal, is probably the most stable cluster. Furthermore, we provide new evidence of a 9-fold complexation geometric arrangement that is competitive (at least energetically) with the 8-fold one and that might suggest the existence of transient structures with higher coordination numbers in the liquid phase.

Computational electrochemistry: prediction of liquid-phase reduction potentials

This article reviews recent developments and applications in the area of computational electrochemistry. Our focus is on predicting the reduction potentials of electron transfer and other electrochemical reactions and half-reactions in both aqueous and nonaqueous solutions. Topics covered include various computational protocols that combine quantum mechanical electronic structure methods (such as density functional theory) with implicit-solvent models, explicit-solvent protocols that employ Monte Carlo or molecular dynamics simulations (for example, Car-Parrinello molecular dynamics using the grand canonical ensemble formalism), and the Marcus theory of electronic charge transfer. We also review computational approaches based on empirical relationships between molecular and electronic structure and electron transfer reactivity. The scope of the implicit-solvent protocols is emphasized, and the present status of the theory and future directions are outlined.

Predicting pKa in Implicit Solvents: Current Status and Future Directions

Computational prediction of condensed phase acidity is a topic of much interest in the field today. We introduce the methods available for predicting gas phase acidity and pKas in aqueous and non-aqueous solvents including high-level electronic structure methods, empirical linear free energy relationships (LFERs), implicit solvent methods, explicit solvent statistical free energy methods, and hybrid implicit–explicit approaches. The focus of this paper is on implicit solvent methods, and we review recent developments including new electronic structure methods, cluster-continuum schemes for calculating ionic solvation free energies, as well as address issues relating to the choice of proton solvation free energy to use with implicit solvation models, and whether thermodynamic cycles are necessary for the computation of pKas. A comparison of the scope and accuracy of implicit solvent methods with ab initio molecular dynamics free energy methods is also presented. The present status of the theory and future directions are outlined.

Electron capture dissociation of trivalent metal ion-peptide complexes

With electrospray ionization from aqueous solutions, trivalent metal ions readily adduct to small peptides resulting in formation of predominantly (peptide + M(T) - H)(2+), where M(T) = La, Tm, Lu, Sm, Ho, Yb, Pm, Tb, or Eu, for peptides with molecular weights below ~1000 Da, and predominantly (peptide + M(T))(3+) for larger peptides. ECD of (peptide + M(T) - H)(2+) results in extensive fragmentation from which nearly complete sequence information can be obtained, even for peptides for which only singly protonated ions are formed in the absence of the metal ions. ECD of these doubly charged complexes containing M(T) results in significantly higher electron capture efficiency and sequence coverage than peptide-divalent metal ion complexes that have the same net charge. Formation of salt-bridge structures in which the metal ion coordinates to a carboxylate group are favored even for (peptide + M(T))(3+). ECD of these latter complexes for large peptides results in electron capture by the protonation site located remotely from the metal ion and predominantly c/z fragments for all metals, except Eu(3+), which undergoes a one electron reduction and only loss of small neutral molecules and b/y fragments are formed. These results indicate that solvation of the metal ion in these complexes is extensive, which results in the electrochemical properties of these metal ions being similar in both the peptide environment and in bulk water.

Coordination numbers of hydrated divalent transition metal ions investigated with IRPD spectroscopy

Hydration of the divalent transition metal ions, Mn, Fe, Co, Ni, Cu, and Zn, with 5-8 water molecules attached was investigated using infrared photodissociation spectroscopy and photodissociation kinetics. At 215 K, spectral intensities in both the bonded-OH and free-OH stretch regions indicate that the average coordination number (CN) of Mn(2+), Fe(2+), Co(2+), and Ni(2+) is ~6, and these CN values are greater than those of Cu(2+) and Zn(2+). Ni has the highest CN, with no evidence for any population of structures with a water molecule in a second solvation shell for the hexa-hydrate at temperatures up to 331 K. Mn(2+), Fe(2+), and Co(2+) have similar CN at low temperature, but spectra of Mn(2+)(H(2)O)(6) indicate a second population of structures with a water molecule in a second solvent shell, i.e., a CN < 6, that increases in abundance at higher temperature (305 K). The propensity for these ions to undergo charge separation reactions at small cluster size roughly correlates with the ordering of the hydrolysis constants of these ions in aqueous solution and is consistent with the ordering of average CN values established from the infrared spectra of these ions.

Ions in size-selected aqueous nanodrops: sequential water molecule binding energies and effects of water on ion fluorescence

The effects of water on ion fluorescence were investigated, and average sequential water molecule binding energies to hydrated ions, M(z)(H(2)O)(n), at large cluster size were measured using ion nanocalorimetry. Upon 248-nm excitation, nanodrops with ~25 or more water molecules that contain either rhodamine 590(+), rhodamine 640(+), or Ce(3+) emit a photon with average energies of approximately 548, 590, and 348 nm, respectively. These values are very close to the emission maxima of the corresponding ions in solution, indicating that the photophysical properties of these ions in the nanodrops approach those of the fully hydrated ions at relatively small cluster size. As occurs in solution, these ions in nanodrops with 8 or more water molecules fluoresce with a quantum yield of ~1. Ce(3+) containing nanodrops that also contain OH(-) fluoresce, whereas those with NO(3)(-) do not. This indirect fluorescence detection method has the advantages of high sensitivity, and both the size of the nanodrops as well as their constituents can be carefully controlled. For ions that do not fluoresce in solution, such as protonated tryptophan, full internal conversion of the absorbed 248-nm photon occurs, and the average sequential water molecule binding energies to the hydrated ions can be accurately obtained at large cluster sizes. The average sequential water molecule binding energies for TrpH(+)(H(2)O)(n) and a doubly protonated tripeptide, [KYK + 2H](2+)(H(2)O)(n), approach asymptotic values of ~9.3 (n ≥ 11) and ~10.0 kcal/mol (n ≥ 25), respectively, consistent with a liquidlike structure of water in these nanodrops.