Electron Hydration and Ion−Electron Pairs in Water Clusters Containing Trivalent Metal Ions

Impact of Nuclear Quantum Effects on the Structural Properties of Protonated Water Clusters

Nuclear quantum effects (NQEs) play a crucial role in hydrogen-bonded systems due to quantum tunneling and proton fluctuation. Our understanding of how NQEs affect microstructures mainly focuses on bulk phases of liquids and solids but remains deficient for water clusters, including their hydrogen nuclei, hydrogen-bonded configurations, and temperature dependence. Here, we conducted ab initio molecular dynamics (MD) and path integral MD simulations to investigate the influence of NQEs on the structural properties of protonated water clusters H+(H2O)n (n = 3, 6, 9, 12). The results reveal that the NQEs become less evident as the cluster size increases due to the competition between NQEs and electrostatic interactions. Simulations of several H+(H2O)6 isomers at different temperatures indicate that the effect of elevated temperature on proton transfer is related to the initial structure. Interestingly, the process of proton transfer also involves the interconversion between Zundel-type and Eigen-type isomers. These findings significantly deepen our understanding of ion-water and water-water interactions, opening new avenues for the study of hydrated ion clusters and related systems.

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.

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.

Ion emission from 1-10 MDa salt clusters: individual charge state resolution with charge detection mass spectrometry

Salt cluster ions produced by electrospray ionization are used for mass calibration and fundamental investigations into cluster stability and charge separation processes. However, previous studies have been limited to relatively small clusters owing to the heterogeneity associated with large, multiply-charged clusters that leads to unresolved signals in conventional m/z spectra. Here, charge detection mass spectrometry is used to measure both the mass and charge distributions of positively charged clusters of KCl, CaCl2, and LaCl3 with masses between ∼1 and 10 MDa by dynamically measuring the energy per charge, m/z, charge, and mass of simultaneously trapped individual ions throughout a 1 s trapping time. The extent of remaining hydration on the clusters, determined from the change in the frequency of ion motion with time as a result of residual water loss, follows the order KCl < CaCl2 < LaCl3, and is significantly lower than that of a pure water nanodrop, consistent with tighter water binding to the more highly charged cations in these clusters. The number of ion emission events from these clusters also follows this same trend, indicating that water at the cluster surface facilitates charge loss. A new frequency-based method to determine the magnitude of the charge loss resulting from individual ion emission events clearly resolves losses of +1 and +2 ions. Achieving this individual charge state resolution for ion emission events is an important advance in obtaining information about the late stages of bare gaseous ions formation. Future experiments on more hydrated clusters are expected to lead to a better understanding of ion formation in electrospray ionization.

On the Role of the Carboxyl Group to the Protective Effect of o-dihydroxybenzoic Acids to Saccharomyces cerevisiae Cells upon Induced Oxidative Stress

In the present work, the role of the carboxyl group of o-dihydroxybenzoic acids (pyrocatechuic, 2,3-diOH-BA and protocatechuic, 3,4-diOH-BA) on the protection against induced oxidative stress in Saccharomyces cerevisiae was examined. Catechol (3,4-diOH-B) was included for comparison. Cell survival, antioxidant enzyme activities, and TBARS level were used to evaluate the efficiency upon the stress induced by H₂O₂ or cumene hydroperoxide. Theoretical calculation of atomic charge values, dipole moment, and a set of indices relevant to the redox properties of the compounds was also carried out in the liquid phase (water). Irrespective of the oxidant used, 2,3-diOH-BA required by far the lowest concentration (3–5 μM) to facilitate cell survival. The two acids did not activate catalase but reduced superoxide dismutase activity (3,4-diOH-BA>2,3-diOH-BA). TBARS assay showed an antioxidant effect only when H₂O₂ was used; equal activity for the two acids and inferior to that of 3,4-diOH B. Overall, theoretical and experimental findings suggest that the 2,3-diOH-BA high activity should be governed by metal chelation. In the case of 3,4-diOH BA, radical scavenging increases, and chelation capacity decreases. The lack of carboxyl moiety (3,4-diOH B) improves to radical scavenging, interaction with lipophilic free radicals, and antioxidant enzymes. The present study adds to our knowledge of the antioxidant mechanism of dietary phenols in biological systems.

Electronic Structure of Water from Koopmans-Compliant Functionals

Obtaining a precise theoretical description of the spectral properties of liquid water poses challenges for both molecular dynamics (MD) and electronic structure methods. The lower computational cost of the Koopmans-compliant functionals with respect to Green's function methods allows the simulations of many MD trajectories, with a description close to the state-of-art quasi-particle self-consistent GW plus vertex corrections method (QSGW + f_xc). Thus, we explore water spectral properties when different MD approaches are used, ranging from classical MD to first-principles MD, and including nuclear quantum effects. We have observed that different MD approaches lead to up to 1 eV change in the average band gap; thus, we focused on the band gap dependence with the geometrical properties of a system to explain such spread. We have evaluated the changes in the band gap due to variations in the intramolecular O-H bond distance and HOH angle, as well as the intermolecular hydrogen bond O···O distance and the OHO angles. We have observed that the dominant contribution comes from the O-H bond length; the O···O distance plays a secondary role, and the other geometrical properties do not significantly influence the gap. Furthermore, we analyze the electronic density of states (DOS), where the KIPZ functional shows good agreement with the DOS obtained with state-of-art approaches employing quasi-particle self-consistent GW plus vertex corrections. The O-H bond length also significantly influences the DOS. When nuclear quantum effects are considered, broadening of the peaks driven by the broader distribution of the O-H bond lengths is observed, leading to a closer agreement with the experimental photoemission spectra.

Interactions between water and rhodium clusters: molecular adsorption versus cluster adsorption

Understanding metal-water interactions and hydrogen-bonding in water droplets is important but highly challenging. Various transition metals may serve as effective coordination centers to water; however, not in all cases is water bonded to a metal center as single molecules. We report here the observations of gas-phase rhodium clusters and their interactions with water. A series of rhodium-water clusters, Rhn±,0(H2O)m (n = 3-30, m = 1-5), with isotope labels were detected by mass spectrometry after exposure to different water concentrations, among which Rh8+(H2O)4 and Rh9+(H2O)3 were prominent in the mass distributions, showing a size-dependent preference of water adsorption on rhodium clusters. Comprehensive density functional theory calculations reveal that the lowest energy structure of Rh9+(H2O)3 possesses a hydrogen-bonded cyclic (H2O)3 water trimer on the top of a tri-capped Rh9+ trigonal prism. The tri-capped Rh9+ trigonal prism and the cyclic (H2O)3 water trimer match in sizes, charge distributions, and orbital symmetries to form effective electrostatic cluster-cluster interactions. In contrast, Rh8+(H2O)4 contains four water molecules separately attached to a bi-capped octahedron, Rh8+, at four corners via single-molecule adsorption. The difference between covalent molecular adsorption and electrostatic cluster-cluster interaction in these two proto-typical rhodium hydrates is further demonstrated by detailed natural bonding orbital, electrostatic surface potential, and charge decomposition analyses.

From absolute potentials to a generalized computational standard hydrogen electrode for aqueous and non-aqueous solvents

We describe a simple and efficient procedure to compute a conversion factor for the absolute potential of the standard hydrogen electrode in water to any other solvent. In contrast to earlier methods our procedure only requires the pKa of an arbitrary acid in water and few simple quantum chemical calculations as input. Thus, it is not affected adversely by experimental shortcomings related to measurements in non-aqueous solvents. By combining this conversion factor with the absolute potential in water, the absolute potential in the solvent of interest is obtained. Based on this procedure a new generalized computational standard hydrogen electrode for the computation of electron transfer and proton-coupled electron transfer potentials in non-aqueous solvents and ionic liquids is developed. This enables for the first time the reliable prediction of redox potentials in any solvent. The method is tested through calculation of absolute potentials in 36 solvents. Using the Kamlet-Taft linear solvation energy model we find that the relative absolute potentials consistently increase with decreasing polarisability and decreasing hydrogen bonding ability. For protic solvents good agreement with literature is observed while significant deviations are found for aprotic solvents. The obtained conversion factors are independent of the quantum chemical method, while minor differences are observed between solvation models. This does, however, not affect the global trends.

Unveiling Zwitterionization of Glycine in the Microhydration Limit

Charge separation under solvation stress conditions is a fundamental process that comes in many forms in doped water clusters. Yet, the mechanism of intramolecular charge separation, where constraints due to the molecular structure might be intricately tied to restricted solvation structures, remains largely unexplored. Microhydrated amino acids are such paradigmatic molecules. Ab initio simulations are carried out at 300 K in the frameworks of metadynamics sampling and thermodynamic integration to map the thermal mechanisms of zwitterionization using Gly(H₂O) _n with n = 4 and 10. In both cases, a similar water-mediated proton transfer chain mechanism is observed; yet, detailed analyses of thermodynamics and kinetics demonstrate that the charge-separated zwitterion is the preferred species only for n = 10 mainly due to kinetic stabilization. Structural analyses disclose that bifurcated H-bonded water bridges, connecting the cationic and anionic sites in the fluctuating microhydration network at room temperature, are enhanced in the transition-state ensemble exclusively for n = 10 and become overwhelmingly abundant in the stable zwitterion. The findings offer potential insights into charge separation under solvation stress conditions beyond the present example.

Theoretical Study on the Antioxidant Activity of Natural Depsidones

Depsidones are secondary metabolites in lichens with a range of potential health benefits. Among others, these compounds are believed to exhibit high hydroxyl radical and superoxide scavenging abilities, warranting a detailed investigation of their antioxidant properties. In this study, the radical scavenging activity of natural depsidones from Ramalina lichenized fungi was investigated in silico. Calculations of the thermodynamic parameters suggested that the main radical scavenging pathway follows the formal hydrogen transfer (FHT) mechanism; however, unexpectedly low rate constants were found in the CH3OO• scavenging reaction. Establishing that the depsidones are mostly ionized in an aqueous environment suggested that the single-electron transfer (SET) mechanism should not be ruled out. Consistently, depsidones were revealed to be excellent HO• and O2 •- scavengers in aqueous solutions (k = 4.60 × 105 - 8.60 × 109 M-1 s-1 and k = 2.60 × 108 - 8.30 × 109 M-1 s-1, respectively) following the sequential proton loss electron transfer (SPLET) mechanism. These results suggest that natural fungal depsidones are potent hydroxyl and superoxide radical scavengers in aqueous solutions.

Hydroxyl Radical Scavenging of Indole-3-Carbinol: A Mechanistic and Kinetic Study

Indole-3-carbinol (I3C) is the product of the enzymatic hydrolysis of glucobrassicin in the human body. I3C exhibits diverse bioactivities. It is used as a supplement to enhance the efficiency of some cancer therapies and is available as an over-the-counter dietary supplement described as a potential antioxidant, among other health benefits. Thus, it is important to develop an in-depth understanding of its antioxidant activity. In this study, the hydroxyl radical scavenging of I3C has been investigated in silico under physiologically relevant conditions (aqueous and lipid-mimetic pentyl ethanoate environment) using thermochemical and kinetic calculations. For benchmarking purposes, the results were compared to known experimental data. The overall reaction rate constant of the HO^• radical scavenging of I3C in water was found to be 2.30 × 10¹⁰ M^-1 s^-1 and over two times lower in lipid-mimetic pentyl ethanoate solvent at 7.74 × 10⁹ M^-1 s^-1. The results also highlighted that the HO^• radical scavenging follows almost exclusively the radical adduct formation mechanism (>94%) in a lipid mimetic medium, whereas this mechanism contributes about 60% in aqueous environments. I3C is considered a dopamine-like antioxidant, its main function being prevention of oxidative degradation of lipids; our study supports this view.

Theoretical Study for Exploring the Diglycoside Substituent Effect on the Antioxidative Capability of Isorhamnetin Extracted from Anoectochilus roxburghii

Radical-scavenging activity of isorhamnetin (1) and its diglycosides, named isorhamnetin-3,5'-O-β-D-diglucoside (2) and isorhamnetin-3,7-O-β-D-diglucoside (3) extracted from Anoectochilus roxburghii, has been studied through three main antioxidant pathways: hydrogen atom transfer (HAT), single electron transfer followed by proton transfer, and sequential proton loss electron transfer (SPLET). All thermodynamic parameters related to these radical-scavenging mechanisms were computed at the B3LYP/6-311G(d,p) level of theory both in the gas phase and in solution. The results suggest that HAT is the predominant mechanism in the gas phase, while SPLET is supported in an aqueous environment. In addition, the stability of radicals has also been explored by electron spin density and intramolecular hydrogen bonding. The potential energy profiles and kinetic calculations for the reactions between the selected compounds and the CH₃OO^• radical were calculated at 298.15 K. Among all investigated, compound 2 has the highest antioxidant activity with the lowest Gibbs free energy (-4.05 kcal/mol) and the highest hydrogen atom transfer rate constant (3.61 × 10⁵ M^-1 s^-1). Substitution of the OH and OMe groups by two glucoses at the 3 and 5' sites of isorhamnetin has a significant impact on its antioxidant activity.

Antioxidant Motifs in Flavonoids: O-H versus C-H Bond Dissociation

Flavonoids provide potential health benefits due to their antioxidant properties. The antioxidant activity of natural flavonoids is primarily exerted by phenolic hydroxyl groups; however, C-H bonds also contribute to these properties. In this study, the contributions of phenolic groups and C-H bonds to the antioxidant properties of 13 flavonoids were investigated by using the (RO)B3LYP/6-311++G(2df,2p)//B3LYP/6-311G(d,p) model chemistry in the gas phase and water and ethanol solvents. It was found that the C-H bonds have lower bond dissociation energies than O-H bonds in the 4-carbonyl and/or 3-hydroxyl group containing flavonoids and hence define antioxidant activity. The HOO· radical scavenging of the selected flavonoids is also investigated in detail through the potential energy surface, natural bond orbitals, and kinetic calculations. It was found that the favored radical scavenging mechanism of the flavonoids is hydrogen atom transfer, with the gas phase rate constants in the range of 7.23 × 103-2.07 × 109 L·mol-1·s-1. The results suggest that the flavonoids, isomelacacidin, isoteracacidin, melacacidin, and teracacidin, have antioxidant properties as high as typical phenolic compounds such as quercetin, trans-resveratrol, trolox, and ascorbic acid.

A theoretical study of the radical scavenging activity of natural stilbenes

Oxidative stress is implicated in aging and aging-related diseases, including cancer.

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.

Density functional theory study of the role of benzylic hydrogen atoms in the antioxidant properties of lignans

Antioxidants are a diverse group of chemicals with proven health benefits and thus potential preventive medicine and therapeutic applications. While most of these compounds are natural products, determining their mechanism of radical scavenging and common motifs that contribute to antioxidant activity would allow the rational design of novel antioxidants. Here the origins of the antioxidant properties of ten natural products of the lignan family were studied in silico by calculating their thermochemical properties by using ROB3LYP/6-311++G(2df,2p)//B3LYP/6-311G(d,p) model chemistry. Three conditions were modelled: gas phase, ethanol and water solvents. The results allowed assigning the antioxidant activity to specific moieties and structural features of these compounds. It was found that the benzylic hydrogen atoms are the most likely to be abstracted to form radicals and hence define antioxidant properties in most of the studied compounds. The results also suggested that the most likely mechanism of HOO^• radical scavenging differs by the key moiety: it is hydrogen atom transfer in case the benzylic C-H bonds, however it is proton coupled electron transfer in case of the compounds where O-H bonds are responsible for radical scavenging.

Probing ionization potential, electron affinity and self-energy effect on the spectral shape and exciton binding energy of quantum liquid water with self-consistent many-body perturbation theory and the Bethe-Salpeter equation

An accurate theoretical prediction of ionization potential (IP) and electron affinity (EA) is key in understanding complex photochemical processes in aqueous environments. There have been numerous efforts in literature to accurately predict IP and EA of liquid water, however with often conflicting results depending on the level of theory and the underlying water structures. In a recent study based on hybrid-non-self-consistent many-body perturbation theory (MBPT) Gaiduk et al (2018 Nat. Commun. 9 247) predicted an IP of 10.2 eV and EA of 0.2 eV, resulting in an electronic band gap (i.e. electronic gap (IP-EA) as measured by photoelectron spectroscopy) of about 10 eV, redefining the widely cited experimental gap of 8.7 eV in literature. In the present work, we show that GW self-consistency and an implicit vertex correction in MBPT considerably affect recently reported EA values by Gaiduk et al (2018 Nat. Commun. 9 247) by about 1 eV. Furthermore, the choice of pseudo-potential is critical for an accurate determination of the absolute band positions. Consequently, the self-consistent GW approach with an implicit vertex correction based on projector augmented wave (PAW) method on top of quantum water structures predicts an IP of 10.2, an EA of 1.1, a fundamental gap of 9.1 eV and an exciton binding (Eb) energy of 0.9 eV for the first absorption band of liquid water via the Bethe-Salpeter equation (BSE). Only within such a self-consistent approach a simultanously accurate prediction of IP, EA, Eg, Eb is possible.

Electron affinity of liquid water

Understanding redox and photochemical reactions in aqueous environments requires a precise knowledge of the ionization potential and electron affinity of liquid water. The former has been measured, but not the latter. We predict the electron affinity of liquid water and of its surface from first principles, coupling path-integral molecular dynamics with ab initio potentials, and many-body perturbation theory. Our results for the surface (0.8 eV) agree well with recent pump-probe spectroscopy measurements on amorphous ice. Those for the bulk (0.1-0.3 eV) differ from several estimates adopted in the literature, which we critically revisit. We show that the ionization potential of the bulk and surface are almost identical; instead their electron affinities differ substantially, with the conduction band edge of the surface much deeper in energy than that of the bulk. We also discuss the significant impact of nuclear quantum effects on the fundamental gap and band edges of the liquid.

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.

The reaction of CF2Cl2 with gas-phase hydrated electrons

The reaction of dichlorodifluoromethane (CF2Cl2) with hydrated electrons (H2O)n(-) (n = 30-86) in the gas phase was studied using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The hydrated electron reacts with CF2Cl2, forming (H2O)mCl(-) with a rate constant of (8.6 ± 2.2) × 10(-10) cm(3) s(-1), corresponding to an efficiency of 57 ± 15%. The reaction enthalpy was determined using nanocalorimetry, revealing a strongly exothermic reaction with ΔHr(CF2Cl2, 298 K) = -208 ± 41 kJ mol(-1). The combination of the measured reaction enthalpy with thermochemical data from the condensed phase yields a C-Cl bond dissociation enthalpy (BDE) ΔHC-Cl(CF2Cl2, 298 K) = 355 ± 41 kJ mol(-1) that agrees within error limits with the predicted values from quantum chemical calculations and published BDEs.

Insight into the antioxidant properties of non-phenolic terpenoids contained in essential oils extracted from the buds of Cleistocalyx operculatus: a DFT study

The antioxidant properties of 21 non-phenolic terpenoids contained in essential oil extracted from the buds of Cleistocalyx operculatus have been investigated using density functional theory (DFT)-based computational methods.

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.

Extremely supercharged proteins in mass spectrometry: profiling the pH of electrospray generated droplets, narrowing charge state distributions, and increasing ion fragmentation

High-performance solutions for supercharging proteins in electrospray ionization were optimized and the origin of the strong dependence of supercharging on acid strength was investigated.

Combining gas phase electron capture and IRMPD action spectroscopy to probe the electronic structure of a metastable reduced organometallic complex containing a non-innocent ligand

Gas-phase reduction of a Zn(ii) complex followed by IR spectroscopy shows that the incoming electron is localized on the metal rather than on the ligand.

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.

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.

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.

An improved cluster pair correlation method for obtaining the absolute proton hydration energy and enthalpy evaluated with an expanded data set

An improved cluster pair correlation method that is based on the method originally introduced by Tuttle et al. ( Tuttle et al. J. Phys. Chem. A 2002 , 106 , 925 - 932 ) was developed and evaluated using a significantly larger data set than used previously. With this larger data set, values for the absolute proton hydration free energy of -259.3 and -265.0 kcal/mol were obtained using the original and improved method, respectively. The former value is ∼4.5 kcal/mol less negative than previously reported values obtained with the same method but with smaller data sets. The dependence of this value on data set size indicates that the uncertainty in the original method may be greater than previously realized. The improved method has the advantages of higher precision, and the effects of cluster size on the proton hydration free energy and enthalpy values can be more readily evaluated. Data for ions with extreme pK(a)s, many of which were included in previous estimates of the proton hydration free energy, were found to be unreliable and were eliminated from the extended data set. There is only a subtle effect of cluster size on the Gibbs free energy values, and within the limits of the approximation inherent in the cluster pair correlation method, the "best" value for the standard absolute proton hydration free energy obtained with this new method and larger data set is -263.4 kcal/mol (average for clusters with 4-6 water molecules). The absolute proton hydration enthalpy values decrease from -273.1 to -275.3 kcal/mol with increasing cluster size (one to six water molecules, respectively). This trend, along with an anomalously high value for the absolute proton hydration entropy, indicates that the enthalpy obtained with this method may not have converged for these relatively small clusters.

Average sequential water molecule binding enthalpies of M(H2O)(19-124)2+ (M = Co, Fe, Mn, and Cu) measured with ultraviolet photodissociation at 193 and 248 nm

The average sequential water molecule binding enthalpies to large water clusters (between 19 and 124 water molecules) containing divalent ions were obtained by measuring the average number of water molecules lost upon absorption of an UV photon (193 or 248 nm) and using a statistical model to account for the energy released into translations, rotations, and vibrations of the products. These values agree well with the trend established by more conventional methods for obtaining sequential binding enthalpies to much smaller hydrated divalent ions. The average binding enthalpies decrease to a value of ~10.4 kcal/mol for n > ~40 and are insensitive to the ion identity at large cluster size. This value is close to that of the bulk heat of vaporization of water (10.6 kcal/mol) and indicates that the structure of water in these clusters may more closely resemble that of bulk liquid water than ice, owing either to a freezing point depression or rapid evaporative cooling and kinetic trapping of the initial liquid droplet. A discrete implementation of the Thomson equation using parameters for liquid water at 0 °C generally fits the trend in these data but provides values that are ~0.5 kcal/mol too low.