Observing the Role of Electron Delocalization in Electronic Transport by Incorporating Actinides into Ligated Metal-Chalcogenide Superatoms.

Since delocalization of electronic states is a prerequisite for exerting unique electron transport properties, early actinides (An) with highly delocalized 5f/6d orbitals are natural candidates. However, given the experimental difficulties of such radioactive compounds and the complex relativistic effects in theoretical studies, understanding the electronic structure and bonding of actinides is underdeveloped on the periodic table. A further challenge is the very complicated electronic structures encountered in the confinement of actinides, as vividly illustrated by the weakly radioactive Th(Thorium)-encapsulated metal chalcogenide clusters, Th@Co6Te8L6 (L = PH3, PMe3, PEt3). Here we report the electronic structure and the electron transport properties of the Th@Co6Te8L6 clusters and compare them with those of the hollow Co6Te8L6 clusters using the nonequilibrium Green's function combined with relativistic density functional theory (NEGF-DFT). We found that the equilibrium conductance in Th@Co6Te8(PH3)6 (0.76 G0) has been greatly improved over that in Co6Te8(PH3)6 (0.03 G0), which has also been verified under an applied different bias voltage. The covalent bonding character between 6d (Th) and 3d (Co) atomic orbitals resulting from steric confinement is the source of the performance enhancement and a most important factor governing the accessibility of such 5f/6d orbitals. The results are of significance to the rapidly developing field of molecular nanoelectronics.

Bis(acyl)phosphide Complexes of U(III)/U(IV): A Case of a Hidden Redox-Active Ligand.

The recently reported tris(bis(2,4,6-triisopropylbenzoyl)-phosphide)uranium (UIII(trippBAP)3, 2) complex (Inorg. Chem. 2022, 61 (32), 12508-12517) demonstrated a silent 31P NMR spectrum. This complex was described as a U(III) complex with an organic radical ligand fragment. Moreover, the EPR spectrum of 2 was indicative of an organic radical in the ligand framework complexed to uranium, in contrast to that of UIV(mesBAP)4, 1. Herein, with the help of relativistic density functional theory (DFT) calculations, the electronic structures of 1, 2, and U(mesBAP)3 (4) are examined in an effort to understand the unusual 31P NMR spectrum of 2. Results indicate the reduction of the carbonyl bonds and delocalization of the electrons over the ligands, indicative of U → L backbonding. Additionally, the reduced acyl carbons are found to exist as ketyl radicals [O═C• -] that are responsible for the silent 31P NMR spectra of 2. These findings demonstrate the redox noninnocent nature of BAP- in 2 and 4, causing uranium to exist in a formal oxidation state of +4.

Actinium coordination chemistry: A density functional theory study with monodentate and bidentate ligands.

In the current study, the coordination chemistry of nine-coordinate Ac(III) complexes with 35 monodentate and bidentate ligands was investigated using density functional theory (DFT) in terms of their geometries, charges, reaction energies, and bonding interactions. The energy decomposition analysis with naturals orbitals for chemical valence (EDA-NOCV) and the quantum theory of atoms in molecules (QTAIM) were employed as analysis methods. Trivalent Ac exhibits the highest affinities toward hard acids (such as charged oxophilic donors, fluoride), so its classification as a hard acid is justified. Natural population analysis quantified the involvement of 5f orbitals on Ac to be about 30% of total valence electron natural configuration indicating that Ac is a member of the actinide series. Pearson correlation coefficients were used to study the pairwise correlations among the bond lengths, ΔG reaction energies, charges on Ac and donor atoms, and data from EDA-NOCV and QTAIM. Strong correlations and anticorrelations were found between Voronoi charges on donor atoms with ΔG, EDA-NOCV interaction energies and QTAIM bond critical point densities.

Selective Chelation of the Exotic Meitner-Auger Emitter Mercury-197 m/g with Sulfur-Rich Macrocyclic Ligands: Towards the Future of Theranostic Radiopharmaceuticals.

Mercury-197 m/g are a promising pair of radioactive isomers for incorporation into a theranostic as they can be used as a diagnostic agent using SPECT imaging and a therapeutic via Meitner-Auger electron emissions. However, the current absence of ligands able to stably coordinate 197m/g Hg to a tumour-targeting vector precludes their use in vivo. To address this, we report herein a series of sulfur-rich chelators capable of incorporating 197m/g Hg into a radiopharmaceutical. 1,4,7,10-Tetrathia-13-azacyclopentadecane (NS4 ) and its derivatives, (2-(1,4,7,10-tetrathia-13-azacyclopentadecan-13-yl)acetic acid (NS4 -CA) and N-benzyl-2-(1,4,7,10-tetrathia-13-azacyclopentadecan-13-yl)acetamide (NS4 -BA), were designed, synthesized and analyzed for their ability to coordinate Hg2+ through a combination of theoretical (DFT) and experimental coordination chemistry studies (NMR and mass spectrometry) as well as 197m/g Hg radiolabeling studies and in vitro stability assays. The development of stable ligands for 197m/g Hg reported herein is extremely impactful as it would enable their use for in vivo imaging and therapy, leading to personalized treatments for cancer.

Interactions between Metals and Eudistomins of Ascidian Origin: A Computational Study.

Ascidians are marine animals that adopt unusual techniques to deter predation. The three main methods are sequestration of unusual metals, high concentrations of sulfuric acid/sulfate ions in tunicate cells, and the presence of eudistomins. In this study, we hypothesize that ascidians sequester metals in their sulfate form, and the complexation of eudistomins with the metals could liberate the sulfate ion. Three representative metal aqua ions were chosen, viz., vanadyl, uranyl, and thorium ions, as well as four simple eudistomins which act as bidentate ligands, viz., eudistomin-W, debromoeudistomin-K, eudistomidin-C, and eudistomidin-B. By designing 7 model reactions, we tested our hypothesis using density functional theory (DFT) methods PBE-D3, BLYP, and B3LYP. The ΔG values of the model reactions provide strong support for our hypothesis. To verify the hypothesis further, we calculated the metal-eudistomin interactions with Be, Zn, and Pb. Based on our results, we suggest that ascidians may not prefer any particular metal. In addition, despite using different DFT functionals, we have observed similar ΔG values for each case. With our work, we have successfully used computational tools in our attempt to understand the unique behavior of ascidians.

Understanding the Coordination Chemistry of AmIII/CmIII in the DOTA Cavity: Insights from Energetics and Electronic Structure Theory.

The minor actinides Am/Cm show multiple possibilities for coordination, providing great opportunities for their extraction and adsorption separation. Herein, we report complexation in an aqueous medium of AmIII/CmIII in the DOTA (H4DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) cavity with axial ligands (OH-, F-, and H2O), based on the energetics and electronic structure properties using density functional theory (DFT). The formation and substitution reactions of OH--capped complexes are more likely to occur due to their enhanced hydration Gibbs free energies, followed by F-, and then H2O. Both the longer An-ODOTA bond lengths and the larger bite angle (∠O-An-O) in the OH--capped complexes reflect the enhanced coordination provided by the axial ligand, slightly less so for F-. Energy decomposition analysis based on the electronic structure supports the preference for OH--capped complexes with a near-perfect balance between attractive and repulsive contributions toward the interaction. Furthermore, molecular orbital analysis revealed that the frontier molecular orbitals of Am and Cm complexes are substantially different; that is, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) compositions of the Am complexes are all contributed by 5f, while the HOMO and LUMO compositions of the Cm complexes are derived from 5f and 6d, respectively. Finally, the metal-exchange reactions demonstrate competitive complexation of DOTA toward AmIII over CmIII for the OH--capped system. These results imply the importance of coordination chemistry in actinide chemistry in general and specifically in AmIII/CmIII solution chemistry.

Periodic Trends in the Stabilization of Actinyls in Their Higher Oxidation States Using Pyrrophen Ligands.

Owing to the prominent existence and unique chemistry of actinyls, their complexation with suitable ligands is of significant interest. The complexation of high-valent actinyl moieties (An = U, Np, Pu and Am) with the acyclic sal-porphyrin analogue called "pyrrophen" (L(1)) and its dimethyl derivative (L(2)) with four nitrogen and two oxygen donor atoms was studied using relativistic density functional theory. Based on the periodic trends, the [UVO2-L(1)/L(2)]1- complexes show shorter bond lengths and higher bond orders that increase across the series of pentavalent actinyl complexes mainly due to the localization of the 5f orbitals. Among the hexavalent complexes, the [UVIO2-L(1)/L(2)] complexes have the shortest bonds. Following the uranyl complex, due to the plutonium turn, the [AmVIO2-L(1)/L(2)] complexes exhibit comparable properties with those of the former. Charge analysis suggests the complexation to be facilitated through ligand-to-metal charge transfer (LMCT) mainly through σ donation. Thermodynamic feasibility of complexation was modeled using hydrated actinyl moieties in aqueous medium and was found to be spontaneous. The dimethylated pyrrophen (L(2)) shows higher magnitudes of thermodynamic parameters indicating increased feasibility compared to the unsubstituted ligand (L(1)). Energy decomposition analysis (EDA) along with extended transition-state-natural orbitals for chemical valence theory (ETS-NOCV) analysis shows that the dominant electrostatic contributions decrease across the series and are counteracted by Pauli repulsion. Slight but considerable covalency is provided to hexavalent actinyl complexes by orbital contributions; this was confirmed by molecular orbital (MO) analysis that suggests strong covalency in americyl (VI) complexes. In addition to the pentavalent and hexavalent actinyl moieties, heptavalent actinyl species of neptunyl, plutonyl, and americyl were studied. Beyond the influence of the charges, the geometric and electronic properties point to the stabilization of neptunyl (VII) in the pyrrophen ligand environment, while the others shift to a lower (+VI) and relatively stable OS on complexation.

Comparative Study of a Decadentate Acyclic Chelate, HOPO-O10, and Its Octadentate Analogue, HOPO-O8, for Radiopharmaceutical Applications.

Radiolanthanides and actinides are aptly suited for the diagnosis and treatment of cancer via nuclear medicine because they possess unique chemical and physical properties (e.g., radioactive decay emissions). These rare radiometals have recently shown the potential to selectively deliver a radiation payload to cancer cells. However, their clinical success is highly dependent on finding a suitable ligand for stable chelation and conjugation to a disease-targeting vector. Currently, the commercially available chelates exploited in the radiopharmaceutical design do not fulfill all of the requirements for nuclear medicine applications, and there is a need to further explore their chemistry to rationally design highly specific chelates. Herein, we describe the rational design and chemical development of a novel decadentate acyclic chelate containing five 1,2-hydroxypyridinones, 3,4,3,3-(LI-1,2-HOPO), referred to herein as HOPO-O10, based on the well-known octadentate ligand 3,4,3-(LI-1,2-HOPO), referred to herein as HOPO-O8, a highly efficient chelator for 89Zr[Zr4+]. Analysis by 1H NMR spectroscopy and mass spectrometry of the La3+ and Tb3+ complexes revealed that HOPO-O10 forms bimetallic complexes compared to HOPO-O8, which only forms monometallic species. The radiolabeling properties of both chelates were screened with [135La]La3+, [155/161Tb]Tb3+, [225Ac]Ac3+ and, [227Th]Th4+. Comparable high specific activity was observed for the [155/161Tb]Tb3+ complexes, outperforming the gold-standard 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, yet HOPO-O10 surpassed HOPO-O8 with higher [227Th]Th4+ affinity and improved complex stability in a human serum challenge assay. A comprehensive analysis of the decadentate and octadentate chelates was performed with density functional theory for the La3+, Ac3+, Eu3+, Tb3+, Lu3+, and Th4+ complexes. The computational simulations demonstrated the enhanced stability of Th4+-HOPO-O10 over Th4+-HOPO-O8. This investigation reveals the potential of HOPO-O10 for the stable chelation of large tetravalent radioactinides for nuclear medicine applications and provides insight for further chelate development.

Precise Recognition in Water by an Endo-Functionalized Cavity: Tuning the Complementarity of Binding Sites.

Precise binding towards structurally similar substrates is a common feature of biomolecular recognition. However, achieving such selectivity-especially in distinguishing subtle differences in substrates-with synthetic hosts can be quite challenging. Herein, we report a novel design strategy involving the combination of different rigid skeletons to adjust the distance between recognition sites within the cavity, which allows for the highly selective recognition of hydrogen-bonding complementary substrates, such as 4-chromanone. X-ray single-crystal structures and density functional theory calculations confirmed that the distance of endo-functionalized groups within the rigid cavity is crucial for achieving high binding selectivity through hydrogen bonding. The thermodynamic data and molecular dynamics simulations revealed a significant influence of the hydrophobic cavity on the binding affinity. The new receptor possesses both high selectivity and high affinity, which provide valuable insights for the design of customized receptors.

Advancing the Am Extractant Design through the Interplay among Planarity, Preorganization, and Substitution Effects.

Advancing the field of chemical separations is important for nearly every area of science and technology. Some of the most challenging separations are associated with the americium ion Am(III) for its extraction in the nuclear fuel cycle, 241Am production for industrial usage, and environmental cleanup efforts. Herein, we study a series of extractants, using first-principle calculations, to identify the electronic properties that preferentially influence Am(III) binding in separations. As the most used extractant family and because it affords a high degree of functionalization, the polypyridyl family of extractants is chosen to study the effects of the planarity of the structure, preorganization of coordinating atoms, and substitution of various functional groups. The actinyl ions are used as a structurally simplified surrogate model to quickly screen the most promising candidates that can separate these metal ions. The down-selected extractants are then tested for the Am(III)/Eu(III) system. Our results show that π interactions, especially those between the central terpyridine ring and Am(III), play a crucial role in separation. Adding an electron-donating group onto the terpyridine backbone increases the binding energies to Am(III) and stabilizes Am-terpyridine coordination. Increasing the planarity of the extractant increases the binding strength as well, although this effect is found to be rather weak. Preorganizing the coordinating atoms of an extractant to their binding configuration as in the bound metal complex speeds up the binding process and significantly improves the kinetics of the separation process. This conclusion is validated by the synthesized 1,2-dihydrodipyrido[4,3-b;5,6-b]acridine (13) extractant, a preorganized derivative of the terpyridine extractant, which we experimentally showed was four times more effective than terpyridine at separating Am3+ from Eu3+ (SFAm/Eu ∼ 23 ± 1).

Methods for Interpreting the Partitioning and Fate of Petroleum Hydrocarbons in a Sea Ice Environment.

Decreases in Arctic Sea ice extent and thickness have led to more open ice conditions, encouraging both shipping traffic and oil exploration within the northern Arctic. As a result, the increased potential for accidental releases of crude oil or fuel into the Arctic environment threatens the pristine marine environment, its ecosystem, and local inhabitants. Thus, there is a need to develop a better understanding of oil behavior in a sea ice environment on a microscopic level. Computational quantum chemistry was used to simulate the effects of evaporation, dissolution, and partitioning within sea ice. Vapor pressures, solubilities, octanol-water partition coefficients, and molecular volumes were calculated using quantum chemistry and thermodynamics for pure liquid solutes (oil constituents) of interest. These calculations incorporated experimentally measured temperatures and salinities taken throughout an oil-in-ice mesocosm experiment conducted at the University of Manitoba in 2017. Their potential for interpreting the relative movements of oil constituents was assessed. Our results suggest that the relative movement of oil constituents is influenced by differences in physical properties. Lighter molecules showed a greater tendency to be controlled by brine advection processes due to their greater solubility. Molecules which are more hydrophobic were found to concentrate in areas of lower salt concentration.

Computational Characterization of AcIII-DOTA Complexes in Aqueous Solution.

The 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) aqueous complexes of AcIII with H2O, dimethyl sulfoxide (DMSO), OH-, and F- as axial ligands were studied using density functional theory. Formation of the [AcIII(DOTA)(OH)]2- and [AcIII(DOTA)(F)]2- complexes is predicted to be significantly more favorable than that of [AcIII(DOTA)(H2O)]- and [AcIII(DOTA)(DMSO)]- because of the enhanced relative Gibbs free energies. Further electronic structure analyses demonstrate that the type and nature of the bond between Ac and the ligand donor atom is the main driving force that determines the thermodynamic stability of the complexes. Specifically, the [AcIII(DOTA)]- complex strongly binds to OH- and F- via covalent bonds, while the bonding to H2O and DMSO is ionic and relatively weaker.

Prediction of beryllium clusters (Ben; n = 3-25) from first principles.

Evolutionary searches using the USPEX method (Universal Structure Predictor: Evolutionary Xtallography) combined with density functional theory (DFT) calculations were performed to obtain the global minimum structures of beryllium (Ben, n = 3-25) clusters. The thermodynamic stability, optoelectronic and photocatalytic properties as well as the nature of bonding are considered for the most stable clusters. It is found that the cluster with n = 15 is the transition point at which the configurations change from 3D hollow cages to filled cage structures (with an interior atom appearing in the structure). All the ground state structures are energetically favorable with negative binding energies, suggesting good synthetic feasibility for these structures. The calculated relative stabilities and electronic structure show that the Be4, Be10 and, Be17 clusters are the most stable structures and can be considered as superatoms. The electron configurations of Be4, Be10 and Be17 clusters with 8, 20 and 34 electrons are identified as 1S2 1P6, 1S2 1P6 1D10 2S2, 1S2 1P6 1D10 2S2 1F14, respectively. Theoretical simulations determined that all the ground state structures exhibit excellent thermal stability, where the upper-limit temperature that the structures can tolerate is 900 K. During AIMD simulation of O2 adsorption onto the Be17 cluster an interesting phenomenon was happening in which the pristine Be17 cluster becomes a new stable Be17O16 cluster. Based on ELF (electron localization function) analysis, it can be concluded that the Be-Be bonds in the small clusters are primarily of van der Waals type, while for the larger clusters, the bonds are of metallic nature. The Ben clusters show very strong absorption in the UV and visible regions with absorption coefficients larger than 105 cm-1, which suggests a wide range of potential advanced optoelectronics applications. The Be17 cluster has a suitable band alignment in the visible-light excitation region which will produce enhanced photocatalytic activities (making it a promising material for water splitting).

Computational Study of Actinyl Ion Complexation with Dipyriamethyrin Macrocyclic Ligands.

Relativistic density functional theory has been employed to characterize [AnO2(L)]0/-1 complexes, where An = U, Np, Pu, and Am, and L is the recently reported hexa-aza porphyrin analogue, termed dipyriamethyrin, which contains six nitrogen donor atoms (four pyrrolic and two pyridine rings). Shorter axial (An═O) and longer equatorial (An-N) bond lengths are observed when going from AnVI to AnV. The actinide to pyrrole nitrogen bonds are shorter as compared to the bonds to the pyridine nitrogens; the former also play a dominant role in the formation of the actinyl (VI and V) complexes. Natural population analysis shows that the pyrrole nitrogen atoms in all the complexes carry higher negative charges than the pyridine nitrogens. Upon binding actinyl ions with the ligand a significant ligand-to-metal charge transfer takes place in all the actinyl (VI and V) complexes. The formation energy of the actinyl(VI,V) complexes in the gas-phase is found to decrease in the order of UO2L > PuO2L > NpO2L > AmO2L. This trend is consistent with results for the formation of complexes in dichloromethane solution. The calculated ΔG and ΔH values are negative for all the complexes. Energy decomposition analysis (EDA) indicates that the interactions between actinyl(V/VI) and ligand are mainly controlled by electrostatic components over covalent orbital interactions, and the covalent character gradually decreases from U to Am for both pentavalent and hexavalent actinyl complexes.

Environmental Mercury Chemistry - In Silico.

Mercury (Hg) is a global environmental contaminant. Major anthropogenic sources of Hg emission include gold mining and the burning of fossil fuels. Once deposited in aquatic environments, Hg can undergo redox reactions, form complexes with ligands, and adsorb onto particles. It can also be methylated by microorganisms. Mercury, especially its methylated form methylmercury, can be taken up by organisms, where it bioaccumulates and biomagnifies in the food chain, leading to detrimental effects on ecosystem and human health. In support of the recently enforced Minamata Convention on Mercury, a legally binding international convention aimed at reducing the anthropogenic emission of-and human exposure to-Hg, its global biogeochemical cycle must be understood. Thus, a detailed understanding of the molecular-level interactions of Hg is crucial. The ongoing rapid development of hardware and methods has brought computational chemistry to a point that it can usefully inform environmental science. This is particularly true for Hg, which is difficult to handle experimentally due to its ultratrace concentrations in the environment and its toxicity. The current account provides a synopsis of the application of computational chemistry to filling several major knowledge gaps in environmental Hg chemistry that have not been adequately addressed experimentally. Environmental Hg chemistry requires defining the factors that determine the relative affinities of different ligands for Hg species, as they are critical for understanding its speciation, transformation and bioaccumulation in the environment. Formation constants and the nature of bonding have been determined computationally for environmentally relevant Hg(II) complexes such as chlorides, hydroxides, sulfides and selenides, in various physical phases. Quantum chemistry has been used to determine the driving forces behind the speciation of Hg with hydrochalcogenide and halide ligands. Of particular importance is the detailed characterization of solvation effects. Indeed, the aqueous phase reverses trends in affinities found computationally in the gas phase. Computation has also been used to investigate complexes of methylmercury with (seleno)amino acids, providing a molecular-level understanding of the toxicological antagonism between Hg and selenium (Se). Furthermore, evidence is emerging that ice surfaces play an important role in Hg transport and transformation in polar and alpine regions. Therefore, the diffusion of Hg and its ions through an idealized ice surface has been characterized. Microorganisms are major players in environmental mercury cycling. Some methylate inorganic Hg species, whereas others demethylate methylmercury. Quantum chemistry has been used to investigate catalytic mechanisms of enzymatic Hg methylation and demethylation. The complex interplay between the myriad chemical reactions and transport properties both in and outside microbial cells determines net biogeochemical cycling. Prospects for scaling up molecular work to obtain a mechanistic understanding of Hg cycling with comprehensive multiscale biogeochemical modeling are also discussed.

Proton affinities of pertechnetate (TcO4-) and perrhenate (ReO4-).

The anions pertechnetate, TcO4-, and perrhenate, ReO4-, exhibit very similar chemical and physical properties. Revealing and understanding disparities between them enhances fundamental understanding of both. Electrospray ionization generated the gas-phase proton bound dimer (TcO4-)(H+)(ReO4-). Collision induced dissociation of the dimer yielded predominantly HTcO4 and ReO4-, which according to Cooks' kinetic method indicates that the proton affinity (PA) of TcO4- is greater than that of ReO4-. Density functional theory computations agree with the experimental observation, providing PA[TcO4-] = 300.1 kcal mol-1 and PA[ReO4-] = 297.2 kcal mol-1. Attempts to rationalize these relative PAs based on elementary molecular parameters such as atomic charges indicate that the entirety of bond formation and concomitant bond disruption needs to be considered to understand the energies associated with such protonation processes. Although in both the gas and solution phases, TcO4- is a stronger base than ReO4-, it is noted that the significance of even such qualitative accordance is tempered by the very different natures of the underlying phenomena.

Adsorption of Actinide (U-Pu) Complexes on the Silicene and Germanene Surface: A Theoretical Study.

Adsorption of actinide (Ac = U, Np, Pu) complexes with environmentally relevant ligands on silicene and germanene surfaces has been investigated using density functional theory to determine the geometrical, energetic, and electronic properties. Three types of ligands for each central metal atom are considered: OH-, NO3-, and CO32- with common oxo ligands in all cases. Among these, carbonate complexes show the strongest adsorption followed by hydroxide and nitrate. Two types of model, cluster and periodic models, have been considered to include the short- and long-range effects. The cluster and periodic models are complementary, although the former has not yet been widely used for studies of 2D materials. Two cluster sizes have been investigated to check size dependency. Calculations were performed in the gas phase and water solvent. On the basis of the adsorption energy, for the CO32- and OH- ligands, the bond position between two Si atoms in the silicene sheet is the most strongly adsorbed site in the cluster model for silicene whereas in the periodic model these complexes exhibit strong binding on the Si atom of the silicene surface. The Ac complexes with the NO3- ligand show strong affinity at the hollow space at the center of a hexagonal ring of silicene in both models. The H site is most favorable for the binding of complexes on the germanene cluster whereas these sites vary in the periodic model. Electronic structure calculations have been performed that show a bandgap range from 0.130 to 0.300 eV for the adsorption of actinide complexes on silicene that can be traced to charge transfer. Density of states calculations show that the contribution of the nitrate complexes is small near the Fermi level, but it is larger for the carbonate complexes in the silicene case. Strong interactions between Ac complexes and silicene are due to the formation of strong Si-O bonds upon adsorption which results in reduction of the actinide atom. Such bonding is lacking in germanene.

Molecular Recognition of Hydrophilic Molecules in Water by Combining the Hydrophobic Effect with Hydrogen Bonding.

During the last half a century, great achievements have been made in molecular recognition in parallel with the invention of numerous synthetic receptors. However, the selective recognition of hydrophilic molecules in water remains a generally accepted challenge in supramolecular chemistry but is commonplace in nature. In an earlier Communication [ Huang et al. J. Am. Chem. Soc. 2016 , 138 , 14550 ], we reported a pair of endo-functionalized molecular tubes that surprisingly prefer highly hydrophilic molecules over hydrophobic molecules of a similar size and shape. The hydrophobic effect and hydrogen bonding were proposed to be responsible, but their exact roles were not fully elucidated. In this Article, we present a thorough study on the binding behavior of these molecular tubes toward 44 hydrophilic molecules in water. Principal component analysis reveals that the binding strength is weakly correlated to the hydrophobicity, volume, surface area, and dipole moment of guests. Furthermore, molecular dynamics simulations show the hydrophobic effect through releasing the poorly hydrogen-bonded cavity water contributes to the binding of all the hydrophilic molecules, while hydrogen bonding differentiates these molecules and is thus the key to achieve a high selectivity toward certain hydrophilic molecules over other molecules with a similar size and shape. Therefore, a good guest for these molecular tubes should meet the following criteria: the hydrogen-bonding sites should be complementary, and the molecular volume should be large enough to expel all the cavity water but not too large to cause steric hindrance. This rule of thumb may also be used to design a selective receptor for certain hydrophilic molecules. Following these guidelines, a "best-fit" guest was found for the syn-configured molecular tube with a binding constant as high as 106 M-1.

Differential Solvation.

Solvation effects influence reaction equilibria by preferentially stabilizing reactants or products (differential solvation). We propose using simple electrostatic concepts to qualitatively interpret and understand these effects, applying the Born and Kirkwood-Onsager equations. Scenarios include, for charged species, redox potentials, different total absolute charges between reactants and products, and size differences between reactants and products. In addition, for neutral species, they are differences in total dipole moment and size differences. These scenarios are illustrated with several examples from different areas of chemistry.

Interfacial Interaction of Titania Nanoparticles and Ligated Uranyl Species: A Relativistic DFT Investigation.

To understand interfacial behavior of actinides adsorbed onto mineral surfaces and unravel their structure-property relationship, the structures, electronic properties, and energetics of various ligated uranyl species adsorbed onto TiO2 surface nanoparticle clusters (SNCs) were examined using relativistic density functional theory. Rutile (110) and anatase (101) titania surfaces, experimentally known to be stable, were fully optimized. For the former, models studied include clean and water-free Ti27O64H20 (dry), partially hydrated (Ti27O64H20)(H2O)8 (sol) and proton-saturated [(Ti27O64H20)(H2O)8(H)2]2+ (sat), while defect-free and defected anatase SNCs involving more than 38 TiO2 units were considered. The aquouranyl sorption onto rutile SNCs is energetically preferred, with interaction energies of -8.54, -10.36, and -2.39 eV, respectively. Energy decomposition demonstrates that the sorption is dominated by orbital attractive interactions and modified by steric effects. Greater hydrogen-bonding involvement leads to increased orbital interactions (i.e., more negative energy) from dry to sol/sat complexes, while much larger steric interaction in the sat complex significantly reduces the sorption interaction (i.e., more positive energy). For dry SNC, adsorbates were varied from aquo to aquo-carbonato, to carbonato, to hydroxo uranyl species. Longer U-Osurf/U-Ti distances and more positive sorption energies were calculated upon introducing carbonato and hydroxo ligands, indicative of weaker uranyl sorption onto the substrate. This is consistent with experimental observations that the uranyl sorption rate decreases upon raising solution pH value or adding carbon dioxide. Anatase SNCs adsorbing aquouranyl are even more exothermic, because more bonds are formed than in the case of rutile. Moreover, the anatase sorption can be tuned by surface defects as well as its Ti and O stoichiometry. All the aquouranyl-SNC complexes show similar character of molecular orbitals and energetic order although differing in highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps and orbital energy levels, but changes can be accomplished by adding carbonato and hydroxo ligands.