Effect of Alkali Metal Cations on Length and Strength of Hydrogen Bonds in DNA Base Pairs

For many years, non-covalently bonded complexes of nucleobases have attracted considerable interest. However, there is a lack of information about the nature of hydrogen bonding between nucleobases when the bonding is affected by metal coordination to one of the nucleobases, and how the individual hydrogen bonds and aromaticity of nucleobases respond to the presence of the metal cation. Here we report a DFT computational study of nucleobase pairs interacting with alkali metal cations. The metal cations contribute to the stabilization of the base pairs to varying degrees depending on their position. The energy decomposition analysis revealed that the nature of bonding between nucleobases does not change much upon metal coordination. The effect of the cations on individual hydrogen bonds were described by changes in VDD charges on frontier atoms, H-bond length, bond energy from NBO analysis, and the delocalization index from QTAIM calculations. The aromaticity changes were determined by a HOMA index.     

A DFT Investigation on Hydrogen Adsorption Based on Alkali-metal Organic Complexes

Using density functional theory calculation based on the B3LYP method,we have studied the interactions of H2 molecules with alkali-metal organic complexes C6H6-nLin(n = 1～3),C6H5Na and C6H5K.A significant part of the electronic charge of M s orbital(Li 2s,Na 3s,K 4s) is donated to phenyl and is accommodated by H2 bonding orbital.For all the complexes considered,each bonded alkali-metal atom can adsorb up to five H2 in molecular form with the mean binding energy of 0.59,0.55 and 0.56 eV/H2 molecule for C6H6-nLin(n = 1～3),C6H5Na and C6H5K,respectively.The kinetic stability of these hydrogen-covered organometallic complexes is discussed in terms of energy gap between HOMO and LUMO.It is remarkable that these alkali-metal organic complexes can store up to 23.80 wt% hydrogen.Therefore,the complexes studied may be used as hydrogen storage materials.     

Fixing the Chirality and Trapping the Transition State of Helicene with Atomic Metal Glue

By combining the intriguing geometrical properties of two classes of well‐established molecules, the metallocenes and the helicenes, we propose a hybrid class of structures—the metallohelicenes. In these, the outer most aryl groups of a specific helicene are glued together by a complexing metal atom. This effectively fixes the chirality of the parent helicene, which otherwise easily undergoes thermal racemization. The fixed chirality suggests several interesting applications, ranging from building blocks of stable molecules with high circular dichroism and optical activity to chiral ligands and catalysts. Alternatively, the metal glue can trap the non‐chiral transition state structure of helicene. High‐level quantum chemical calculations show the readiness of formation and stability of the proposed complexes.     

How Absorbed Hydrogen Affects the Catalytic Activity of Transition Metals

Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show that a sufficient concentration of subsurface hydrogen, H^sub, may either significantly increase or decrease the bond energy and the reactivity of the adsorbed hydrogen, H^ad, depending on the metal. We predict a representative reaction, ethyl hydrogenation, to speed up on Pd and Pt, but to slow down on Ni and Rh in the presence of H^sub, especially on metal nanoparticles. The identified effects of subsurface H on surface reactivity are indispensable for an atomistic understanding of hydrogenation processes on transition metals and interactions of hydrogen with metals in general.     

Theoretical Study of Electron Density Delocalization in the Excited States of Transition Metal Complexes

This paper presents a theoretical study of electron density delocalization effects over an electron-accepting ligand in metal-to-ligand charge-transfer (MLCT) complexes in the excited states, where the ligand is 4,4'-X₂-2,2'-bpy (X = H, NH₂, CH₃, Ph, Cl, CO₂Et, NO₂, bpy = 2,2'-bipyridine) or terpy (2,2':6',2'-terpyridine). Optimal geometry calculations are performed for neutral ligand molecules and their radical anions modeling the state of the ligands during MLCT excitations. Spin density distribution over atoms in the radical anions is used as a measure of the degree of delocalization. The role of spin density distribution in excitation-induced changes of geometrical parameters of the ligands is considered.     

析氢过电位对金属与酸释出氢气的影响

基于析氢过电位解释了以下问题：Zn（s）|H2SO4（aq）|Cu（s）原电池实验中,如何避免锌表面产生H2?为什么纯锌、纯铁分别与稀硫酸反应速率几乎一样?如何解释铅蓄电池中铅与硫酸难以反应的事实?电解FeCl2溶液其阴极一定得到H2吗?     

On the Nature of Blueshifting Hydrogen Bonds

The block‐localized wave function (BLW) method can derive the energetic, geometrical, and spectral changes with the deactivation of electron delocalization, and thus provide a unique way to elucidate the origin of improper, blueshifting hydrogen bonds versus proper, redshifting hydrogen bonds. A detailed analysis of the interactions of F₃CH with NH₃ and OH₂ shows that blueshifting is a long‐range phenomenon. Since among the various energy components contributing to hydrogen bonds, only the electrostatic interaction has long‐range characteristics, we conclude that the contraction and blueshifting of a hydrogen bond is largely caused by electrostatic interactions. On the other hand, lengthening and redshifting is primarily due to the short‐range n(Y)→σ*(X?H) hyperconjugation. The competition between these two opposing factors determines the final frequency change direction, for example, redshifting in F₃CH ??? NH₃ and blueshifting in F₃CH ??? OH₂. This mechanism works well in the series F_nCl_3?nCH ??? Y (n=0–3, Y=NH₃, OH₂, SH₂) and other systems. One exception is the complex of water and benzene. We observe the lengthening and redshifting of the O?H bond of water even with the electron transfer between benzene and water completely quenched. A distance‐dependent analysis for this system reveals that the long‐range electrostatic interaction is again responsible for the initial lengthening and redshifting.     

CO Adsorption on a LaNi5 Hydrogen Storage Alloy Surface: A Theoretical Investigation

Density functional theory calculations are carried out to study CO adsorption on the (001) surface of a LaNi₅ hydrogen storage alloy. At low coverages, CO favors adsorption on Ni? Ni bridge sites. With an increase in CO coverage, the decrease in the adsorption energy is much larger for Ni? Ni? CO bridge adsorption than that for Ni? CO on‐top adsorption. Thus, the latter sites in the relatively stable adsorption structure are preferentially utilized at high CO coverages. The nature of the bonding between CO and the LaNi₅ (001) surface is analyzed in detail.     

Enhanced Hydrogen Evolution in Neutral Water Catalyzed by a Cobalt Complex with a Softer Polypyridyl Ligand

To explore the structure–function relationships of cobalt complexes in the catalytic hydrogen evolution reaction (HER), we studied the substitution of a tertiary amine with a softer pyridine group and the inclusion of a conjugated bpy unit in a Co complex with a new pentadentate ligand, 6‐[6‐(1,1‐di‐pyridin‐2‐yl‐ethyl)‐pyridin‐2‐ylmethyl]‐[2,2′]bipyridinyl (Py3Me‐Bpy). These modifications resulted in significantly improved stability and activity in both electro‐ and photocatalytic HER in neutral water. [Co(Py3Me‐Bpy)(OH₂)](PF₆)₂ catalyzes the electrolytic HER at ?1.3 V (vs. SHE) for 20 hours with a turnover number (TON) of 266 300, and photolytic HER for two days with a TON of 15 000 in pH 7 aqueous solutions. The softer ligand scaffold possibly provides increased stability towards the intermediate Co^I species. DFT calculations demonstrate that HER occurs through a general electron transfer/proton transfer/electron transfer/proton transfer pathway, with H₂ released from the protonation of Co^II?H species.     

Metal‐Functionalized Silicene for Efficient Hydrogen Storage

First‐principles calculations based on density functional theory are used to investigate the electronic structure along with the stability, bonding mechanism, band gap, and charge transfer of metal‐functionalized silicene to envisage its hydrogen‐storage capacity. Various metal atoms including Li, Na, K, Be, Mg, and Ca are doped into the most stable configuration of silicene. The corresponding binding energies and charge‐transfer mechanisms are discussed from the perspective of hydrogen‐storage compatibility. The Li and Na metal dopants are found to be ideally suitable, not only for strong metal‐to‐substrate binding and uniform distribution over the substrate, but also for the high‐capacity storage of hydrogen. The stabilities of both Li‐ and Na‐functionalized silicene are also confirmed through molecular dynamics simulations. It is found that both of the alkali metals, Li⁺ and Na⁺, can adsorb five hydrogen molecules, attaining reasonably high storage capacities of 7.75 and 6.9 wt %, respectively, with average adsorption energies within the range suitable for practical hydrogen‐storage applications.     

Hydrogen in Porous Tetrahydrofuran Clathrate Hydrate

The lack of practical methods for hydrogen storage is still a major bottleneck in the realization of an energy economy based on hydrogen as energy carrier. ¹ Storage within solid‐state clathrate hydrates, ² – ⁴ and in the clathrate hydrate of tetrahydrofuran (THF), has been recently reported. ⁵ , ⁶ In the latter case, stabilization by THF is claimed to reduce the operation pressure by several orders of magnitude close to room temperature. Here, we apply in situ neutron diffraction to show that—in contrast to previous reports^{[5, 6]}—hydrogen (deuterium) occupies the small cages of the clathrate hydrate only to 30 % (at 274 K and 90.5 bar). Such a D₂ load is equivalent to 0.27 wt. % of stored H₂. In addition, we show that a surplus of D₂O results in the formation of additional D₂O ice Ih instead of in the production of sub‐stoichiometric clathrate that is stabilized by loaded hydrogen (as was reported in ref. ⁶ ). Structure‐refinement studies show that [D₈]THF is dynamically disordered, while it fills each of the large cages of [D₈]THF?17D₂O stoichiometrically. Our results show that the clathrate hydrate takes up hydrogen rapidly at pressures between 60 and 90 bar (at about 270 K). At temperatures above ≈220 K, the H‐storage characteristics of the clathrate hydrate have similarities with those of surface‐adsorption materials, such as nanoporous zeolites and metal–organic frameworks, ⁷ , ⁸ but at lower temperatures, the adsorption rates slow down because of reduced D₂ diffusion between the small cages.     

Directionality of Dihydrogen Bonds: The Role of Transition Metal Atoms

A theoretical study on two series of electron‐rich group 8 hydrides is carried out to evaluate involvement of the transition metal in dihydrogen bonding. To this end, the structural and electronic parameters are computed at the DFT/B3PW91 level for hydrogen‐bonded adducts of [(PP₃)MH₂] and [Cp*MH(dppe)] (M=Fe, Ru, Os; PP₃=κ⁴‐P(CH₂CH₂PPh₂)₃, dppe= κ²‐Ph₂PCH₂CH₂PPh₂) with CF₃CH₂OH (TFE) as proton donor. The results are compared with those of adduct [Cp₂NbH₃] ? TFE featuring a “pure” dihydrogen bond, and classical hydrogen bonds in pyridine ? TFE and Me₃N ? TFE. Deviation of the H ??? H? A fragment from linearity is shown to originate from the metal participation in dihydrogen bonding. The latter is confirmed by the electronic parameters obtained by NBO and AIM analysis. Considered together, orbital interaction energies and hydrogen bond ellipticity are salient indicators of this effect and allow the MH ??? HA interaction to be described as a bifurcate hydrogen bond. The impact of the M ??? HA interaction is shown to increase on descending the group, and this explains the experimental trends in mechanisms of proton‐transfer reactions via MH ??? HA intermediates. Strengthening of the M ??? H interaction in the case of electron‐rich 5d metal hydrides leads to direct proton transfer to the metal atom.     

Catalytic Potential of Post-Transition Metal Doped Graphene-Based Single-Atom Catalysts for the CO2 Electroreduction Reaction

Catalysts are required to ensure electrochemical reduction of CO₂ to fuels proceeds at industrially acceptable rates and yields. As such, highly active and selective catalysts must be developed. Herein, a density functional theory study of p-block element and noble metal doped graphene-based single-atom catalysts in two defect sites for the electrochemical reduction of CO₂ to CO and HCOOH is systematically undertaken. It is found that on all of the systems considered, the thermodynamic product is HCOOH. Pb/C₃, Pb/N₄ and Sn/C₃ are identified as having the lowest overpotential for HCOOH production while Al/C₃, Al/N₄, Au/C₃ and Ga/C₃ are identified as having the potential to form higher order products due to the strength of binding of adsorbed HCOOH.     

N‐Doped Carbon Networks: Alternative Materials Tracing New Routes for Activating Molecular Hydrogen

The fragmentation of molecular hydrogen on N‐doped carbon networks was investigated by using molecular (polyaromatic macrocycles) as well as truncated and periodic (carbon nanotubes) models. The computational study was focused on the ergonicity analysis of the reaction and on the properties of the transition states involved when constellations of three or four pyridinic nitrogen atom defects are present in the carbon network. Calculations show that whenever N‐defects are embedded in species characterized by large conjugated π‐systems, either in polyaromatic macrocycles or carbon nanotubes, the corresponding H₂ bond cleavage is largely exergonic. The fragmentation Gibbs free energy is affected by the final arrangement of the hydrogen atoms on the defect and by the extension of the π‐electron cloud, but it is not influenced by the curvature of the system.     

Influence of Solvent Polarity and Hydrogen Bonding on the Electronic Transition of Coumarin 120: A TDDFT Study

The characteristics of the electronic transition energy of Coumarin 120 (C120) and its H‐bonded complexes in various solvents have been examined by time‐dependent density functional theory (TDDFT) in combination with a polarizable continuum solvent model (PCM). Molecular structures of C120 and its H‐bonded complexes are optimized with the B3LYP method in PCM solution, and the dihedral angle H14? N13? C7? H15 is dependent on solvent polarity and the type of H‐bond. A linear correlation of the absorption maximum of C120 with the solvent polarity function is revealed with the PCM model for all solvents except DMSO. The experimental absorption maximum of C120 in nine solvents is well described by a PCM–TDDFT scheme augmented with explicit inclusion of a few H‐bonded solvent molecules, and quantitative agreement between our calculated results and experimental measurements is obtained with an average error of less than 2 nm. H‐bonding at three different sites shifts the absorption wavelength of C120 either to the blue or to the red, that is, a significant role is played by solvent molecules in the first solvation shell in determining the electronic transition energy of C120. The dependence on the H‐bonding site and solvent polarity is examined by using the Kamlet–Taft equation for solvatochromism.     

Transition metal hydrazide-based hydrogen-storage materials: the first atoms-in-molecules analysis of the Kubas interaction

Molecular models of the M-H(2) binding sites of experimentally characterised amorphous vanadium hydrazide gels are studied computationally using gradient corrected density functional theory, to probe the coordination number of the vanadium in the material and the nature of the interaction between the metal and the H(2) molecules. The H(2) is found to bind to the vanadium through the Kubas interaction, and the first quantum theory of atoms-in-molecules analysis of this type of interaction is reported. Strong correlation is observed between the electron density at the H-H bond critical point and the M-H(2) interaction energy. Four coordinate models give the best reproduction of the experimental data, suggesting that the experimental sites are four coordinate. The V-H(2) interaction is shown to be greater when the non-hydrazine based ligand, THF, of the experimental system is altered to a poorer π-acceptor ligand. Upon altering the metal to Ti or Cr the M-H(2) interaction energy changes little but the number of H(2) which may be bound decreases from four (Ti) to two (Cr). It is proposed that changing the metal from V to Ti may increase the hydrogen storage capacity of the experimental system. A 9.9 wt% maximum storage capacity at the ideal binding enthalpy for room temperature performance is predicted when the Ti metal is combined with a coordination sphere containing 2 hydride ligands.     

金属元素在氢能利用过程中的作用

将金属元素分为主族、Ⅷ、镧系、其他d区等4个类别,综述了它们在氢气的生产、储存、氢燃料电池等3个氢能利用的主要过程中的作用。对现有研究结果的不完全统计表明,在元素周期表中除放射性金属元素之外的65种金属元素中,有46种金属元素以单质、合金、氧化物、盐等多种形式参与了氢能利用过程,包括作为载体、反应物、催化剂等,其中以铂系为代表的d区金属元素和以镧为代表的轻稀土金属元素应用最为广泛。