Computational Study of Metal–Dinitrogen Keggin-Type Polyoxometalate Complexes [PW₁₁O₃₉M^IIN₂)]^5– (M = Ru, Os, Re, Ir): Bonding Nature and Dinitrogen Splitting

Molecular geometry, electronic structure, and metal–dinitrogen bonding nature of a series of metal–dinitrogen derivatives of Keggin-type polyoxometalates (POMs) [PW₁₁O₃₉M^IIN₂)]^5– (M = Ru, Os, Re, Ir) have been studied by using a density functional theory (DFT) method with the M06L functional. Among these Keggin-type POM complexes, Os- and Re-substituted POM complexes are the most active for N₂ adsorption with considerable adsorption energy. The electronic structure analysis shows that Os^II and Re^II centers in their metal–dinitrogen POM complexes possess π²_<i>xz</i>π²_<i>yz</i>π²_<i>xy</i> and π²_<i>xz</i>π²_<i>yz</i>π¹_<i>xy</i> configurations, respectively. DFT-M06L calculations show that the possible synthesis routes proposed in this work for the Ru–, Os–, and Re–dinitrogen POM complexes are thermodynamically feasible under various solvent environments. Meanwhile, the Re–dinitrogen POM complex was assessed for the direct cleavage of dinitrogen molecule. In the reaction mechanism, a dimeric Keggin-type POM derivative of rhenium could represent the intermediate which undergoes N–N bond scission. The calculated free energy barrier (Δ<i>G</i>^⧧) for a transition state with a zigzag conformation is 16.05 kcal mol^–1 in tetrahydrofuran, which is a moderate barrier for the cleavage of the N–N bond when compared with the literature values. In conclusion, regarding the direct cleavage of the dinitrogen molecule, the findings would be very useful to guide the search for a potential N₂ cleavage compound into totally inorganic POM fields

Computational Study on Redox-Switchable 2D Second-Order Nonlinear Optical Properties of Push−Pull Mono-tetrathiafulvalene-Bis(Salicylaldiminato) Zn(II) Schiff Base Complexes

The redox-switchable 2D second-order nonlinear optical (NLO) property of a series of tetrathiafulvalene (TTF) derivatives has been studied based on the density functional theory (DFT) calculations. The redox-active TTF unit has been considered as a manipulative center for switching the 2D second-order NLO properties. Our DFT calculations show that introduction of the TTF unit cannot effectively enhance the second-order NLO properties relative to the reference system 1 because the nonplane embowed arrangement of the TTF unit reduces the electron donor capacity. The electronic structure analysis shows that the TTF unit acts as the oxidized center in one- and two-electron-oxidized processes for 5. A significant transformation on the structure of the TTF unit, the TTF unit changes from the embowed structure to a planar structure, has been found in the series of oxidized processes according to DFT-optimized calculations. This leads to the low excited energy and different charge transfer features of the oxidized species relative to its reduced parents, and thus enhances the static first hyperpolarizabilities. The β value of one- and two-electron-oxidized species is at least ∼15 and ∼8.6 times as large as that of its reduced parents according to our DFT calculations. Simultaneously, the oxidized process increases the contributions from the y-polarized transition, and thus improves the 2D second-order NLO property

A Systematic Theoretical Study on Electronic Interaction in Cu-based Single-Atom Alloys

A meticulous understanding of the electronic structure of catalysts may provide new insight into catalytic performances. Here, we present a d–d interaction model to systematically study the electronic interaction in Cu-based single-atom alloys. We refine three types of electronic interactions according to the position of the antibonding state relative to the Fermi level. Moreover, we also find a special phenomenon in Mn-doped single-atom alloys in which no obvious electronic interaction is found, and the doped Mn metal seems to be a free atom. Then, taking Hf/Mn-doped single-atom alloys as an example, we discuss the electronic structure based on the density of states, charge transfer, crystal orbital Hamilton population, and wavefunctions. To support the proposed model and help analyze the data, we perform an energetic analysis of water dissociation in the water-gas shift reaction. The calculation results well confirm the d–d interaction model, where alloys with the position of the antibonding state close to the Fermi level exhibit excellent water dissociation ability in the water-gas shift reaction. However, the catalytic performance of the Mn-doped alloy is unsatisfactory, which is caused by its own special phenomenon

Redox and Photoisomerization Switching the Second-Order Nonlinear Optical Properties of a Tetrathiafulvalene Derivative Across Six States: A DFT Study

The switching of second-order nonlinear optical (NLO) properties for a tetrathiafulvalene (TTF) derivative across the six stable states has been studied by using the density functional theory (DFT) calculations. The redox-active TTF unit and a photoisomerized chromophore 1,2-dithienylperfluorocyclopentene (DTE) have been implemented to switch the second-order NLO responses. Our DFT calculations with three functionals demonstrate that introduction of the DTE moiety into the π-conjugated bridge can significantly enhance the second-order NLO response relevant to the donor/acceptor end in this work. Our DFT calculations illustrate that photoisomerization bring forth a large change in the geometry of the series of compounds. The closed-ring form possesses a good π-conjugation relative to the open-ring form and thus a large second-order NLO response. The electronic structure analysis shows that the TTF unit will perform as an oxidation center in the one- and two-electron-oxidation processes. The one- and two-electron-oxidized species have better planar structures of TTF unit than its neutral compound, which ultimately leads to the low excited energy and enhances the static first hyperpolarizability. Our present DFT calculations using three functionals show that the TTF derivative 4 can switch the second-order NLO properties across six stable states, which is a rare example in previously reported second-order NLO switches

Jahn–Teller Distorted Effects To Promote Nitrogen Reduction over Keggin-Type Phosphotungstic Acid Catalysts: Insight from Density Functional Theory Calculations

Molecular geometry, electronic structure, and possible reaction mechanism of a series of mono-transition-metal-substituted Keggin-type polyoxometalate (POM)–dinitrogen complexes [PW11O39M­(N2)]n− (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg) have been investigated by using density functional theory (DFT) calculations with M06L functional. The calculated adsorption energy of N2 molecule, N–N bond length, N–N stretching frequency, and the NBO charge on the coordinated N2 moiety indicate that MoII-, TcII-, WII-, ReII-, and OsII-POM complexes are significant for binding and activation of the inert N2 molecule. The degree of the N2 activation can be classified into the “moderately activated” category according to Tuczek’s sense [J. Comput. Chem. 2006, 27, 1278]. Electronic structure and NBO analysis indicate that the terminal N atom of the coordinated N2 molecule in these POM–dinitrogen complexes possesses more negative charge relative to the bridge N atom because Jahn–Teller distorted effects lead to an effective orbital mixture between σ2s* orbital of N2 and dz2 orbital of transition metal center. And the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Meanwhile, a catalytic cycle for direct conversion of N2 into NH3 has been systematically investigated based on a Re-POM complex along distal, alternating, and enzymatic pathways. The calculated free energy profile of the three catalytic cycles indicates that the distal mechanism is the favorable pathway in the presence of proton and electron donors

Quantum Chemical Studies on High-Valent Metal Nitrido Derivatives of Keggin-Type Polyoxometalates ([PW₁₁O₃₉{M^VIN}]⁴⁻ (M = Ru, Os, Re)): M^VI−N Bonding and Electronic Structures

High-valent MVIN (M = Ru, Os) species are important reagents in nitrogen transfer reactions; the unique withdrawing properties of polyoxometalate (POMs) ligands would possibly modify the reactivity of the MVIN functional group. In the present paper, density functional theory (DFT) and natural bond orbital (NBO) analysis have been employed to calculate electronic structures, MVI−N bonding, and redox properties of high-valent metal nitrido derivatives of Keggin-type POMs, [PW11O39 {MVIN}]4− (M = Ru, Os, Re). Our calculations show that [PW11O39{RuN}]4− possesses stronger antibonding interaction between metal and nitrogen atoms compared with anions [PW11O39{OsN}]4− and [PW11O39{ReN}]4−. A large increase in the Ru−N bond length of anion [PW11O39{RuN}]4− in the excited states has been found; the effective order and composition of the molecular orbital in anion [PW11O39{RuN}]4− is a key factor in determination of the increase of the Ru−N bond length in the excited states. The substitution effects of central tetrahedron heteroatoms (XO4, X = Al, Si, P, As) in anions [XW11O39{RuN}]4− affect the relative energy of the LUMO; the relevant orbital energy increases in the order Al(III) 11O39{RuVIN}]4− shows that the Ru−N bond possesses a covalent feature and displays triple-, double-, and single-bond character when moving along the change of spin state (11 → 31 → 51)

Estimation of various chemical bond dissociation enthalpies of large-sized kerogen molecules using DFT methods

<p>Kerogen is irregularly connected by a variety of chemical bonds including C–C bond, C=C bond, C–O bond, C–N bond, C–H bond and C–S bond. It is difficult to identify selective bond breaking events using the existing experimental conditions. In this study, we predict dissociation tendencies of chemical bonds in various kerogen molecules by using 554 small molecular model compounds based on the density functional theoretical (DFT) calculations. Results from our DFT study indicate that the calculated various subtypes of bond dissociation enthalpies (BDEs) changing tendency is that C–S bond < C–N bond < C–O bond < C–C bond < C–H bond < C=C bond. However, for each bond type, the BDE value has a large range and overlaps each other, and the BDE mostly relies on the local environment (the functional groups and position) of chemical bonds and radical stability. For the C–C bond, a linear relationship (<i>R</i> = 0.85) between C–C bond distances and BDE has been achieved. Thus, the bond distance can serve as a good indicator of bond strength for C–C bond linkages. All results support that the BDE is a good choice to evaluate the dissociation tendencies of kerogen and this work provides an effective path to reveal the nature of chemical bonding for large-sized kerogen molecules.</p

Synergistic Effects of Keggin-Type Phosphotungstic Acid-Supported Single-Atom Catalysts in a Fast NH₃‑SCR Reaction

Fast selective catalytic reduction of nitrogen oxide with ammonia (NH3-SCR) (2NH3 + NO2 + NO → 2N2 + 3H2O) has aroused great interest in recent years because it is inherently faster than the standard NH3-SCR reaction (4NO + 4NH3 + O2 → 4N2 + 6H2O). In the present paper, the mechanism of the fast NH3-SCR reaction catalyzed by a series of single-atom catalysts (SACs), M1/PTA SACs (PTA = Keggin-type phosphotungstic acid, M = Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, and Pt), has been systematically studied by means of density functional theory (DFT) calculations. Molecular geometry and electronic structural analysis show that Jahn–Teller distortion effects promote an electron transfer process from N–H bonding orbitals of the NH3 molecule to the symmetry-allowed d orbitals (dxy and dx2–y2) of the single metal atom, which effectively weakens the N–H bond of the adsorbed NH3 molecule. The calculated free energy profiles along the favorable catalytic path show that decomposition of NH3 to *NH2 and *H species and decomposition of *NHNOH into N2 and H2O have high free energy barriers in the whole fast NH3-SCR path. A good synergistic effect between the Brønsted acid site (surface oxygen atom in the PTA support) and the Lewis acid site (single metal atom) effectively enhances the decomposition of NH3 to *NH2 and *H species. M1/PTA SACs (M = Ru, Rh, Pd, and Pt) were found to have potential for fast NH3-SCR reaction because of the relatively small free energy barrier and strong thermodynamic driving forces. We hope our computational results could provide some new ideas for designing and fabricating fast NH3-SCR catalysts with high activity