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.

Stable Dioxin-Linked Metallophthalocyanine Covalent Organic Frameworks (COFs) as Photo-Coupled Electrocatalysts for CO2 Reduction.

In this work, we rationally designed a series of crystalline and stable dioxin-linked metallophthalocyanine covalent organic frameworks (COFs; MPc-TFPN COF, M=Ni, Co, Zn) under the guidance of reticular chemistry. As a novel single-site catalysts (SSCs), NiPc/CoPc-TFPN COF exhibited outstanding activity and selectivity for electrocatalytic CO2 reduction (ECR; Faradaic efficiency of CO (FECO )=99.8(±1.24) %/ 96.1(±1.25) % for NiPc/CoPc-TFPN COF). More importantly, when coupled with light, the FECO and current density (jCO ) were further improved across the applied potential range (-0.6 to -1.2 V vs. RHE) compared to the dark environment for NiPc-TFPN COF (jCO increased from 14.1 to 17.5 A g-1 at -0.9 V; FECO reached up to ca. 100 % at -0.8 to -0.9 V). Furthermore, an in-depth mechanism study was established by density functional theory (DFT) simulation and experimental characterization. For the first time, this work explored the application of COFs as photo-coupled electrocatalysts to improve ECR efficiency, which showed the potential of using light-sensitive COFs in the field of electrocatalysis.

The use of main-group elements to mimic catalytic behavior of transition metals I: reduction of dinitrogen to ammonia catalyzed by bis(Lewis base)borylenium diradicals.

The use of boron (B) atoms as transition metal mimics opens the door to new research in catalytic chemistry. An emerging class of compounds, bis(Lewis base)borylenes with an electron-rich B(i) center, are potential metal-free catalysts for dinitrogen bonding and reduction. Here, the molecular geometry, electronic structure, and possible reaction mechanism of a series of bis(Lewis base)borylene-dinitrogen compounds corresponding to the nitrogen reduction reaction have been investigated by using density functional theory (DFT) calculations. Our DFT calculations show that these free borylene compounds possess radical features and have the capability to activate N2 molecules via an effective combination of π(B → N2), π(N2 → B), and σ(N2 → B) electron transfer processes. The possible reaction mechanisms for direct conversion of N2 into NH3 for these bis(Lewis base)borylene-dinitrogen compounds have been systematically investigated along distal and alternating paths. The calculated free energy profiles indicate that the limiting potential of a bis(phosphine)borylene-dinitrogen compound is comparable to that of metal-based catalysts, which is the most promising candidate for the reduction of N2 to NH3via the alternating mechanism among all compounds studied here. The electronic structure analysis shows that the B center plays the role of an electron donor and acceptor alternatively in the consecutive six protonation and reduction processes, and thus acts as the electron transfer medium.

Reduction of N2O by CO via Mars-van Krevelen [corrected] Mechanism over Phosphotungstic Acid Supported Single-Atom Catalysts: A Density Functional Theory Study.

In general, reduction of N2O by CO is first performed by N2O decomposition over a catalyst surface to release N2 and form an active oxygen species, and subsequently CO is oxidized by the active oxygen species to produce CO2. However, the strong adsorption behavior of CO on the catalyst surface usually inhibits adsorption and decomposition of N2O, which leads to a low activity or poisoning of catalysts. In the present paper, a Mars-van Krevelen (MvK) [correction] mechanism has been probed based on a series of phosphotungstic acid (PTA) supported single-atom catalysts (SACs), M1/PTA (M = Fe, Co, Mn, Rh, Ru, Ir, Os, Pt, and Pd). Although the calculated adsorption energy of CO is exceedingly higher than N2O for our studied systems, the adsorbed CO could react with the surface oxygen atom of the PTA support through the MvK mechanism to form an oxygen vacancy on the PTA surface. N2O acts as an oxygen donor to replenish the PTA support and release N2 in the whole reaction process. This proposed reaction mechanism avoids competitive adsorption and poisoning of the catalyst caused by CO. The calculated adsorption energy, oxygen vacancy formation energy, and the free energy profiles show that the catalytic activity of Pd1/PTA, Rh1/PTA, and Pt1/PTA SACs is quite high, especially for Pt1/PTA and Pd1/PTA systems. Meanwhile, molecular geometry and electronic structure analysis along the favorable reaction pathway indicates that the metal single atom not only plays the role of adsorbing CO and activating surface atoms of the PTA support but also works as an electron transfer media in the whole reaction process. We expect that the present calculated results could provide some clues for the search for appropriate catalyst for reduction of N2O to N2 by CO at low temperature.

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 d z2 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.

Reduction of N2O by H2 Catalyzed by Keggin-Type Phosphotungstic Acid Supported Single-Atom Catalysts: An Insight from Density Functional Theory Calculations.

In the present paper, the mechanisms of N2O reduction by H2 were systemically examined over various polyoxometalate-supported single-atom catalysts (SACs) M1/PTA (M = Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; PTA = [PW12O40]3-) by means of density functional theory calculations. Among these M1/PTA SACs, Os1/PTA SAC possesses high activity for N2O reduction by H2 with a relatively low rate-determining barrier. The favorable catalytic pathway involves the first and second N2O decomposition over the Os1/PTA SAC and hydrogenation of the key species after the second N2O decomposition. Molecular geometry and electronic structure analyses along the favorable reaction pathway indicate that a strong charge-transfer cooperative effect of metal and support effectively improves the catalytic activity of Os1/PTA SAC. The isolated Os atom not only plays the role of adsorption and activation of the N2O molecule but also works as an electron transfer medium in the whole reaction process. Meanwhile, the PTA support with very high redox stability has also been proven to be capable of transporting the electron to promote the whole reaction. We expect that our computation results can provide ideas for designing new SACs for N2O reduction by using H2 selective catalytic reduction technology.

Calculations of NO reduction with CO over a Cu1/PMA single-atom catalyst: a study of surface oxygen species, active sites, and the reaction mechanism.

Density functional theory (DFT) calculations have been employed to probe the reaction mechanism of NO reduction with CO over a Cu1/PMA (PMA is the phosphomolybdate, Cs3PMo12O40) single-atom catalyst (SAC). Several important aspects of the catalytic system were addressed, including the generation of oxygen vacancies (Ov), formation of N2O2 intermediates, scission of the N-O bond of N2O2 intermediates to form N2O or N2, and decomposition of N2O to form N2. Unlike most previous theoretical studies, which tend to explore the reaction mechanism of polyoxometalate (POM) systems based on the isolated anionic unit, here, we build a model of the catalytic system with neutral species by introduction of counter cations to model the solid structure of the Cu1/PMA SAC. The major findings of our present study are: (1) CO adsorption on Cu sites leads to the formation of cationic Cu carbonyl species; (2) the Oc atom at the surface of the PMA support can easily react with the adsorbed CO to generate a Cu-Ov pair; (3) the Cu-Ov pair embedded on PMA is found to be the active site, not only for the formation of N2O2* by the reaction of two NO molecules via an Eley-Rideal pathway but also for the decomposition of N2O to form N2; (4) the adsorption of a NO molecule on the Cu-Ov pair with a bridging model results in charge transfer from the Cu atom to the π* antibonding orbital of the NO molecule; (5) IR spectroscopy of the key intermediates has been identified based on our DFT calculations; and (6) the Cu atom serves as an electron acceptor in Ov formation steps and an electron donor in N2O2 decomposition steps, and thus represents an electron reservoir. These results suggest that the POM-supported SAC with the cheaper Cu element is an efficient catalyst for the reaction between CO and NO.

Computational Study on M1/POM Single-Atom Catalysts (M = Cu, Zn, Ag, and Au; POM = [PW12O40]3-): Metal-Support Interactions and Catalytic Cycle for Alkene Epoxidation.

Geometrical structures, metal-support interactions, and infrared (IR) spectroscopy of a series of M1/POM (M = Cu, Zn, Ag, and Au; POM = [PW12O40]3-) single-atom catalysts (SACs), and catalytic cycle for alkene epoxidation catalyzed by M1/POM SACs were studied using density functional theory (DFT) calculations. The calculations demonstrate that the most probable anchoring sties for the isolated single atoms studied here in the M1/POM SACs are the fourfold hollow sites on the surface of POM support. The bonding interaction between single metal atom and surface of POM support comes from the molecular orbitals with a mixture of d atomic orbital of metal and 2p group orbital of surface oxygen atoms of POM cage. The calculated adsorption energy of isolated metal atoms in these M1/POM SACs indicates that the early transition metals (Cu and Zn) have high thermal stability. The DFT-derived IR spectra show that the four characteristic peaks of free Keggin-type POM structure split into six because of introduction of isolated metal atom. Compared with other metal atoms, the Zn1/POM SAC has the high reactivity for activity of dioxygen molecule, because the dioxygen moiety in Zn1/POM SAC displays O2-· radical feature with [POM4-·Zn2+O2-·]3- configuration. Finally, a catalytic cycle for ethylene epoxidation by O2 catalyzed by Zn1/POM SAC was proposed based on our DFT calculations. Supported noble-metal SACs are among the most important catalysts currently. However, noble metals are expensive and of limited supply. Development of non-noble-metal SACs is of essential importance. Therefore, the reported Zn1/POM SAC would be very useful to guide the search for SACs into non-noble metals.

Proteomics of methyl jasmonate induced defense response in maize leaves against Asian corn borer.

BACKGROUND: Jasmonic acid (JA) and methyl jasmonate (MeJA) regulate plant development, resistance to stress, and insect attack by inducing specific gene expression. However, little is known about the mechanism of plant defense against herbivore attack at a protein level. Using a high-resolution 2-D gel, we identified 62 MeJA-responsive proteins and measured protein expression level changes. RESULTS: Among these 62 proteins, 43 proteins levels were increased while 11 proteins were decreased. We also found eight proteins uniquely expressed in response to MeJA treatment. Data are available via ProteomeXchange with identifier PXD001793. The proteins identified in this study have important biological functions including photosynthesis and energy related proteins (38.4%), protein folding, degradation and regulated proteins (15.0%), stress and defense regulated proteins (11.7%), and redox-responsive proteins (8.3%). The expression levels of four important genes were determined by qRT-PCR analysis. The expression levels of these proteins did not correlate well with their translation levels. To test the defense functions of the differentially expressed proteins, expression vectors of four protein coding genes were constructed to express in-fusion proteins in E. coli. The expressed proteins were used to feed Ostrinia furnacalis, the Asian corn borer (ACB). Our results demonstrated that the recombinant proteins of pathogenesis-related protein 1 (PR1) and thioredoxin M-type, chloroplastic precursor (TRXM) showed the significant inhibition on the development of larvae and pupae. CONCLUSIONS: We found MeJA could not only induce plant defense mechanisms to insects, it also enhanced toxic protein production that potentially can be used for bio-control of ACB.

Computational Study of Metal-Dinitrogen Keggin-Type Polyoxometalate Complexes [PW11O39M(II)N2)](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(II)N2)](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 π(2)xzπ(2)yzπ(2)xy and π(2)xzπ(2)yzπ(1)xy 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 (ΔG(⧧)) 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.

A fresh perspective on dissociation mechanism of cellulose in DMAc/LiCl system based on Li bond theory.

In this case, various characterization technologies have been employed to probe dissociation mechanism of cellulose in N,N-dimethylacetamide/lithium chloride (DMAc/LiCl) system. These results indicate that coordination of DMAc ligands to the Li+-Cl- ion pair results in the formation of a series of Lix(DMAc)yClz (x = 1, 2; y = 1, 2, 3, 4; z = 1, 2) complexes. Analysis of interaction between DMAc ligand and Li center indicate that Li bond plays a major role for the formation of these Lix(DMAc)yClz complexes. And the saturation and directionality of Li bond in these Lix(DMAc)yClz complexes are found to be a tetrahedral structure. The hydrogen bonds between two cellulose chains could be broken at the nonreduced end of cellulose molecule via combined effects of basicity of Cl- ion and steric hindrance of [Li (DMAc)4]+ unit. The unique feature of Li bond in Lix(DMAc)yClz complexes is a key factor in determination of the dissociation mechanism.

Carbon-metal versus metal-metal synergistic mechanism of ethylene electro-oxidation via electrolysis of water on TM2N6 sites in graphene.

Acetaldehyde (AA) and ethylene oxide (EO) are important fine chemicals, and are also substrates with wide applications for high-value chemical products. Direct electrocatalytic oxidation of ethylene to AA and EO can avoid the untoward effects from harmful byproducts and high energy emissions. The most central intermediate state is the co-adsorption and coupling of ethylene and active oxygen intermediates (*O) at the active site(s), which is restricted by two factors: the stability of the *O intermediate generated during the electrolysis of water on the active site at a certain applied potential and pH range; and the lower kinetic energy barriers of the oxidation process based on the thermo-migration barrier from the *O intermediate to produce AA/EO. The benefit of two adjacent active atoms is more promising, since diverse adsorption and flexible catalytic sites may be provided for elementary reaction steps. Motivated by this strategy, we explored the feasibility of various homonuclear TM2N6@graphenes with dual-atomic-site catalysts (DASCs) for ethylene electro-oxidation through first-principles calculations via thermodynamic evaluation, analysis of the surface Pourbaix diagram, and kinetic evaluation. Two reaction mechanisms through C-TM versus TM-TM synergism were determined. Between them, a TM-TM mechanism on 4 TM2N6@graphenes and a C-TM mechanism on 5 TM2N6@graphenes are built. All 5 TM2N6@graphenes through the C-TM mechanism exhibit lower kinetic energy barriers for AA and EO generation than the 4 TM2N6@graphenes through the TM-TM mechanism. In particular, Pd2N6@graphene exhibits the most excellent catalytic activity, with energy barriers for generating AA and EO of only 0.02 and 0.65 eV at an applied potential of 1.77 V vs. RHE for the generation of an active oxygen intermediate. Electronic structure analysis indicates that the intrinsic C-TM mechanism is more advantageous than the TM-TM mechanism for ethylene electro-oxidation, and this study also provides valuable clues for further experimental exploration.

Preparation of Cellulose Nanofibers from Corn Stalks by Fenton Reaction: A New Insight into the Mechanism by an Experimental and Theoretical Study.

Agricultural biomass wastes are an abundant feedstock for biorefineries. However, most of these wastes are not treated in the right way. Here, corn stalks (CSs) were assigned as the raw material to produce cellulose nanofibers (CNFs) via in situ Fenton oxidation treatment. In order to probe the formation mechanism of an in situ Fenton reactor, the bonding interaction of hydrated Fe2+ ions and fiber has been systemically studied based on adsorption experiments, IR spectroscopy, density functional theory (DFT) calculations, and Raman spectroscopy. The results indicate that the coordination of the hydrated Fe2+ ion to the fiber generates a quasi-octahedral-coordinated sphere around the Fe center. The Jahn-Teller distortion effect of the Fe center promotes the Fe-O2H2 bonding interaction via reduction of the energy gap of the dz2 orbital of the Fe center and π2py/π2pz orbitals of the H2O2 molecule. The oxidation treatment of the pretreated CS by the in situ Fenton process shows the formation of a new carboxyl group on the fiber surface. The scanning electron microscopy image shows that the Fenton-treated fiber was scattered into the nanosized CNFs with a diameter of up to 50 nm. Both experimental and theoretical studies show that the pseudo-first-order kinetic reaction could describe the in situ Fenton kinetics well. Moreover, the proposed catalytic cycle shows that the large thermodynamic barrier is the cleavage of the O-O bond of H2O2 to generate the •OH radical, and the whole catalytic cycle is found to be spontaneous at room temperature.

Redox and photoisomerization switching of the second-order optical nonlinearity of a tetrathiafulvalene derivative of spiropyran across five states: a DFT study.

Second-order nonlinear optical properties of a tetrathiafulvalene (TTF) derivative of spiropyran have been studied based on density functional theory (DFT) combined with the finite field (FF) calculations. Our DFT-FF calculations confirm a switching behavior of the static first hyperpolarizability caused by the redox and photochromic reaction. The photochromic reaction generates spiropyran-merocyanine conversion by reversible cleavage of the C-O bond, which is relative to the close- and open-ring forms 1-c and 1-o. The open-ring form 1-o displays the large static first hyperpolarizability relative to its close-ring form 1-c according to our DFT-FF calculations with three functionals. The electronic structure analysis and spin unrestricted calculations show that the redox processes significantly affect the geometrical structure of the TTF unit, and thus enhance the static first hyperpolarizabilities. The one-electron-oxidized species having good planar structure of the TTF unit are ~30 and ~200 times as large as that of the neutral compounds 1-c and 1-o, respectively. But the difference in the static first hyperpolarizability between one- and two-electron-oxidized states of spiropyran species is not substantial according to our DFT-FF calculations, and the spiropyran-merocyanine conversion of two-electron-oxidized species does not largely affect their static first hyperpolarizability. On the basis of the large change in the static first hyperpolarizability, our DFT-FF calculations support a five-state switching of the static first hyperpolarizability based on the redox and photoisomerization.

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.

Rhizobacteria communities reshaped by red mud based passivators is vital for reducing soil Cd accumulation in edible amaranth.

Red mud (RM) was constantly reported to immobilize soil cadmium (Cd) and reduce Cd uptake by crops, but few studies investigated whether and how RM influenced rhizobacteria communities, which was a vital factor determining Cd bioavailability and plant growth. To address this concern, high-throughput sequencing and bioinformatics were used to analyze microbiological mechanisms underlying RM application reducing Cd accumulation in edible amaranth. Based on multiple statistical models (Detrended correspondence analysis, Bray-Curtis, weighted UniFrac, and Phylogenetic tree), this study found that RM reduced Cd content in plants not only through increasing rhizosphere soil pH, but by reshaping rhizobacteria communities. Special taxa (Alphaproteobacteria, Gammaproteobacteria, Actinobacteriota, and Gemmatimonadota) associated with growth promotion, anti-disease ability, and Cd resistance of plants preferentially colonized in the rhizosphere. Moreover, RM distinctly facilitated soil microbes' proliferation and microbial biofilm formation by up-regulating intracellular organic metabolism pathways and down-regulating cell motility metabolic pathways, and these microbial metabolites/microbial biofilm (e.g., organic acid, carbohydrates, proteins, S2-, and PO43-) and microbial cells immobilized rhizosphere soil Cd via the biosorption and chemical chelation. This study revealed an important role of reshaped rhizobacteria communities acting in reducing Cd content in plants after RM application.

Quantum chemical study of redox-switchable second-order nonlinear optical responses of D-π-A system BNbpy and metal Pt(II) chelate complex.

A second-order nonlinear optical (NLO) molecular switching with redox has been investigated in the present paper. The static first hyperpolarizabilities of 5-(BMes(2))-5'-(NPh(2))-2,2'-bipyridine (BNbpy) containing three-coordinate organoboron, Pt(II) chelate complex Pt(BNbpy)Ph(2), and their reduced forms have been calculated by density functional theory (DFT) combined with the analytic derivatives method. There is an enhancement of static first hyperpolarizabilities in the reduced form according to the calculations. That is, the ß(vec) value of one-electron-reduced form is ~7 times as large as that of neutral form BNbpy; the ß(vec) values of one- and two-electron-reduced forms are ~3 and ~4 times as large as that of neutral form Pt(BNbpy)Ph(2), respectively. In particular, the ß(vec) value of two-electron-reduced form (3)Pt(BNbpy)Ph(2)(2-) is 1349 × 10(-30) esu, ~286 times larger than its neutral form. Moreover, the component ß(z) value of the metal chelate complex Pt(BNbpy)Ph(2) is 25 × 10(-30) esu, which is ~14 times as large as that of ligand BNbpy; the corresponding F(-)/CN(-) compounds show a decrease in ß(x) values compared with the case of the ligand and Pt(II) complex. Analyses of geometries, density of states (DOS), and time-dependent DFT (TDDFT) calculations reveal that the one-electron reduction promotes the molecular conjugation in the x-axis and intensifies the interaction between the metal Pt(II) and ligand and then results in an enhancement of the static first hyperpolarizability, whereas the binding of F(-)/CN(-) to the B atom turns off the p(π)-π* conjugation and has no effect on the conjugation of bipyridine, which leads to a decreasing ß value in the x-axis.

Degradation Mechanism of Benzo[a]pyrene Initiated by the OH Radical and 1O2: An Insight from Density Functional Theory Calculations.

The degradation mechanism of benzo[a]pyrene (BaP) initiated by •OH and 1O2 in aqueous solution is investigated by density functional theory calculations. The main degradation products are BaP-1,6-quinone, BaP-3,6-quinone, BaP-4,6-quinone, and BaP-6,12-quinone. •OH and HO2 are the main intermediate radical species. At a low initial concentration of •OH, 1O2 could be a primary driver for BaP degradation. The degradation mechanism includes six consecutive elementary reactions: (1) 1O2 initiation forming BaP-6-OO. (2) 1,3 H-shift (H atom shifts to the OO group) that is promoted by H2O, forming BaP-6-OOH. (3) BaP-6-OOH decomposes into the •OH radical and BaP-6-O. (4) •OH addition to BaP-6-O forming BaP-6-O-1(3,4,12)-OH. (5) Extracting the H atom from the carbon with the OH group by 1O2. (6) Extracting the H atom from the OH group by HO2. At a high initial concentration of •OH, the •OH-initiated and 1O2-initiated degradation reactions of BaP are both feasible. The degradation mechanism includes six consecutive elementary reactions: (1) •OH initiation forming BaP-6-OH or 1O2 initiation forming BaP-6-OO. (2) 1O2 addition to BaP-6-OH forming BaP-6-OH-12(1,3,4)-OO or •OH addition to BaP-6-OO forming BaP-6-OO-12(1,3,4)-OH. (3) Extracting the H atom from the carbon with the OH group by 1O2, forming HO2. (4) 1,3 H-shift (H-shift from the carbon to the OO group), promoted by H2O. (5) The loss of the OH radical. (6) Abstracting the H atom from the OH group by HO2. In this paper, the formation of BaP-4,6-quinone via the BaP degradation is first reported. Water participates in the elementary reaction in which the H atom attached on the aromatic ring shifts to the OO group, serving as a bridge that stabilizes the transition state and transports the proton. A comprehensive investigation explains the degradation mechanism of BaP initiated by •OH and 1O2 in aqueous solution.

Redox-switchable second-order nonlinear optical responses of push-pull monotetrathiafulvalene-metalloporphyrins.

The redox-active tetrathiafulvalene (TTF) is a good electron donor, and porphyrin is highly delocalized in cyclic pi-conjugated systems. The direct combination of the two interesting building units into the same molecule provides an intriguing molecular system for designing nonlinear optical (NLO) molecular materials. In the present paper, the second-order NLO properties of a series of monoTTF-porphyrins and metalloporphyrins have been calculated by density functional theory (DFT) combined with the finite field (FF) method. Our calculations show that these compounds possess considerably large static first hyperpolarizabilities, approximately 400 x 10(-30) esu. Since the TTF unit is able to exist in three different stable redox states (TTF, TTF(*+), and TTF(2+)), the redox switching of the NLO response of the zinc(II) derivative of monoTTF-metalloporphyrin has been studied, and a substantial enhancement in static first hyperpolarizability has been obtained in its oxidized species according to our DFT-FF calculations. The beta values of one- and two-electron-oxidized species are 3.6 and 8.7 times as large as that of the neutral compound, especially for two-electron-oxidized species, with a value of 3384 x 10(-30) esu. This value is about 3 times that for a push-pull metalloporphyrin, which has an exceptionally large hyperpolarizability among reported organic NLO chromophores. Meanwhile, to give a more intuitive description of band assignments of the electron spectrum and trends in NLO behavior of these compounds, the time-dependent (TD)DFT method has been adopted to calculate the electron spectrum. The TDDFT calculations well-reproduce the soret band and Q-type bands of the monoTTF-porphyrin, and these absorption bands can be assigned to the pi --> pi* transition of the porphyrin core. On the other hand, the oxidized process significantly affects the geometrical structures of the TTF unit and porphyrin ring, and the two-electron-oxidized species has a planar TTF unit and a high conjugative porphyrin ring. This effect reduces the excited energy, changes the CT feature, and thus enhances its static first hyperpolarizability.