Withdrawal Of Electron Density Research Articles

Sensing potential of C6N8 for ammonia (NH3) and nitrogen triflouride (NF3): A DFT study

The detection of toxic gases (NH3 and NF3) in regulating and monitoring air quality in the atmosphere has drawn a lot of attention. Herein, we explored a novel material (C6N8) for the detection of the important but toxic gases (NH3 and NF3). We investigated the interactions of the NH3 and NF3 with C6N8 through DFT at B3LYP, ωB97XD, and non-DFT M06-2X. Counterpoise interaction energy values (Eint. cp.) of NH3@C6N8 and NF3@C6N8 are −0.45 eV and −3.51 eV (for B3LYP), −0.42 eV and 2.11 eV (for ωB97XD) and −0.44 eV and −3.41eV (for M06-2X), respectively. Complexes having the most stable configurations were then subjected to further analyses including frontier molecular orbitals, H-L gap, and conductivity of complexes. An increase in the H-L gap in complexes (NH3@C6N8 and NF3@C6N8) is observed. The conductivity of NH3@C6N8 and NF3@C6N8 decreases as compared to C6N8. A considerable change in dipole moment was seen in C6N8 before and after complex formation. This is because of the shifting of charge between C6N8 and gases (NH3 and NF3). CHELPG and NBO charge analysis were used to evaluate the amount of charge transfer between C6N8 and gases. These analyses demonstrate that NH3 and NF3 withdraw electron density from C6N8. It was found that NH3 tends to be physically adsorbed on C6N8 while NF3 adsorbs chemically on C6N8. NCI and QTAIM analyses were performed to investigate the kind of interactions between the surface (C6N8) and gases (NH3 and NF3). Furthermore, the recovery time of NH3@C6N8 and NF3@C6N8 shows that C6N8 can be a better choice for sensing NH3 and NF3 gases.

A DFT study of ethanol interaction with the bimetallic clusters of PtSn and its implications on reactivity

The comprehension of the factors affecting the adsorption of ethanol over metals and metal alloys is a crucial step for the rational development of new catalysts for hydrogen production through ethanol reforming. In this work, we analyze the effect of combining Pt and Sn on a metal cluster on the complexation energy and reactivity for OH dehydrogenation of ethanol. Metal clusters of Pt10, Sn10 and Pt5Sn5 had their putative minimum located with the help of the artificial bee colony algorithm. Whereas the isolated Pt cluster shows a high degree of polarization (ESP surface), the Sn cluster shows a quite uniform electron density surface. The PtSn cluster is strongly polarized, with Pt atoms withdrawing electron density of Sn atoms. Complexation occurs with the oxygen atom of ethanol directed towards the point of highest electron potential in the ESP surface. Pt presents the highest complexation energy, −20.90 kcal/mol, against only −7.83 kcal/mol (at the B97-3c level). For the PtSn cluster, the value is intermediate, namely −12.39 kcal/mol. The more malleable electron density of Pt and its electron affinity are responsible for its highest complexation energy. These characteristics are partially transferred to the PtSn cluster. QTAIM results show that, for the PtSn cluster, the O–H bond in ethanol is somewhat weaker than for pure Pt and Sn. As a consequence, the energy barrier for the O–H dehydrogenation has its lowest value for the PtSn cluster, which shows that the alloying of two metals can lead to quite quite unexpected results opening the perspective for a more rational fine tuning of catalysts properties.

Catalytic stannane–fluorine activation triggered by Pd → Sn–F interaction

The chemistry of σ-acceptor (Z-type) ligands has been attracting significant interest for some time, and the geometric and electronic structures featuring Z-type ligands have provided a profound understanding of their distinctive chemical bonds, chemical properties, and reactivities. We herein report that the coordination of Lewis acidic fluorostannane as Z-type ligands enhances their nucleophilicities, opening new opportunities for the development of Pd-catalyzed cross-coupling reaction of fluorostannanes as coupling reagents. Thanks to phosphine chelation, fluorostannane serves as a Lewis acid (Z-type ligand) toward Pd atom and withdraws electron density from the Pd center via the σ*(Sn–F) orbital. Thus, the strong and polar Sn–F bond is efficiently weakened via σ-donation from the Pd center. Accordingly, the F atom in fluorostannane was readily abstracted by even moderate Lewis acidic boranes, leading to the conversion of Z-type stannane to X-stannyl complexes. This key conversion enabled the catalytic arylation of fluorostannane for the first time.

Synthesis and Unexpected Optical Properties of Ionic Phosphorus Heterocycles with P-Regulated Noncovalent Interactions.

Optoelectronic properties of organic chromophores (OCPs) are to a large extent dictated by the chemical structures. Herein, we synthesized a new series of ionic phosphorus(P)-heteropines via the methylation of the P(III) center. Our studies revealed that methylation is highly dependent on the P(III) environments (NPN, NPC, and CPC), in which adjacent nitrogen atoms greatly withdraw electron density of the P(III) center. The observation of noncovalent interactions between solvent molecules and the molecular backbones of the related P-heterocycle in the single crystal structure implied tunable molecular conformations. Different from the red-shifted absorption and emission spectra of ionic P-OCPs induced by either decreased lowest unoccupied molecular orbital (LUMO) or intramolecular charge transfer (ICT) state in previous studies, current ionic P-heterocycles exhibit blue-shifted absorption and emission spectra compared to the nonionic counterparts. Our experimental and theoretical studies suggest that the unexpected photophysical characters are probably due to the counter-anion induced structure twisting via intermolecular noncovalent interactions between NH-indole and O(OTf), and/or strong intermolecular O···F bonding between O(MI) and F(OTf). Our studies also revealed that the P-environments (NPN, NPC, and CPC) conjunctly impact the photophysical properties of the ionic P-heteropines. Overall, the fact that the P-environment-regulated noncovalent interactions induce the rich structure dynamics and photophysics offers us with a new and effective strategy to fine-tune the optical properties of OCPs.

Substitution reactions of cis-platinum(II) complexes containing bidentate N,N-donor pyridinecarboxamide ligands with different substituents

Substitution reactions of [2-(pyridinecarboxamide)dichloride Pt(II)] [PtCl2 ], [N-phenyl-(2-pyridinecarboxamide)dichloride Pt(II)] [PhPtCl2 ], [N-(4-methylphenyl)-2-pyridinecarboxamide)dichloride Pt(II)] [CH3PhPtCl2 ], [N-(4-methoxyphenyl)-(2-pyridine-carboxamide)dichloride Pt(II)] [CH3OPhPtCl2 ] and [N-(4-fluorophenyl)-(2-pyridinecarboxamide)dichloride Pt(II)] [FPhPtCl2 ], with nucleophiles; thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU) were studied under pseudo first-order conditions. The rates of substitution were investigated as a function of nucleophile concentration and temperature using stopped-flow and UV-visible absorption spectrophotometers. Substitutions of the two coordinated chloride ligands of the Pt(II) complexes occur consecutively, with the first substitution occurring opposite the coordinated pyridyl. The observed pseudo first-order rate constants regressed linearly with concentration of the incoming nucleophiles according to the equation k obs = k 2[Nu]. The highest substitution rates were measured for PtCl2, which is attributed to the unsubstituted non-leaving carboxamide ligand. The Pt(II) center of this complex is the most electrophilic as a result of the strong withdrawal of electron density through π-resonance by the carboxamide group. The introduction of a 4′-substituted phenyl group on the amido N of the carboxamide reduces the reactivity of the complexes due to proportional electronic effects from the ancillary substituents on the phenyl ring as well as its conformational disposition with respect to the plane of the complex. The order of reactivity of studied nucleophiles is TU > DMTU > TMTU. The substitution is associatively activated as supported by the negative entropy of activation values for the reactions.

Photoinduced loading of electron-rich Cu single atoms by moderate coordination for hydrogen evolution

Single-atom catalysts offer maximal atom utilization efficiencies and high-electronegativity heteroatoms play a crucial role in coordinating reactive single metal atoms to prevent agglomeration. However, these strong coordination bonds withdraw electron density for coordinated metal atoms and consequently affect their catalytic activity. Herein we reveal the high loading (11.3 wt%) and stabilization of moderately coordinated Cu-P3 structure on black phosphorus support by a photochemical strategy with auxiliary hydrogen. Single-atom Cu sites with an exceptional electron-rich feature show the triangle {G}_{{{{{{rm{H}}}}}}*} close to zero to favor catalysis. Neighboring Cu atoms work in synergy to lower the energy of key water adsorption and dissociation intermediates. The reported catalyst shows a low overpotential of only 41 mV at 10 mA cm−2 and Tafel slope of 53.4 mV dec−1 for the alkaline hydrogen evolution reaction, surpassing both isolated Cu single atoms and Cu nanoclusters. The promising materials design strategy sheds light on the design and fabrication of high-loading single metal atoms and the role of neighboring single atoms for enhanced reaction kinetics.

Electro-inductive effect: Electrodes as functional groups with tunable electronic properties

In place of functional groups that impose different inductive effects, we immobilize molecules carrying thiol groups on a gold electrode. By applying different voltages, the properties of the immobilized molecules can be tuned. The base-catalyzed saponification of benzoic esters is fully inhibited by applying a mildly negative voltage of -0.25 volt versus open circuit potential. Furthermore, the rate of a Suzuki-Miyaura cross-coupling reaction can be changed by applying a voltage when the arylhalide substrate is immobilized on a gold electrode. Finally, a two-step carboxylic acid amidation is shown to benefit from a switch in applied voltage between addition of a carbodiimide coupling reagent and introduction of the amine.

A potential method to tune reactivity

Organic Chemistry In organic chemistry, reaction rates are often sensitive to substituents that either donate or withdraw electron density from a reactive carbon center. Heo et al. now report that when reactants are tethered to an electrode, the polarization effect of an applied potential can exert an analogous influence, essentially acting as a tunable functional group. The authors demonstrate rate modulations in ester hydrolysis and arene cross-coupling, as well as a two-step acid-to-amide conversion that benefits from inversion of the polarization midway through the reaction. Science , this issue p. [214][1] [1]: /lookup/doi/10.1126/science.abb6375

Grids I: Electrophilic Aromatic Substitution

El efecto de los sustituyentes en la reactividad de la sustitución electrofílica aromática ha sido organizado en cuadrículas para demostrar los efectos de inducción y resonancia. Este esquema podría incorporar la competencia entre donación y extracción de la densidad electronica. Proponemos actividades en clase, en las cuales los estudiantes pueden practicar los efectos mientras están jugando.

Enhancing the Acceptor Character of Conjugated Organoborane Macrocycles: A Highly Electron‐Deficient Hexaboracyclophane

AbstractWe introduce a new boron‐doped cyclophane, the hexabora[16]cyclophane B6‐FMes, in which six tricoordinate borane moieties alternate with short conjugated p‐phenylene linkers. Exocyclic 2,4,6‐tris(trifluoromethyl)phenyl (FMes) groups serve not only to further withdraw electron density but at the same time sterically shield the boron atoms, resulting in a macrocycle that is both highly electron‐deficient and stable. The optical and electronic properties are compared with those of related linear oligomers and the electronic structure is further evaluated by computational methods. The studies uncover unique properties of B6‐FMes, including a low‐lying and extensively delocalized LUMO and a wide HOMO–LUMO gap, which arise from the combination of a cyclic π‐system, strong electronic communication between the closely spaced borons, and the attachment of electron‐deficient pendent groups. The binding of small anions to the electron‐deficient macrocycle and molecular model compounds is investigated and emissive exciplexes are detected in aromatic solvents.

Influence of pH and Divalent Metals Relevant to California Rice Fields on the Hydroxide-Mediated Hydrolysis of the Insecticide Chlorantraniliprole

The hydrolysis of chlorantraniliprole (3-bromo-N-[4-chloro-2-methyl-6-(methylcarbamoyl)phenyl]-1-(3-chloro-2-pyridine-2-yl)-1H-pyrazole-5-carboxamide; CAP) was investigated over the pH range of 6-10, reflective of California rice field conditions, with variable additions of Cu2+, Zn2+, Mn2+, or Ni2+. Dissipation accelerated as pH increased with half-lives ranging from 26.9 to 2.2 days with slight inhibition in rice field water. The addition of divalent metals was not observed to catalyze the hydrolysis of CAP at pH 6, indicating that the insecticide is likely to remain recalcitrant to hydrolysis in neutral or acidic surface waters. However, Mn2+ and Ni2+ were observed to inhibit hydrolysis at pH 8 and 9. Attenuated total reflectance Fourier transform infrared analysis supports the conclusion that divalent metals may withdraw electron density from the amide nitrogen via interaction with the amide oxygen, though additional quantum chemical modeling is necessary to provide further mechanistic insights. Overall, the hydrolysis of CAP in California rice fields and their surrounding surface waters will be dominated by pH and inhibited by dissolved metal species.

Cp2Mo(OH)(OH2)]+‐Catalyzed Hydrolysis of Mono‐ and Difunctional Ethers: Theoretical Understanding of Their Divergent Reactivity

The reaction mechanism of [Cp2Mo(OH)(OH2)]+‐catalyzed hydrolysis of ethyl vinyl, divinyl, and diethyl ethers was theoretically investigated. The reaction pathway evolving through the coordination of the ether oxygen to Mo is notably disfavored, as experimentally suggested. This is the only operative coordination mode for diethyl ether and explains why this monofunctional ether is not hydrolyzed by molybdocenes. However, difunctional ethers containing a functionality susceptible to activation like the vinyl group present an alternative reaction pathway proceeding through the coordination of the vinyl terminal carbon to Mo with accessible rate‐determining Gibbs energy barriers of 29.3 kcal/mol (divinyl ether) and 22.2 kcal/mol (ethyl vinyl ether). In these cases, the catalyst withdraws electron density from the unsaturated bonds and prepares the group for an easy OH‐nucleophilic attack. This explains the molybdocene‐catalyzed hydrolysis of ethyl vinyl ether experimentally observed and, more interestingly, reveals what the crucial role of the functional groups linked to the ether oxygen really is in the viability of these reactive processes.

Evolution of Oxygen–Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X‐Ray Spectroscopy

AbstractManganese oxide (MnOx) electrocatalysts are examined herein by in situ soft X‐ray absorption spectroscopy (XAS) and resonant inelastic X‐ray scattering (RIXS) during the oxidation of water buffered by borate (pH 9.2) at potentials from 0.75 to 2.25 V vs. the reversible hydrogen electrode. Correlation of L‐edge XAS data with previous mechanistic studies indicates MnIV is the highest oxidation state involved in the catalytic mechanism. MnOx is transformed into birnessite at 1.45 V and does not undergo further structural phase changes. At potentials beyond this transformation, RIXS spectra show progressive enhancement of charge transfer transitions from oxygen to manganese. Theoretical analysis of these data indicates increased hybridization of the Mn−O orbitals and withdrawal of electron density from the O ligand shell. In situ XAS experiments at the O K‐edge provide complementary evidence for such a transition. This step is crucial for the formation of O2 from water.

Evolution of Oxygen-Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X-Ray Spectroscopy.

Manganese oxide (MnOx ) electrocatalysts are examined herein by in situ soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) during the oxidation of water buffered by borate (pH 9.2) at potentials from 0.75 to 2.25 V vs. the reversible hydrogen electrode. Correlation of L-edge XAS data with previous mechanistic studies indicates MnIV is the highest oxidation state involved in the catalytic mechanism. MnOx is transformed into birnessite at 1.45 V and does not undergo further structural phase changes. At potentials beyond this transformation, RIXS spectra show progressive enhancement of charge transfer transitions from oxygen to manganese. Theoretical analysis of these data indicates increased hybridization of the Mn-O orbitals and withdrawal of electron density from the O ligand shell. In situ XAS experiments at the O K-edge provide complementary evidence for such a transition. This step is crucial for the formation of O2 from water.

(Invited) Computational Screening of Single Ion Conducting Polymer Electrolytes Based on sp3 Boron-Poly (ethylene glycol)

The application of polymer electrolytes in Lithium Ion batteries (LIB), has been proposed as an effective method to alleviate the problems associated to the limited voltage window and safety issues associated to the traditional liquid organic electrolytes. However, polymer electrolytes studied so far based on poly (ethylene glycol) and its derivatives have been unable to reach the required Li+ ionic conductivity needed for a broader application for commercial devices. Common polymer electrolytes contain a polymer with specific chain length at a defined C:O ratio, the conducting salt is added either during the polymerization process or afterwards by a dissolution process; however, the salt is often not completely dissociated while the conduction mechanism has been described as hoping of Li+ (often Li+ and counter ions) between oxygen functional groups and between chains induced by its intrinsic mobility. In order to increase the polymer ionic conductivity, it has been recently proposed the modification of the polymer for the introduction of negatively charge moieties which allows the direct interaction of Li+ with the polymer having no free counter ions; these structures present two big advantages, the Li+ transport number is close to unity and the mobility of Li is not lowered by the presence of the counter Ion. In order to identify the possible modifications to the polymer which allows higher Li conductivity, a detailed study of sp3-Boron Poly (ethylene-glycol) is reported. In this work, by means of density functional theory simulations, different electron withdrawing/donating ligands have been included as modifiers of the Boron site electron density. It was found that the effect of the ligand on the electron density of the Boron site has a great influence on the Bader charge of the boron site and also in the interaction energy with Lithium Ions was. A further study by ab-initio molecular dynamics (AIMD) shows that the mobility of Lithium ion can be modified up to two-fold by the inclusion of withdrawing electron density ligands. This methodology might lead to the discovery of new polymer electrolytes with enhanced Li+ conductivity.

An Experimental Validated Computational Method for pKa Determination of Substituted 1,2-Dihydroxybenzenes

1,2-dihydroxybenzenes (DHBs) are organic compounds which are widely studied as they are applied to advanced oxidation processes (AOPs). These compounds are also related to the development of oxidative stress, wood biodegradation, and neuronal disease in humans. DHBs are metal ligands with pro-oxidant and antioxidant properties. These activities are related to their chelation properties and a consequence of the deprotonation of their hydroxyl groups. In literature, there are several pKa values for the hydroxyl groups of DHBs. These values vary depending on the experimental conditions or the algorithm used for calculation. In this work, an experimentally validated computational method was implemented in aqueous solution for pKa determination of 24 DHBs. The deprotonation order of the hydroxyl groups in DHB was determined observing a selective deprotonation, which depended on the ability of the substituent to donate or withdraw electron density over the ring.

Hyperconjugation Promotes Catalysis in a Pyridoxal 5′-Phosphate-Dependent Enzyme

Pyridoxal 5′-phosphate (PLP)-dependent enzymes facilitate reaction specificity by aligning the scissile σ-bond of the PLP-substrate covalent complex perpendicular to the ring of the cofactor. Current models propose that this alignment causes a destabilization of the ground state. To test this hypothesis, quantum chemical calculations, utilizing our recent neutron diffraction models of aspartate aminotransferase, were performed. The calculations reveal that the scissile σ-bond orbital overlaps significantly with the π* orbital of the Schiff base. This σ → π* hyperconjugation interaction stabilizes the ground state of the external aldimine and substantially contributes to transition-state stabilization by withdrawing electron density from the Cα-H σ bond into the π system of PLP, enhancing the rate of catalysis.

Stability and Effects of Subsurface Oxygen in Oxide-Derived Cu Catalyst for CO2 Reduction

Oxide-derived copper (OD-Cu) catalysts are promising candidates for the electrochemical CO2 reduction reaction (CO2RR) due to the enhanced selectivity toward ethylene over methane evolution, which has been linked to the presence of subsurface oxygen (Osb). In this work, Osb is investigated with theoretical methods. Although Osb is unstable in slab models, it becomes stabilized within a “manually” reduced OD-Cu nanocube model which was calculated by self-consistent charge density functional tight binding (SCC-DFTB). The results obtained with SCC-DFTB for the full nanocube were confirmed with subcluster models extracted from the nanocube, calculated with both density functional theory (DFT) and SCC-DFTB. The higher stability of Osb in the nanocube is attributed to the disordered structure and greater flexibility. The adsorption strength of CO on Cu(100) is enhanced by Osb withdrawing electron density from the Cu atom, resulting in reduction of the σ-repulsion. Hence, the coverage of CO may be increased, fac...

Theoretical insights into the sites and mechanisms for base catalyzed esterification and aldol condensation reactions over Cu.

Condensation and esterification are important catalytic routes in the conversion of polyols and oxygenates derived from biomass to fuels and chemical intermediates. Previous experimental studies show that alkanal, alkanol and hydrogen mixtures equilibrate over Cu/SiO2 and form surface alkoxides and alkanals that subsequently promote condensation and esterification reactions. First-principle density functional theory (DFT) calculations were carried out herein to elucidate the elementary paths and the corresponding energetics for the interconversion of propanal + H2 to propanol and the subsequent C-C and C-O bond formation paths involved in aldol condensation and esterification of these mixtures over model Cu surfaces. Propanal and hydrogen readily equilibrate with propanol via C-H and O-H addition steps to form surface propoxide intermediates and equilibrated propanal/propanol mixtures. Surface propoxides readily form via low energy paths involving a hydrogen addition to the electrophilic carbon center of the carbonyl of propanal or via a proton transfer from an adsorbed propanol to a vicinal propanal. The resulting propoxide withdraws electron density from the surface and behaves as a base catalyzing the activation of propanal and subsequent esterification and condensation reactions. These basic propoxides can readily abstract the acidic Cα-H of propanal to produce the CH3CH(-)CH2O* enolate, thus initiating aldol condensation. The enolate can subsequently react with a second adsorbed propanal to form a C-C bond and a β-alkoxide alkanal intermediate. The β-alkoxide alkanal can subsequently undergo facile hydride transfer to form the 2-formyl-3-pentanone intermediate that decarbonylates to give the 3-pentanone product. Cu is unique in that it rapidly catalyzes the decarbonylation of the C2n intermediates to form C2n-1 3-pentanone as the major product with very small yields of C2n products. This is likely due to the absence of Brønsted acid sites, present on metal oxide catalysts, that rapidly catalyze dehydration of the hemiacetal or hemiacetalate over decarbonylation. The basic surface propoxide that forms on Cu can also attack the carbonyl of a surface propanal to form propyl propionate. Theoretical results indicate that the rates for both aldol condensation and esterification are controlled by reactions between surface propoxide and propanal intermediates. In the condensation reaction, the alkoxide abstracts the weakly acidic hydrogen of the Cα-H of the adsorbed alkanal to form the surface enolate whereas in the esterification reaction the alkoxide nucleophilically attacks the carbonyl group of a vicinal bound alkanal. As both condensation and esterification involve reactions between the same two species in the rate-limiting step, they result in the same rate expression which is consistent with experimental results. The theoretical results indicate that the barriers between condensation and esterification are within 3 kJ mol-1 of one another with esterification being slightly more favored. Experimental results also report small differences in the activation barriers but suggest that condensation is slightly preferred.

Glycine Polymerization on Oxide Minerals.

It has long been suggested that mineral surfaces played an important role in peptide bond formation on the primitive Earth. However, it remains unclear which mineral species was key to the prebiotic processes. This is because great discrepancies exist among the reported catalytic efficiencies of minerals for amino acid polymerizations, owing to mutually different experimental conditions. This study examined polymerization of glycine (Gly) on nine oxide minerals (amorphous silica, quartz, α-alumina and γ-alumina, anatase, rutile, hematite, magnetite, and forsterite) using identical preparation, heating, and analytical procedures. Results showed that a rutile surface is the most effective site for Gly polymerization in terms of both amounts and lengths of Gly polymers synthesized. The catalytic efficiency decreased as rutile > anatase > γ-alumina > forsterite > α- alumina > magnetite > hematite > quartz > amorphous silica. Based on reported molecular-level information for adsorption of Gly on these minerals, polymerization activation was inferred to have arisen from deprotonation of the NH3+ group of adsorbed Gly to the nucleophilic NH2 group, and from withdrawal of electron density from the carboxyl carbon to the surface metal ions. The orientation of adsorbed Gly on minerals is also a factor influencing the Gly reactivity. The examination of Gly-mineral interactions under identical experimental conditions has enabled the direct comparison of various minerals' catalytic efficiencies and has made discussion of polymerization mechanisms and their relative influences possible Further systematic investigations using the approach reported herein (which are expected to be fruitful) combined with future microscopic surface analyses will elucidate the role of minerals in the process of abiotic peptide bond formation.