Ehsan Zahedi

The cross-substitution effect of tantalum on the visible-light-driven water oxidation activity of BaNbO2N crystals grown directly by an NH3-assisted flux method

Various perovskite-type transition metal oxynitride photocatalysts have been intensively investigated for photocatalytic water splitting under visible light. The band gap engineering and the formation of solid solution have been demonstrated to be one of the beneficial approaches to achieve a high solar energy conversion efficiency. As a member of the niobium-based oxynitride family, BaNbO2N has a small band gap energy of 1.79 eV and light absorption up to 690 nm. Here, we have investigated the cross-substitution effect of tantalum on the photocatalytic water oxidation activity of BaNbO2N crystals grown directly by an NH3-assisted flux method. It was found that with increasing the amount of tantalum substituted for niobium in BaNb1−xTaxO2N (x = 0, 0.25, 0.50, 0.75, and 1), the average crystal size and the intensity of background absorption gradually reduced and the O2 evolution rate increased monotonically for BaTaO2N crystals due to the shift of the top of the valence band to a more positive side and the lowered densities of mid-gap states associated with defects. The CoOx/flux-grown-BaTaO2N/Ta photoanode exhibited a photocurrent of about 0.85 mA cm−2 at 1.2 VRHE, which decreased gradually by about 35% after 24 h, evidencing that the flux-grown BaTaO2N crystals are highly stable for photoelectrochemical water oxidation. BaTaO2N modified with CoOx (2 wt% Co) as an O2 evolution cocatalyst exhibited an apparent quantum yield (AQY) of 0.24% at 420 nm for an O2 evolution reaction in the presence of Ag+ ions as sacrificial electron acceptors. In addition, the results from the first-principles DFT calculations conform to the experimentally obtained data on the photocatalytic water oxidation activity of the BaNb1−xTaxO2N crystals.

Mechanism and regioselectivity of 1,3-dipolar cycloaddition reactions of sulphur-centred dipoles with furan-2,3-dione: A theoretical study using DFT

AbstractThe mechanism and regioselectivity of 1,3-dipolar cycloaddition reactions of sulphur-centred 1,3-dipoles including thiocarbonyl S-imide (D1), thiocarbonyl S-oxide (D2) and thiocarbonyl S-sulphide (D3) with an electron-deficient dipolarophile, furan-2,3-dione (DPh), were studied in the light of some theoretical approaches, namely, activation energy, density functional theory (DFT) reactivity indices and Houk’s rule based on the frontier molecular orbital (FMO) theory at the B3LYP/6-311+ +G** level. The present analysis reveals that the cycloaddition reactions under study can be classified in the normal electron demand category. An excellent agreement was observed between the kinetic results and the electronic approaches; in fact, maximum hardness principle (MHP), Chattaraj’s polar model, Houk’s rule and the Gazquez–Mendez rule confirm the resultant regioselectivity based on the calculated activation energies. Graphical AbstractThe mechanism and regioselectivity of 1,3-dipolar cycloaddition reactions were examined. Based on several theoretical approaches, all reactions were found to have NED character. These reactions can take place with considerable rates at room temperature. The kinetic results and electronic approaches are in good agreement.

Electronic, optical and photocatalytic properties of three-layer perovskite Dion–Jacobson phase CsBa2M3O10 (M = Ta, Nb): a DFT study

Structure, electronic and optical properties have been studied to consider the photocatalytic activity of the three-layer perovskite Dion–Jacobson phase CsBa2M3O10 (M = Ta, Nb) using the density functional theory (DFT) method with the plane-wave pseudo-potential in the frame of the GGA proposed by Perdew, Burke, and Ernzerhof (PBE). CsBa2Ta3O10 and CsBa2Nb3O10 are indirect band gap semiconductors with different types of M–O bonds, from pure covalent to pure ionic. According to the symmetry breaking of MO6 octahedra from the Oh point group to centrosymmetric (D4h) and non-centrosymmetric (C4v) point groups, the generation of local internal fields, the promotion of electron–hole pairs separation in the very initial process of photo-oxidation and the improvement in photocatalytic activity are suggested. Site and angular projected densities of state for CsBa2M3O10 show that the valence and conduction bands are made mainly of O and M orbitals, therefore the presence of a dipole moment at the MO6 octahedra will play an important role in separating electrons and holes. According to the optical properties simulation, the origin of interband transitions for CsBa2Ta3O10 and CsBa2Nb3O10 are from O-p and M-d orbitals. CsBa2Ta3O10 and CsBa2Nb3O10 are UV-light-responsive photocatalysts with plasmon energies of 13.16 and 12.07 eV, respectively, which can possibly oxidize/decompose organic pollutants and generate hydrogen from water splitting.

DFT STUDY OF HYDROGEN STORAGE ON Li- AND Na-DOPED C59B HETEROFULLERENE

Effect of light alkali metal (Li and Na) decorated on the C59B heterofullerene for hydrogen storage is considered using DFT-MPW1PW91 method. Results show that Li and Na atoms strongly prefer to adsorb on top of five-member and six-member ring where a carbon atom is replaced by a boron atom. Significant charge transfer from the alkali metal to the C59B compensates for the electron deficiency of C59B and makes the latter aromatic in nature. Corrected binding energies of hydrogen molecule on the alkali-doped C59B using counterpoise method, structural properties and NBO analysis indicate that first hydrogen molecule is adsorbed physically and does not support minimal conditions of DOE requirement. Finally, positive values of binding energies for the adsorption of a second hydrogen molecule show that alkali doped C59B are capable of storing a maximum of one hydrogen molecule.

NBO and NICS analysis of the allylic rearrangements (the Cope and 3-aza-Cope rearrangements) of hexa-1,5-diene and N-vinylprop-2-en-1-amine: A DFT study

AbstractIn this work, ab initio density functional theory (DFT) calculations have been performed on the 3,3-sigmatropic rearrangements of hexa-1,5-diene (Cope) and N-vinylprop-2-en-1-amine (3-aza-Cope) in the gas phase. The barrier heights and heats of reactions calculated at the B3LYP/6-311G** level of theory were in good agreement with experimental data. Transition states optimized with B3LYP/6-311G** theory were used for calculating the nucleus independent chemical shift (NICS) and, a natural bond orbital (NBO) analysis was also performed at the same level of theory. Our results indicate that the aromaticities of the transition states are controlled by the out-of-plane component and that the chair-like transition state of the Cope rearrangement exhibits the strongest aromatic character. Analysis of donor-acceptor (bonding and anti-bonding) interactions of σ3–4 → π*1–2 suggests that the TS structure in the hexa-1,5-diene reaction (the Cope rearrangement) has more aromatic character than the N-vinylprop-2-en-1-amine reaction (the 3-aza-Cope rearrangement). The NBO results show that in the hexa-1,5-diene and N-vinylprop-2-en-1-amine rearrangements, activation energies are controlled by σ3–4 → π*1–2 and σ3–4 → π*1–2 resonance energies.

Following the Molecular Mechanism of Decarbonylation of Unsaturated Cyclic Ketones Using Bonding Evolution Theory Coupled with NCI Analysis

The synergetic use of bonding evolution theory (BET) and noncovalent interaction (NCI) analysis allows to obtain new insight into the bond breaking/forming processes and electron redistribution along the reaction path to understand the molecular mechanism of a reaction and recognize regions of strong and weak electron pairing. This viewpoint has been considered for cheletropic extrusion of CO from unsaturated cyclic ketones cyclohepta-3,5-dien-1-one CHD, cyclopent-3-en-1-one CPE, and bicyclo[2.2.1]hept-2-en-7-one BCH by using hybrid functional MPWB1K in conjugation with aug-cc-pVTZ basis set. Decarbonylation of CHD, CPE, and BCH are nonpolar cyclo-elimination reactions that are characterized by the sequence of turning points (TPs) as CHD, 1-11-C[CC]C†C†FFFTSC†C†C†-0:HT + CO; CPE, 1-8-CC[C†C†F†][FF][FF]FTS[C†C†]-0:BD + CO; and BCH, 1-8-CC[C†C†]F[FF]FTS[C†C†]-0:CD + CO. Breaking of C-C bond between the terminal carbon atoms of diene/triene framework and carbon atom of CO fragment starts at a distance of ca. 1.9-2.0 Å in the vicinity of the transition structure where the transition states are not reached yet. NCI analysis explains that the noncovalent interactions between two fragments appeared after the breaking of C-C bonds.

A DFT Study of NBO and NICS Analysis of the Allylic Rearrangements (the Claisen and Thio-Claisen Rearrangements) of 3-(Vinyloxy)prop-1-ene and Allyl Vinyl Sulfide

Abstract Ab initio density functional theory (DFT) calculations have been performed on the 3,3-sigmatropic rearrangements of 3-(vinyloxy)prop-1-ene (Claisen) and allyl vinyl sulfide (thio-Claisen) in the gas phase. The barrier height of the Claisen rearrangement calculated at the B3LYP/6-311G** level of theory was in good agreement with the corresponding experimental value. Optimized transition states at the B3LYP/6-311G** level were used for calculating of nucleus independent chemical shift (NICS) and also natural bond orbital (NBO) analysis at the same level. Our results indicate that aromaticities of the transition states are controlled by the out-of-plane component and that the strongest aromatic character is for the chair-like transition state of the thio-Claisen rearrangement. Analysis of donor-acceptor (bonding and antibonding) interactions suggests that the aromatic character of TS structure in the allyl vinyl sulfide reaction (the thio-Claisen rearrangement) is more than the 3-(vinyloxy)prop-1-ene reaction (the Claisen rearrangement). The NBO results show that in these rearrangements, activation energies are controlled by resonance energies. GRAPHICAL ABSTRACT

Understanding the kinetics of thermal decomposition of 2,3-epoxy-2,3-dimethylbutane using RRKM theory

The thermal decomposition kinetics of 2,3-epoxy-2,3-dimethylbutane have been studied computationally using density functional theory, along with various exchange–correlation functionals and an extremely large basis set. The calculated energy profiles have been supplemented with calculations of kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been supplied, using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been supplied, using transition state and RRKM theories. By comparison with experiment, all our calculations indicate that, from a kinetic viewpoint, the most favorable process is thermal decomposition of 2,3-epoxy-2,3-dimethylbutane into the 2,3-dimethylbut-3-en-2-ol, whereas under thermodynamic control of the reactions, the most abundant product derived from the 2,3-epoxy-2,3-dimethylbutane species will be the 3,3-dimethylbutan-2-one species. The regioselectivity of the decomposition decreases with increasing temperatures and decreasing pressures. In line with rather larger energy barriers, pressures larger than 10−6 bar are in general sufficient for ensuring a saturation of the computed unimolecular kinetic rate constants compared with the high-pressure limit (TST) of the RRKM unimolecular rate constants. The bonding evolution theory indicated that thermal decomposition of 2,3-epoxy-2,3-dimethylbutane into the 2,3-dimethylbut-3-en-2-ol takes place along three differentiated successive structural stability domains after passing the reactant from the associated transition state.

A computational investigation of carbon-doped beryllium monoxide nanotubes

To investigate the influence of C-doping on the electrostatic structure properties in the frame work of density functional theory (DFT), we considered beryllium monoxide nanotubes (BeONTs), consisting of 60 Be and 60 O atoms. Full geometry optimizations are performed for all structures, i.e., all atoms are allowed to relax. Afterwards, the chemical shielding (CS) tensors are calculated for Be-9, O-17 and C-13 nuclei in the C-doped forms and also pristine models of the (10, 0) zigzag and (5, 5) armchair BeONTs. Formation energies indicate that C-doping of Be atom (CBe form) could be more favorable than C-doping of O atom (CO form) in both zigzag and armchair BeONTs. Gap energies and dipole moments detected the effects of dopant in the (5, 5) armchair models; however, those parameters did not indicate any significant changes in the C-doped (10, 0) zigzag BeONT models. The results show that the CS values for the Be and O atoms-contributed to the Be-C bonds or those atoms close to the C-doped region-in the CO form of BeONTs (zigzag and armchair) are changed. The same values only for the O atoms-contributed to the O-C bonds- in the CBe form of BeONTs (zigzag and armchair) are changed.

Kinetic and thermodynamic study of the substituent effect on the amino-Claisen rearrangement of para-substituted N-allyl-N-arylamine: a Hammett study via DFT

In order to find the susceptibility of the amino-Claisen rearrangement and the next proton shift reaction of N-allyl-N-arylamine to the substituent effects in the para position, the kinetic and thermodynamic parameters were calculated at the B3LYP level using the 6-31G** basis set. The calculated activation energies for the rearrangements and proton shift reactions are close to 44.4 and 49.5 kcal mol− 1, respectively. The transition states of the rearrangement with electron-donor substituents are more stable than those with electron-withdrawing substituent groups, but for the proton shift reaction, this situation is reversed (with the exception of fluorine atom for the rearrangement and fluorine and chlorine atoms for the proton shift reaction). Negative values for the activation entropy confirm the concerted mechanism for the amino-Claisen rearrangement and proton shift reaction. The Hammett ρ values of − 2.4172 and − 1.7791 are obtained for σp and σ− (enhanced sigma) in the amino-Claisen rearrangement, respectively. The correlation between log(k X/k H) and σp is weaker than that with σ− (enhanced sigma). A negative Hammett ρ value indicates that the electron-donating groups slightly increase the rate of amino-Claisen rearrangement. A positive Hammett ρ value (2.4921) for the proton shift reaction indicates that electron-withdrawing groups increase the rate of reaction.