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The 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic polychlorinated dibenzo-p-dioxin. The OH –initiated oxidation of TCDD has been studied using the density functional, canonical transition state, and canonical Rice-Ramsperger-Kassel-Marcus theories. The kinetic data were corrected for quantum tunneling by the Wigner and Eckart models. All OH addition and hydrogen atom abstraction channels were thermodynamically exergonic. The kinetic and thermodynamic data analysis at the reliable level MPWB1K/MG3S//M06-2X/MG3S indicate that the addition of OH to the carbon atom adjacent to the oxygen atom in dioxin ring leads to the formation of predominant adduct. The calculated bimolecular rate constant for the formation of predominant adduct was ~5.97–6.75 × 10−13 cm3 molecule−1 s−1, its branching ratio was ~0.955, and the overall rate constant for the OH –initiated oxidation of TCDD was ~6.25–7.08 × 10−13 cm3 molecule−1 s−1. The atmospheric lifetime of TCDD determined by OH was ~8.17–9.26 days indicating the TCDD can be categorized as medium lifetime organic pollutant.

The gas-phase β-elimination kinetics of 2,2-difluoroethyltrifluorosilane (1), 2,2-difluoroethylmethyldifluorosilane (2), 2,2-difluoroethyldimethylfluorosilane (3), and 2,2-difluoroethyltrimethylsilane (4) have been investigated computationally using M06-2x exchange-correlation functional as well as the benchmark CBS-QB3 quantum chemical approach. The obtained energy profile has been enhanced with kinetic calculations using statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory and transition state theory (TST). The calculated results are in good agreement with the available experimental data which obtained by the CBS-QB3 approach. The comparison between all our calculations and experiments indicates that a thermodynamically-controlled reaction that gives more stable products derived from the compound 2 species will be the vinyl fluoride and methyltrifluorosilane species, whereas the elimination of compound 1 into the vinyl fluoride and silicon tetrafluoride species is favorable process from kinetic point of view. In proportion to rather larger barrier heights, pressures where P > 10―4 bar are insufficient to ensure a saturation of the calculated rate constant compared with the RRKM unimolecular rate kinetics (in high-pressure limit). Natural bond orbital analysis revealed that in accordance with an increase of barrier height from compounds 1 to 4, the HOMO-LUMO energy-gaps decreases. Furthermore, the obtained order of barrier heights could be explained by the number of electron-withdrawing fluorine atoms attached to the silicon atom. The occupancies of σC1―F3 bonding orbital for the studied compounds are as follows: 1>2>3>4 and those of σ*C1―F3 antibonding orbital increase in the opposite order (4>3>2>1) by NBO analysis. This fact explains a comparatively easier elimination of the σC1―F3 bond in compound 1 compared to the other compounds. The calculated data reveal that the polarization of the C1―F3 bond in the sense C1δ+–F3δ− is the determining factor in the elimination reaction of the studied compounds.

The approach presented here is an unprecedented insight into the understanding of kinetics and molecular mechanism of thermal Curtius rearrangement of 3-oxocyclobutane-1-carbonyl azide. Curtius rearrangement can proceed via concerted and stepwise mechanisms. The CBS-QB3 and CBS-APNO composite methods indicated that concerted pathway is dominant and 104–105 times faster than stepwise path. The bonding evolution theory analysis at the B3LYP/6-311G(d,p) revealed that the reaction via concerted pathway can be described with catastrophe sequence 9-CF†C†TSFC†FC†C-0 by the following chemical events: (a) change of topological signature of N2 N3 bond; (b) increasing the number of non-bonding monosynaptic attractor on N1 atom; (c) breaking of N1 N2 bond and extrusion of nitrogen molecule; (d) decreasing the number of non-bonding monosynaptic attractors on N1 atom; (e) breaking of C4 C5 bond and formation of pseudoradical centers on the C4 and C5 atoms; (f) annihilation of pseudoradical center on the C5 atom; (g) change of topological signature of N1 C5 bond; and (h) formation of N1 C4 bond. Along the reaction course electron flow redistribution is asynchronous and bond breaking/forming do not takes place simultaneously demonstrating that the reaction is concerted yet highly asynchronous process.

The crystal structures, electro–optical properties, and charge carrier effective masses of Cs 2 TeW 3 O 12 and Cs 2 TeMo 3 O 12 with hexagonal, polar and non–centrosymmetric crystal structure were investigated based on density functional theory. Cs 2 TeW 3 O 12 and Cs 2 TeMo 3 O 12 are found to be indirect K (1/3, 1/3, 0) → G (0, 0, 0) band gap semiconductors ( E g > 3 eV) with small effective masses of photogenerated charge carriers. The mixing of octahedrally coordinated d ° transition metal cations (W 6+ and Mo 6+ ) with the filled p orbitals of the oxygen ligands leads to the formation of some W 5+ /Mo 5+ sites and splitting of d orbitals into the partially filled t 2g ( dxy , dyz , and dzx ) orbitals and empty e g ( dz 2 and dx 2 – y 2 ) orbitals. The top of the valence bond is mainly contributed by O 2 p orbital of the oxygen ligands mixed with the partially filled t 2g orbitals of W 5 d /Mo 4 d , while the conduction band mainly consists of empty e g orbitals of W 5 d /Mo 4 d with a little contribution of O 2 p orbitals. The dielectric function exhibits a slight anisotropic behavior and optical absorption peak for Cs 2 TeW 3 O 12 and Cs 2 TeMo 3 O 12 belonging to the strong electronic transition O 2 p → W 5 d /Mo 4 d within the octahedral units. According to the estimated valence band and conduction band edges, Cs 2 TeW 3 O 12 and Cs 2 TeMo 3 O 12 can be applied as visible−light−responsive photocatalysts for the decomposition of organic pollutants and dye molecules. Also, Cs 2 TeMo 3 O 12 can be used in water splitting for hydrogen generation but Cs 2 TeW 3 O 12 requires further experimental studies to confirm its ability for water splitting.

The thermal decomposition kinetics of 2-chloroethylsilane and derivatives in the gas phase has been studied computationally using density functional theory, along with various exchange-correlation functionals (UM06-2x and ωB97XD) and the aug-cc-pVTZ basis set. The calculated energy profile has been supplemented with calculations of kinetic rate constants under atmospheric pressure and in the fall-off regime, using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Activation energies and rate constants obtained using the UM06-2x/aug-cc-pVTZ approach are in good agreement with the experimental data. The decomposition of 2-chloroethyltriethylsilane species into the related products [C2H4 + Et3SiCl] is characterized by 6 successive structural stability domains associated to the sequence of catastrophes C8H19SiCl: 6-C†FCC†[FF]-0: C6H15SiCl + C2H4. Breaking of Si–C bonds and formation of Si–Cl bonds occur in the vicinity of the transition state. All calculations presented in this work have been performed at the Flemish Supercomputer Center (Vlaams Supercomputer Centrum). This cluster has been financed by budgets obtained from the Katholieke Universiteit Leuven, as well as from individual contributions by users, and funding obtained from the Hercules foundation and the Flemish government.

The kinetics and mechanisms of pyrolysis of 1,3-dihydroisothianaphthene-2,2-dioxide toward benzocyclobutene have been theoretically studied using canonical transition state theory (CTST), statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and bonding evolution theory (BET) in conjugation with M06-2X/aug-cc-pVTZ calculations. The CTST slightly breaks down to estimate the reaction rate of the cheletropic extrusion. RRKM results indicated that the cheletropic extrusion and electrocyclic reaction require energy barriers of 171.3 and 122.2 kJ/mol to be overcome; and can be characterized respectively by 7 and 3 phases associated to the sequence of catastrophes C 8 H 8 SO 2 ( 1 ): 7-[FF]C † C † FFF-0: C 8 H 8 + SO 2 and C 8 H 8 ( 2 ): 3-[F † F † ]C-0: C 8 H 8 ( 3 ). For the cheletropic extrusion, breaking of the C 7 –S and C 8 –S bonds begins respectively at Rx = −2.7434 amu 1/2 Bohr and Rx = −1.7458 amu 1/2 Bohr, and formation of the sulfur dioxide is completed at Rx = −0.2494 amu 1/2 Bohr. For the electrocyclic reaction, formation of new C 7 –C 8 bond occurs at Rx = 1.6214 amu 1/2 Bohr from C- to C- coupling between the generated pseudoradical centers at Rx = 0.1474 amu 1/2 Bohr on the terminal carbon atoms.

To expand the family of transition metal oxynitride perovskites, the two-step synthesis of a series of tungsten-based metal oxynitride perovskites (EuW(O,N)3, NdW(O,N)3, PrW(O,N)3, LaW(O,N)3, and SrW(O,N)3) and their visible-light-driven photocatalytic water oxidation activity with the assistance of CoOx (2 wt% Co) cocatalyst were studied in this work. The XRD results revealed that the cubic perovskite LnW(O,N)3 (Ln = Pr, Nd, and Eu) and SrW(O,N)3 phases and tetragonal perovskite LaW(O,N)3 phase were successfully synthesized by nitriding their corresponding oxide precursors at 900 °C for 10–25 h under an NH3 flow, with minor secondary phases in only PrW(O,N)3 and NdW(O,N)3. The highly porous structures of EuW(O,N)3, LaW(O,N)3, and SrW(O,N)3 were formed from the segregation of nanocrystals with average sizes of 140, 92, and 160 nm, respectively. The surfaces of the NdW(O,N)3 and PrW(O,N)3 crystal structures were covered with plate-like crystals which can be identified as W5N4. No clear absorption edges were observed in the UV–Vis diffuse reflectance spectra of the tungsten-based metal oxynitrides owing to the extensive amount of reduced tungsten species (W5+ and W4+) or metallic tungsten and anion deficiency. Within 5 h of the photocatalytic water oxidation half-reaction, the CoOx-loaded SrW(O,N)3 crystal structures exhibited the highest photostability and O2 evolution rate of 3.3 μmol h−1 compared with CoOx-loaded LnW(O,N)3 (Ln = La, Pr, Nd, and Eu) crystal structures due possibly to the highest O/N ratio and more positively positioned top of valence band of SrW(O,N)3. The present work is expected to stimulate research into the development of more stable and efficient tungsten-based metal oxynitride perovskites in the future.

A theoretical study of the thermal decomposition of β-propiolactone is carried out using ab initio molecular orbital (MO) methods at the MP2/6-311+G ∗∗ level and Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The reported experimental results showed that decomposition of β-propiolactone occurred by three competing homogeneous and first order reactions. For the three reactions, the calculation was also performed at the MP2/6-311+G ∗∗ level of theory, as well as by single-point calculations at the B3LYP/6-311+G ∗∗ //MP2/6-311+G ∗∗ , and MP4/6-311+G ∗∗ //MP2/6-311+G ∗∗ levels of theory. The fall-off pressures for the decomposition in these reactions are found to be 2.415, 9.423 × 10 −2 and 3.676 × 10 −3 mmHg, respectively.

The electronic, optical, and photocatalytic properties of ribbon-like Bi 2 S 3 have been investigated at different external applied pressures 0–9.18 GPa using the density functional theory (DFT) method within the generalized gradient approximation. The electronic structures and optical properties indicate that ribbon-like Bi 2 S 3 is p-type semiconductor with indirect band gap. The photocatalytic activity of ribbon-like Bi 2 S 3 decreases with increasing in the hydrostatic pressure. Based on band structure and density of states (DOS) calculations the conduction and valence bands of ribbon-like Bi 2 S 3 are mostly derived from Bi 6p and S 3p states. The charge density distribution indicates that the nature of Bi–S bonds is covalent. The calculated optical properties indicate that ribbon-like Bi 2 S 3 is ultra violet light response photocatalyst. Calculated position of valence band and conduction band potentials indicate that ribbon-like Bi 2 S 3 has strong oxidation ability for water splitting into O 2 , but for water splitting into H 2 and subsequently increasing of photocatalytic activity co-catalyst should be added.

The pressure dependencies of structural properties, stability, mechanical properties, mechanical anisotropic, linear compressibility, Debye temperature, minimum thermal conductivity and thermal expansion coefficient of the orthorhombic Bi 2 S 3 in the Pnma structure have been investigated using density functional theory. All calculated properties are in excellent agreement with experimental results, which imply the reliability of the present calculation method. The obtained elastic constants satisfying that the Bi 2 S 3 crystals are mechanically stable up to 9.18 GPa and its hardness is improved under compression. The surface constructions and planar contours of bulk and Young’s moduli at (1 0 0), (0 1 0) and (0 0 1) crystal planes indicate that bulk modulus is more isotropic than Young’s modulus, and anisotropies in both moduli decrease under compression. Furthermore sound velocity, Debye temperature and minimum thermal conductivity are found to be increasing with pressure. The thermal expansion coefficient of Bi 2 S 3 has a strong pressure dependence and its thermal conductivities are extraordinary low which demonstrate its technological application as novel thermal barrier coating materials. Unfortunately, there is currently no experimental measurements of elastic constants and other related properties for comparison.

The nonequilibrium Green׳s function technique combined with density functional theory were used to investigate the transport properties of 1,2-bis(5-methyl-[2,2′-bithiophen]-4-yl)cyclopent-1-ene optical molecular switch. Both of its closed and open forms have two S-linkers and translated into the Gold junction with the (1 1 1) surfaces. I–V characteristics, differential conductance, on-off ratio, electronic transmission coefficients, spatial distribution of molecular projected self-consistent Hamiltonian (MPSH) orbitals and projected of the density of states spectrums corresponding to the closed and open forms have been calculated and analyzed. The influences of the delocalization degree of MPSH states in the bias window and coupling degree between molecule orbitals and electrodes levels, on the electronic transport of two systems were discussed in detail. Meantime, larger current through the closed form and negative differential resistance behavior were observed and considered.

Adsorption of nitrogen dioxide on the exterior surface of C30B15N15 through B atom of adsorbent is studied using the DFT-B3LYP/6-31G* level. Independent to the orientation, the NO2 molecule presents strong chemisorption with large charge transfer from C30B15N15 to adsorbed gas. NBO (natural electron configuration of adsorbing B atom), AIM analysis and spin density distributions indicate that the chemisorption of NO2 on the C30B15N15 in all studied complexes are covalent in nature.

The nonequilibrium Green's function approach in combination with density-functional theory is used to perform quantum mechanical calculations of the electron transport properties of furan and thiophene dimmers. Both the molecular systems have two S-linker and translated into the Gold junction with (1 1 1) surfaces. The studied molecular junctions at zero bias voltage are HOMO-based junctions and currents through these systems are driven by hole transport. The current–voltage characteristics of the both studied molecular junctions illustrate that negative differential resistance (NDR) feature is observed over the bias voltage of 2.0 V. Higher conductivity of fused furan dimmer and NDR character have been explained by the monitoring of the transmission resonance peak across the bias window against varying bias voltages.

The interaction between zigzag BNNTs with chirality index n=3–10 and ammonia has been studied at the level of B3LYP/6-31G⁎. Ammonia can be chemically adsorbed on (3,0) to (7,0) BNNTs and physically adsorbed on other studied BNNTs. From NBO analysis charge transfer occur from NH3 to BNNTs and change in the natural electron configuration of B atom of BNNTs at adsorption site for the (3,0) and (4,0) BNNTs cases is larger than others. The DOS result show that after functionalization of BNNTs with NH3 molecules electronic properties of tubes are largely preserved and can be viewed as some kind of harmless modification. Electronic analysis revealed that the interaction of zigzag BNNTs with ammonia is more electrostatic (ionic) in nature, rather than the sole covalent and electrostatic nature increased with increasing of tube diameter.

Theoretical study of exohedral chemical functionalization of C48B6N6 with NH3 molecules has been investigated using DFT. It was found that NH3 molecule can be chemically adsorbed on boron sites of C48B6N6, with a charge transfer from NH3 to C48B6N6. Adsorption energy and the quantity of electron charge transfer from latest adsorbed ammonia to C48B6N6 decreased with increasing in the adsorbed NH3 molecules. Despite the strong adsorption energies, electronic properties of C48B6N6 is preserved after modification(s) with NH3 molecule(s) and chemical modification of C48B6N6 with NH3 molecules can be viewed as some kind of safe modification.

The electronic structures and physical properties of zigzag BC2N (n,0; n = 4–10) and armchair BC2N (n,m; n = m = 4–10) nanotubes (type III) are studied by using density functional theory with the generalized gradient approximation. From a comparison of the binding energies, it is inferred that in the large diameter BC2N nanotubes, the zigzag form is thermally more stable than the armchair form. BC2N nanotubes (with the exception of (4,0) which is conductor) are gapless semiconductors. Depend on the chirality index, the zigzag forms of BC2N nanotubes have narrower band gap than the armchair form. Semiconductor character in the studied BC2N nanotubes is due to contribution of p electrons in the Fermi level. Mulliken population analyses show that significant amounts of electron charge are transferred between atoms; which suggests the existence of polar covalent bonds in the BC2N nanotubes.

The electron transport properties of furan, thiophene and selenophene dithiols based molecular wires through two electrodic systems using non-equilibrium Green’s functions technique (NEGF) are investigated. The electron transport of the above systems is systematically studied by analysis of transmission function, density of states, current–voltage characteristics, and conductance of the systems. The maximum current is occurred at the vicinity of 2.0 V and the values are 90.37, 98.82 and 100.31 μA for furan, thiophene and selenophene dithiols, respectively. These results can be attributed to the molecular projected self consistent Hamiltonian (MPSH) of two electrodic systems with different molecules at different bias voltage and also to quality of resonance of π electrons of heterocyclic ring. We can foresee that the furan, thiophene, and selenophene dithiols can be applied at electronic devices because of switching the high and low current.

Adsorption of NH 3 and NO 2 molecules on the external surface of C 48 B 6 N 6 heterofullerene is investigated using DFT method. Attachment of NH 3 and NO 2 on C 48 B 6 N 6 heterofullerenes are compared with the bare C 48 B 6 N 6 model optimized at the B3LYP/6-31G ⁎ level. The high surface binding energies indicates that ammonia undergoes chemical adsorption and could be compatible with the long recovery time but C 48 B 6 N 6 should be good NO 2 sensors with quick response as well as short recovery time. Total (TDOS) and partial (PDOS) density of state calculations is also considered to elucidate the difference in the NH 3 and NO 2 gas detection mechanism of C 48 B 6 N 6 . The overlap population density of state (OPDOS) indicated that the chemical adsorption is due to the overlap of atomic orbitals below the Fermi level. The calculated results suggest that the C 48 B 6 N 6 heterofullerene is a suitable sensor material for NO 2 and is an ideal material for elimination and filtering of ammonia.

Electronic and magnetic properties of finite-boron nitride zigzag nanotubes (BNNTs) with n =3–10 are investigated using density functional theory (DFT) calculations. All structures are optimized at the B3LYP/6-31 G ⁎ level, then chemical shielding (CS) and electric field gradient (EFG) tensors are calculated at the B3LYP/6-31 G ⁎⁎ level. Density of state (DOS) diagrams and calculation of the population of conduction electrons indicated that the conduction of BNNTs decreased with the increase in tube radius. In these models, CS tensors and quadrupole coupling constants (QCC) of nitrogen and boron nuclei are clearly sensitive to the nanotube radius.

In nuclear reactor core design, achieving the optimized arrangement of fuel assemblies (FAs) is the most important step towards satisfying safety and economic requirements. In most studies, nuclear fuel optimizations have been performed by using a finite number of different types of FAs. However the effect of FA numbers with different enrichments and the difference between their maximum and minimum enrichment values can be important and should be evaluated in the optimization process. This research is aimed at evaluating the effect of using different enrichment values for FAs. This issue has been investigated by focusing on two parameters, namely, the initially selected enrichment and the difference between the minimum and maximum enrichments applied in the core design. In the previous studies of nuclear fuel management, these parameters have been kept as fixed quantities and considered as initial assumptions in the optimization process. Therefore, to achieve an optimized arrangement of the core, the proper values of these parameters have to be determined. For this purpose a parameter ( δ ) served through the optimization process to show the effect of the difference between the enrichment values of FAs. Another parameter named e 0 shows the minimum enrichment of FAs. These parameters are defined based on a factor named Fuel Quality Factor (FQF) as a characteristic of fuel composition. FQF is shown by Z ( r ) is also used through the optimization process for achieving the smooth distribution of power. The values of Z ( r ) are calculated using the MCNP code. This methodology is applied to a VVER-1000 nuclear reactor core in order to minimize the local power peaking factor ( P q ). For finding the best configuration of FAs in the core, Cellular Automata (CA) is applied as a powerful and reliable tool. The computer codes WIMS and CITATION are used for core calculations. The results provide a comprehensive view of VVER-1000 reactor core configuration for different groups of δ and e 0 and provide a useful database for designers and engineers.

Ammonia adsorption on the external surface of C 30 B 15 N 15 heterofullerene was studied using density functional calculations. Three models of the ammonia-attached C 30 B 15 N 15 together with the perfect model were optimized at the B3LYP/6-31G ⁎ level. The optimization process reveals that dramatic influences occurred for the geometrical structure of C 30 B 15 N 15 after ammonia adsorption; the B atom relaxes outwardly and consequently the heterofullerene distorts from the spherical form in the adsorption sites. The chemical shielding (CS) tensors and nuclear quadrupole coupling constants of B and N nuclei were calculated at the B3LYP/6-311G ** level. Our calculations reveal that the B atom is chemically bonded to NH 3 molecule. The B atom in the NH 3 -attached form has the largest chemical shielding isotropic (CSI) value among the other boron nuclei. The C Q parameters of B nuclei at the interaction sites are significantly decreased after ammonia adsorption.

Two models of (10, 0) boron nitride nanotubes (BNNTs), perfect and Ammonia-attached, were studied in order to evaluate the influence of NH 3 -attaching on the B-11 and N-15 nuclear magnetic resonance in the (10, 0) boron-nitride nanotube (BNNT) for the first time. At first, based on density functional theory (DFT) each of the structures was optimized using B3LYP/6-31G (d) model chemistry. At the next step, the chemical-shielding (CS) tensors were calculated using the B3LYP/6-31G (d, p) level of theory in both of the relaxed forms and were converted to experimentally measurable nuclear magnetic resonance (NMR) parameters, i.e. chemical-shielding isotropic (CSI) and chemical-shielding anisotropic (CSA). Our calculation revealed that in the NH 3 -attached BNNT (the most stable model) the B atom chemically bonded to the NH 3 molecule has the largest chemical-shielding isotropic (CSI) and the smallest chemical-shielding anisotropic (CSA) values among the other boron nuclei. Additionally, the NMR parameters of those nuclei directly bonded to the boron dramatically change while those of the other B nuclei remain almost unchanged.

The NBO (natural bond orbital analysis), NICS (nucleus independent chemical shift) and 14N NQR parameters of the most stable tautomers of 5-methylcytosine in the gas phase were predicted using density functional theory (DFT) method. The NBO analysis revealed that the resonance interaction between lone pair of the nitrogen atom and empty (σ* or π*) non-Lewis NBO increases with increasing the p character of the nitrogen lone pair. The present investigation indicated the π clouds in the considered heterocyclic ring contained six electrons, and these tautomers have the aromatic character. The NICS study utilizing the gauge-invariant atomic orbital (GIAO) method showed that there are diatropic currents in the heterocyclic rings of the tautomers, so we determined the order of aromaticity of these tautomers. The results of NQR parameter calculations showed three parameters are effective on nuclear quadrupole coupling constant; the p character value of lone pair electrons of nitrogens, and the related occupancies and the portion of lone pair electrons interfering in magnetic currents of ring and whenever, the lone pair electrons of nitrogens participate in the formation of chemical bond and/or π system of the ring, the qzz and consequently its χ decreases.

The structure and energies of the tautomers of 5-methylcytosine in gas phase were predicted using Density Functional Theory (DFT) method. Solvent-induced effects on stability and 15N NMR shielding on the most stable tautomers of 5-methylcytosine were calculated using DFT combined with the polarizable continuum model (PCM) and using the gauge-invariant atomic orbitals (GIAO). In a wide range of solvent dielectrics, the 1-H-oxo-amino form (T6) is predicted as the most stable tautomer and the total electronic energy values of the more stable tautomers in the liquid phase decrease with an increase in the dielectric constant. Direct and indirect solvent effects on 15N NMR shielding of the pyrimidine ring of three dominant tautomers are also calculated. It has been shown that in trivalent nitrogens, the observed solvent-induced shielding variation is more strongly related to the intensity of the solvent reaction field rather than on the change of molecular geometry induced by the solvent.