Changes In Bond Lengths Research Articles

Computational studies on the rotamers of 4-aryl-2-oxo-1,2,3,4-tetrahydropyrimidines and 4-aryl-2-oxo-1,2-dihydropyrimidines

The potential energy surface of 2-oxo-1,2,3,4-tetrahydropyrimidines (THPMs) is explored by density functional theory at the B3LYP level with a 6-31++G** basis set. All minimum and maximum points display a flat boat conformation of the heterocyclic ring in which the C4-aryl ring occupies a pseudo-axial position. In order to elucidate the comparative effect of the space orientation of the C4-aryl ring or the 5-CO group towards the heterocyclic ring on the total energy content of THPM and also the changes in some characteristic bond lengths and angles, these groups are rotated around the C4—C11 and C5—C7 single bonds, respectively. The results indicate an effect of the orientation of the 5-CO group on the investigated parameters compared to that of the C4-aryl ring. Natural bond orbital (NBO) analysis is carried out to investigate the effect of various intramolecular interactions and also intermolecular interactions with a medium in the calculation on the stabilization energy compared to those in the gas phase calculation. The results indicate that the dipole moments and also the charge densities on the specific atoms are effectively changed by the orientation of the C4-aryl ring and the 5-CO group towards the heterocyclic ring. In addition, bond rotations around the C4—C11 and C5—C7 single bonds in the oxidized compounds 2-oxo-1,2-dihydropyrimidines (DHPMs) are also carried out to compare the effects of these changes with those in THPMs.

Neutral and cationic phosphine and arsine complexes of tin(iv) halides: synthesis, properties, structures and anion influence.

The reaction of trans-[SnCl4(PR3)2] (R = Me or Et) with trimethylsilyltriflate (TMSOTf) in CH2Cl2 solution substitutes one chloride to form [SnCl3(PR3)2(OTf)]; addition of excess TMSOTf does not substitute further chlorides. The complexes have been fully characterised by microanalysis, IR and multinuclear NMR (1H, 13C{1H}, 19F{1H}, 31P{1H}, 119Sn) spectroscopy. The crystal structure of [SnCl3(PMe3)2(OTf)] revealed mer-chlorines and trans-phosphines. In contrast, trans-[SnBr4(PR3)2], [SnCl4{Et2P(CH2)2PEt2}], [SnCl4{o-C6H4(PMe2)2}] and [SnCl4{o-C6H4(AsMe2)2}] did not react with TMSOTf in CH2Cl2 solution even after 3 days. The arsine complexes, [SnX4(AsEt3)2] (X = Cl, Br), were confirmed as trans-isomers by similar spectroscopic and structural studies, while attempts to isolate [SnI4(AsEt3)2] were unsuccessful and reaction of SnX4 with SbR3 (R = Et, iPr) resulted in reduction to SnX2 and formation of R3SbX2. trans-[SnCl4(AsEt3)2] is converted by TMSOTf into [SnCl3(AsEt3)2(OTf)], whose X-ray structure reveals the same geometry found in the phosphine analogues, with the triflate coordinated. The salts, [SnCl3(PEt3)2][AlCl4] and [SnCl2(PEt3)2][AlCl4]2 were made by treatment of [SnCl4(PEt3)2] with one and two mol. equivalents, respectively, of AlCl3 in anhydrous CH2Cl2, whereas reaction of [SnCl4(AsEt3)2] with AlCl3 produced a mixture including Et3AsCl2 and [Et3AsCl][Sn(AsEt3)Cl5] (the latter identified crystallographically). In contrast, using Na[BArF] (BArF = [B{3,5-(CF3)2C6H3}4]-) produced [SnCl3(PEt3)2][BArF] and also allowed clean isolation of the arsine analogue, [SnCl3(AsEt3)2][BArF]. [SnCl4{o-C6H4(PMe2)2}] also reacts with AlCl3 in CH2Cl2 to form [SnCl3{o-C6H4(PMe2)2}][AlCl4] and [SnCl2{o-C6H4(PMe2)2}][AlCl4]2. Multinuclear NMR spectroscopy on the [AlCl4]- salts show that δ31P and δ119Sn move progressively to high frequency on conversion from the neutral complex to the mono- and the di-cations, whilst 1J(119Sn-31P) follow the trend: [SnCl3{o-C6H4(PMe2)2}]+ > [SnCl4{o-C6H4(PMe2)2}] > [SnCl2{o-C6H4(PMe2)2}]2+. DFT studies on selected complexes show only small changes in ligand geometries and bond lengths between the halide and triflate complexes, consistent with the X-ray crystallographic data reported and the HOMO and LUMO energies are relatively unperturbed upon the introduction of (coordinated) triflate, whereas the energies of both are ca. 4 eV lower in the cationic species and reveal significant hybridisation across the pnictine ligands.

On understanding bandgap bowing and optoelectronic quality in Pb–Sn alloy hybrid perovskites

Experimental insights regarding bandgap evolution in hybrid perovskite alloys and the optimal small-bandgap absorber composition desired for next-generation perovskite tandems.

Structural studies of spray pyrolysis synthesized oxygen deficient anatase TiO2 thin films by using X-ray absorption spectroscopy.

The present work focuses on synthesis and X-ray absorption studies of single phase oxygen deficient anatase TiO2 thin films. These films are prepared in a two-step method viz. the synthesis of near stoichiometric anatase TiO2 films using an open atmospheric spray pyrolysis method followed by vacuum annealing at their corresponding synthesis temperatures (Ts = 450 °C, 500 °C) for different time durations (t = 2, 4, 6, 8 hours). XRD and Raman studies of these films ascertained the formation and retention of the anatase phase post annealing, indicating that there was no phase change due to prolonged annealing. Extended X-ray absorption spectra (EXAFS) and X-ray absorption near edge spectra (XANES) revealed the presence of an oxygen vacancy and its effect on the local co-ordination. The co-ordination number obtained from EXAFS analysis revealed that the number density of the oxygen vacancy is higher in the case of thin films synthesized at 450 °C than in the case of the films synthesized at 500 °C. As the oxygen vacancy leads to changes in local co-ordination, which in turn have a profound effect on the pre-edge features of the X-ray absorption spectra (XAS), theoretically simulated XAS spectra of pure anatase TiO2 and oxygen deficient anatase TiO2 were generated using FEFF and were found to match with the experimentally observed spectra. In addition, the ambiguities in whether a change in the metal-oxygen bond length has any effect on the pre-edge features or not were delineated in the present study by comparing the pre-edge peak positions of the oxygen deficient TiO2 films. Our results matched with those of some of the researchers who have studied the rutile phase TiO2, wherein it was concluded that in the case of the titanium-oxygen system, the mean Ti-O bond length does have an effect on the pre-edge peak position. It was observed that as the Ti-O mean bond length increases, the pre-edge peak positions shift towards lower energy, which is in concurrence with the literature available for other Ti-O systems with similar geometry. The second pre-edge peak intensity, which is a measure of disorder, is higher for TiO2450 °C-2 h and TiO2500 °C-2 h thin films. This is attributed to the annealing effect, which suggests that TiO6 octahedrons are the most disordered for thin films annealed for 2 h and become more ordered upon annealing for longer times.

Electronic substituent effect on Se-H⋯N hydrogen bond: A computational study of para-substituted pyridine-SeH2 complexes

Complexes between para-substituted pyridine and SeH2 have been investigated at the MP2/aug-cc-pVTZ level. Various electron donating and withdrawing substituents (-NH2, -OH, -CH3, -H, -F, -Cl, -CN, and -NO2) are chosen in order to characterize their influence on Se-H⋯N intermolecular hydrogen-bonding interaction. The electron donating substituents lead to an increase of the stabilization energy along with elongation in the Se-H bond length and red-shift in Se-H stretching frequency. Conventional electronic substitution effect has been observed on various hydrogen-bond parameters, such as, stabilization energy, change in Se-H bond length and stretching frequency, charge transfer, bond order, electron density at hydrogen-bond critical point.

Evidence for non-thermal microwave effect in processing of tailing-based glass-ceramics

Tailing-based glass-ceramics were crystallized via conventional and microwave heating at 720 °C for 30 min and 820 °C without holding and compared in order to obtain evidence for a non-thermal microwave effect. The comparative analytical results showed that the microstructural uniformity was greatly enhanced and the crystallization activation energy was significantly reduced by microwave processing compared to conventional heating, suggesting accelerated grain growth during crystallization. Microwave radiation affected the crystal orientation and induced corresponding changes in the SiO bond lengths and SiOSi angles, thus enhancing the formation of the diopside crystal structure. In addition, the samples under microwave processing exhibited superior physicochemical performance, including greater relative density, bending strength, microhardness, and resistance to acids and alkali, compared to conventionally processed samples. All these results provided positive evidence supporting the existence of a genuine microwave non-thermal effect, allowing deepened understanding for fine control over microwave-assisted metallurgy.

A proposal based on quantum phenomena for the ORR mechanism on nitrogen-doped carbon-based electrocatalysts

Abstract Several theories have tried to explain the role of nitrogen-doped carbon-based electrocatalysts during the oxygen reduction reaction (ORR). However, there is no consensus about which of the existing theories is most accurate. In this work, we propose a hypothesis presented for the first time, which consists on the hybridization change of carbon atoms in the graphene lattice, from sp2 to sp3. As shown in several studies, carbons with sp3 hybridization can store electrons (i.e. work as capacitors), this phenomenon, along with nitrogen inversion, due to lattice defects, would allow the nitrogen atom to works as a “switch”, enabling or stopping the flow of electrons, generating active sites that enhance the ORR. Computational chemistry was used to prove this hypothesis. A graphene of 92 atoms doped and undoped has been constructed using the Gauss View 5.0® program, the geometries were optimized by DFT in Gaussian 0.9® software, in order to compare changes in bond angles and lengths of the lattices, and map the electrostatic potential distribution (ESP). The results obtained in the measurement of the angles and charges around the doping nitrogen presented interesting results that can help to demonstrate part of the established hypothesis about nitrogen inversion. The study shows that it is possible that this phenomenon increases when there are a greater number of defects in the lattice. Understanding this phenomenon will help to develop and know the best synthesis conditions for doped carbon electrocatalysts.

Investigations on structural, optical and dielectric properties of Mn doped ZnO nanoparticles synthesized by co-precipitation method

Mn doped ZnO (ZnO:Mn) nanopowders with Mn:ZnO ratio from 0 to 3% were successfully synthesized by co-precipitation method. Their structural, optical, dielectric and conducting properties were investigated. All samples crystallize in the hexagonal wurtzite structure. The change in lattice parameters, bond length and preferential crystalline orientation for Mn doped ZnO samples indicates Mn2+ ions incorporation in ZnO lattice. Both micro-strain and dislocation density decrease due to doping whereas the average crystallite size increases suggesting the improvement of crystalline quality. The optical transmission in the visible region was improved as a result of doping. The observed decrease in ac conductivity (σac) with increasing Mn content is associated with the increase of defects amount. Cole-Cole plots of all samples were fitted to a circuit consisting in a parallel combination of a resistance and a constant phase element (CPE).

Influence of intermolecular interactions on molecular geometry and physical quantities in electrolyte systems

ABSTRACTIn this paper, using Fourier transform infrared (FTIR) spectroscopy, ion conductivity measurements and first-principle density functional theory (DFT) calculations, we study intermolecular interactions between three molecules (methyl tetrahydrophthalic anhydride (MeTHPA), succinonitrile (SN) plastic crystal, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt) constituting the lithium-ion battery electrolyte. The C–O stretching band position in MeTHPA shifts to a lower frequency in the order of MeTHPA–SN < MeTHPA < MeTHPA–LiTFSI/SN < MeTHPA–LiTFSI; the average C–O bond length in MeTHPA increases in the same order, which reveals the linear correlation between the vibration frequency shift and bond length change. Furthermore, the lithium ionic conductivities of MeTHPA–LiTFSI/SN and MeTHPA–LiTFSI are consistent with this linear relationship, which confirms that the bond length, vibration frequency and lithium-ion transport are strongly influenced by molecular-level interactions. Our results provide fundamental insights valuable for the understanding of the effect of intermolecular interactions on molecular geometry and physical quantities in different electrolytes, and could be utilized to guide the design of high-performance electrolyte materials.

Dependence of NMR chemical shifts upon CH bond lengths of a methyl group involved in a tetrel bond

Four different Lewis acids that might participate in a tetrel bond with a nucleophile (SEt2Me+, NMe4+, SMe2, NMe3) are examined. The NMR chemical shifts of the methyl C and H atoms are calculated as the CH bond lengths are systematically stretched and contracted, in the absence of a base. The C shielding diminishes by roughly 2 ppm for a stretch of 0.01 Å, while that of H drops by only 0.3 ppm. The deshieldings caused purely by the bond length changes are far too small to account for the amounts that are computed when the nucleophile is actually present.

Tuning the Anomeric Effect in Sulfamide with Superacids.

This study shows that the anomeric effect (negative hyperconjugation) that arises in sulfamide, as a result of the relatively short S-N bonds, can be tuned by the utilization of superacidic media. Sulfamide was reacted in binary superacidic systems XF/MF5 (M=As, Sb; X=H, D) and HF/BF3 . The colorless salts formed, [X2 NSO2 NX3 ]+ [MF6 ]- and [H2 NSO2 NH3 ]+ [BF4 ]- were characterized by low-temperature vibrational spectroscopy. In the case of [H2 NSO2 NH3 ]+ [BF4 ]- , a single crystal X-ray diffraction study was performed. The salt crystallizes in the monoclinic space group P21 /c with four formula units per unit cell. An exclusive N,N'-diprotonation was observed in the superacidic system HF/SbF5 when using several equivalents of the Lewis acid. Low-temperature vibrational spectra as well as a single-crystal X-ray structure of [H3 NSO2 NH3 ]2+ 2 [SbF6 ]- ⋅2 HF are reported. The salt crystallizes in the orthorhombic space group Pna21 with four formula units per unit cell. Upon mono- or diprotonation of sulfamide, remarkable structural changes of the sulfur-nitrogen bond lengths were observed. Herein, these changes are discussed together with quantum chemical calculations.

Investigation of uniaxial and biaxial strains on the band gap modifications of monolayer MoS2 with tight-binding method

In this article a tight-binding model is used to study the changes in crystal structure, hopping parameters, and electronic band gap of semiconductor monolayer MoS2 under uniaxial strain in zigzag (x-) and armchair (y-) directions, and biaxial strain in xy-direction. The change in crystal structure modifies the reciprocal lattice structure, the hopping parameters, and the Hamiltonian matrix elements. So, by determining the nearest neighbor atoms and orbital analyzing, we have calculated the modified hopping parameters and the Hamiltonian matrix elements under various types of strains, up to first-order of corrections due to the changes in the interatomic bond lengths. Our calculations show that by applying the x-direction (y-direction) strain up to 4% (up to 3%) the direct band gap of monolayer MoS2 increases. With further increasing the strain, the direct band gap changes into an indirect band gap and starts to decrease. On the other hand, by applying the biaxial strain, the direct band gap decreases and after a 5% strain the direct band gap changes to an indirect band gap. In this paper, we also demonstrate the shift of the position of the minimum of conduction and the maximum of valence bands due to both uniaxial and biaxial strains.

Predicting the Initial Thermal Decomposition Path of Nitrobenzene Caused by Mode Vibration at Moderate-Low Temperatures: Temperature-Dependent Anti-Stokes Raman Spectra Experiments and First-Principals Calculations.

The lack of understanding of the initial decomposition micromechanism of energetic materials subjected to external stimulation has hindered its safe storage, usage, and development. The initial thermal decomposition path of nitrobenzene triggered by molecular thermal motion is investigated using temperature-dependent anti-Stokes Raman spectra experiments and first-principles calculations to clarify the initial thermal decomposition micromechanism. The experiment shows that the symmetric nitro stretching, antisymmetric nitro stretching, and phenyl ring stretching vibration modes are active as increasing temperature below 500 K. The DFT method is used to examine the effects of the three mode vibrations on the initial decomposition of nitrobenzene by relaxed scan for each relevant change in bond lengths and bond angles to obtain the optimal reaction channel leading to initial thermal decomposition of nitrobenzene. The results demonstrate that the initial thermal decomposition is the isomerization of nitrobenzene to phenyl nitrite. The optimal reaction channel leading to the initial isomerization is the increase or decrease of angle O-N-C from the antisymmetric nitro stretching vibration, which causes the torsion of nitro group and the subsequent oxygen atom attacking carbon atom. The scanning energy barrier related to angle O-N-C is about 62.1 kcal/mol, which is very consistent with the calculated activation barrier of isomerization of nitrobenzene. This proves the reliability of our conclusions.

Bond disproportionation, charge self-regulation, and ligand holes in s−p and in d -electron ABX3 perovskites by density functional theory

Some $AB{X}_{3}$ perovskites exhibit different local environments (DLE) for the same $B$ atoms in the lattice, an effect referred to as disproportionation, distinguishing such compounds from common perovskites that have single local environments (SLE). The basic phenomenology associated with such disproportionation involves the absence of $B$-atom charge ordering, the creation of different B-X bond length (``bond alternation'') for different local environments, the appearance of metal (in SLE) to insulator (in DLE) transitions, and the formation of ligand holes. We point out that this phenomenology is common to a broad range of chemical bonding patterns in $AB{X}_{3}$ compounds, either with s-p electron $B$-metal cations (${\mathrm{BaBiO}}_{3}$, ${\mathrm{CsTlF}}_{3}$) or with noble-metal cations (${\mathrm{CsAuCl}}_{3}$), as well as with $d$-electron cations (${\mathrm{SmNiO}}_{3}$, ${\mathrm{CaFeO}}_{3}$). We show that underlying much of this phenomenology is the ``self-regulating response,'' whereby in strongly bonded metal-ligand systems with high-lying ligand orbitals, the system protects itself from creating highly charged cations by transferring ligand electrons to the metal, thus preserving a nearly constant metal charge in different local environments, while creating $B$-ligand bond alternation and ligand-like conduction band (``ligand hole'' states). We are asking what are the minimal theory ingredients needed to explain the main features of this SLE-to-DLE phenomenology, such as its energetic driving force, bond length changes, possible modifications in charge density, and density of state changes. Using as a guide the lowering of the total energy in DLE relative to SLE, we show that density functional calculations describe this phenomenology across the whole chemical bonding range without resort to special strong correlation effects, beyond what DFT naturally contains. In particular, lower total energy configurations (DLE) naturally develop bond alternation, gapping of the metallic SLE state, and absence of charge ordering with ligand hole formation.

Effect of defect states and oxygen vacancies on optical transitions due to Co2+ substitution in CeO2

CeO2 has cubic fluorite structure which is modified due to oxygen content as well as external substituents. The oxidation state of Ce plays an important role in strain and related physical properties. Ce3+ being larger in size than the Ce4+ ion, one expects a change in band structure due to changes in bond length; substitution of Ce4+ by Co2+ in sol–gel prepared, homogeneous, single-phase Ce1−xCoxO2 (x ≤ 10) nanopowders. The lower valence states of Co2+ induces oxygen vacancies which transforms some Ce4+ to Ce3+. A careful study of oxygen vacancies, strain, bond length and related band structure changes, have been targeted in this study. The possibility of phonon participation in electronic transition has been discussed using Tauc plot. Ce3+ forms defect states, between valence and conduction bands. Lattice parameters decrease, but strain increases with substitution.

Charge-Transfer Salts of 6,6-Dicyanopentafulvenes: From Topology to Charge Separation in Solution.

6,6-Dicyanopentafulvene derivatives and metallocenes with redox potentials appropriate for forming their radical anions form highly persistent donor-acceptor salts. The charge-transfer salts of 2,3,4,5-tetraphenyl-6,6-dicyanofulvene with cobaltocene (1⋅Cp2 Co) and 2,3,4,5-tetrakis(triisopropylsilyl)-6,6-dicyanofulvene with decamethylferrocene (2⋅Fc*) have been prepared. The X-ray structures of the two salts, formed as black plates, were obtained and are discussed herein. Compared with neutral dicyanopentafulvenes, the chromophores in the metallocene salts show substantial changes in bond lengths and torsional angles in the solid state. EPR, NMR, and optical spectroscopy, as well as superconducting quantum interference device (SQUID) measurements, reveal that charge-separation in the crystalline states and in frozen and fluid solutions depends on subtle differences of redox potentials, geometry, and on ion pairing. Whereas 1⋅Cp2 Co reveals paramagnetic character in the crystalline state and in solution, compound 2⋅Fc* shows a delicate balance between para- and diamagnetism, depending on the temperature and solvent characteristics.

ATR-FTIR study of the isosbestic point in water solution of electrolytes

Here we report an ATR-FTIR study on aqueous electrolytic solutions aimed to investigate the nature of the isosbestic point observed in the stretching mode broad band. This feature reveals that the properties of the system are described by the superposition of two components at equilibrium whose relative abundance depends on the running parameter. Here we show that the OH intramolecular bond length changes differently in the two components. It has been observed that, surprisingly, the OH bond length of highly correlated water molecules decreases with the concentration of the solute. On the opposite, it expands in molecules belonging to small cluster. As the shortening of the OH bond shows that the water-water interaction prevails on water-ion interaction, we make the hypothesis that the decrease of the bond length is due to a collective behaviour of the water molecules replacing the classic concept of H-bond in water.

Geometrical and electronic properties of C-doped graphyne-like BN sheets

ABSTRACTIt has been previously indicated that graphyne-like BN (BNyne) sheets have distinct and interesting properties with graphyne and h-BN sheets. Herein, the geometric and electronic properties of C-doped graphyne-like BN (C-BNyne) sheets (including α-, β- and γ-) were systematically investigated by the first-principles calculations. The results reveal that C impurity is more likely to substitute the linear-B and corner-N sites due to the lower formation energy. The C atom incorporation could lead to a change in electron transfer and local bond lengths and a pronounced structural distortion was observed in α C-doped linear-B BNyne and in γ C-doped linear-B BNyne configurations. When a single C impurity atom was doped in linear-B site, it introduced donor levels near the bottom of the conduction band and thus the structures show n-type semiconductor character. As for C-doped corner-N site BNynes, electronic structure calculation showed p-type semiconductor character. Meanwhile, strong spin splitting phenomenon occurs in β C-doped linear-B BNyne and the spin splitting character originated from the impurity C atom and its first neighboring B atom. However, the other doped configurations are nonmagnetic semiconductor.

Temperature evolution of the structure parameters and exchange interactions in BaFe12−xInxO19

Indium doped barium hexaferrites were synthesized by usual ceramic technology. They were investigated by the high resolution neutron powder diffraction and vibration sample magnetometry in temperatures up to 730 K. Analysis of NPD data was performed by FullProf in classical approximation. Structure parameters as well as bond lengths and bond angles were determined versus the substitution level and the temperature. Main attention was paid to the distribution of indium cations between different crystallographic positions and temperature change of bond lengths and bond angles. Effect of these two structure parameters on Fe3+(i) – O− – Fe3+(j) (i, j = 1, 2, 3, 4, 5) indirect superexchange interactions with the substitution level and temperature was established. A dominant effect of a positive intrasublattice Fe3+(i) – O2− – Fe3+(i) (i = 1, 2, 3, 4, 5) indirect superexchange interactions on a magnetic ordering was detected.

Twisted angle effects in the absorption spectra of carbon nanotube

The spectral characteristics of carbon nanotubes is essential for the development of optoelectronic components and sensors with application in many fields. The linear polarizability absorption spectra of the carbon nanotube with a twisted angle have been calculated by using the tight-binding model and sum-over-state method. First, a structural model of carbon nanotubes based on PPP(Pariser-Parr_Pople) was established, then a certain carbon atom was moved to a position where the carbon nanotubes are angled, and the torsional carbon nanotube structure model was calculated based on the change in bond length. Finally, the absorption spectrum at different twisted angles based on SOS(Sum-over-State) was calculated, and then the detailed analysis was made. It is found that the twisted angle has real impact on the absorption spectra of different types carbon nanotubes. The torsional angle leads to changes in the position and number of absorption peaks of the carbon nanotubes. Its influence on absorption spectra of different types of carbon nanotubes is different, but mainly concentrated in the low frequency region. Exactly, altering the twisted angle has an important effect on the absorption spectra of the carbon nanotube. This work offers a guidance of fabricating carbon nanotubes in the applications of photonic device in many fields.