A computational study on the mechanism and the kinetics of urethane formation

The reaction of n-butanol with several aromatic diisocyanates – 2,4 toluene diisocyanate (2,4 TDI), 2,6 toluene diisocyanate (2,6 TDI), 2,4′ dibenzyl diisocyanate (2,4′ DBDI), 4,4′ stilbene diisocyanate, (4,4′ SBDI), m-phenylene diisocyanate (m-PDI), and p-phenylene diisocyanate (p-PDI) – resulting in the production of urethane is studied by using density functional theory (DFT) calculations (B3LYP/6-31 + G(d,p)). The addition of alcohol to isocyanate follows either a concerted or stepwise path. The obtained product differs in two mechanisms. Single point solvent calculations were carried out in benzene, by means of the polarizable continuum model (PCM) at the B3LYP/6-31 + G(d,p) level of theory. The calculated free energy profiles showed that the concerted path is more likely to occur than the stepwise route. Natural bond orbital analysis was performed to provide an insight into charge delocalization and the stabilization energies of the molecules. The calculated rate constant ratios (k₁/k₂) of the two consecutive alcoholysis reactions of aromatic diisocyanates agree strongly with the experimental results.     

Monoheteroatom substituted six-membered carbenes: A computational survey of stability and reactivity

1-(X)-3,3-dimethyl-2-cyclohexanylidenes, where X = amino, oxy, phosphino, and thio, are compared and contrasted to probe the effect of monoheteroatom substitution on the stability, multiplicity and reactivity of six-membered saturated and unsaturated cyclic carbenes. The stabilizing effects of substituents are discussed through geometrical parameters, and AIM analysis. The strong π-donor/σ-acceptor amino group exerts singlet–triplet energy separation (ΔE_S–T) of 38.4 kcal/mol while more electronegative oxy group induces a ΔE_S–T of 23.0 kcal/mol. Aminoalkylcarbenes rebuff dimerization while oxyalkylcarbenes show a high dimerizing affinity. The reactivity of species is discussed in terms of nucleophilicity and electrophilicity indices as well as proton affinities. It seems that our six-membered carbenes are more nucleophilic than common five-membered N-heterocyclic carbenes.     

Gas-phase Smiles rearrangement in structural analysis of a pseudo-oxidative impurity generated in the pharmaceutical synthesis of S-(thiobenzoyl)thioglycolic acid

Several mass spectrometry (MS) techniques including accurate MS and MS/MS, as well as hydrogen/deuterium (H/D) exchange, were utilized to characterize a pseudo-oxidative reaction by-product (impurity I) in the pharmaceutical synthesis of S-(thiobenzoyl)thioglycolic acid. The negative ion MS/MS data provided complementary structural information to the positive ion MS/MS data. An understanding of the gas-phase Smiles rearrangement upon collision-induced dissociation (CID) in the negative ion MS/MS mode played an important role in structural elucidation of impurity I. The theoretical calculations by density functional theory (DFT) at the B3LYP/6-311G(d,p) level provided insights into the thermochemistry of the Smiles rearrangement reaction. This pseudo-oxidative impurity is proposed to be generated via the base-catalyzed hydrolysis in solution.     

Synthesis,molecular structure and urease inhibitory activity of novel bis-Schiff bases of benzyl phenyl ketone: A combined theoretical and experimental approach

BackgroundUrease belongs to the family of amid hydrolases with two nickel atoms in their core structure. On the basis of literature survey, this research work is mainly focused on the study of bis-Schiff base derivatives of benzyl phenyl ketone nucleus.ObjectiveSynthesis of benzyl phenyl ketone based bis-Schiff bases in search of potent urease inhibitors.MethodIn the current work, bis-Schiff bases were synthesized through two steps reaction by reacting benzyl phenyl ketone with excess of hydrazine hydrate in ethanol solvent in the first step to get the desired hydrazone. In last, different substituted aromatic aldehydes were refluxed in catalytic amount of acetic acid with the desired hydrazone to obtain bis-Schiff base derivatives in tremendous yields. Using various spectroscopic techniques including FTIR, HR-ESI-MS, and ¹H NMR spectroscopy were used to clarify the structures of the created bis-Schiff base derivatives.ResultsThe prepared compounds were finally screened for their in-vitro urease inhibition activity. All the synthesized derivatives (3–9) showed excellent to less inhibitory activity when compared with standard thiourea (IC₅₀ = 21.15 ± 0.32 µM). Compounds 3 (IC₅₀ = 22.21 ± 0.42 µM), 4 (IC₅₀ = 26.11 ± 0.22 µM) and 6 (IC₅₀ = 28.11 ± 0.22 µM) were found the most active urease inhibitors near to standard thiourea among the synthesized series. Similarly, compound 5 having IC₅₀ value of 34.32 ± 0.65 µM showed significant inhibitory activity against urease enzyme. Furthermore, three compounds 7, 8, and 9 exhibited less activity with IC₅₀ values of 45.91 ± 0.14, 47.91 ± 0.14, and 48.33 ± 0.72 µM respectively. DFT used to calculate frontier molecular orbitals including; HOMO and LUMO to indicate the charge transfer from molecule to biological transfer, and MEP map to indicate the chemically reactive zone suitable for drug action. The electron localization function (ELF), non-bonding orbitals, AIM charges are also calculated. The docking study contributed to the analysis of urease protein binding.     

Quantum-Chemical Consideration of Al2M2 Tetranuclear Metal Clusters (M–3d-Element): Molecular/Electronic Structures and Thermodynamics

Quantum-chemical calculation of most important parameters of molecular and electronic structures of tetra-nuclear (pd) metal clusters having Al₂M₂ composition, where M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn (bond lengths, bond and torsion angles), and HOMO and LUMO of these compounds by means of DFT OPBE/QZVP method, have been carried out. It has been found that, for each of these metal clusters, an existence of rather large amount of structural isomers different substantially in their total energy, occurs. It has been noticed that molecular structures of metal clusters of the given type differ significantly between them in terms of geometric parameters, as well as in geometric form, wherein the most stable modifications of metal clusters considered are similar between themselves in geometric form. In addition, the standard thermodynamic parameters of formation of metal clusters considered here, and namely standard enthalpy Δ_fH⁰(298 K), entropy S_f⁰(298 K), and Gibbs’ energy Δ_fG⁰(298 K) of formation for these metal clusters, were calculated.     

Vacancy Defects in Ga2O3: First-Principles Calculations of Electronic Structure

First-principles density functional theory (DFT) is employed to study the electronic structure of oxygen and gallium vacancies in monoclinic bulk β-Ga₂O₃ crystals. Hybrid exchange–correlation functional B3LYP within the density functional theory and supercell approach were successfully used to simulate isolated point defects in β-Ga₂O₃. Based on the results of our calculations, we predict that an oxygen vacancy in β-Ga₂O₃ is a deep donor defect which cannot be an effective source of electrons and, thus, is not responsible for n-type conductivity in β-Ga₂O₃. On the other hand, all types of charge states of gallium vacancies are sufficiently deep acceptors with transition levels more than 1.5 eV above the valence band of the crystal. Due to high formation energy of above 10 eV, they cannot be considered as a source of p-type conductivity in β-Ga₂O₃.     

Interlayer and Intralayer Excitons in AlN/WS2 Heterostructure

The study of intra and interlayer excitons in 2D semiconducting vdW heterostructures is a very hot topic not only from a fundamental but also an applicative point of view. Due to their strong light–matter interaction, Transition Metal Dichalcogenides (TMD) and group-III nitrides are particularly attractive in the field of opto-electronic applications such as photo-catalytic and photo-voltaic ultra-thin and flexible devices. Using first-principles ground and excited-state simulations, we investigate here the electronic and excitonic properties of a representative nitride/TMD heterobilayer, the AlN/WS2. We demonstrate that the band alignment is of type I, and low energy intralayer excitons are similar to those of a pristine WS2 monolayer. Further, we disentangle the role of strain and AlN dielectric screening on the electronic and optical gaps. These results, although they do not favor the possible use of AlN/WS2 in photo-catalysis, as envisaged in the previous literature, can boost the recently started experimental studies of 2D hexagonal aluminum nitride as a good low screening substrate for TMD-based electronic and opto-electronic devices. Importantly, our work shows how the inclusion of both spin-orbit and many-body interactions is compulsory for the correct prediction of the electronic and optical properties of TMD/nitride heterobilayers.     

Thermal Expansion of 3C-SiC Obtained from In-Situ X-ray Diffraction at High Temperature and First-Principal Calculations

In situ X-ray crystallography powder diffraction studies on beta silicon carbide (3C-SiC) in the temperature range 25–800 °C at the maximum peak (111) are reported. At 25 °C, it was found that the lattice parameter is 4.596 Å, and coefficient thermal expansion (CTE) is 2.4 ×10−6/°C. The coefficient of thermal expansion along a-direction was established to follow a second order polynomial relationship with temperature (α11=−1.423×10−12T2+4.973×10−9T+2.269×10−6). CASTEP codes were utilized to calculate the phonon frequency of 3C-SiC at various pressures using density function theory. Using the Gruneisen formalism, the computational coefficient of thermal expansion was found to be 2.2 ×10−6/°C. The novelty of this work lies in the adoption of two-step thermal expansion determination for 3C-SiC using both experimental and computational techniques.