Analysis of pseudo jahn–teller distortion based on natural bond orbital theory: Case study for silicene

Ground state (GS) instability of nondegenerate molecules in high symmetric structures is understood through Pseudo Jahn–Teller mixing of the electronic states through the vibronic coupling. The general approach involves setting up of a Pseudo Jahn–Teller (PJT) problem wherein one or more symmetry allowed excited states couple to the GS to create vibrational instability along a normal mode. This faces two major complications namely (1) estimating the adiabatic potential energy surfaces for the excited states which are often difficult to describe in case the excited states have charge-transfer or multi-excitonic (ME) character and (2) finding out how many such excited states (all satisfying the symmetry requirements for vibronic coupling) of increasing energies need to be coupled with the GS for a particular PJT problem. An analogous alternative approach presented here for the well-known case of symmetry breaking of planar (D_6h) hexasilabenzene (Si₆H₆) to the buckled (D_3d) structure involves identifying the second-order donor–acceptor, hyperconjugative interactions (E²_{i → j}) that stabilize the distorted structure. Following the recent work of Nori-Shargh and Weinhold, one observes that the orbitals involved in the vibronic coupling between the S₀/S_n states and those for the donor (filled)–acceptor (empty) interactions are identical. In fact, deletion of any particular pair of E²_{i → j} interaction creates vibrational instability in the buckled structure and as a corollary, deleting it for the planar structure removes its instability. The one-to-one correlation between the natural bond orbital theory and PJT theory assists in an intuitive identification of the relevant (few) excited states from a manifold of computed ones that cause symmetry breaking by vibronic coupling. © 2019 Wiley Periodicals, Inc.     

Theoretical investigation of the stereoselectivity of some Diels＇Alder reactions involving cyclopentadiene

The stereoselectivity of some Diels-Alder reactions was investigated by means of ABEEM-oTr model. Combined with local hard-soft and acid-base （HSAB） principle, we made reasonable explanation by calculating local softness of atom and bond regions for the stereoselectivity of four Diels-Alder reactions involving cyclopentadiene.     

Theoretical investigation of the decomposition of monofluoromethanol

Ab initio calculations have been used to study the decomposition pathways of monofluo-roinethanol. Equilibrium geometries and transition state structures were optimized at the HF/6-31G(d) and MP2/6-31G(d) levels. Single point energies were obtained at different levels of theory. The most favorable reaction to dissociation is the 1,2-HF elimination which is consistent with the experimental results.     

Theoretical study of some bis-verdazyl diradicals: singlet–triplet energy gap

The bis-verdazyl diradical (BVD) system is closely examined by using the multiconfiguration wavefunctions as well as the density functional theory (DFT). The totally symmetric singlet ground state turns out to have strong multiconfiguration character at all levels of theory. The singlet ground state takes on the planar structure while the most stable triplet state corresponds to the twisted form. The MCSCF+MCQDPT2 calculations are shown to be sufficient to predict the singlet–triplet energy gap which is insensitive to the electronic characters of the ring substituents.     

A new insight on hydrogen bond donor property of bridged B–H–B protons in the interaction of diborane with some hydrogen bond acceptor molecules

Structural Chemistry - Ab initio calculations at the MP2/6-311++G(d,p) computational level were used to analyze the interactions between a molecule of B2H6 with some small molecules which often...     

Semi-empirical molecular orbital calculations of the infrared band intensities of some simple polyatomic molecules—III. Halogenated germanes

Using a CNDO/2 semi-empirical molecular orbital computer program, the i.r. band intensities and equilibrium dipole moments and molecular geometries of germyl fluoride, chloride and bromide, dichloro- and dibromogermane, and trichlorogermane have been calculated. These have been compared with a similar set of calculations on the halogenated methanes and silanes, in an effort to establish any periodic trends in the calculated band intensifies among these series of molecules. It is concluded that such trends exist only for some normal modes, or only among limited groups of molecules.The calculated dipole moments, bond lengths and bond angles have been compared with such experimental results as have been reported. The same broad level of agreement as was found in the case of the halogenated methanes and silanes has been found here also.     

Theoretical study of the intermolecular hydrogen bond interaction for furan-HCl and furan-CHCl_3 complexes

The importance of intermolecular interactions in biology and material science has prompted chemists to explore the nature of the variety of such interactions. The strongest of these interac-tions are the hydrogen bonds, which play an important role in determining the molecular confor-mation, crystal packing, and the structure of biological systems such as nucleic acids. Extensive experimental and theoretical efforts[1—5] have been devoted to the studies of this type of interac-tions, such as …     

Theoretical study of H⋯P and X⋯P interactions of methylphosphines with HSX molecules

Ab initio calculations were used to analyze the interactions between thiohypohalous acids (HSX; X = F, Cl, Br, I) and methylphosphine derivatives (PH _n Me_3?n , n = 0–3) at the MP2/aug-cc-pVDZ level of theory. Interaction of HSX with PH _n Me_3?n leads to both hydrogen bond (HSX–PH _n Me_3?n –HB) as well as halogen bond (HSX–PH _n Me_3?n –XB) complexes. Stabilities of both HB and XB complexes increase with basicity of the phosphines. However, HB complexes of a phosphine molecule with different HSX have the same order of stabilities, but XB complexes of heavier thiohypohalous acids are more stable. Electron densities of complexes were characterized with the atoms in molecules methodology. The charge transfer within dimers was analyzed by means of natural bond orbitals.     

Theoretical study of the excited states and the redox potentials of unusually distorted ��-trifluoromethylporphycene

Quantum chemical calculations were performed to analyze the excited states and the redox potentials of a recently synthesised fluorine-containing porphycene, 2,7,12,17-tetraethyl-3,6,13,16-tetrakis(trifluoromethyl) porphycene. The reduction and oxidation potentials of the porphycenes measured by cyclic voltammetry were semiquantitatively reproduced using density-functional theory (DFT) and polarizable continuum model calculations, in which solvent effect and basis-sets extension effect were indispensable. Symmetry-adapted cluster configuration interaction and time-dependent (TD) DFT calculations were performed to analyze the visible region of the absorption spectra, and the results were in good agreement with the experimental data. The results of the calculations showed that both structural distortion and electronic effects cause specifically large stabilization of the LUMO level and become the origin of the particularly large positive-side shift of the reduction potential and the red-shift in the Q band absorption.     

Role of CH�C�� interaction energy in self-assembled gear-shaped amphiphile molecules: correlated ab initio molecular orbital and density functional theory study

Ab initio molecular orbital and density functional theory calculations with inclusion of dispersion interaction effect are employed to reveal the characteristic features of intermolecular interactions for the molecular capsule (1 ₆) consisting of six gear-shaped amphiphile molecules (1) discovered by Hiraoka et al. (J Am Chem Soc 130:14368?C14369, 2008). The contributions of CH?C?? and ?ШC?? type dispersion energies are found to be indispensable for the formation of hexameric capsule 1 ₆ by the analysis of decomposed interaction energies between fragmented-model species in the 1 molecule. We have also calculated the hexameric capsule (2 ₆) from demethylated 1 molecule (2). Such subtle structural difference induces the different characters of intermolecular interactions, in which the stabilization energy of hexameric 2 ₆ capsule is about 40?kcal/mol smaller than that of the original 1 ₆ capsule, due to the lack of three methyl groups for the CH?C?? interactions in 2 molecules.     

A density functional theory study of the ��mythic�� Lindlar hydrogenation catalyst

A Lindlar catalyst is a popular heterogeneous catalyst that consists of 5?wt.% palladium supported on porous calcium carbonate and treated with various forms of lead and quinoline. The additives strategically deactivate palladium sites. The catalyst is widely used for the partial hydrogenation of acetylenic compounds in organic syntheses. Alkyne reduction is stereoselective, occurring via syn addition to give the cis-alkene. Even if it has been employed for about 60?years, there is a lack of molecular level understanding of the Lindlar catalyst. We have applied density functional theory simulations to understand the structure and the nature of the interplay between the multiple chemical modifiers in the Lindlar catalyst. Indeed, the poisons influence different parameters controlling selectivity to the alkene: Pb modifies the thermodynamic factor and hinders the formation of hydrides, while quinoline isolates Pd sites thus minimizing oligomerization.     

Theoretical study of the C�CF bond activation in methyl fluoride by alkaline-earth metal monocations

Reactions of methyl fluoride with bare alkaline-earth metal monocations (Mg⁺, Ca⁺, Sr⁺, and Ba⁺) were studied using theoretical methods. Thermochemical data were calculated using density functional theory in conjunction with polarized 3-?? and 4-?? basis sets. Variational/conventional microcanonical transition state theory was used for the calculation of the reaction rate constants over a large range of temperatures. According to our calculations, the Ca⁺, Sr⁺, and Ba⁺ reactions with CH₃F proceed to yield CaF⁺, SrF⁺, and BaF⁺, in agreement with the experimental observation. The theoretically predicted global rate constants are in reasonable agreement with the experimental data. In the case of Mg⁺, the large value of the computed energy barrier associated with the ??inner?? transition structure is fully consistent with the limited progress experimentally observed for this reaction. The importance of bottlenecks other than the ??inner?? transition state is highlighted and its mechanistic implications discussed. Particularly, our calculations suggest that the studied processes proceed through a ??harpoon-like?? mechanism.     

Theoretical study of association of acetylene,benzene, and carbonyl molecules “squeezed” inside fullerene cages

The equilibrium geometries, normal mode frequencies, magnetic shielding constants, and energetic characteristics of model endohedral 2(C₂H₂)@C₇₀, (C₄H₄)@C₇₀, 2(C₆H₆)C₈₄, (C₁₂H₁₂)@C₈₄, 6(CO)@C₈₄, and (C₆O₆)@C₈₄ clusters, mimicking the structure and properties of the guest molecules under “ultrahigh” pressures inside the fullerene cages, were calculated at the density functional theory B3LYP/6-31G and B3LYP/6-31G* levels. According to the calculations, all the structures under consideration correspond to local minima of the corresponding potential energy surfaces. The 2(C₂H₂)@C₇₀ isomer with two separated endohedral acetylene molecules turns out to be considerably less favorable than the (C₄H₄)@C₇₀ isomer with endohedral cyclobutadiene. The 2(C₆H₆)C₈₄ isomer with two separated endohedral benzene rings is almost 30 kcal/mol less favorable than the (C₁₂H₁₂)@C₈₄) isomer with distorted endohedral prismane. The 6(CO)@C₈₄ isomer with six separated carbonyl molecules is almost 45 kcal/mol less favorable than the (C₆O₆)@C₈₄ isomer in which the carbonyls are associated to form a more compact cyclic hexamer C₆O₆. The barriers separating the “associated” and “dissociated” (consisting of monomers) isomers are estimated at 10–15 kcal/mol. Calculations show that, in extremely tight endoclusters, the increase in compression and strain energy caused by the repulsion of the electronic shells of guest molecules and wall atoms is accompanied by a sharp energetic stabilization of compact associated isomers (including those poorly stable or unstable in the free state) as compared with the dissociated isomers, on the one hand, and by a sharp decrease in activation barriers, on the other hand. Both factors should favor the realization of association processes unlikely or impossible under common conditions.     

Theoretical study of intramolecular proton transfer of perylenequinone

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A comparative study of complexation methods for cefdinir-hydroxypropyl-��-cyclodextrin system

The aim of this study was to investigate the effect of hydroxypropyl-??-cyclodextrin (HP??CD) on the solubility and dissolution rate of Cefdinir (CEF). The methods that were employed to prepare CEF?CHP??CD complexes were Kneading (KN), Co-evaporation (CE), Spray drying (SD) and a novel approach of Microwave irradiation (MWI). The formation of inclusion complexes with HP??CD in the solid state, were characterized by Differential Scanning Calorimetry (DSC), Fourier Transformation Infrared spectroscopy (FTIR), Proton Nuclear Magnetic Resonance Spectroscopy (NMR), X Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) studies, and comparative studies on the in vitro dissolution of CEF were carried out. Phase solubility profile with HP??CD was classified as A_L type, indicating the formation of 1:1 stoichiometric inclusion complexes. Characterization of binary systems by DSC, FTIR, NMR, XRD and SEM indicated that SD and MWI method resulted in formation of true complexes. Binary systems showed significant increase in dissolution rate as compared to plain drug. Amongst the various binary systems, MWI products were prepared in least time with better yield and highest dissolution rate.     

A study on the inhibition mechanism of ��-cyclodextrin on pullulanase

Effects of ??-cyclodextrin on the activity and structure of pullulanase were investigated in this work. Inhibition of ??-cyclodextrin on pullulanase depended on concentration and tended to be enhanced with the increase of ??-cyclodextrin. The increase of intrinsic fluorescence induced by ??-cyclodextrin was observed, which was probably due to the formation of inclusion complexes between aromatic amino acid residues of pullulanase and ??-cyclodextrin. The ??-helix content in pullulanase decreased from 15 to 9% but the ??-sheet content increased from 35 to 43% with the increase of the concentration of ??-cyclodextrin from 0 to 10 mM. The ratio variation of ??-helix to ??-sheet was almost accordance with that of relative residual activity of pullulanase, especially in the presence of high concentration of ??-cyclodextrin. These results showed that ??-cyclodextrin changed the secondary structure and microenvironment of pullulanase, accordingly leading to the loss of enzymatic activity.     

Development of a natural ingredient – Natural preservative: A case study

Lately, the cosmetic and personal care market has been more and more driven toward natural ingredients by the rising consumers' awareness about personal health and safety and their will for safer cosmetics free of harmful chemicals. Preservatives are no exception to the rule: evidence or suspicion of the toxicity of certain synthetic preservatives that have been around for decades pushed the cosmetic industry forward to seek for natural alternatives, as the selection of natural preservatives already available is quite limited. Sourcing active metabolites and developing new natural ingredients are long-term procedures that are thoroughly described in the present paper, via the example of the design of a natural preservative based on the Santolina chamaecyparissus extract, and of the assessments of its preservative effectiveness.