Journal of Molecular Structure-theochem最新文献

Theoretical calculations were performed at the modest B3LYP/6-31G(d) level of DFT to interpret the regioselectivity and enantioselectivity and also to predict diastereofacial selecitivity for 1,3-dipolar cycloaddition reaction of two known nitrones leading to a diastereomeric excess of the products. In this respect, N-substituted and C-substituted nitrones were considered for reactions with the substituted dipolarophiles like styrene and allyl alcohol. The reactions were studied by calculating the potential energy surface of the addition process and rationalizing in terms of the reactivity index of global electrophilicity. For the reactions considered, trends in reactivity, i.e., the regio- and exo/endo-selectivity and product ratios have been explained in terms of frontier orbitals interaction, electrophilicity difference, the rate constant values and an analysis of Pauling’s bond order and Wiberg bond index in the transition state. All these were found to be in good agreement with the experimental findings.

Adsorption of molecular and atomic oxygen on neutral and negatively charged Pd_xCu_3−x (x = 0–3) nano-clusters have been studied by density functional theory. It has been observed that modes and energies of adsorption strongly depend on the charge and composition of the nano-clusters. The most stable adsorption mode for molecular oxygen on neutral Pd/Cu nano-clusters is to bridge Pd–Cu site (adsorption energy = −103.7 kJ mol⁻¹) while on negatively charged nano-clusters bridging of Pd–Pd or Cu–Cu are more stable adsorption modes (adsorption energies = −140.9 and −172.9 kJ mol⁻¹). Also, the most stable adsorption mode for atomic oxygen on neutral Pd/Cu nano-clusters is on the hollow site (adsorption energy = −111.9 kJ mol⁻¹) while on negatively charged nano-clusters adsorption on top of Pd or Cu are the most stable adsorption modes (adsorption energies = −289.7 and −370.2 kJ mol⁻¹). With increasing copper content the adsorption energy of molecular and atomic oxygen on neutral nano-clusters decrease while on negatively charged nano-clusters increase. Also, it has been shown that (I) increasing of copper content in nano-cluster and (II) adding of negative charge to nano-clusters weaken O–O bond which can increase the activity of the nano-cluster for dissociation of molecular oxygen. Moreover, it has been shown that dissociation of molecular oxygen on nano-clusters is exothermic and occurs more favorably on Pd/Cu nano-clusters compared to on Pd nano-cluster.

The reaction of Pt (³D, ¹S) with CH₂FCl was theoretically investigated using B3LYP method. The results indicate that C–F, C–Cl and C–H bonds can be activated by Pt atom. C–F bond activation has the highest energy barriers on both triplet and singlet surfaces (154.6 and 58.5 kJ/mol, respectively). C–Cl bond activation has lower barrier on triplet surface (98.3 kJ/mol), and it is barrierless process on singlet surface. C–H activation was only found on triplet surface with a barrier of 92.3 kJ/mol. Pathways for the HF, HCl and H₂-elimination were proposed and the experimentally observed product CHF–PtHCl is in fact reaction intermediate.

Computations using B3LYP density functional method with 6-31G (d,p) and 6-311 + G (d,p) basis sets were carried out on a cationic cyanine dye at various locations of the Br⁻ counter ion. It was found that the Br⁻ position and its accompanying electric field have a significant affect on the charge distribution within the cationic dye. Furthermore, the enthalpy of anion/cation interaction was highly sensitive to the Br⁻ location. Bringing the Br⁻ either closer or further than the optimum is substantially destabilizing. The flexible cyanine dye molecule tends to fold to accommodate electrostatic interactions. On the other hand, less activation energy was required for the approach of F⁻ towards the cyanine dye (6 kcal/mol vs. 15 kcal/mol for the Br⁻ approach).

The local structures and the electron paramagnetic resonance (EPR) g factors $g_{\parallel }$ and $g_{\perp }$ for the substitutional V⁴⁺ and Cr⁴⁺ at the Ge⁴⁺ sites in Bi₄Ge₃O₁₂ are theoretically investigated from the perturbation formulas of the g factors for 3d¹ and 3d² ions under tetragonally elongated tetrahedra. In the calculations, the ligand orbital and spin–orbit coupling contributions are taken into account from the cluster approach in view of the covalency of the systems. The local impurity-ligand bond angles β related to the C₄ axis in the V⁴⁺ and Cr⁴⁺ centers are found to be about 5° and 6°, respectively, lower than the host angle β_H in the pure crystal. So the ligand tetrahedra transform from original compression at the host Ge⁴⁺ site into elongation in the impurity centers due to the size mismatching substitution of the smaller Ge⁴⁺ by the larger impurities. Meanwhile, the Jahn–Teller effect also brings forward some contribution to the local structure of the V⁴⁺ center. The calculated g factors are in good agreement with the experimental data. The local structures of both centers are discussed.

The conformations of cycloheptanone and cycloheptanethione were studied at the B3LYP and CCSD(T) levels of theory using the 6–311 + G(d,p) basis set. Both molecules are remarkably similar in their conformational properties and are characterized by the presence of a broad potential well centred around the symmetrical twist-chair conformation. Pseudorotational barriers are found to be higher than in cycloheptane, whereas the transition from the chair family to the twist-boat conformation requires less energy.

The electronic isomerization arising from the switch of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) has been investigated within density functional theory. The switch of the HOMO and LUMO changes the electronic structures, thus the physicochemical properties of isomers. The predicted geometry, infrared spectra, and electronic spectra of fullerenes investigated show significant change upon electronic isomerization.

The non-linear optical properties of Acenaphthene (ANP) and its derivative Acenaphthenequinone (ANPQ) have been evaluated and compared using the Hartree Fock (HF) and density functional theory (DFT). According to the calculations, the β value of ANP is more than five times and ANPQ is almost five times than that of urea. The molecular HOMO, LUMO composition, their respective energy gaps, MESP contours/surfaces have also been drawn and compared to explain the activity of ANPQ over ANP. For a complete description of molecular dynamics, vibrational wavenumber calculation along with normal mode analysis have been carried out at the DFT level.

We present conformational preferences, energies and dipole moments for six-membered ring cyclic phosphine oxides with phenyl substitution on phosphorus. Conformers found by molecular dynamics were fully optimized at the RHF B3LYP/6-31G∗ and MP2/6-31G∗ levels of theory. We compare the results of calculations against some experimental results. We discuss the reasons for the perpendicular vs. parallel preference of the phenyl group either in axial or equatorial orientation.

Self-dimerisation of the three isomeric semiquinone radicals is considered herein. Optimised geometries and thermochemical parameters, in terms of heat of formation, entropy, heat capacity and Gibbs free energy of formation are provided for all possible cross coupling products. It is found that self-dimerisation of the three semiquinone radicals is not as thermodynamically favoured as the self-dimerisation of the phenoxy radical. Accordingly, the three isomeric semiquinone radicals can be regarded as less active precursors for the formation of dioxin compounds than phenoxy radicals.