Journal of Molecular Structure-theochem最新文献

The π-halogen bond interactions are found between the BB triple bond and X1X2 (X1, X2 = F, Cl, Br) employing MP2(full) method at 6-311+G(2d), aug-cc-pVDZ and aug-cc-pVTZ levels according to the “CP (counterpoise) corrected potential energy surface (PES)” methodology, accompanied by the BB bond contraction. The (2, 3) extrapolated energies using the two-point approximation are also reported. All the π-halogen complexes are of electronic state ¹A₁ with the C_2V symmetry. The dipole moment of dihalogen, the effects of the polarization of the halogen atom X1 and the electron withdrawing of X2 influence the strength of π-halogen bond interaction. The analyses of the natural charges, natural bond orbital (NBO), atoms in molecules (AIM) theory and electron density shifts reveal the nature of the π-halogen bond interactions, and explain the origin of the BB bond contraction. The energy decomposition analysis at B3LYP/TZ2P level shows that the interaction energy in the OCBBCO⋯X1X2 is mainly determined by the orbital energy. The values of ΔE_int, ΔE_elstat, ΔE_pauli and ΔE_orb are all arranged in the order of OCBBCO⋯BrF > OCBBCO⋯ClF ≈ OCBBCO⋯FCl > OCBBCO⋯BrCl > OCBBCO⋯Br₂ > OCBBCO⋯Cl₂ > OCBBCO⋯ClBr > OCBBCO⋯FBr. The binding energy of the complex of OCBBCO with X1X2 is stronger than that of the corresponding HCCH⋯X1X2 complex. OCBBCO⋯F₂ is indicative of covalent interaction. These results confirm that OCBBCO can be as π-electron donor to form the π-halogen bond interaction.

Segmented all-electron contracted double zeta valence plus polarization function (DZP) basis sets for the element Pt were constructed for use in conjunction with the non-relativistic and Douglas–Kroll–Hess (DKH) Hamiltonians. The DZP–DKH set is loosely contracted and thus offer computational advantages compared to the generally contracted relativistic basis sets, while their sufficiently small size allows it to be used in place of effective core potentials (ECP) for routine studies of molecules. Using the one-parameter hybrid functional mPW1PW, the performance of the basis sets is assessed for predicting the molecular structures and atomic charges of platinum(II) antitumor drugs, cisplatin and carboplatin. These results can be used as reference values to calibrate further ECP calculations. Despite their compact size, the DZP sets demonstrate consistent, efficient, and reliable performance and will be especially useful in calculations of molecular properties that require explicit treatment of the core electrons.

A theoretical study was conducted on the effects of three substituents in the Ru(II) polypyridyl complex, [Ru(phen)₂(6-R-dppz)]²⁺, (R = H for complex 1, OH for complex 2, and NO₂ for complex 3), on the photocleavage of DNA. The geometric and electronic structures of these complexes in the ground (S₀), first singlet (S₁), and triplet (T₁) excited states, were calculated using the density functional theory (DFT), Hartree–Fock (HF), and configuration interaction singles (CIS) methods. The excited reduction potentials of the Ru(II) complexes in aqueous solution, which appear to be primarily responsible for the DNA-photocleavage behavior, were calculated by thermodynamic cycle, and determined to be higher than the oxidation potentials of some DNA-bases. The order of the reduction potentials changes from 3 > 2 > 1 in the ground state to 2 > 1 > 3 in the excited state, suggesting that the substituent on the main ligand has a significant effect on the electrochemical properties of the excited Ru(II) polypyridyl complexes. In addition, these theoretical results can explain two experimentally-observed phenomena: (a) the cleavage of DNA by complex 3 in the absence of light irradiation; (b) the inability of complex 3 to emit luminescence in its excited triplet state in aqueous solution.

The uncatalyzed Johnson–Claisen rearrangement has been investigated at the B3LYP/6-311G(d,p) level of theory. The effect of electron donating and electron withdrawing substitutions in different positions on the transition state has been studied. Our results show that electron-donating substituents accelerate rearrangement while electron-withdrawing substituents act in opposite direction and decelerate the reaction. The amount of acceleration or deceleration depends on substituent position. In addition to mono-substituted compounds, di-substituted compounds have been also investigated. All of the calculations have been carried out in gas phase.

Halophilic reactions have been modeled theoretically by employing CBr₄, Cl₃CCN, Cl₃CCOCl, CCl₄ and Cl₃CF as substrates and Cl⁻ as a nucleophile. It was found that the formation of the strong halogen bond is a necessary but not sufficient condition for the occurring of the halophilic reaction: the strong red-shifting halogen bond facilitates the reaction whereas the strong blue-shifting halogen bond retards it. The theoretical results are in good agreement with the experimental results.

Density functional calculations of the structure, molecular electrostatic potential and thermodynamic functions have been performed at B3LYP/6-31G(d) level of theory for the title compound of N-2-Methoxyphenyl-2-oxo-5-nitro-1-benzylidenemethylamine. To investigate the tautomeric stability, optimization calculations at B3LYP/6-31G(d) level were performed for the enol and keto forms of the title compound. Calculated results reveal that the enol form of the title compound is more stable than its keto form. The predicted non-linear optical properties of the title compound are much greater than ones of p-Nitroaniline. The changes of thermodynamic properties from the monomers to title compound with the temperature ranging from 200 K to 450 K have been obtained using the statistical thermodynamic method. At 298.15 K the change of Gibbs free energy for the formation reaction of the title compound is 30.654 kJ/mol. The title compound cannot be spontaneously produced from the isolated monomers at room temperature. The tautomeric equilibrium constant is computed as 0.0192 at 298.15 K for enol-imine ↔ keto-amine tautomerization of the title compound. In addition, natural bond orbital analysis of the title compound were performed using the B3LYP/6-31G(d) method.

The dumbbell-shaped dimers constructed from C₅₀ cages are investigated using self-consistent field molecular orbital method based on density functional theory. Our study focuses on the structures, stabilities, electronic and vibrational properties of the C₅₀ dumbbell-shaped dimers. It is found that the stability of these C₅₀ dimers is related to bonding positions and linking patterns. For the dimers by [2 + 2] cycloaddition, a simple rule is proposed to predict the stabilities of these additive products of fullerenes according to the environment around the C–C bonds on the addition position. Moreover, higher thermodynamic stability is accompanied with larger HOMO–LUMO gaps for these dimers. The vibrational properties of the C₅₀ dimers are also discussed in this paper.

The kinetic and mechanism of the unimolecular gas-phase elimination of 2-(dimethylamino)ethyl chloride were examined by using density functional theory methods to explain the enhanced reactivity in gas-phase elimination compared to the parent compound ethyl chloride. The plausible anchimeric assistance of the dimethylamino proposed in the literature was investigated. The theoretical calculations were carried out at B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p), PBEPBE/6-31G(d,p), and PBEPBE/6-31++G(d,p) levels of theory. The previous proposed reaction path of anchimeric assistance has an energy of activation 60 kJ/mol higher than the experimental value. The located transition state in the minimum energy path is a four-centered cyclic configuration comprising chlorine, hydrogen and two carbon atoms. Calculation results give a lower energy of activation of 2-(dimethylamino)ethyl chloride when compared to the parent compound ethyl chloride. This result is due to the stabilization of the transition state because of electron delocalization involving the dimethylamino substituent.

The 8-hydroxyquinoline (8HQ) molecule and its four derivatives used as medicines: chloroxine, clioquinol, iodoquinol, and nitroxoline (8HQ substituted in the benzene ring by 5,7-dichloro-, 5-chloro-7-iodo-, 5,7-diiodo-, and 7-nitro-, respectively), were studied using the DFT/B3LYP/6-311G∗∗ method. Three forms of each molecule were considered: (OH⋯N) with the intramolecular OH⋯N hydrogen bond, (OH;N) with the broken intramolecular hydrogen bond, and (NH), the tautomer with the H-atom attached to the pyridine N-atom. Regardless of the substitution, the (OH⋯N) form, with the intramolecular OH⋯N hydrogen bond, was the most stable form. Breaking the intramolecular bond led to the formation of (OH;N), which was less stable by at least 25 kJ/mol. The NH tautomer was higher in energy than the (OH⋯N) tautomer by at least 40 kJ/mol. Based on AIM analysis, it was found that the intramolecular OH⋯N bond was the weakest in 8HQ, stronger in chloroxine, clioquinol and iodoquinol, and it was the strongest in nitroxolin. The benzene ring aromaticity decreased from 8HQ, through halogenosubstituted 8HQ, to nitroxoline, which was in line with the decrease of the π-electron population in the benzene ring. The sum of aromaticities of the two rings was largest for the (OH⋯N) tautomers, significantly lower for the (OH;N) tautomers, and the smallest for (NH) tautomers. From the electron population in σ and π valence orbitals of the two quinoline rings it appears that, for the benzene ring, the halogens acted as σ-electron withdrawing and π-electron donating substituents, whereas NO₂ was a σ- and π-electron withdrawing substituent. The σ substituent effect almost solely influenced the substitution site, i.e., the benzene ring, whereas the π substituent effect was extended to the pyridine ring. Here, we also present changes in the σ and π-electron populations resulting from tautomerization and breaking of the intramolecular OH⋯N hydrogen bonds.