Journal of Molecular Structure-theochem最新文献

The dynamic reaction pathways after passing the initial barrier for the reaction of atomic oxygen radical anion (O⁻) with ethylene (CH₂CH₂) have been investigated with Born–Oppenheimer molecular dynamics (BOMD) simulations. The BOMD simulations initiated at this [O⋯H⋯CHCH₂]⁻ barrier on the exit-channel potential energy surface (PES) reveal several different types of dynamic reaction pathways leading to various anionic products. In particular, as the energy added on the transition vector of the [O⋯H⋯CHCH₂]⁻ transition state increases remarkably, the OH⁻ and CH₂CH become the dominant products instead of the CH₂CHO⁻ and H. As a result, animated images are displayed and more extensive reaction mechanisms are illuminated for the title reaction from the perspective of the dynamic reaction pathways.

In order to understand cellulose pyrolysis mechanism, the pyrolysis of β-d-glucopyranose was investigated using density functional theory methods at B3LYP/6-31++G(d,p) level. Four possible pyrolytic pathways were proposed and geometries of reactants, transition states, intermediates and products were fully optimized. In pathway 1, the products are glycolicaldehyde, acetol, CO and H₂O; in pathway 2, the products are 5-hydroxymethylfurfural and H₂O; in pathway 3, the products are levoglucosan and H₂O; in pathway 4, the products are 3,4-anhydroaltrose and H₂O. The standard thermodynamic and kinetic parameters in each reaction pathway were calculated at different temperatures. The calculation results show that all reactions are endothermic and can take place spontaneously when reaction temperature exceeds 550 K. The changes of Gibbs free energies and the activation energies of rate-determining steps in reaction pathways 1 and 2 are less than that in reaction pathways 3 and 4. The activation energy of rate-determining step in pathway 1 is 297.0 kJ/mol and the activation energy of rate-determining step in pathway 2 is 284.5 kJ/mol. Based on thermodynamics and kinetic analysis, reaction pathways 1 and 2 are major pyrolysis reaction channels and the major products of β-d-glucopyranose pyrolysis are low molecular weight compounds such as glycolicaldehyde, 5-hydroxymethylfurfural, acetol and CO. The above results are in accordance with the related experimental results.

The reaction mechanism of atomic oxygen radical anion (O⁻) with pyridine (C₅H₅N) has been investigated at the G3MP2B3 level of theory. Three different entrance potential energy surfaces are explored, respectively, as atomic oxygen radical anion attacks γ-, β- and α-H atoms of pyridine. Possible thermodynamic product channels are examined subsequently. Based on the calculated G3MP2B3 energies and optimized geometries of all species for the title reaction, it has been demonstrated that the oxide anion formation channel is dominant, and the C₅H₃N⁻ + H₂O channel is also favorable in thermodynamics, whereas the H-abstraction and H⁺-abstraction channels are inaccessible at room temperature. The present conclusions are consistent qualitatively with the previous experimental results. The secondary reactions of the anionic products are expected to be responsible for the contradiction of branching ratios between present calculation and previous experiments.

The reactions of allylic and aliphatic alcohols with ethyl acetoacetate have been investigated at the DFT/B3LYP level using 6-311G(d) basis set. Analysis results of frontier molecular orbital (FMO) interactions, chemical potential μ and electrophilicity index ω according to Fukui and Pearson respectively, provide a good prediction of experimental results. Alkylation or transesterification reaction is predicted reliably by the calculations.

The electronic structure and geometry of clusters of the type K_n, ${\text{K}}_n^{+}$, K_nCu_m and ${\text{K}}_n{\text{Cu}}_m^{+}$ (n, m ⩽ 4) were theoretically investigated and compared with similar clusters containing lithium atoms, using density functional methods. The K_nCu_m bimetallic system is important to understand the promotion effects of the alkali atoms on the copper surface. The inclusion of potassium atoms on a bare copper cluster tends to break the Cu–Cu bond when n ⩾ m favors the formation of polar K–Cu bonds. The geometrical shape of K_n and K_nCu_m clusters follow the same trend, but the bimetallic clusters are more stable than K_n clusters. However, the global stability of K_n and K_nCu_m clusters is minor in comparison with corresponding lithium clusters.

The equilibrium geometries, energies, charge transfer, and dipole moments of small MgSi_n (n = 2–10) species and their anions have been systematically investigated at the highest level of Gaussian-3 (G3) theory. For neutral MgSi_n clusters, the ground-state structures are found to be “attaching structure” in which the Mg atom is bound to Si_n clusters. The lowest-energy structures for their anions, however, are found to be “substitutional structures”, which are derived from Si_n+1 by replacing a Si atom with a Mg atom. The reliable adiabatic electron affinities of MgSi_n have been predicted to be 1.84 eV for MgSi₂, 1.90 eV for MgSi₃, 2.17 eV for MgSi₄, 2.35 eV for MgSi₅, 2.45 eV for MgSi₆, 2.18 eV for MgSi₇, 2.98 eV for MgSi₈, 3.00 eV for MgSi₉, and 2.00 eV for MgSi₁₀. The dissociation energies of Mg atom from the lowest-energy structure of MgSi_n clusters have been evaluated to examine relative stabilities. The charge transfer and dipole moments have also been calculated to further understand the interaction between the Mg atom and the silicon clusters.

A theoretical calculation of MP2/6-311++G** and MP2/aug-cc-pVDZ was used to predict the single-electron lithium bond system of H–Be⋯Li–Y (Y = H, OH, F, CCH, CN and NC). The results confirmed H–Be and Li–Y could interact to form the complexes I (HBe⋯Li–H), II (HBe⋯Li–OH), III (HBe⋯Li–F), IV (HBe⋯Li–CCH), V (HBe⋯Li–CN), and VI (HBe⋯Li–NC). The binding energies for these complexes ranged from −22.06 to −27.69 kJ mol⁻¹ at the CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ level. Complex strength was in the order I < II < III < IV < V < VI. Analysis of the stretching frequencies showed that complexes I–III were red-shifting single-electron lithium bonds complexes. In contrast, complexes IV–VI were abnormal blue-shifting complexes, and the Li–Y bonds were longer in the complexes than in the corresponding monomers. The characteristics of the complexes were investigated using natural bond orbital (NBO), atoms-in-molecules (AIM) and electrostatic potential map (EPM) theories.

Density functional theory (DFT) calculations at the B3LYP/aug-cc-pVDZ level have been performed on the condensed polynitroazoles based on diazole and triazole skeletons. Energy of explosion (≈1.60 kcal/g), density (≈1.92 g/cm³), detonation velocity (≈9.30 km/s), and detonation pressure (≈ 39 GPa) of model molecules are found to be promising compared to the well known explosives 1,3,5-trinitro-1,3,5-triazinane (RDX), octahydro-1,3,5,7-tetranitro-l,3,5,7-tetraazocane (HMX), and 4,4′,5,5′-tetranitro-2,2′-bi-1H-imidazole (TNBI). Presumably, the relative positions of nitro groups and the nature of azole ring determines the geometry, stability, sensitivity, density and thus detonation performance.

The singlet and triplet potential surfaces of the ²NCO + ²N₂H reaction have been investigated at the B3LYP/6-311G (d,p) level. The single-point energy calculations are performed at the high-level CCSD (T)/6-311G (d,p) for more accurate energy values. DFT calculations reveal the reaction mechanism to be mainly a barrierless addition of ²NCO to ²N₂H leading to an intermediate ¹im3 (OCN–N₂H) on the singlet potential surface. The adduct ¹im3 goes through an H shift from N₂H to NCO, forming the product of HNCO and N₂. Due to the higher barrier of initial association, the reaction is more difficult on the triplet potential surface.

Ab initio calculations at MP2/6-311++G(d,p) computational level were used to analyze the interaction between a molecule of the nitrosyl hydride with 1 up to 4 molecules of ammonia. Three minima were found for 1:2 and 1:4 complexes of HNO and NH₃. Four complexes were located as minima on the potential energy surface of 1:3 complexes. Particular attention is given to existence and magnitude of NH⋯N blue-shifting hydrogen bonds. Blue shifts of the N–H stretching frequency upon complex formation in the ranges between 33 and 105 cm⁻¹ are predicted. Cooperative effect in terms of stabilization energy is calculated for the studied clusters. The cooperative effect is increased with the increasing size of studied clusters. The Quantum Theory Atoms in Molecules (QTAIM) theory was also applied to explain the nature of the complexes.