Journal of Molecular Structure-theochem最新文献

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 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.

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.

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.

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.

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.

Density functional theory (B3LYP) calculation is used to determine the gas-phase proton affinities of four series of nano-size diamines (1–2.3 nm) including –(CH₂)_n–C₆₀–(CH₂)_n–, –(C₆H₄)_n–, –(CHCH)_n–, and –(CH₂)_n– spacers, where the maximum value of n is 5, 5, 9 and 15, respectively. The results showed that, in the case of diamines with two former spacers the first proton affinity, PA₁, decreases by increasing the value of n, while in the case of two latter diamines it increases by increasing the value of n. However, the second proton affinity, PA₂, and proton overallaffinity, PA_ov, of all above molecules, as expected, increases with increasing the value of n. Among all above compounds the greatest amounts of the PA₁ and PA₂ were calculated for H₂NCH₂–C₆₀–CH₂NH₂ and H₂N–(CH₂)₁₅–NH₂ molecules, respectively. It seems that the C₆₀ molecule has an increasing effect on PA₁ of diamines, while a saturated long aliphatic chain which separates well the positively charged nitrogen atoms has an increasing effect on PA₂ of diamines. The above results has led us to design new nano-size superbases with –N(CH₃)₂ or –NC{N(CH₃)₂}₂ basic groups on fullerene molecule. The amounts of PA₁, PA₂ and PA_ov, for one of these superbases are 1126.3, 786.6 and 1912.9 kJ mol⁻¹, respectively, indicating that it is one of the strongest superbases known so far.

Density function theory (DFT) has been applied to investigate the interactions between NO and N₂O with Cu⁺ species in a beta zeolite (BEA) at sites T1 and T9. The geometries for Cu-BEA represented as 10T cluster, and complexes of NO and N₂O adsorptions on them in η¹-O and η¹-N modes have been completely optimized. The calculated results showed that NOx could be adsorbed on Cu⁺ species in two modes, and N–O bond distances of NOx increase after adsorption. The adsorption energies of NO and N₂O molecules in η¹-N mode on Cu-BEA are larger than in η¹-O mode, and the interactions between N₂O or NO and Cu-BEA at site T9 are stronger than at T1 site.

The enthalpies of formation of brominated benzenes and phenols were predicted using Gaussian-4 (G4), G3X, and G3XMP2 model chemistries and a few popular density functional methods, coupled with homodesmic reactions (HR1), (HR2) in which C₆H₆, C₆H₅Br, and C₆H₅OH are used as reference compounds. The results from G4, G3X, and G3XMP2 agree closely within 2 kJ/mol for all brominated benzenes and phenols; while the results from density functional methods are systematically higher than the G4 ones. The predicted enthalpies of formation for 2- and 4-bromophenols are in close agreement with the recent experimental measurements. Three reactions (R1), (R2), (R3) were also used to derive ${\Delta }_{\text{f}}{H}_{298\text{K}}^{°}$(g, C₆H₅Br) = 98.7 ± 1.0 kJ/mol by using CH₄, CH₃Br, CH₂Br₂, CH₂CHBr, and C₆H₆ as reference compounds at G4 and G3X levels. The value is significantly lower than the only experimental value of 105.4 kJ/mol given by Cox and Pilcher in 1970, and a re-determination is called.