Journal of Molecular Structure-theochem最新文献

Density functional theory was applied to study the effects of H-bonds N⋯H–X between pyridine and H₂O, HCONH₂ and CH₃COOH on normal vibrational modes of pyridine at the B3LYP/AUG-cc-pVDZ and B3LYP/AUG-cc-pVTZ levels. The results show that the formation of H-bonds leads to an increase in frequencies of the ring breathing mode v₁, the N-para-C stretching mode v_6a and the meta-CC stretching mode v_8a of pyridine but there was no change in triangle mode v₁₂. The natural bond orbital analysis shows that the frequency blue shift in the ring stretching modes of pyridine is a corporate result of the intermolecular charge transfer caused by the intermolecular hyperconjugation n(N) → σ∗(HX) and the intramolecular charge redistribution caused by intramolecular hyperconjugation n(N) → σ∗(meta-CC) in the pyridine ring. We also found that the magnitude of the frequency blue shift increases with the strength of the hydrogen bonding.

To understand the energetic properties of 2,3,4-Trinitrotolune (TNT) molecule, a quantum chemical calculation and the electronic charge density analysis have been performed. The density functional theory (B3P86/6-311G∗∗) calculation was carried out using Gaussian03 software. The energy-minimized wave function obtained from DFT was used for the charge density analysis. The inductive and steric effects of methyl and nitro substituents are not showing any unique geometric and bond topological features on C–C bonds of phenyl ring. A large charge accumulation (∼3.49 eÅ⁻³) is found in NO bonds; its corresponding Laplacian of electron density is ∼−27.6 eÅ⁻⁵, this indicates that the charges of the bonds are highly concentrated. Comparatively, the Laplacian of electron density of C–NO₂ (∼−17.1 eÅ⁻⁵) and C–CH₃ (−14.7 eÅ⁻⁵) bonds are found very less, confirm that the bond charges are significantly depleted; hence these bonds are considered as the weak bonds in the molecule. The isosurface of electrostatic potential of the molecule displays high electronegative region around the nitro groups, which are the reaction surface of the molecule. Present study predicts the relationship between the bond charge depletion and the bond sensitivity. Further, it proposes that, if the highly charge depleted bonds exhibit positive V_mid values, which are the sensitive bonds. We found, C–N bonds are the sensitive bonds in the molecule.

The hydrogen abstraction reactions of CF₃CHCl₂ + F (R1) and CF₃CHClF + F (R2) are investigated by dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) levels. Higher-level energies are obtained at the G3(MP2) method using the B3LYP and MP2-optimized geometries, respectively. Complexes with energies lower than those of the reactants are located at the entrance of these two reactions at the B3LYP level, respectively. Using the variational transition-state theory (VTST) with the inclusion of the small-curvature tunneling correction, the rate constants are calculated over a wide temperature range of 200–2000 K. The agreement between theoretical and experimental rate constants is good. In addition, the effect of fluorine substitution on reactivity of the C–H bond is discussed. Our calculations show that the fluorine substitution deactivates the C–H bond reactivity.

Density functional theory calculations were carried out for hydrogen atom binding on small Ag_nCu_m clusters (n + m ⩽ 5). It was found that hydrogen prefers to bind with Cu atoms when both Ag and Cu site co-exist in the cluster. In general the binding energies increase with the increasing Cu content for the given cluster size. The metal–H frequencies vary according to the way the metal atoms bound with hydrogen. The most favorable dissociation channels and the corresponding dissociation energies for the most stable bare clusters and cluster hydrides are determined.

Electronic structure of hydrogen bonded complexes of several organoselenium compounds was studied by means of the quantum chemistry methods (HF, DFT, MP2). Energy, geometric and spectral properties of the complexes justify the formation of weak hydrogen bond with selenium atom. However, the detailed analysis of orbital characteristics and the features of electronic distribution by means of topological parameters and integrated atomic properties included in Koch–Popelier criteria of hydrogen bond formation revealed similarities and differences in the properties of complexes with the Se…H bonds in comparison with traditional H-complexes, in which electronegative atoms of the second-row act as proton acceptors. The observed peculiarities of Se…H contacts can be explained by unusual electrostatic repulsion between Se and H atoms opposite to classical N…H or O…H contact and greater stabilization effect of charge transfer in the former. Se…H interaction is also characterized by greater covalence in comparison with conventional H-bonds.

The energies and structures of various peroxynitric acid (HOONO₂) isomers as well as their isomerization and dissociation reactions have been investigated at the CBS-QB3 level of theory. One HOONO₂, five HOOONO, three HONO₂O, and one HONO₃ isomers were found here. Among them, the HOONO₂ configuration (isomer 1) is the most stable one in both the gas phase and water, while the configuration HONO₃ (isomer 9) is the energetically highest. Moreover, four HONO⋯O₂ complexes, i.e., isomers a, b, c, and d, were found. Calculated results indicate that different isomers of HOONO₂ can rearrange into each other via one-step or multi-step isomerization. The isomerization and dissociation reactions involving isomer 1 were found to be hard to occur, which implies that isomer 1 is kinetically stable in the above reactions. For other isomers, their isomerization is relatively easy to occur. Additionally, the effects of aqueous solvation of water on the isomerization and dissociation reactions were also investigated.

The structure and stability of a series of porphyrin–nanotube complexes (ZnP–SWNT, H₂P–SWNT, ZnP–pp–SWNT and H₂P–pp–SWNT) were computed at the density functional theory (DFT) level. In addition, the first hyperpolarizability (β) was calculated using a coupled-perturbed-HF approach. The results indicate that complex stability is mainly dictated by the presence of Zn(II), with push–pull substituents also improving the stability. By taking the average interaction energy throughout the series of isomers found on the PES, the following stability order was predicted: ZnP–pp–SWNT > ZnP–SWNT ∼ H₂P–pp–SWNT > H₂P–SWNT. In addition, the push–pull groups, namely NH₂ and NO₂ in the present work, are essential to the first hyperpolarizability enhancement. For the free porphyrins ZnP–pp and H₂P–pp, the β values were (in 10⁻³⁰ esu⁻¹ cm⁵) 55 and 68, respectively. These values rose to 93 and 121 (increasing by around 40%) when the complexes with SWNT were formed. Thus, these results indicate that the hybrid nanocomposites represented by H₂P–pp–SWNT and ZnP–pp–SWNT might be interesting systems to investigate as lead compounds for NLO properties.

The hetero Diels–Alder (HDA) reactions of 1-diethoxyphosphonyl-1,3-butadiene with various nitroso dienophiles have been studied at the B3LYP/6-31G∗∗ level. Structural, energetic and electronic properties are discussed. These cycloadditions with nitroso dienophiles are characterized by a total proximal regioselectivity and an endo selectivity. The influence of the nitroso substitution on the activation barrier and the regiochemistry of the reaction is presented. The analysis of the chemical rearrangement along the intrinsic reaction pathway (IRC), based on bond order and on natural bond orbital (NBO) calculations, emphasizes the polar nature of these cycloadditions. Despite the early and the cyclic nature of the corresponding transition states, a two-center interaction governs this mechanism: these cycloadditions are Polar Diels–Alder reactions (P-DA).

The density functional theory (DFT) at the B3LYP/6-31G++ (d p) basis set level, ab initio calculations using the HF/6-31G++ (d p) and semi-empirical PM3 methods were performed on four triazole derivatives used as corrosion inhibitors, namely (5-amino 1,2,4 triazole (5-ATA), 5-amino-3-mercapto 1,2,4 triazole (5-AMT), 5-amino-3-methyl thio 1,2,4 triazole (5-AMeTT), 1-amino-3-methyl thio 1,2,4 triazole (1-AMeTT)), to investigate the correlation between its molecular structure and the corresponding inhibition efficiency (%IE). Quantum chemical parameters such as the highest occupied molecular orbital energy (E_HOMO), the lowest unoccupied molecular orbital energy (E_LUMO), energy gap (ΔE), dipole moment (μ), sum of total negative charges (TNC), molar volume (MV), electronegativity (χ), hardness (η), softness (σ) and the fraction of electrons transferred from the inhibitor molecule to the metal surface (ΔN), have been calculated. Furthermore, the adsorption energies of the inhibitors with the copper (1 1 0) surface were studied using molecular dynamic (MD) method. A good correlation between the theoretical data and the experimental results was found.

The effect of methanol as a solvent on 1,3-dipolar cycloaddition reaction of methyldiazoacetate to alkenes has been studied by DFT calculations. Reaction reactivity has been qualitatively analyzed by the frontier molecular orbital (FMO) approach and quantitatively estimated by hard and soft acid base (HSAB) theory. Application of the FMO theory to the (3 + 2)-cycloaddition process testifies that the specific solvation of methanol with reagents increases reaction rate in comparison to the gas phase, since it is reflected in Δε reduction. From the point of view of the HSAB theory was shown that the complexation of methanol with reagents plays a predominating role in the acceleration of the (3 + 2)-cycloaddition reaction, however, the neglecting of the solvent influence as polarized continuum will cause an appreciable error in the calculation results and, consequently, is not justified.