Ab Initio Calculation of the Structure of 5-Chloro-1,2,4-triazole

Three tautomeric forms of 5-chloro-1,2,4-triazole were calculated using the Hartree-Fock (ab initio) and Meller-Plesset methods in the 6-31G(d) basis. The ³⁵Cl NQR parameters were calculated using the occupancies of the 3p-components of the valence p-orbitals of the chlorine atom. The structure of this triazole was derived from the data obtained.     

Structure and dynamics of the CrIII ion in aqueous solution: Ab initio QM/MM molecular dynamics simulation

Structural and dynamical properties of the Cr(III) ion in aqueous solution have been investigated using a combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation. The hydration structure of Cr(III) was determined in terms of radial distribution functions, coordination numbers, and angular distributions. The QM/MM simulation gives coordination numbers of 6 and 15.4 for the first and second hydration shell, respectively. The first hydration shell is kinetically very inert but by no means rigid and variations of the first hydration shell geometry lead to distinct splitting in the vibrational spectra of Cr(H(2)O)(6) (3+). A mean residence time of 22 ps was obtained for water ligands residing in the second hydration shell, which is remarkably shorter than the experimentally estimated value. The hydration energy of -1108 +/- 7 kcal/mol, obtained from the QM/MM simulation, corresponds well to the experimental hydration enthalpy value.     

含磺酸,羧酸双功能基树脂对混合氨基酸的分离性能研究

对苯乙烯-丙烯腈-二乙烯基苯(S/AN/DVB)交联共聚机理研究曾发现,丙烯腈作为第三单体,可使DVB竞聚率比在S/DVB共聚体系中降低,相互分离显著,从而有利于合成性能较好的离子交换树脂;本文在此基础上,将S/AN/DVB共聚体中的氰基水解成羧基来合成含磺酸、羧酸双功能基树脂,解决了前人研究中仅有50％的氰基水解成羧基的问题,并首次将这种树脂应用于混合氨基酸的分离。     

Fragmentations of Hydroxymethyl Radical Cation: An Ab Initio Study

The probable fragmentation channels of hydroxymethyl radical cation were studied through the H‐and H₂‐abstraction and C‐O bond breaking reactions including their related isomerization reactions. The energy barriers for hydroxymethyl cation undergoing isomerization reactions are generally higher than those undergoing the concerted 1,2‐elimination reactions to generate CHO⁺ and H₂. The fragmentation reaction to form CHO⁺ and H₂ through the 1,2‐elimination pathways is the major fragmentation channel for hydroxymethyl cation, consistent with the experimental observation. H abstraction from the hydroxyl group of CH₂OH⁺ is more difficult than that from the methylene group. The feasible path to lose H is to generate CHOH₂⁺ through hydrogen transfer reaction as the first step and then to undergo H‐elimination to generate trans‐CHOH⁺. Among all the reactions found in this study, the OH‐elimination to generate CH₂⁺ has the highest energy barrier. Our calculation results indicate that the major signals contributed from the related species of hydroxymethyl cation found in the mass spectrum should be m/e 29, m/e 30.     

An Ab Initio Study of the Complexes Formed by the Positive Acetylene Ion with the Hydrogen and Nitrogen Molecules and the Argon Atom

The complexes formed by the positive acetylene ion with the hydrogen molecule, the nitrogen molecule, and the argon atom are investigated with ab initio calculations using the 6-311G** and the 6-31+G(2df,2pd) basis sets. MP2/6-311G** energies and optimum geometries are obtained, as well as single-point MP3, MP4, and QCISD(T) energies with the MP2/6-311G** optimized geometries. Single-point calculations are performed with the 6-31+G(2df,2pd) basis set at MP2/6-311G** optimized geometries.     

Anomalous Density and Permittivity Effects on the Structure of Water

The relationship between the structural peculiarities of the hydrogen bond net and the anomalous behavior of density and dielectric permittivity of water is investigated. The degree of ordering in the network of hydrogen bonds is described in terms of the structural functions, among which the most important ones are tetrahedricity and the mean number of hydrogen bonds per molecule. The temperature dependence of the number of hydrogen bonds per molecule is discussed in terms of the analysis of the experimental temperature dependences of density and permittivity of water on the saturation line. The estimated density of hexagonal ice at the melting point is reproduced in terms of the concepts suggested for analysis; the estimate is rather close to the experimental value. Possible applications of the new approach are discussed.     

Molecular Conformation of 2,2'-Dinitrodiphenylamine According to Dielectric Studies and Ab Initio Quantum Chemical Calculations

According to the results of ab initio calculations employing the HF/6-31G* approach, the isolated 2,2'-dinitrodiphenylamine molecule exists as an sc, sc conformer stabilized by a symmetric intramolecular bifurcated (three-center) hydrogen bond. In protophilic solvents (1,4-dioxane), the conformational equilibrium is also shifted in the direction of this rotational isomer.     

一维链状银配合物的合成及晶体结构

The complex [Ag(L)]NO₃ (1) [L=1,5-bis(imidazol-1-yl)pentane] was synthesized in ethanol and water, and characterized by elemental analysis and X-ray crystal structural analysis. The result shows that the complex crystallizes in monoclinic, space group P2₁/c with a=0.695 7(4) nm, b=0.988 7(4) nm, c=1.974 8(7) nm, β=91.934(19)°, V=1.357 5(10) nm³, Z=4, D_calc=1.831 g·cm^-3, F(000)=752, μ=1.50 cm^-1, the final R=0.036 6, wR=0.091 0. The coordination environment of Ag(Ⅰ) is nearly linear with two N atoms, and each L ligand links two Ag(Ⅰ) atoms using its two imidazolyl N atoms to generate an infinite one-dimensional (1D) chain structure. The 1D chains are further connected by C-H…O hydrogen bonds to give a three-dimensional structure. CCDC: 609514.     

电子转移的外电场效应──苯分子与苯正离子自由基间电子转移的从头算研究

基于半经典电子转移理论,结合量子化学计算,在HF/DZP水平上,研究外电场作用下平行的苯分子-苯正离子自由基体系(C₆H₄)₂⁺的分子内电子转移问题.在给体和受体几何构型优化的基础上,用线性反应坐标确定电子转移过渡态,分别用两态变分方法和基于Koopmans定理的分子轨道跃迁能方法计算电子转移矩阵元V_AB,讨论了V_AB对给体和受体中心距d的指数衰减关系.取中心距为0.6nm,研究了外电场对反应热的影响,计算得到在不同外电场强度下分子内气相电子转移的速率常数k.     

Ab Initio and MNDO Calculation of the Intermediates of N-Diisopropylphosphoryl L-lspartic Acid

Many biological processes are regulated by the phosphorylation and dephosphorylation of amino acid residues in the proteins^[1]. High-coordinate phosphorus intermediates forming from amino acid residues in protein are of great importance in the process of phosphorylation and dephosphorylation of protein. It has been proposed that the mechanisms of phosphorylation through penta-coordinate intramolecular mixed carboxylic-phosphoric anhydride intermediates^[2]. It was found that α-COOH group^[3] and β-COOH group^[4] in aspartic acid had different activities in the ester exchange on phosphorus. So it is important to discuss which carboxylic acids is involved in the penta-coordinate phosphorus intermediate of aspartic acid.     

Re_2(edt)_4的电子结构和电子吸收光谱的从头算研究

用密度泛函理论中的B3LYP/CEP-4G方法对Re2（edt）4进行了结构优化和自然键轨道计算,并用单电子激发组态相互作用方法（CIS）计算了Re2（edt）4的分子轨道和激发态。结果表明Re-Re之间不存在明显的三重键,进而很好地解释了Re2（edt）4分子的电子吸收光谱。     

甲醛与乙醛,甲醚,硝基甲烷相互作用的从头算研究

用6-31G、全构型优化,研究了甲醛与乙醛、甲醚、硝基甲烷的相互作用。结果表明所有超分子稳定构型都包含2个C—H—O氢键的平面环状结构。H—O距离为0.228～0.264 nm,作用能为—19～—24 kJ/mol,与二聚水的作用能接近。稳定性主要取决于甲基上取代基Y吸电子能力以及环状结构中氢键张力。STO-3G不很适用于研究这类分子的相互作用。     

Ab Initio MP2 and Density Functional Theory Computational Study of AcAlaNH2 Peptide Hydration: A Bottom‐Up Approach

AcAlaNH₂?n H₂O (n=1–13) complexes have been proposed as models to account for water solvent effects on the molecular properties of N‐acetyl‐L ‐alanine amide. Ab initio computations are planned to evaluate peptide–water interactions and to provide a means for approximating relative effects of the short‐range many‐body interactions arising in real solution without introducing any external parameters intended to quantify empirical or semiempirical potential‐energy functions. The present bottom‐up approach reveals the formation of compact ring clusters of water molecules strongly bonded to peptidic polar groups throughout hydrogen bonds. The explicit coordination of water molecules around the peptide renders the fully extended (FE) and polyproline II (PPII) conformers more stable with respect to the 3₁₀ helix or γ turn. The alternance of donor and acceptor groups on both sides of the FE and PPII conformers allows for synergy and extensive H‐bonding.     

Preparation of Phosphoric-Carboxylic Cation Exchangers from Wood Cellulose

Phosphorus-containing cellulose cation exchangers were synthesized by reaction of wood cellulose with orthophosphoric acid and the ternary polymer from glycidylmethacrylate, styrene, and maleic anhydride. The effects of the ratio of reactants, temperature, and duration of the reaction on the phosphorylation and exchange capacity of the modified cellulose material were studied.     

Ab Initio Dot Structures Beyond the Lewis Picture

The empirical Lewis picture of the chemical bond dominates the view chemists have of molecules, of their stability and reactivity. Within the mathematical framework of quantum mechanics, all this chemical information is hidden in the many-particle wave function Ψ. Thus, to reveal and understand it, there is great interest in enhancing the Lewis model and connecting it to computable quantities. As has previously been shown, the Lewis picture can often be recovered from the probability density |Ψ|2 with probabilities in agreement with valence bond weights: the structures appear as most likely positions in the all-electron configuration space. Here, we systematically expand this topological probability density analysis to molecules with multiple bonds and lone pairs, employing correlated Slater-Jastrow wave functions. In contrast to earlier studies, non-Lewis structures are obtained that disagree with the prevalent picture and have a potentially better predictive capability. While functional groups are still recovered with these ab initio structures, the boundary between bonds and lone pairs is mostly blurred or non-existent. In order to understand the newly found structures, the Lewis electron pairs are replaced with spin-coupled electron motifs as the fundamental electronic fragment. These electron motifs—which coincide with Lewis’ electron pairs for many single bonds—arise naturally from the generally applicable analysis presented. An attempt is made to rationalize the geometry of the newly-found structures by considering the Coulomb force and the Pauli repulsion.     

Structures and energies of seleno derivatives of biuret.Ab Initio comparative studies of diselenobiuret, selenobiuret, and selenothiobiuret

Ab initio studies (LCAO-MO method) on conformers of three seleno derivatives of the biuret molecules diselenobiuret [I], selenobiuret [II], and selenothiobiuret [III] were carried out at the Hartree-Fock (HF) and MP2 levels. The molecular geometries of these species were fully optimized at the HF level and characterized by analysis of the harmonic vibrational frequencies using a split-valence triple-zeta basis set augmented by a set ofd polarization functions on heavy atoms andp polarization functions on hydrogen atoms [TZP(d, p)]. The total energies of the HF-optimized structures were calculated at the MP2 (frozen core) level using a larger TZP (2df, 2pd) basis set. The potential energy searches revealed a total of 11 minimum-energy conformers (assigned astrans-trans, trans-cis, cis-trans, andcis-cis) and seven transition-state species for the title molecules. The two predicted conformers for diselenobiuret (Ia=trans-trans andIc=cis-cis) are characterized byC ₂ and the third byC _s symmetry. For selenothiobiuret two forms (IIIa=trans-trans andIIId=cis-cis) possessC ₁ and two (IIIb=trans-cis andIIIc=cis-trans) possessC _s symmetries, respectively. For selenobiuret, four formsIIa=trans-trans (C₁),IIb=trans-cis (C _s),IIc=cis-trans (C ₁), andIId=cis-cis (C₁), were obtained as a result of gradient optimization. Comparison of the relative energies for the considered species indicated that thecis-trans forms are the most stable conformations for all three systems at both the HF and MP2 levels of theory.     

Molecular Structure of ortho-Fluoronitrobenzene Studied by Gas Electron Diffraction and Ab Initio MO Calculations

The molecular structure of ortho-fluoronitrobenzene (o-FNB) has been investigated by gas-phase electron diffraction and ab initio MO calculations. The geometrical parameters and force fields of o-FNB were calculated by ab initio and DFT methods. The obtained force fields were used to calculate vibrational amplitudes required as input parameters in an electron diffraction analysis. Within the experimental error limits, the geometrical parameters obtained from the gas-phase electron diffraction analysis are mostly in agreement with the results obtained from the ab initio calculations. The main results are: the molecular geometry of o-FNB is nonplanar with a dihedral angle about C–N of 38(3)°. The r _g (C–F) bond is shortened to 1.307(13) Å in comparison with r _g (C–F) = 1.356(4) Å in C₆H₅F.     

草酰胺桥联双核铜配合物结构单元的从头算

运用Gaussian 94W量子化学程序包 ,采用LanL2DZ基组 ,对草酰胺桥联双核铜配 (聚 )合物结构单元Cu2 (oxen) (OH) 2 [H2 oxen =N ,N' 二 ( 2 胺乙基 )草酰胺 ](包括顺、反构型及其单、三重态电子组态 )进行从头算研究 ,探讨该配合物结构单元的稳定性 ,并从电荷布居及分子轨道组成等电子结构特征分析这种配合物反式三重态比较稳定的原因 .计算结果与实验规律相符合 .