An ab initio Molecular Orbital Study of the Conformational Properties of all-(Z)-Cyclododeca-1,4,7,10-tetraene

Summary.  Ab initio HF/6-31G^* and MP2/6-31G^*//HF/6-31G^* methods were used to calculate the structure optimization and conformational interconversion pathways for all-(Z )-cyclododeca-1,4,7,10-tetraene. This compound adopts the symmetrical crown (C _4v) conformation. Ring inversion takes place via symmetrical intermediates, such as boat-chair (BC, C _s) and twist (C _2h) conformers and requires about 22.3 kJ · mol⁻¹. The calculated strain energies for BC and twist conformers are 5.9 and 13.5 kJ · mol⁻¹. The results of semiempirical AM1 calculations for structural parameters and relative energies of the important geometries of the title compound are in good agreement with the results of ab initio methods. Received July 9, 2001. Accepted September 26, 2001     

The Keto-Enol Equilibrium of Pentane-2,4-dione Studied by ab initio Methods

 The mechanism of the keto-enol interconversion of pentane-2,4-dione (trivial name: acetylacetone, acac) was examined at the restricted Hartree-Fock (HF) level and the DFT correlation functional BLYP method using the 6-311G^** basis, both included in the program GAUSSIAN 98. Two initial enol forms are considered: the omega and sickle forms, related by a rotation of 180° around the OC*CC bond. The study is restricted to the through-space transfer of the hydroxyl proton to C(2). The two geometry-optimized enol forms are planar; the geometry optimization of the diketone forms leads to the same non-planar structure, regardless of the starting enol geometry. The transition state of the through-space omega-enol→diketone conversion has also a non-planar structure, indicating that the hydroxyl proton moves outside of the CCC plane. The BLYP-calculated energy barrier of the forward (omega-enol→diketone) conversion is 245 kJ·mol⁻¹, that of the reverse (diketone→omega-enol) conversion 222 kJ·mol⁻¹; thus, an almost symmetric barrier, which is not thermally accessible, is defined. The energy barrier for the sickle-enol→diketone conversion is considerably lower (187 kJ·mol⁻¹), to access the sickle form from the more stable omega form, a rotation is needed (energy barrier: 88 kJ·mol⁻¹). The HF-calculated barriers are 1.3–1.4 times higher than those obtained with the BLYP method.     

A Molecular Orbital Approach to Conformational Study of the Polyisocyanates

A conformational study of poly(alkyl isocyanates) with both non‐chiral and chiral side groups (R) is presented. For this purpose the conformational preferences of model compounds CH₃—(CONR)_m—CH₃, where m is the number of monomers, were investigated by means of quantum mechanical methods. The influence of the number of monomers and the side chain conformation on the relative stability of the different helical minima has been systematically studied. Finally, the influence of the solvent chloroform has been examined by using a self‐consistent reaction‐field. The results provide a detailed picture of the modulation exerted by these factors on the helical preferences of these compounds.     

Configurational and Conformational Properties of 1,3,7,9-Tetraphospha-Cyclododeca-1,2,7,8-tetraene: An Ab Initio Study and NBO Analysis

An investigation employing the ab initio molecular orbital (MO) and density functional theory (DFT) methods to calculate structural optimization and conformational interconversion pathways for the two diastereoisomeric forms, (±) and meso configurations of 1,3,7,9-tetraphospha-cyclododeca-1,2,7,8-tetraene (1) was undertaken. Two axial symmetrical conformations are found for (±)-1 configuration. (±)-1-TB axial symmetrical form is found to be about 0.35 and 0.99 kcal mol^?1 more stable than (±)-1-Crown axial symmetrical conformation, as calculated by HF/6-31G*//HF/6-31G* and B3LYP/6-31G*//HF/6-31G* levels of theory, respectively. The unsymmetrical meso-1-TBCC form is found to be the most stable geometry, among the various conformations of meso-1 configuration. HF/6-31G*//HF/6-31G* and B3LYP/6-31G*//HF/6-31G* results showed that between the two most stable conformations of (±) and meso configurations, (±)-1-TB is more stable than meso-1-TBCC by about 3.35 and 2.43 kcal mol^?1, respectively. In addition, MP2/6-31G* and B3LYP/6-311+G** results showed that the (±)-1-TB form is about 1.10 and 2.36 kcal mol^?1 more stable than the meso-1-TBCC form. Further, NBO results revealed that in the most stable form of meso configuration (meso-1-TBCC), the sum of the π* allenic antibonding orbital occupancies (Σ π *_occupancy) is greater than dl configuration ((±)-1-TB). Also, NBO results indicated that in the (±)-1-TB conformer, the sum of σ and π allenic moieties bonding orbital deviations (Σ σ _dev+Σ π _dev) from their normal values, is lower than in the meso-1-TBCC form.     

Reactions of N , N -Disubstituted 5-Arylmethylidene-2-amino-thiazol-4(5 H )-ones with CH Acids

 N,N-Disubstituted 5-arylmethylidene-2-aminothiazol-4(5H)-ones reacted with diethyl malonate, ethyl benzoylacetate, acetylacetone, or cyclopentadiene in refluxing toluene and in presence of powdered sodium to give the respective 5-arylmethylidene-2′-amino-2,5′-bithiazolylidene-4,4′-dione derivatives in moderate yields. 5-Benzylidene-2-morpholin-4-yl-2-thiazol-4(5H)-one reacted with malononitrile in toluene and in presence of powdered sodium under mild conditions to afford the 1:1 adduct, benzylmalononitrile, and 2-morpholin-4-yl-2-thiazol-4(5H)-one. However, similar treatment of 5-(4-methoxyphenylmethylidene)-2-morpholin-4-yl-2-thiazol-4(5H)-one with malononitrile yielded the 2,5′-bithiazolylidene-4,4′-dione derivative together with 4-methoxyphenylmethylidene malononitrile. Treatment of 5-benzylidene- and 5-(4-methoxyphenylmethylidene)-2-morpholin-4-yl-2-thiazol-4(5H)-ones with 3-phenyl-4-oxo-2-thioxo-1,3-thiazolidine in refluxing toluene and in presence of powdered sodium produced 5-arylmethylidene-3-phenyl-4-oxo-2-thioxo-1,3-thiazolidines in good yields. The structures of all products were deduced from microanalytical and spectroscopic data, mechanistic details are discussed.     

Bis -( bis -(1,3-diaminopropane)-copper(II) Tetracyanonickellate(II)) Tetrahydrate: Preparation, Characterization, and Crystal Structure of a Novel Molecular Tetracyanonickellate

 [Cu(tn)₂Ni(CN)₄]₂ċ4H₂O and Cu(tn)₂Ni(CN)₄ (tn = 1,3-diaminopropane) were prepared and characterized. The hydrate is unstable on air and readily dehydrates to Cu(tn)₂Ni(CN)₄. Crystal structure analysis of the hydrate at 150 K revealed a novel tetranuclear molecular structure of the tetracyanonickellate. The building elements are two [Cu(tn)₂]²⁺ cations (coordination numbers of Cu: 5 and 6, respectively), two [Ni(CN)₄)²⁻ anions, and crystal water. The two cations are linked by one tetracyanonickellate anion via bridging cyano groups placed in cis positions. The second anion is bound weakly (Cu-N = 2.82 ?) via one μ₂-bridging cyano ligand. The tetranuclear molecules and pairs of solvate water molecules are linked by strong hydrogen bonds, thus forming infinite planes which are linked in the third dimension by considerably weaker hydrogen bonds.     

The Unusual Thermodynamic Stability Order of the First-series Transition Metal Complexes with N,N,N′-tri(2-pyridylmethyl)glycinamide and the Crystal Structure of the Nickel(II) Complex

A new pentadentate tripodal peptide ligand N,N,N′-tri(2-pyridylmethyl)glycinamide (L) has been synthesized. The crystal structure of its nickel(II) complex, [NiL(H₂O)] · 1.17ClO₄ · 0.17H₃O · 0.03H₂O (1), has been determined by X-ray diffraction. In the complex, the deprotonated ligand L acts in a pentadentate fashion and coordinates to the nickel(II) ion through five nitrogen atoms, while the sixth position is occupied by a water molecule. The units of the complex are connected as a 3D honeycomb network by intermolecular hydrogen bonds. The thermodynamic properties of the ligand L with the first-series transition metal ions Co(II), Ni(II), Cu(II) and Zn(II) have been investigated by potentiometric titration and the results show that the order of their stability constants does not conform to the Irving–Williams serial. The reason why the stability constants of the Cu(II) complex are unconventionally small is proposed.     

Polymorphism and Pressure Driven Thermal Spin Crossover Phenomenon in [Fe( abpt )2(NC X )2] ( X =?S, and Se): Synthesis, Structure and Magnetic Properties

Summary.  The monomeric compounds [Fe(abpt)₂(NCX)₂] (X = S (1), Se (2) and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) have been synthesized and characterized. They crystallize in the monoclinic P2₁/n space group with a = 11.637(2) ?, b = 9.8021(14) ?, c = 12.9838(12) ?, β = 101.126(14)°, and Z = 2 for 1, and a = 11.601(2) ?, b = 9.6666(14) ?, c = 12.883(2) ?, β = 101.449(10)°, and Z = 2 for 2. The unit cell contains a pair mononuclear [Fe(abpt)₂(NCX)₂] units related by a center of symmetry. Each iron atom, located at a molecular inversion center, is in a distorted octahedral environment. Four of the six nitrogen atoms coordinated to the Fe(II) ion belong to the pyridine-N(1) and triazole-N(2) rings of two abpt ligands. The remaining trans positions are occupied by two nitrogen atoms, N(3), belonging to the two pseudo-halide ligands. The magnetic susceptibility measurements at ambient pressure have revealed that they are in the high-spin range in the 2 K–300 K temperature range. The pressure study has revealed that compound 1 remains in high-spin as pressure is increased up to 4.4 kbar, where an incomplete thermal spin crossover appears at around T _1/2 = 65 K. Quenching experiments at 4.4 kbar have shown that the incomplete character of the conversion is a consequence of slow kinetics. Relatively sharp spin transition takes place at T _1/2 = 106, 152 and 179 K, as pressure attains 5.6, 8.6 and 10.5 kbar, respectively. Corresponding author. E-mail: jose.a.real@uv.es Received June 12, 2002; accepted July 1, 2002     

Interaction of Dimethyltin(IV) with DNA Constituents

 The interaction of dimethyltin(IV) (DMT) with some selected DNA constituents was investigated potentiometrically. The stepwise formation constants of the complexes were determined, and the concentration distribution of the various complex species was evaluated as a function of pH. The effect of dioxane on the protonation constants of the ligands and the formation constants of dimethyltin(IV) complexes are discussed. The thermodynamic parameters Δ H° and ΔS° were calculated from the temperature dependence of the equilibrium constants.     

An Ab Initio Molecular Orbital Study of Isophosphinines

Ab initio molecular orbital calculation at HF/6-31G*, HF/6-31G**, HF/6-311G**, HF/6-311++G**, RMP2-FC/6-31G*, and B3LYP/6-31G* levels of theory for geometry optimization and MP4(SDQ)/6-31G* for a single-point total energy calculation are reported for phosphinine and 13 isophosphinines 7-19 . Isomers 7-11 with an allenic system are calculated to be 8-18 kcal mol m 1 more stable than structures 12-17 with an acetylenic moiety. The calculated energy difference (66.19 kcal mol m 1 ) between phosphinine and the most stable isophosphinine (1-phospha-1,2,4-cyclohexatriene, 10 ) is smaller than the difference (78.96 kcal mol m 1 ) between benzene and the most stable isobenzene (cyclohexa-1,2,4-triene, 2 ). The isophosphinines 18 and 19 , with a butatriene moiety, are calculated to be the least stable isomers.     

Synthesis and structural studies of (2 E ,3 E ,6 R ,9 E ,11 E )-6-isopropenyl-3,6,10-trimethyl-5,8-dioxa-4,9-diazadodeca-3,9-diene-2,11-dione dioxime, ligand and its Mono-, homo, and heterodinuclear copper(II)/nickel(II) complexes

This article describes the synthesis of a new (2E,3E,6R,9E,11E)-6-isopropenyl-3,6,10-trimethyl-5,8-dioxa-4,9-diazadodeca-3,9-diene-2,11-dione dioxime (H₂hmdm), and its mono-, homo, and heterodinuclear copper(II)/nickel(II) complexes. Elemental analyses, stoichiometric and spectroscopic data of the metal complexes indicated that the metal ions are coordinated to the oxime and imine nitrogen atoms (C=N). The Cu(H₂hmdm), molecule coordinates to the second copper(II) ion through the oximate oxygens to afford a binuclear structure doubly bridged by the oximate groups in the cis arrangement. In the dinuclear complexes, in which the first Cu^II ion was complexed with nitrogen atoms of the oxime and imine groups, the second Cu^II ion is ligated with the 1,10-phenanthroline nitrogen atoms. Ligand and its mononuclear copper(II), homo and heterodinuclear copper(II)–nickel(II) complexes of (H₂hmdm) were characterized by elemental analyses, magnetic moments, ¹H-n.m.r. and ¹³C-n.m.r., i.r., and mass spectral studies. The data support the proposed structure of H₂hmdm and its complexes.     

Cyclododecatetraene tetraepoxide: highly diastereofacial epoxidation of all-(Z)-1,4,7,10-cyclododecatetraene and selective synthesis of bridged bis-oxocanes

Epoxidation of all-(Z)-1,4,7,10-cyclododecatetraene has been investigated with use of m-CPBA and dimethyldioxirane in anhydrous solvent. The diastereoselectivity of multiple epoxidation steps is complete affording only exo,exo,exo,endo-1,4,7,10-tetraepoxide. To understand this result, the step-by-step epoxidation reaction was investigated and each step was found to be highly regio- and stereoselective. Finally, a Lewis acid-catalyzed ethanolysis or treatment with HBr/KBr of the tetraepoxide gave rearranged diepoxy-oxabicyclo[5.5.1]tridecane (bridged bis-oxocanes) in which eight stereocenters were controlled.     

苯分子与Rh(111)面相互作用的分子轨道研究

用群分解EHMO法研究了苯在Rh(111)面上的化学吸附和成键性质。以Rh_7原子簇模型模拟Rh(111)表面,得到结合能为41kJ/mol,吸附高度2.60。被吸附的苯分子通过π键和金属原子的d轨道相互作用而受到活化,有利于在(111)表而上的化学反应。     

超细CeM(M=Mg,Sr)Ox复合氧化物的性能研究

采用聚合物前驱体法首次合成了超细粒子CeM(M=Mg，Sr)O_x复合氧化物粉体。通过XRD、TEM、BET等技术对合成的粉体进行了表征，结果表明CeO₂与碱土金属形成的CeM(M=Mg，Sr)O_x新粉体材料比表面积为66～69 m²·g^-1，经800 ℃、1 000 ℃焙烧2 h后，粉体平均晶粒仍保持在10～20 nm，基本不发生团聚现象，具有较好的抗烧结能力。用甲烷催化燃烧为模型反应，发现该粉体催化剂具有很高的催化活性，优于单组分CeO₂。