Remote substituent effects on bond dissociation energies of para-substituted aromatic silanes

UB3LYP/6-31G(d) and ROMP2/6-311++G(d,2p) methods were used to calculate the Si-X bond dissociation energies (BDEs) of a number of para-substituted aromatic silanes (4-Y-C(6)H(4)-SiH(2)X, where X = H, F, Cl, or Li). It was found that the substituent effect on the Si-H BDE of 4-Y-C(6)H(4)-SiH(3) was small, as the slope (rho(+)()) of the BDE- regression was only 0.09 kJ/mol. In comparison, the substituent effect on the Si-F BDE of 4-Y-C(6)H(4)-SiH(2)F was much stronger, whose rho(+ )()value was -2.34 kJ/mol. The substituent effect on the Si-Cl BDE of 4-Y-C(6)H(4)-SiH(2)Cl was also found to be strong with a rho(+)() value of -1.70 kJ/mol. However, the substituent effect on the Si-Li BDE of 4-Y-C(6)H(4)-SiH(2)Li was found to have a large and positive slope (+9.12 kJ/mol) against. The origin of the above remarkably different substituent effects on the Si-X BDEs was found to be associated with the ability of the substituent to stabilize or destabilize the starting material (4-Y-C(6)H(4)-SiH(2)X) as well as the product (4-Y-C(6)H(4)-SiH(2)* radical) of the homolysis. Therefore, the direction and magnitude of the effects of Y-substituents on the Z-X BDEs in compounds such as 4-YC(6)H(4)Z-X should have some important dependence on the polarity of the Z-X bond undergoing homolysis. This conclusion was in agreement with that from earlier studies (for example, J. Am. Chem. Soc. 1991, 113, 9363). However, it indicated that the proposal from a recent work (J. Am. Chem. Soc. 2001, 123, 5518) was unfortunately not justified.     

Spontaneous fractal aggregation of gold nanoparticles and controlled generation of aggregate-based fractal networks at air/water interface

The spontaneous fractal aggregation of as-prepared cetytrimethylammonium bromide (CTAB)-capped gold nanoparticles was found to happen at the air/water interface after spreading their chloroform solution on water surfaces. Aided by Langmuir-Blodgett techniques, these fractal aggregates can be interconnected with each other to form aggregate-based fractal networks.     

Exploration of the nature of active Ti species in metallic Ti-doped NaAlH4

Clarification of the nature of active Ti species has been a key challenge in developing Ti-doped NaAlH(4) as a potential hydrogen storage medium. Previously, it has been greatly hindered by the invisibility of Ti-containing species in conventional analysis techniques. In the present study, for the first time, the catalytically active Ti-containing species have been definitely identified by X-ray diffraction in the hydrides doped with metallic Ti. It was found that mechanical milling of a NaH/Al mixture or NaAlH(4) with metallic Ti powder resulted in the formation of nanocrystalline Ti hydrides. The variation of the preparation conditions during the doping process leads to a slight composition variation of the Ti hydrides. The catalytic enhancement arising upon doping the hydride with commercial TiH(2) was quite similar to that achieved in the hydrides doped with metallic Ti. Moreover, the cycling stability that was previously established in metallic Ti-doped hydrides was also observed in the hydrides doped with TiH(2). These results clearly demonstrate that the in situ formed Ti hydrides act as active species to catalyze the reversible dehydrogenation of NaAlH(4). The mechanism by which Ti hydrides catalyze the reversible de-/hydrogenation reactions of NaAlH(4) was discussed.     

Longitudinal study of iodine in market milk and infant formula via epiboron neutron activation analysis

A global radionuclide monitoring system is being engineered as part of a multi-technology verification system for the Comprehensive Nuclear Test Ban Treaty. The system detects airborne radioactive aerosols and gases that can indicate nuclear weapons test debris. The backbone of the system is a network of 80 remote detection stations that utilize high-volume air sampling and high-resolution gamma spectrometry to provide in-situ assay and near-real time reporting. These stations are linked to the International Data Centre, which is a central data processing hub where raw spectral data is automatically processed, analyzed, and disseminated to the states parties. Measurements are categorized based on spectral content to determine which contain anomalous anthropogenic radionuclides that require intensive radiochemical analysis at a certified laboratory. The resulting system has the capability to measure microbecquerel concentrations of radionuclides and provide accessible data products within minutes of field measurements. During the past year of international operations, the minimum detectable concentrations and spectroscopy processing statistics were recorded as a function of geographical location and time. The results show that this system is an effective tool for nuclear test monitoring, as well as other applications such as radiological emergency response, public health monitoring, and scientific research.     

Palladium complexes with ethylene-bridged bis(N-heterocyclic carbene) for C-C coupling reactions

A series of new ethylene-bridged bis(imidazolium) halides with various N-substitutions were synthesized. Complexation of these imidazolium halides with Pd(OAc)₂ produced new Pd(II) ethylene-bridged bis(carbene) complexes. Crystallographic analyses of some of the new imidazolium salts and Pd(II) complexes were determined. Applications of these seven-member palladacycles in Suzuki and Heck coupling reactions produced comparable catalytic activities to those of six-member analogs.     

两个双核铁（Ⅲ）和三足配体的配合物的合成和晶体结构

运用三足四齿配体三（2－甲基吡啶）胺（TPA）或三（2－甲基苯丙咪唑）胺（TBA）,得到两个双核铁(III)配合物，[Fe₂L₂(μ₂-O)(μ₂-p-NH₂-C₆H₄COO)]³⁺ (L = TPA, 1 和 L = TBA, 2)。两个配合物均为单斜晶系，空间群为P2(1)/c.晶胞参数 1: a = 1.4529(4), b = 1.6622(5), c = 2.0625(6) nm, β= 100.327(5)º, V = 4.900(3) nm³, z = 4, F(000) = 2344, 分子量M_r = 1142.91, Dc = 1.549 g/cm³, R₁ = 0.0544, R₂ = 0.0962. 2: a = 1.3378(4), b = 2.1174(7), c = 2.4351(7) nm, β= 97.315(6)º, V = 6.842(4) nm³, z = 4, F (000) = 3116, 分子量M_r = 1505.08, Dc = 1.444 g/cm³, R₁ = 0.0793, R₂ = 0.1623. 在两个双核铁(III)配合物中，中心的三价铁和配体TPA或TBA上的四个氮原子和两个氧原子通过不同的桥形成一个畸变的八面体构型。     

Distorted wave Born and three-body distorted wave Born approximation calculations of the fully differential cross section for electron-impact ionization of nitrogen molecules

Currently there are no reliable theoretical approaches for calculating fully differential cross sections (FDCSs) for low-energy electron-impact ionization of large molecules. We have recently proposed the orientation-averaged molecular orbital (OAMO) for calculating cross sections averaged over molecular orientations. In this paper, we use the OAMO to calculate distorted wave Born approximation (DWBA) and molecular three-body distorted wave (M3DW) Born approximation FDCS for electron-impact ionization of the nitrogen molecule. Both coplanar symmetric and asymmetric FDCSs are investigated in the energy range of 35.6-400 eV. By comparing with the experimental data, we found that the M3DW is reasonably accurate in this energy range. We also found that the postcollision interaction plays a sufficiently important role and that the DWBA is not reliable.     

Synthesis of Iso-condensed Heteroaromatic Pyrroles and Their Application in the Preparation of Conducting Polymers

Iso-condensed heteroaromatic pyrroles 1 are 10 -electron aromatic compounds. They are of interest from both theoretical and synthetic points of view. They are the cyclic analogues of heteroaromatic ortho-quinodimethanes 2, and can react with dienophiles in a Diels-Alder reaction to give the synthetically useful cycloadducts 3. Many of them are also of potential pharmaceutical importance because they are isosteric with indoles. Iso-condensed heteroaromatic pyrroles 1 can be used also as the monomers for the synthesis of new conducting polymers 5 with special properties and characteristics. However, the methods for the efficient preparation of the iso-condensed heteroaromatic pyrroles are quite limited. Iso-condensed heteroaromatic pyrroles are generally unstable in acidic conditions and are easily oxidized by air. In our laboratories, we developed three methods for the preparation of this labile heterocyclic ring system under acidic, neutral, and basic conditions.1 Synthesis of the conjugated systems such as 6 for OLED applications will also be discussed.     

Influence of pretreatment conditions on low-temperature CO oxidation over Au/MOx/Al2O3 catalysts

Composite oxide MO_x/Al₂O₃ supported gold catalysts for low-temperature CO oxidation were prepared and investigated. The presence of transition metal oxide was proved to be beneficial to the improvement of catalytic performance of Au/Al₂O₃ catalysts for low-temperature CO oxidation. Furthermore, the influence of various pretreatment conditions on Au/MO_x/Al₂O₃ catalysts was studied carefully. The image of TEM showed that gold catalyst with small gold particles only in the form of a fine dispersion exhibited highly catalytic activity. The XPS, Fourier transform infrared (FTIR) spectroscopy characterization results of Au/FeO_x/Al₂O₃ catalyst showed that gold catalysts having partially oxidized gold species have the best catalytic performance. One possible pathway for CO oxidation on Au/FeO_x/Al₂O₃ catalyst is that the CO adsorbed on gold particles reacts with adsorbed oxygen, which is possible to occur on oxygen vacancies on the support or at the metal–support interface.     

Photochemical Reaction of Decacarbonyldirhenium with Thiophene and Crystal Structure of Tetradecacarbonylhydridotrirhenium

The photochemical reaction of Re₂(CO)₁₀ with thiophene in hexane solution was investigated under vacuum. Three rhenium clusters: H₂Re₃(CO)₁₂, HRe₃(CO)₁₄ and Re₃(CO)₁₄(OH)₄, were isolated. The structure of Fellmann-Kaesz cluster Complex HRe₃(CO)₁₄ was determined by use of the X-ray diffraction method. The three rhenium atoms form a plane of symmetry and L: Re₁Re₂Re₃ is 107°. The ten carbonyl groups bonded to the two terminal rhenium atoms Re₁ and Re₃, are staggered with respect to the central rhenium atoms. The bond lengths are 3.10 Å for Re₂-Re₃ and 3.34 Å for Re₁-Re₂. The bridging hydride is between Re₁ and Re₂.