An investigation of hydrogen-bonding effects on the nitrogen and hydrogen electric field gradient and chemical shielding tensors in the 9-methyladenine real crystalline structure: a density functional theory study

Hydrogen-bonding effects in the real crystalline structure of 9-methyladenine, 9-MA, were studied using calculated electric field gradient, EFG, and chemical shielding, CS, tensors for nitrogen and hydrogen nuclei via density functional theory. The calculations were carried out at the B3LYP and B3PW91 levels with the 6-311++G basis set via the Gaussian 98 package. Nuclear quadrupole coupling constants, C(Q), and asymmetry parameters, eta(Q), are reported for (14)N and (2)H. The chemical shielding anisotropy, Deltasigma, and chemical shielding isotropy, sigma(iso), are also reported for (15)N and (1)H. The difference between the calculated parameters of the monomer and heptameric layer-like cluster 9-MA shows how much H-bonding interactions affect the EFG and CS tensors of each nucleus. This result indicates that N(10) (imino nitrogen) has a major role in H-bonding interactions, whereas that of N(9) is negligible. There is good agreement between the present calculated parameters and reported experimental data. Although some discrepancies were observed, this could be attributed to the different conditions which were applied for calculation and the experiments.     

Synthesis of methylene- and difluoromethylenephosphonate analogues of uridine-4-phosphate and 3-deazauridine-4-phosphate

Cytidine triphosphate synthetase (CTPS) catalyzes the formation of cytidine triphosphate from glutamine, uridine-5'-triphosphate (UTP), and adenosine-5'-triphosphate. Inhibitors of CTPS are of interest because of their potential as therapeutic agents. One approach to potent enzyme inhibitors is to use analogues of high energy intermediates formed during the reaction. The CTPS reaction proceeds via the high energy intermediate UTP-4-phosphate (UTP-4-P). Four novel analogues of uridine-4-phosphate (U-4-P) and 3-deazauridine-4-phosphate (3-deazaU-4-P) were synthesized in which the labile phosphate ester oxygen was replaced with a methylene and difluoromethylene group. The methylene analogue of U-4-P, compound 1, was prepared by a reaction of the sodium salt of tert-butyl diethylphosphonoacetate with protected, 4-O-activated uridine followed by acetate deprotection and decarboxylation. It was found that this compound undergoes relatively facile dephosphonylation presumably via a metaphosphate intermediate. The difluoromethylene derivative, compound 2, was prepared by electrophilic fluorination of protected 1. This compound was stable and did not undergo dephosphonylation. Synthesis of the methylene analogue of 3-deazaU-4-P, compound 3, was achieved by ribosylation of protected 4-(phosphonomethyl)-2-hydroxypyridine. Electrophilic fluorination was also employed in the preparation of protected 4-(phosphonodifluoromethyl)-2-hydroxypyridine which was used as the key building block in the synthesis of difluoro derivative 4. These compounds represent the first examples of a nucleoside in which the base has been chemically modified with a methylene or difluormethylenephosphonate group.     

An unsymmetrical approach to the synthesis of bismethylene triphosphate analogues

A protected, unsymmetrical bismethylene triphosphate analogue was prepared by sequential Michaelis-Arbuzov reactions on ethyl bis(halomethyl)phosphinates. This species was monodeprotected at one of the terminal phosphonate groups in high yield. The resulting monodeprotected compound was used to achieve the first syntheses of the bismethylene triphosphate analogues of UTP and CTP.     

[Omim][BF4], a green and recyclable ionic liquid medium for the one-pot chemoselective synthesis of benzoxazinones

An efficient procedure for the one-pot chemoselective synthesis of 2H-benzo[b][1,4]oxazin-3(4H)-one derivatives from their corresponding o-aminophenols is developed using DBU in the ionic liquid [omim][BF₄]. Upon completion of the reaction and separation of the product, the ionic liquid is recovered and successfully reused over nine recycles without any noticeable loss of performance.     

Electronic structure of sulfur terminated zigzag boron nitride nanotube: A computational study

Density functional theory (DFT) calculations have been performed to investigate the electronic and structural properties of sulfur (S) terminated models of zigzag boron nitride (BN) nanotube. Four models including pristine, boron (B) tip terminated by S, nitrogen (N) tip terminated by S, and both of B and N tips terminated by S have been considered for optimizations and chemical shielding (CS) parameters calculations. The results indicate that the B–N bond lengths do not detect any changes due to the S-termination but the band gaps and dipole moments detect notable changes especially for the model of the N-tip terminated by the S atoms. The CS parameters also indicate that the atoms of the models are divided into layers with similar parameters in each layer. In the model of the B-tip terminated by the S atoms, the CS parameters indicate strong chemical bonding of N- and S-layers; however, only some attractions between the B- and S-layers of the model of the N-tip terminated by the S atoms have been detected. In the model of B and N tips terminated by the S atoms, the most significant changes among the models are detected.     

Diastereoselective synthesis of spiro-functionalized tetraalkyl benzoisoquinopyrrolonaphthyridine-tetracarboxylates from isoquinoline, dialkyl acetylenedicarboxylates, and indane-1,3-dione

An effective route to spiro-functionalized fused polycyclic derivatives of isoquinoline is described via tandem reaction of isoquinoline, dialkyl acetylenedicarboxylates, and indane-1,3-dione.     

Radiosynthesis of [18F]-5-[2-(2-chlorophenoxy)phenyl]-1,3,4-oxadiazole-2-yl-4-fluorobenzoate: A labeled ligand for benzodiazepine receptors

5-[2-(2-Chlorophenoxy)phenyl]-1,3,4-oxadiazole-2-yl-4-fluorobenzoate 6a, the non-classic benzodiazepine ligand, has been shown to elicit a significant anticonvulsant activity against pentylenetetrazole-induced convulsion. In order to perform biological studies, we decided to prepare the [¹⁸F]-labeled compound. This compound was prepared in no-carrier-added (n.c.a) form from 5-[2-(2-chlorophenoxy)phenyl]-1,3,4-oxadiazole-2-yl-4-N,N,N-trimethylanilinium triflate 5 in one step at 125 °C in Kryptofix 2.2.2/[¹⁸F] and DMSO as the solvent followed by column chromatography. The synthesis took 20 minutes with an overall radiochemical yield of 70-75% (EOS) and a specific activity about 74 GBq/mmole and chemical-radiochemical purity more than 95%. This revised version was published online in July 2006 with corrections to the Cover Date.     

三维百合花状的CoNi2S4作为先进的双功能电催化剂用于氢析出和氧析出反应

为简化电解水催化剂的合成过程和优化电解水操作系统, 双功能电解水催化剂的研究, 特别是在碱性条件下同时具有优异催化氢析出和氧析出反应性能的双功能电催化剂的研究尤为重要. 其中, 过渡金属硫化物, 特别是 CoNi 硫化物, 被报道有与氢化酶类似的催化活性中心, 从而具有优异的催化氢析出和催化氧析出反应性能. 虽然有关对过渡金属硫化物的研究很多, 但主要集中在具有一维纳米线和二维纳米片形貌结构的过渡金属硫化物. 不幸的是, 这些形貌结构的过渡金属硫化物在电催化过程中容易聚集和受限于电荷传输能力. 三维纳米结构的材料具有较大的比表面积以分布更多的活性位点和拥有良好的电子传输能力, 所以, 开发三维纳米结构的过渡金属硫化物材料可能是改进其催化电解水性能的一个好途径. 本文采用简单的两步水热法, 通过硫化合成的 CoNi 前体得到了长于泡沫镍上的三维百合花状的 CoNi2S4(Co-Ni2S4/Ni). 它只需要 54 mV 的过电位即可获得 10 mA cm-2的催化氢析出反应电流, 是最好的碱性催化氢析出反应电极材料之一. 它在驱动 100 mA cm-2的催化氧析出反应电流时也只需要 328 mV 的过电位. 另外, 把 CoNi2S4/Ni 分别作为阴极和阳极组装成双电极碱性水电解槽时, 它只需要 1.56 V 的电压即可获取 10 mA cm-2的催化全电解水电流并具有良好的催化全电解水稳定性.扫描电子显微镜、透射电子显微镜和 N2吸脱附曲线测试结果表明, 该三维百合花状的 CoNi2S4/Ni 的表面粗糙度高和拥有多孔特性. 多孔结构的 CoNi2S4/Ni 可提供更多可接触的催化活性位点, 也有利于催化过程中的电解质和生成的气体的扩散与传递. 交流阻抗图谱测试结果表明, CoNi2S4/Ni 具有良好的电子传输能力. 另外, 不同于前期对尖晶石结构的硫化物 AB2S4的研究结果, XPS 结果表明, CoNi2S4/Ni 中含有 Niб+和 Sб-活性物种, 表明 CoNi2S4具有与活性氢化酶类似的活 性中心. Niδ+和 Sδ-可分别作为氢氧根和质子的接收体, 协助促进吸附的水分子的分离, 从而提高材料的催化性能. 所以, Niδ+和 Sδ-活性物种的出现, 大比表面积的三维百合花状多孔结构和良好的电荷传输能力等特性集合于 CoNi2S4/Ni 上使得CoNi2S4/Ni 具有优异的催化氢析出和催化氧析出反应性能.     

Investigation the chemical reactivity of positions N‐3, C‐5 and C6‐methyl group in biginelli type compounds and synthesis of new dihydropyrimidine derivatives

The compounds 5‐ethoxycarbonyl‐1,6‐dimethyl‐4‐(3‐nitrophenyl)‐3,4‐dihydropyrimidin‐2(lH)‐one (5) and 5‐ethoxycarbonyl‐1‐phenyl‐6‐methyl‐4‐(3‐nitrophenyl)‐3,4‐dihydropyrimidin‐2(lH)‐one (1) were prepared by the Biginelli condensation method and they converted to eight N‐3 substituted dihydropyrimidines using NaH and various electrophiles (ClCO₂Et, TsCl, Ac₂O, AcCl and PhCOCl). Compound (1) was mono‐brominated at the C₆‐methyl group using bromine solution. Reaction of the bromo derivative with amino nucleophiles such as methyl amine and cyclohexyl amine produced two pyrrolo‐pyrimidine derivatives. All the compounds except 5‐ethoxycarbonyl‐1‐phenyl‐6‐methyl‐4‐(3‐nitrophenyl)‐3,4‐dihydropyrimidin‐2(l_H)‐one ( 4 ) were purified by recrystallization methods. The structure of all the new compounds was confirmed using FT‐ir,¹H nmr, ¹³C nmr spectral and elemental analyses methods.     

Fluidized Bed Polyethylene Reactor Modeling in Condensed Mode Operation

Summary: Modeling of fluidized bed reactor for polyethylene production in the condensed mode operation is proposed in this paper. A two-phase model including the emulsion and bubble phases with the constant bubble size is employed to describe hydrodynamic behavior of the reactor. The kinetics of Ziegler-Natta polymerization is also modeled with a two active site model. The phase behavior and solubility of low molecular weight components in polyethylene are estimated with the Sanchez-Lacombe equation of state. The validation of the model is carried out with industrial data for an ethylene and 1-butene copolymerization with the isopentane as a condensable component. The simulation results are in good agreement with industrial data. The model is also used to study quantitatively the effect of the inlet stream temperature to the reactor and isopentane concentration in the reactor on the production rate. For instance, in a typical process, when the inlet stream temperature changes 10 °C, the production rate will alter about 40%. Furthermore, the change of the isopentane concentration around 1 mole percentage leads to a variation of production rate about 14%.