Independent sets,cliques and hamiltonian graphs

LetG be ak-connected (k 2) graph onn vertices. LetS be an independent set ofG. S is called essential if there exist two distinct vertices inS which have a common neighbor inG. LetV _r, be a clique which is a complete subgraph ofG. In this paper it is proven that if every essential independent setS ofk + 1 vertices satisfiesS V _r , thenG is hamiltonian, orG{u} is hamiltonian for someu V _r, orG is one of three classes of exceptional graphs. Our theorem generalizes several well-known theorems.     

ULTRASONICALLY DISPERSED POTASSIUM AS DEPROTONATING AND COUPLING AGENT──SYNTHESIS OF 1，6－DINITRILES IN SITU

ＵＬＴＲＡＳＯＮＩＣＡＬＬＹ　ＤＩＳＰＥＲＳＥＤ　ＰＯＴＡＳＳＩＵＭ　ＡＳ　ＤＥＰＲＯＴＯＮＡＴＩＮＧ　ＡＮＤ　ＣＯＵＰＬＩＮＧ　ＡＧＥＮＴ──ＳＹＮＴＨＥＳＩＳ　ＯＦ　１，６－ＤＩＮＩＴＲＩＬＥＳ　ＩＮ　ＳＩＴＵＵＬＴＲＡＳＯＮＩＣＡＬＬＹＤＩＳＰ...     

Re(CO)5Br-catalyzed addition of carboxylic acids to terminal alkynes: a high anti-Markovnikov and recoverable homogeneous catalyst

The addition of carboxylic acids to terminal alkynes is efficiently catalyzed by the early transition-metal complex Re(CO)5Br in toluene or n-heptane at 110 degrees C in an air atmosphere, affording the anti-Markovnikov adducts in good yields with high selectivity. In most cases, the reactions afford unusual Z-adduct predominantly. When n-heptane was used as solvent, Re(CO)5Br can be partly recovered from the reaction mixture.     

A solvothermal synthesis and the structure of K4Ag2Sn3S9 x 2KOH

A novel layered K(4)Ag(2)Sn(3)S(9) x 2KOH was synthesized solvothermally. A mixture of ethanol and HSCH(2)CH(SH)CH(2)OH was used for the synthesis. The HSCH(2)CH(SH)CH(2)OH plays an important role and appears to serve as a mineralizer for the solvothermal reaction. A layer comprises Sn(3)S(9)(6)(-) clusters linked by Ag(+) ions and possesses two types of 1D channels in which potassium ions are located. The compound crystallizes in the monoclinic P2(1)/m space group [a = 7.8071(2) A, b = 27.3508(1) A, c = 10.5008(2) A, alpha = 90 degrees, beta = 103.874(1) degrees, gamma = 90 degrees, Z = 4]. Its crystal structure, composition analysis, and IR are presented.     

Electric Potential (psi(delta) and psi(d)) at Outer Helmholtz Plane and Midplane on the Clay Colloid Surface with Overlapping Flat Double Layers

An anion negative adsorption equation in the condensed colloidal suspension with overlapping flat double layers was derived according to Gouy-Chapman theory. The electric potential at the outer Helmholtz plane (OHP), psi(delta), and the electric potential at the midplane, psi(d), were numerically solved by computer using the anion negative adsorption equation on the basis of experiments. The results showed that psi(delta) and psi(d) increase with the decrease of the distance between two clay plates, lambda, at first in the given electrolyte concentration. When lambda is smaller than 50-70 ?, psi(d) remains almost unchanged while psi(delta) declines remarkably with the further decrease of lambda. The change of psi(d)/psi(delta) with lambda can explain and manifest overlapping degree of flat double layers more appropriately than psi(d) in previous works. Due to compression of the flat double layer on the clay colloid surface at increasing electrolyte concentration, the magnitude of the electrical potentials at OHP and midplane is considerably reduced at a given lambda. Copyright 2000 Academic Press.     

An aldol approach to the total synthesis of (+)-brefeldin A

A convergent selective route to (+)-brefeldin A (BFA) and 7-epi-BFA was developed, with the crucial C-4/C-5 stereogenic centers were established using Crimmins asymmetric aldolization.     

β-D-核糖(RI)与一价、二价金属离子相互作用的理论研究

在HF和MP2水平用全电子(AE)和相对论有效芯势(RECP)方法研究了Ⅰa、Ⅰb、Ⅱa和Ⅱb族金属离子与β-D-核糖(RI)的相互作用.结果表明,RECP能可靠地用于重金属离子;二价金属离子(M2+)比一价金属离子(M+)更易使β-D-核糖(RI)变形;二价金属离子络合物(RI-M2+)比一价金属离子络合物(RI-M+)稳定.电荷布居分析的结果支持上述结论.     

DETECTION OF T_g BY MODULATED DIFFERENTIAL SCANNING CALORIMETRY

The glassy transition of the polyethylene terephthalate (PET) sampleswhich have been subjected to solvent induced crystallization (SINC) was investigated bymodulated differential scanning calorimetry (MDSC) and density measurement. The dif-ferential of heat capacity signal, d C_p/dT from MDSC, was used to monitor the SINCprocess. It reveals that the T_g temperature shifts to higher value with the advancement ofSINC. When the toluene-immersing time was longer (168h), the detection of T_g becomemore difficult, because some smaller peaks emerged at the lower temperatures and theseare explained as the movement of small segments in the amorphous region. These observedresults are due to the morphology and structure introduced by the SINC process.     

Extensive Studies on the AlEt3/THF-Promoted Diastereoselec-tive Tandem Rearrangement/Reduction of a-Hydroxy (Amino) Heterocyclopropane: An Efficient Approach to 2-Quaternary 1,3-Diheteroatom Units

Introduction Diastereoselective construction of 1,3-diheteroatom units, especially 1,3-amino alcohols and 1,3-diols, has always been in high demands in synthetic organic chemistry, as they are widely present in many natural products and potent drugs, such as nucleoside antibiot-ics and the HIV protease inhibitors.1,2 They have also been used as chiral ligands for asymmetric catalysts and as synthetic intermediates.3 Although several methods for the construction of 1,3-amino alcohols have been…     

羟基自由基引发的马来酸酐聚合反应机理AM1研究

用AM1方法计算了马来酸酐、羟基自由基及其加成产物α-羟基丁二酸酐基自由基的电子结构、电荷分布和键级.应用前线轨道理论和成键三原则研究了羟基自由基引发下马来酸酐聚合过程中α-羟基丁二酸酐基自由基活性中间体参与反应的可能性及其自由基聚合反应机理.计算结果表明:马来酸酐基态分子的HOMO和LUMO分别对应于双键CC的成键π-MO和反键π -MO;马来酸酐的羟基自由基加成反应活化能计算值为55 7kJ/mol;马来酸酐在羟基自由基引发下的自由基聚合产物是链式结构,与实验事实相符.