Etude du mecanisme de la cyclotrimerisation catalytique de l'hexyne-3 par l'intermediaire d'un complexe cyclobutadienique du nickel

The reduction of nickel halide by metals in low oxidation states, in a boiling solution of 3-hexyne in tetrahydrofuran leads to catalytic and quantitative cyclotrimerisation of the unsaturated compound. During the reaction, tetraethylcyclobutadienenickel(II) dibromide can be isolated in appreciable yield. A mechanism is proposed for the cyclotrimerisation.     

Blocker sets, orthogonal arrays and their application to combination locks

Let A denote a set of order m and let X be a subset of A^k+1. Then X will be called a blocker (of A^k+1) if for any element say (a₁,a₂,…,a_k,a_k+1) of A^k+1, there is some element (x₁,x₂,…,x_k,x_k+1) of X such that x_i equals a_i for at least two i. The smallest size of a blocker set X will be denoted by α(m,k)and the corresponding blocker set will be called a minimal blocker. Honsberger (who credits Schellenberg for the result) essentially proved that α(2n,2) equals 2n² for all n. Using orthogonal arrays, we obtain precise numbers α(m,k) (and lower bounds in other cases) for a large number of values of both k and m. The case k=2 that is three coordinate places (and small m) corresponds to the usual combination lock. Supposing that we have a defective combination lock with k+1 coordinate places that would open if any two coordinates are correct, the numbers α(m,k) obtained here give the smallest number of attempts that will have to be made to ensure that the lock can be opened. It is quite obvious that a trivial upper bound for α(m,k) is m² since allowing the first two coordinates to take all the possible values in A will certainly obtain a blocker set. The results in this paper essentially prove that α(m,k) is no more than about m²/k in many cases and that the upper bound cannot be improved. The paper also obtains precise values of α(m,k) whenever suitable orthogonal arrays of strength two (that is, mutually orthogonal Latin squares) exist.     

Protein Folding and β‐Sheet Proteins

The aim of this comprehensive review is to critically evaluate the progress in research in the area of protein folding. In the first section, we discuss the various models proposed to explain the protein folding paradox. In the succeeding section of the review, a detailed account of the developments in our understanding of the folding path ways of β‐sheet proteins is provided.     

填充型具有微孔结构的磺化聚芳醚砜/聚醚砜复合质子交换膜的制备及性能

制备了基于磺化聚芳醚砜(SPAES)及聚醚砜(PES)的填充型复合质子交换膜, 研究了其吸水率、 尺寸变化、 热-机械特性、 质子电导率、 甲醇透过性及稳定性等性能. 通过浸入沉淀相转化法, 采用磺化度分别为30%(S30), 40%(S40)及50%(S50)的SPAES与PES制备了系列微孔型复合质子交换膜 Sx-y(x为SPAES的磺化度, y为SPAES的质量分数); 然后利用真空抽滤法在微孔中填充S50制备了相应的填充型复合质子交换膜Sx-y+F50. 结果表明, 由于微孔的引入及皮层结构的存在, Sx-y膜在低离子交换容量(IEC)条件下仍具有较高的电导率、 优良的机械强度、 优异的化学稳定性及较低的甲醇透过性. 经S50填充后, Sx-y+F50膜的IEC及电导率明显提升, 甲醇透过率大幅下降, 但机械强度及化学稳定性未见劣化. 其中S30-40+F50膜(IEC=0.69 mmol/g)的综合性能最佳, 其质子电导率在90 ℃水中达到50.4 mS/cm; 经140 ℃水处理24 h后失重率仅为8.2%, 质子电导率降低仅9%; 经过芬顿试剂(3% H₂O₂, 20 mg/L FeSO₄, 80 ℃, 1 h)处理后失重率仅为0.66%; 甲醇透过率仅为6.8×10^-8 cm²/s.     

Suppression of mode partition noise in a multiwavelength semiconductor laser through hybrid mode locking

We demonstrate the suppression of intensity fluctuations, which are known as mode partition noise, in a multiwavelength semiconductor laser by using a hybrid mode-locking scheme. The laser design incorporates a saturable absorber and a gain-modulated semiconductor optical amplifier, along with spectral filtering, in an external cavity to achieve multiwavelength hybrid mode locking. The mode-locked laser produces an error-free (pulse Q>13) 300-MHz optical pulse train in each of three wavelength channels.     

Dispersion-managed semiconductor mode-locked ring laser

A novel breathing-mode external sigma-ring-cavity semiconductor mode-locked laser is developed. Intracavity pulse compression and stretching produce linearly chirped pulses with an asymmetric exponential temporal profile. External dispersion compensation reduces the pulse duration to 274 fs (within 10% of the bandwidth limit).     

The preparata codes and a class of 4-designs

An extension theorem for t-designs is proved. As an application, a class of 4-(4^m + 1,5,2) designs is constructed by extending designs related to the 3-designs formed by the minimum weight vectors in the Preparata code of length n = 4^m, m ≥ 2. © 1994 John Wiley & Sons, Inc.     

Quasi-residual quasi-symmetric designs

It is shown that for each λ ? 3, there are only finitely many quasi-residual quasi-symmetric (QRQS) designs and that for each pair of intersection numbers (x, y) not equal to (0, 1) or (1, 2), there are only finitely many QRQS designs.A design is shown to be affine if and only if it is QRQS with x = 0. A projective design is defined as a symmetric design which has an affine residual. For a projective design, the block-derived design and the dual of the point-derivate of the residual are multiples of symmetric designs.     

关于n-维矩阵群

An n × matrix A = {a_ij} will be called a 2-dimensional matrix of order n . We shall study groups of 2m -dimensional matrices of order n over a field of characteristic O with respect to an associative matrix product and we obtain the dimensions of such matrix groups.     

Effect of pH on the fluorescence and phosphorescence spectra of indolecarboxylic acids. Evidence for the existence of a singlet excited-state indole acid dianion

Room-temperature electronic absorption, fluorescence, and low-temperature (77 K) phosphorescence spectra of a series of indolecarboxylic acids have been measured in predominantly aqueous solutions at pH 1.2, 7.0, 12.5, and in 5 N NaOH. The ¹L_a and ¹L_b π, π ¹ states have been assigned to the electronic transitions for all the compounds under study. The quenching of fluorescence, the appearance of a new fluorescence emission band, red-shifted by 40 nm, in very alkaline media, and the disappearance of the indole carboxylate short-wavelength band have been attributed to the formation of an excited singlet-state indolate carboxylate (dianion) stabilized by hydrogen bonding with the solvent. The existence of two successive excited singlet-state prototropic equilibria is discussed.