Nitrogen NMR spectroscopy: Application to some substituted pyrroles

A series of ¹⁵N labeled 2-acylpyrroles was prepared and the nitrogen and proton n.m.r. spectra obtained. ¹⁵N chemical shifts for these compounds are reported for the first time. No correlation between the nitrogen chemical shift and any Hammett substituent constant could be found. No variation in J(¹⁵N? H) was observed for any compound with changes in solvent, temperature or concentration, ruling out any observable tautomeric equilibria for these systems. An increase in J(¹⁵N? H) with the addition of electron withdrawing groups indicates increasing polarization of the N? H bond and acidity of these molecules. Two and three bond ¹⁵N couplings are also reported.     

On Rusakov's n-ary rs-Groups

Properties of n-ary groups connected with the affine geometry are considered. Some conditions for an n-ary rs-group to be derived from a binary group are given. Necessary and sufficient conditions for an n-ary group -derived from an additive group of a field to be an rs-group are obtained. The existence of non-commutative n-ary rs-groups which are not derived from any group of arity m<n for every n3, r>2 is proved.     

On K s -free subgraphs in K s+k -free graphs and vertex Folkman numbers

Extending the problem of determining Ramsey numbers Erdős and Rogers introduced the following function. For given integers 2 ≤ s < t let f _s,t (n) = min{max{|S|: S ⊆ V (H) and H[S] contains no K _s }}, where the minimum is taken over all K _t -free graphs H of order n. This function attracted a considerable amount of attention but despite that, the gap between the lower and upper bounds is still fairly wide. For example, when t=s+1, the best bounds have been of the form Ω(n ^1/2+o(1)) ≤ f _s,s+1(n) ≤ O(n ^1−ɛ(s)), where ɛ(s) tends to zero as s tends to infinity. In this paper we improve the upper bound by showing that f _s,s+1(n) ≤ O(n ^2/3). Moreover, we show that for every ɛ > 0 and sufficiently large integers 1 ≪ k ≪ s, Ω(n ^1/2−ɛ ) ≤ f _s,s+k (n) ≤ O(n ^1/2+ɛ . In addition, we also discuss some connections between the function f _s,t and vertex Folkman numbers.     

半导体纳米激光

半导体纳米激光的研究是目前纳米技术（或纳米光子学）和半导体激光交汇产生的研究前沿。本文将综述这一领域最近一些最激动人心的进展。我们将集中讨论两种半导体纳米结构的纳米激光：自下而上生长而成的纳米线和自上而下刻蚀制成的纳米柱状结构。本文将综述这些纳米激光器的特殊特征,特别是利用表面等离子效应而成制成的金属{半导体等离子体激光,即目前世界上最小激光器的最新进展。由于这些纳米激光器的微小的尺寸以及对光在空间限制增强,我们必须重新检验半导体激光器中某些熟悉的概念在纳米尺度上的正确性和含义,例如模式增益和光限制因子（CF）的概念。本文将从统一的观点解释光限制因子在电介质和等离子体纳米激光器中的某些似乎反常的行为。在本文的通篇论述中,我们将尝试回答究竟激光器的尺寸能够小到什么程度,或者激光器的尺寸是否存在一个最小极限等基本问题。     

Investigations on the Synthesis of Zeolite ZSM-5 (III)

The effect of the chemical composition of the reacting mixture (RM) on the amount of zeolite ZSM-5 and other crystalline phases and the amorphous phase as well in the obtained preparations was investigated. Ethylenediamine (ED) served as a templating agent. The quantitative estimation of the phase composition was made on the ground of X-ray diffraction measurements. Zeolite ZSM-5 of maximum phase purity of 96%, contaminated mainly with quartz and mordenite, was obtained.