The mechanism of N-haloamide promoted electrophilic additions. high regio and stereoselectivity in the conversion of 2-tert-butyl-3,6-dihydro-2H-pyran into bromohydrins and in the opening of the corresponding epoxides

The reactions of cis- and trans-2-tert-butyl-4,5-epoxytetrahydropyran with HBr and with LAH have been examined as a model for the nucleophilic step of the reaction of the corresponding olefin with NBA in aqueous dioxane. A remarkable 90:10 preference for electrophilic attack syn to the tert -butyl group in the NBA reaction is found and shows that the two epoxides, as well as the intermediate epibromonium ions, undergo nucleophilic attack with high preference for diaxial opening, even when this requires reaction at carbon 5, which is more subject than carbon 4 to the unfavourable inductive effect of the pyran ring oxygen. These results constitute a further proof in favour of a mechanism of N-haloamide promoted electrophilic additions in which the electrophilic step is rapidly reversible and product composition is determined during the nucleophilic step.     

Highly sintered nickel oxide: surface morphology and FTIR investigation of CO adsorbed at low temperature

The monolayer of CO molecules adsorbed at low temperature on highly sintered nickel oxide, gives rise to a very symmetric IR absorption band at 2136 cm⁻¹ (θ_max) with a full-width at half-maximum (FWHM) of 3.7 cm⁻¹. This band shifts to higher frequenc upon decreasing the coverage, reaching the 2152 cm⁻¹ value for θ→0. The observed shift is due to changes in the lateral interactions (dynamic and static) among the adsorbed molecules. The observed spectral simplicity implies that most of the adsorbed CO molecules occupy crystallographically identical sites with a similar environment. Moreover, the remarkably small half-width indicates that inhomogeneous broadening effects, due to surface defects, are very small and that NiO microcrystals behave as single crystals. The morphology of microcrystals has been studied by SEM, AFM and HRTEM techniques: it was concluded that the surface termination of the sample is mainly represented by the (100) and (111) faces.     

The Weyl transform and bounded operators on Lp(Rn)

A multiplier theorem for the Weyl transform is proved. This theorem is used to derive sufficient conditions for the boundedness of a general operator on L^p($\text{R}$ⁿ). An application to multipliers of the Hermite expansion is given.     

Surface relaxation and reconstruction in diamond-like crystals

The MNDO semiempirical method has been used to study the structure of the (111) surface of diamond-like crystals employing two model clusters. Numerical results are in agreement with previous computations for the small X₄H₉ clusters and become very similar to experimental data when employing the larger X₉H₁₅ clusters. These latter clusters allow also the study of surface reconstruction. Our results suggest that the Haneman model of 2 × 1 reconstruction could be equally valid for silicon and diamond, but in the latter case the difference in vertical shifts is too small to be experimentally observed.     

湍流的相干结构:推演方法和结果

本文讨论壁面湍流发展的相干结构的观点.在简要的历史文献综述后,我们回顾一些基本观点, 并且介绍相干结构的思想.基于大量主要是由实验所得的结果,本文通过广泛运用的事件检 测技术,探讨湍流边界层内部和外部区域发生的现象.我们从边界层内部区域发生的现象、 边界层外部大尺度运动的发展和涡结构动力学的角度来描述流动的现象.在文章的 第2部分,介绍从背景流动中推演出湍流相干结构的各种方法以及在各种方法框架下所得到的结果, 讨论速度梯度张量不变量、压力的Hessian矩阵分析和本征正交分解等方法.每一个过程 都有``相干结构'的特定的定义,满足恰当的数学构架,并可以对湍流数据做相干结构动力 学分析.这一工作可能会对当前流体动力学家在湍流研究中用到的最新理论和技术的传播有 所贡献.     

The dynamical problem of an elastic plate resting on a two-parameter foundation which does not react in tension

Summary This paper analyzes the dynamic problem of an elastic plate which is unilaterally constrained by a two-parameter foundation of Pasternak of Hetényi type. It is assumed that no friction exists at the area of contact. A regularizing technique has been used both in formulating the problem and in the subsequent discussion. Some results of existence and uniqueness of the solution are given. In the application example described, a finite element approach is used to numerically analyze an axisymmetric circular plate. From the numerical results obtained, it can be observed how the solution is influenced by the principal parameters involved.

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Sommario Il lavoro esamina il problema dinamico di una piastra elastica vincolata unilatepalmente su una fondazione a due parametri del tipo Hetényi o Pasternak. Il contatto è supposto privo di attrito. Il problema viene formulato e discusso mediante una tecnica di regolarizzazione: se ne forniscono alcuni risultati relativí all'esistenza ed unicità della soluzione. In particolare si analizza dal punto di vista numerico mediante discretizzazione agli elementi finiti il caso di una piastra circolare assialsimmetrica. I risultati numerici mostrano l'influenza sulla soluzione dei principali parametri da cui essa dipende.

     

CFD modelling of a spray deposition process of paint

The paint deposition process by spraying has been studied by means of Computational Fluid Dynamics in order to predict the final thickness of the coating and to determine theoretically the overspraying phenomenon. The VOF model has been used to describe the impact phenomena onto the wall and the Euler-Lagrange approach to simulate droplet trajectories on their way to the surface. Particular attention has been devoted to the prediction of the maximum diameter reached by an impinging droplet at the end of the spreading phase. This diameter is very important in the study of the paint processes because the high viscosity and the small surface tension of paints reduce the impingement practically only to the spreading phase. Two different configurations of atomizers have been considered. The air flux provides a finer atomisation of the liquid, gives to the droplets the necessary velocity to reach the wall, but is also the main cause of overspray since the small droplets tend to follow its deviation near the wall.