Switching on the Metathesis Activity of Re Oxo Alkylidene Surface Sites through a Tailor‐Made Silica–Alumina Support

Re oxo alkylidene surface species are putative active sites in classical heterogeneous Re‐based alkene‐metathesis catalysts. However, the lack of evidence for such species questions their existence and/or relevance as reaction intermediates. Using Re(O)(=CH‐CH=CPh₂)(OtBu_F6)₃(THF), the corresponding well‐defined Re oxo alkylidene surface species can be generated on both silica and silica–alumina supports. While inactive on the silica support, it displays very good activity, even for functionalized olefins, on the silica–alumina support.     

Orbital dependence of the fermi liquid state in Sr2RuO4

We have used angle-resolved photoemission spectroscopy to determine the bulk electronic structure of Sr(2)RuO(4) above and below the Fermi liquid crossover near 25 K. Our measurements indicate that the properties of the system are highly orbital dependent. The quasi-2D gamma band displays Fermi liquid behavior while the remaining low energy bands show exotic properties consistent with quasi-1D behavior. In the Fermi liquid state below 25 K, the gamma band dominates the electronic properties, while at higher temperatures the quasi-1D beta and alpha bands become more important.     

Samarium diiodide-catalyzed diastereoselective pinacol couplings

A complex of samarium diiodide (SmI(2)) with tetraglyme catalyzes the intermolecular pinacol coupling of aromatic or aliphatic aldehydes at loadings of 10 mol % in the presence of Me(2)SiCl(2) and Mg. Diastereoselectivity of up to 95/5 (+/-/meso) has been achieved for aliphatic aldehydes and up to 19/81 (+/-/meso) for aromatic aldehydes. De values of up to 99% have been achieved in intramolecular pinacol coupling reactions using the SmI(2)/tetraglyme/Mg/Me(2)SiCl(2) catalytic system.     

Identifying Sn Site Heterogeneities Prevalent Among Sn‐Beta Zeolites

In recent years, various protocols on preparing Lewis acidic Sn‐β zeolite hydrothermally and postsynthetically have been reported. However, very little is known about the effects of different synthesis protocols on the Sn(IV) speciation in the final material. Even the effects of individual synthesis parameters within a certain preparation method have not been studied systematically. Here, we demonstrate that hydrothermally synthesized Sn‐β zeolites prepared via very similar recipes show significantly different ¹¹⁹Sn‐NMR spectra, suggesting different Sn site speciation. Among postsynthetically prepared Sn‐β zeolites, less variation in the resulting ¹¹⁹Sn‐NMR spectra have been observed, indicating a more reproducible synthesis procedure compared to hydrothermal synthesis in fluoride media. This work highlights the importance of ¹¹⁹Sn‐NMR measurements to elucidate the precise local geometry of the Sn heteroatoms in Sn‐β, and the need to quantify the number of reactive Sn sites on each sample that participate in a given catalytic reaction, in order to accurately compare materials prepared by different routes.     

Soft car sequencing with colors: Lower bounds and optimality proofs

This paper is a study of the car sequencing problem, when feature spacing constraints are soft and colors of vehicles are taken into account. Both pseudo-polynomial algorithms and lower bounds are presented for parts of the problem or family of instances. With this set of lower bounds, we establish the optimality (up to the first non-trivial criteria) of 54% of best known solutions for the benchmark used for the Roadef Challenge 2005. We also prove that the optimal penalty for a single ratio constraint N/P can be computed in O(P) and that determining the feasibility of a car sequencing instance limited to a pair of simple ratio constraints can be achieved by dynamic programming. Finally, we propose a solving algorithm exploiting these results within a local search approach. To achieve this goal, a new meta-heuristic (star relinking) is introduced, designed for the optimization of an aggregation of criteria, when the optimization of each single criterion is a polynomial problem.     

Random walks on finite volume homogeneous spaces

Extending previous results by A. Eskin and G. Margulis, and answering their conjectures, we prove that a random walk on a finite volume homogeneous space is always recurrent as soon as the transition probability has finite exponential moments and its support generates a subgroup whose Zariski closure is semisimple.     

Monochromatic triangles in two-colored plane

We prove that for any partition of the plane into a closed set C and an open set O and for any configuration T of three points, there is a translated and rotated copy of T contained in C or in O.     

Introduction to random walks on homogeneous spaces

Let a ₀ and a ₁ be two matrices in SL(2, ${\mathbb{Z}}$ ) which span a non-solvable group. Let x ₀ be an irrational point on the torus ${\mathbb{T}^2}$ . We toss a ₀ or a ₁, apply it to x ₀, get another irrational point x ₁, do it again to x ₁, get a point x ₂, and again. This random trajectory is equidistributed on the torus. This phenomenon is quite general on any finite volume homogeneous space.