When III-V growth is interrupted for processing, in the ambient laboratory environment for example, regrown heterojunction quality has been rather disappointing in comparison to uninterrupted epitaxial growth. We have conducted a search for surface chemical preparations on In,,53G%.47As which would produce the highest-quality InP/Ino.53Ga,,.47As regrown heterojunction interface, as measured by surface recombination velocity (SRV) . After an extensive survey, we have found that dilute bromine-based etching solutions are best for preparing a free Ino.53G%.47 As surface for subsequent InP regrowth. The resulting InP/Ino.53G%,47As interfacial SRV is 5 20 cm/s, comparable to heterojunctions grown without any interruption at all.
Silicon Siting The synthesis of many pharmaceutical and agrochemical compounds requires selective functionalization of multiple different sites on aromatic ring frameworks. The size and electronic properties of the first substituent added can influence where the next one is likely to end up. Cheng and Hartwig (p. 853 , see the Perspective by Tobisu and Chatani ) discovered a rhodium-catalyzed reaction that is particularly sensitive to size that places a silicon substituent as far away as possible from the largest group already on the ring. The silicon group can then be replaced with carbon, oxygen, nitrogen, or halide substituents as needed.
Many active pharmaceuticals, herbicides, conducting polymers, and components of organic light-emitting diodes contain arylamines. For many years, this class of compound was prepared via classical methods, such as nitration, reduction and reductive alkylation, copper-mediated chemistry at high temperatures, addition to benzyne intermediates, or direct nucleophilic substitution on particularly electron-poor aromatic or heteroaromatic halides. However, during the past decade, palladium-catalyzed coupling reactions of amines with aryl halides have largely supplanted these earlier methods. Successive generations of catalysts have gradually improved the scope and efficiency of the palladium-catalyzed reaction. This Account describes the conceptual basis and utility of our latest, "fourth-generation" palladium catalyst for the coupling of amines and related reagents with aryl halides. In the past five years, we have developed these catalysts using the lessons learned from previous generations of catalysts developed in our group and in other laboratories. The ligands on the fourth-generation catalyst combine the chelating properties of the aromatic bisphosphines of the second-generation systems with the steric properties and strong electron donation of the hindered alkylphosphines of the third-generation systems. The currently most reactive catalyst in this class is generated from palladium and a sterically hindered version of the Josiphos family of ligands that possesses a ferrocenyl-1-ethyl backbone, a hindered di-tert-butylphosphino group, and a hindered dicyclohexylphosphino group. This system catalyzes the coupling of aryl chlorides, bromides, and iodides with primary amines, N-H imines, and hydrazones in high yield. The reaction has broad scope, high functional group tolerance, and nearly perfect selectivity for monoarylation. It also requires the lowest levels of palladium that have been used for C-N coupling. In addition, this latest catalyst has dramatically improved the coupling of thiols with haloarenes to form C-S bonds. Using ligands that lacked one or more of the structural elements of the most active catalyst, we examined the effects of individual structural elements of the Josiphos ligand on catalyst activity. This set of studies showed that each one of these elements contributes to the high reactivity and selectivity of the catalyst containing the hindered, bidentate Josiphos ligand. Finally, we examined the effect of electronic properties on the rates of reductive elimination to distinguish between the effect of the properties of the M-N sigma-bond and the nitrogen electron pair. We have found that the effects of electronic properties on C-C and C-N bond-forming reductive elimination are similar. Because the amido ligands contain an electron pair, while the alkyl ligands do not, we have concluded that the major electronic effect is transmitted through the sigma-bond.
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
We explore the use of certain image features, blockwise histograms of local orientations, used in many current object recognition algorithms, for the task of handwritten digit recognition. Existing approaches find that polynomial kernel SVMs trained on raw pixels achieve state of the art performance. However such kernel SVM approaches are impractical as they have a huge complexity at runtime. We demonstrate that with improved features a low complexity classifier, in particular an additive-kernel SVM, can achieve state of the art performance. Our approach achieves an error of 0.79% on the MNIST dataset and 3.4% error on the USPS dataset, while running at speeds comparable to the fastest algorithms on these datasets which are based on multilayer neural networks and are significantly faster and easier to train.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
The invention provides for a nanostructure, or an array of such nanostructures, each comprising a rough surface, and a doped or undoped semiconductor. The nanostructure is an one-dimensional (1-D) nanostructure, such a nanowire, or a two-dimensional (2-D) nanostructure. The nanostructure can be placed between two electrodes and used for thermoelectric power generation or thermoelectric cooling.
Comments and Discussion Barry Eichengreen and Jeffrey A. Frankel Barry Eichengreen: The first rule of forecasting is, "Give them a forecast or give them a date, but never give them both."1 Michael Dooley and Peter Garber have given us a forecast, namely, that the dollar will fall and U.S. Treasury yields will rise. Bravely, they have also given us a date. Unfortunately for those of us interested in the future, that date is 1971. Like Dooley and Garber, I agree that what cannot go on forever generally will not. But unlike them I do not believe that recent events in financial markets can help us pin down the timing. The middle of March, just before the Brookings Panel meeting, saw an increase in noise about the possibility that foreign central banks might diversify out of dollars. The governor of the Bank of Korea made some widely reported comments about the need for more-active reserve management. Prime Minister Junichiro Koizumi of Japan told a parliamentary committee that reserve diversification was "necessary."2 Y. V. Reddy, governor of the Reserve Bank of India, said that the diversification of reserves was under active discussion.3 Ukrainian economy minister Sergiy Teriokhin argued publicly that the country should diversify its reserves out of dollars and into euros.4 This upsurge in noise was associated with an eight-month high in Treasury yields, reinforcing the belief that reserve diversification could eventually force the dollar down and Treasury yields up. At the same time, that eight-month high in Treasury yields was not all that high. I would acknowledge that this is a troubling point. I am not alone, of [End Page 188] course: Federal Reserve Chairman Alan Greenspan has commented on this issue extensively, to the point where it is now known as the Greenspan conundrum. Factors invoked to help explain it include the relatively short supply of new long-term Treasury debt coming onto the market as the debt managers at the U.S. Treasury shorten maturities, and the inelastic demands of various institutional investors for government securities. In Dooley and Garber's view, the proper interpretation is that financial market participants are attaching a positive probability to Asian central banks continuing to support the dollar by making massive purchases of Treasury bonds. This is the substance of the first of the authors' three notes. Who am I to second-guess the markets, much less to second-guess our esteemed authors? Well, I'm an economic historian who can recall a substantial number of previous episodes where major imbalances leading to sharp changes in exchange rates were not obviously factored into financial markets until immediately before the event. For example, the January March 1933 run on the dollar, a suggestive precedent, had virtually no discernible impact on interest rate differentials or forward exchange rates until almost immediately before it occurred, despite the fact that the possibility had been actively discussed for the better part of a year.5 The 1992 attacks on the pound sterling, a currency that commentators regularly cited as ready for a fall, were similarly not preceded by the emergence of noticeable interest rate differentials or a forward discount in the foreign exchange market until a couple of weeks before the denouement.6 Particularly interesting, given the context, is that in 1968-71, in the run-up to the collapse of the Bretton Woods system, the forward discount on the dollar was very modest, as was the interest rate differential between the United States and Germany.7 Then, in the summer of 1971, the forward discount jumped upward. Although one can always ascribe such behavior to the arrival of new information, it is not as if people failed to see the collapse of the Bretton Woods system and a substantial devaluation of the dollar coming. To the contrary, there was an immense contemporary literature warning [End Page 189] that the system would dissolve and that the dollar would have to fall substantially. Yet there seemed to be a striking reluctance to take a position on this basis until one minute before the clock struck midnight. This behavior may be hard to reconcile with perfect foresight, but, if it exists...
