Low temperature techniques

OC-Peroxylactones (1,2-Dioxetanones). Alkyl-substituted 1,2-dioxetanones (21) are prepared using low temperature techniques (54,55). The... 

Conventional electrodeposition from solutions at ambient conditions results typically in the formation of low-grade product with respect to crystallinity, that is, layers with small particle size, largely because it is a low-temperature technique thereby minimizing grain growth. In most cases, the fabricated films are polycrystalline with a grain size typically between 10 and 1,000 nm. The extensive grain boundary networks in such polycrystalline materials may be detrimental to applications for instance, in semiconductor materials they increase resistivity... 

By carrying out photolyses in liquid nitrogen- or liquid helium-cooled infrared cells using a special low-temperature apparatus (see Figure 4.2), one is often able to obtain direct spectroscopic evidence for intermediates of photochemical reactions. In this section we will briefly review how low-temperature techniques have been used to observe intermediates in type I cleavage reactions. 

Using low-temperature techniques such as those described in the previous section, Haller and Srinivasanmb) obtained direct spectroscopic evidence for the formation of the cyclopropanone. At 4°K infrared bands corresponding to carbon monoxide, dimethylketene, tetramethylethylene, and a compound... 

Until recently, fast time-resolved IR spectroscopy has been a technique fraught with difficulty. Generally it has been easier to use low temperature techniques, particularly matrix isolation (2,4), to prolong the lifetime of the fragments so that conventional spectrometers can be used. In the last 5 years, however, there have been major advances in fast IR spectroscopy. It is now posssible to detect metal carbonyl intermediates at room temperature in both solution and gas phase reactions. In Section II of this article, we explain the principles of these new IR techniques and describe the apparatus involved in some detail. In Section III we give a self-contained summary of the organometallic chemistry that has already been unravelled by time-resolved IR spectroscopy. 

Pereira-Ramos, J.-P., Electrochemical properties of cathodic materials synthesized by low-temperature techniques, J. Power Sources 54, 120-126(1995). 

A varied and productive chemistry is now established for most of the Group IV dihalides. By combining high temperature and low temperature techniques, one may isolate AX2 species and observe molecular parameters as well as physical and chemical properties. The CX2 (carbenes) and SiX2 (silylenes) molecules have a rich chemistry and provide new and unique opportunities for organic and organo-metallic syntheses. 

These experiments were carried out mainly by NMR spectrometry ( H and ljC) and high performance liquid chromatography. Stopped-flow and low temperature techniques were used in some of the NMR experiments. In all cases the reactions were aimed at monitoring the half-life of the starting materials as well as monitoring the products which were formed. Detailed procedures will appear shortly [15]. 

Thus cooling-warming cycles, suitably induced, allowed temporal resolution of the reaction, step by step. The absorption, optical rotary dispersion, and electron spin resonance (ESR) spectra of pure compounds I and II were then recorded and found to be similar to those obtained under fast-reaction conditions (Douzou et al., 1970 Douzou and Leter-rier, 1970). Additional interesting observations were made possible by the low-temperature technique for instance, instead of making the time scale of reactions feasible exp>erimentally by increasing the substrate concentrations as in fast techniques, reactions at low temperatures could be performed with stoichiometric concentrations of enzyme. Such con-... 

Finally, one uses low-temperature techniques for ESR measurements of non-Kekule species not only because of the inherent high reactivity of the species, but also because unless the sample is immobilized or at least hampered in its motion by viscous solvent, rapid tumbling will cause the spectrum to suffer line shape distortions that impede the extraction of the zero-field splitting parameters. 

Particles produced in the gas phase must be trapped in condensed media, such as on solid substrates or in liquids, in order to accumulate, stock, and handle them. The surface of newly formed metallic fine particles is very active and is impossible to keep clean in an ambient condition, including gold. The surface must be stabilized by virtue of appropriate surface stabilizers or passivated with controlled surface chemical reaction or protected by inert materials. Low-temperature technique is also applied to depress surface activity. Many nanoparticles are stabilized in a solid matrix such as an inert gas at cryogenic temperature. At the laboratory scale, there are many reports on physical properties of nanometer-sized metallic particles measured at low temperature. However, we have difficulty in handling particles if they are in a solid matrix or on a solid substrate, especially at cryogenic temperature. On the other hand, a dispersion system in fluids is good for handling, characterization, and advanced treatment of particles if the particles are stabilized. 

Until recently, the synthesis of ionic/covalent nitrides was relatively unexplored except for the pioneering work of Juza on ternary lithium nitrides.11 However, within the last decade, several groups have begun to explore ternary nitride systems, many of which have relied on the inductive effect. The inductive effect is based on the donation of electron density from an electropositive element to an adjacent metal-nitrogen bond, thereby increasing the covalency and stability of that bond and of the nitride material itself. The success of this method is illustrated by the fact that almost all of the known ionic/covalent ternary nitrides contain electropositive elements. Only recently has a small number of transition metal ternary nitrides been synthesized in the absence of the inductive effect at moderate temperatures, by taking advantage of low temperature techniques, such as the ammonolysis of oxide precursors and metathesis reactions.6,12-17... 

The synthesis of borylenes is as yet restricted to the aforementioned high and low temperature techniques, respectively. Related hypovalent compounds of other p-block elements, e.g., Al,17 C,18 22 and Si,23 28 however,... 

The wet procedure described above and based on precipitation of microcrystals from the solution makes possible to introduce gas molecules in the lattice of C60 at room temperature and atmospheric pressure. In contrast to hot pressing method, which could be used only for incorporation of chemically inert molecules to the fullerenes, low temperature technique allow us to synthesize for the first time fullerene microcrystals containing chemically active molecules like oxygen. The molecules of 02, N2 and Ar atoms occupy octahedral lattice sites in C60 microcrystals causing the lattice parameter increase. Ar interstitial fullerene obtained by precipitation was found to exhibit much higher thermal stability then that produced by hot isostatically pressing method. 

The technique of matrix isolation has been shown to produce highly characteristic spectra of individual components of complex samples combination of MI spectroscopy with separation techniques promises to increase further the analytical capabilities of the technique. While our research to date has emphasized FTIR and molecular fluorescence spectrometry, MI as a sampling procedure is not limited to these two forms of spectrometry. For example, some interesting preliminary analytical results by MI Raman spectrometry recently have been described (32). It should also be stressed that the cryogenic procedures required for the vast majority of MI spectral studies are neither difficult nor unduly expensive except in very special cases, closed-cycle cryostats requiring no cryogenic liquids (and no prior experience in low-temperature techniques) are entirely satisfactory for MI... 

As the use of low-temperature techniques becomes more routine, the quality of the analyses in this regard should improve, but this will be affected by a trend toward the study of more complex molecules where the diffraction quality of the crystals and the observation-to-parameter ratio becomes less favorable for locating the hydrogen atoms. 

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