Quenching conditions, effect

In Chapter 6.3, C-J. Sung examines extinction of counterflow premixed flames. He emphasizes flame quenching by stretch and highlights four aspects of counterflow premixed flame extinction limits effect of nonequidiffusion, parf played by differences in boundary conditions, effect of pulsating insfabilify, and relation to the fundamental limit of flammability. 

Sc(OTf)3 is stable in water, and effectively activates carbonyl and related compounds as a Lewis acid in water. This is remarkable, because most Lewis acids react immediately with water rather than the substrates, and are decomposed or deactivated. It has already been found that lanthanide trifiates Ln(OTf)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and yttrium trifiate Y(OTf)3 are stable in water, and can act as Lewis-acid catalysts in aqueous media.46-48 They are used catalytically in many reactions and can often be recovered and reused, because they are stable under the usual water-quenching conditions. 

In some cases, any effect of added LiCl was not noticeable in aldol reactions under external quench conditions. Majewski and coworkers have observed that aldol reaction of tropinone 32 and benzaldehyde using the chiral lithium base 26 in the presence as well as in the absence of added LiCl gave the aldol product with the same 90% ee (Scheme 24)53,56. 

More recently, Amedjkouh has described the use of 48 in deprotonation of 28. Silylenol ethers could be obtained with 85% and 75% ee under internal quench and external quench conditions, respectively (Scheme 32). Mixed dimers 49 and 50 (see Section II.E.2) proved to be effective under external quench conditions and provided silylenol ether in up to 63% ee73. 

There are procedures for making more accurate estimates of quenching conditions than those outlined here. For example, differences between properties of the inert gas and of the combustible may be taken into account [111]. It is interesting that very weak walls can often effectively provide... 

There are notable reactivity differences between iron-vanadium and iron molybdenum nitrogenases. Iron-vanadium nitrogenase allocates electrons less effectively, producing at least three molecules of H2 for every molecule of N2 reduced. The mechanism may be similar, as evidenced by production of hydrazine under similar quenching conditions that are effective for Mo nitrogenase.Acetylene is reduced to mixtures of ethylene and ethane by iron-vanadium nitrogenase, whereas iron-molybdenum nitrogenase forms only ethylene. 

Abstract. The Thermal Analsysis (TA) applies a great variety of techniques suitable for determining the thermophysical properties of solids. Here after a wide and detailed review on more conventional methods of differential thermal analysis (DTA) and differential scanning calorimetry (DSC) to study non-equilibrated materials, the experimental results obtained from Short-Range Ordering (SRO) in a Cu-Al alloys is presented and discussed. The kinetic parameters and laws in these materials are deeply discussed focusing attention also to vacancy behavior and effects of quenching conditions. 

The deprotonation of conformationally locked 4-t-butylcyclohexanone became a kind of benchmark reaction to study the efficiency of appropriate chiral bases. As shown in Scheme 2.20, the enantiotopic axial hydrogen atoms in o-position of the carbonyl group can be removed selectively by the C2-symmetric lithium base R,R) or (S,S)-72a, and the enantiomeric enolates R)-73a and (S)-73a thus formed were trapped with chlorotrimethylsilane to give enantiomeric silyl enol ethers (/ )-73b and (S)-73b, respectively. It turned out that - symptomatically for the chemistry of lithium enolates - the conditions have a dramatic effect on the enantioselectivity. When internal-quench conditions were applied (i.e., chlorotrimethylsilane present in the mixture from the very beginning), R)-73 was obtained in 69% ee. The external-quench protocol (i.e., deprotonation with the lithium amide 72a first, followed by trapping with chlorotrimethylsilane) led to a collapse of enantioselectivity (29% ee). Thus, here again, the idea came up that lithium chloride that forms gradually under the internal-quench conditions influences dramatically the deprotonation mode. Consequently, the enolate formation was performed in the presence of lithium chloride (0.5 equiv.), and chlorotrimethylsilane was added thereafter. The result was an enhancement of the ee value to 83% [75]. 

Ion implantation (qv) has a large (10 K/s) effective quench rate (64). This surface treatment technique allows a wide variety of atomic species to be introduced into the surface. Sputtering and evaporation methods are other very slow approaches to making amorphous films, atom by atom. The processes involve deposition of a vapor onto a cold substrate. The buildup rate (20 p.m/h) is also sensitive to deposition conditions, including the presence of impurity atoms which can faciUtate the formation of an amorphous stmcture. An approach used for metal—metalloid amorphous alloys is chemical deposition and electro deposition. 

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