High stability

The high stability of terminal aeetylenes of the pyrazole series allows the preparation of polyethynyl derivatives like 3,4-diethynyl-, 4,5-diethynyl-, and even 3,4,5-tiiethynyl-l-methylpyrazole in 64%, 78%, and 41% yields, respeetively, in these strong eonditions (71IZV1764) (Sehemes 93 and 94). 

Because of its high stability, the processing window (range of processing conditions) is wider than for many other thermoplastics. The main points to bear in mind are  

W. F. Taylor, Development of High Stability Fuels, Final report. Contract N00140-74-C-0618, Naval Air Propulsion Center, Trenton, N.J., Dec. 1975. W. F. Taylor,/. Chem. 18, 25 (1968). 

The additive characteristic of the scheme now has high stability and prevents the relay from operating on moderate external faults, while it is sensitive to small spill currents for all internal ground and phase faults, including winding faults. 

The results of aetivatioii of aeyl cations led to our study of other carboxonium ions. Carboxonium ions are highly stabilized compared to alkyl cations. As their name indicates, they have both carbocationic and oxonium ion nature. 

Conversions of acid anhydrides to other carboxylic acid derivatives are illustrated m Table 20 2 Because a more highly stabilized carbonyl group must result m order for nucleophilic acyl substitution to be effective acid anhydrides are readily converted to carboxylic acids esters and amides but not to acyl chlorides 

Tin(IV) oxide occurs naturally, clearly indicating its high stability. It can be prepared either by heating tin in oxygen or by heating the 

Diaralkyl peroxides have been prepared by autoxidation. Those compounds which autoxidize to symmetrical diaralkyl peroxides form highly stabilized radical intermediates, eg, triphenylmethane, 9-phenylanthrone, and 2,4,6-tri(/-butyl)phenol (44,66). Compounds that form stable radicals by cleavage of carbon—carbon bonds can be autoxidized to diaralkyl peroxides (69). 

The stability of isothiazole derives from the fact that it has an aromatic delocalized ir-electron system. The NMR chemical shifts, which depend, inter alia, on ring currents, and the high stability of the molecular ions in mass spectrometry, are typical of aromatic compounds, and X-ray measurements confirm the partial double bond character of all the bonds of the ring. 

A hydrogen attached to the a-carbon atom of a p-keto ester is relatively acidic. Typical P-keto esters have values of about 11. Because the a-carbon atom is flanked by two electron-withdrawing carbonyl groups, a carbanion formed at this site is highly stabilized. The electron delocalization in the anion of a p-keto ester is represented by the resonance structures 

The theory predicts high stabilities for hard acid - hard base complexes, mainly resulting from electrostatic interactions and for soft acid - soft base complexes, where covalent bonding is also important Hard acid - soft base and hard base - soft acid complexes usually have low stability. Unfortunately, in a quantitative sense, the predictive value of the HSAB theory is limited. Thermodynamic analysis clearly shows a difference between hard-hard interactions and soft-soft interactions. In water hard-hard interactions are usually endothermic and occur only as a result of a gain in entropy, originating from a liberation of water molecules from the hydration shells of the 

Fig. 1. The rotating-cathode DC arc method [6a]. The cathode deposit is immediately taken out of the discharge by rotation and cropped within one turn. This method offers high stability and reliability of the handling and makes the continuous mass production possible.

The l,2,3-tri-/-butylcyclopropenium cation is so stable that the perchlorate salt can be recrystallized from water. An X-ray study of triphenylcyclopropenium perchlorate has verified the existence of the carbocation as a discrete species. Quantitative estimation of the stability of the unsubstituted ion can be made in terms of its pXn+ value of —7.4, which is intermediate between those of such highly stabilized ions as triphenylmethyl 

For a reconstmcted surface, the effect of an adsorbate can be to provide a more bulk-like enviromnent for the outemiost layer of substrate atoms, thereby lifting the reconstmction. An example of this is As adsorbed onto Si(l 11)-(7 X 7) [37]. Arsenic atoms have one less valence electron than Si. Thus, if an As atom were to replace each outemiost Si atom in the bulk-temiinated stmcture, a smooth surface with no impaired electrons would be produced, with a second layer consisting of Si atoms in their bulk positions. Arsenic adsorption has, in fact, been found to remove the reconstmction and fomi a Si(l 11)-(1 x l)-As stmcture. This surface has a particularly high stability due to the absence of dangling bonds. 

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