Alternative Cores

Two alternate core structures of the ordinary 1/2[110] dislocation, shown schematically in 1 gs. 2a amd b, respectively, were obtained using different starting configurations. The core shown in Fig. 2a is planar, spread into the (111) plame, while the core shown in Fig. 2b is non-plamar, spread concomitcmtly into the (111) amd (111) plames amd thus sessile. The sessile core is energetically favored since when a shear stress parallel to the [110] direction was applied in the (111) plane the planar core transformed into the non-plamar one. However, in a similar study emplo3dng EAM type potentials (Rao, et al. 1991) it was found that the plamar core configuration is favored (Simmons, et al. 1993 Rao, et al. 1995). 

In the early clinical trials of Pravadoline (248), the Sterling-Winthrop Research group observed CNS side effects. In an effort to circumvent these issues, indenes were explored as an alternative core [197]. The rationale behind this approach came from the observation that sulindac (308), an indene analogue of indomethacin (309), had anti-inflammatory properties comparable to (309) but without the CNS-associated side effects. 

Current AMI and PM3 methods use an alternative core repulsion function which differs from that used in MNDO in that an additional term involving one to four Gaussian functions is used (defined by parameters a-c, Eq. 5-6) [20, 21], These extra terms help to reduce the excessive core-core repulsions just outside bonding distances. 

In cooperation with Teles and colleagues, our research group has studied the triazole heterocycle as an alternative core structure of nucleophilic carbenes. First, the triazol-5-ylidene 12 (Fig. 9.3 see also Scheme 9.2) was synthesized and shown to be stable at temperatures up to 150 °C in the absence of air and moisture [22]. Compound 12 exhibited the typical behavior of a nucleophilic N-heterocyclic car-bene, and was found to be sufficiently stable to become the first commercially available carbene [23]. As shown in Scheme 9.2, the crystalline carbene was obtained from the corresponding triazolium salt precursor 13 by the addition of methanolate and subsequent thermal decomposition of the adduct 14 in vacuo via a-elimination of methanol [24]. 

With respect to the application of asymmetric carbene catalysis as a tool for enantioselective synthesis, the last decade s major success is based on substantial improvements in catalyst development. Early reports dealt with implementing chirality in thiazolium scaffolds (Sheehan and Hunneman 1966 Sheehan and Hara 1974 Dvorak and Rawal 1998), but their catalytic performance suffered from either low yields or low ee-values. In this regard, the investigation of triazole heterocycles as an alternative core structure (Enders et al. 1995) has played a crucial role to provide heterazolium precatalysts improving both asymmetric benzoin and Stetter reactions. An intramolecular Stetter reaction yielding chromanones upon cyclization of salicylaldehyde-derived substrates is commonly used as a benchmark reaction to compare catalyst efficiency (Scheme 1 Ciganek 1995 Enders et al. 1996 Kerr et al. 2002 Kerr and Rovis 2004). 

Precipitation polymerization allows preparation of microgel particles with a coreshell structure. This can be achieved by using monomers of different reactivity or hydrophilicity in a batch polymerization process. Alternatively, core-shell microgels can be prepared by seed polymerization techniques or by stepwise addition of co-monomers to the reaction mixture. 

Raine, A. R., Scrutton, N. S., and Mathews, F. S., 1994, On the evolution of alternate core packing in eightfold beta/alpha- barrels, Protein Sci 3 1889-1892. 

What stiU makes socio-material design a potentially good alternative core of engineering education is not that this perspective is seen as a substitute to math and science in any banal way. Rather it is because it emphasizes competencies that... 

L. B. Le.sem and P. H. Harley, Scale-up of Alternate IIRT Core, USAEC Report AErD-3971, Oak Ridge National Laboratory, May 7. 1954. L. B. Le.sem and I. Spiewak, Alternate Core Proposal for the IIRT, US.VEC Report CF-.54-1-80, Oak Ridge National Laboratory,. Ian. 28. 1954. 

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