Isolated turning point

As the number of atoms involved in a reaction (or the number of degrees of freedom allowed to them) increases, the calculations become more and more complex, and more ingenuity is needed to isolate the essential turning points of the process under consideration. 

The turning point in the understanding of the chemical nature of S-S dications came in 1976 when Musker reported synthesis, isolation and characterization of disulfonium dication 12 formed by the two-electron oxidation of 1,5-dithiacyclooctane 10 (Scheme 4).29,30... 

Fig. 12.2 Left The ground (X, solid line), excited (6, dashed line) and dissociative [a1g(3II), dotted line] electronic state potentials of the iodine molecule. The arrow indicates the electronic excitation. The initial excited wave packet is located in the Franck-Condon region near to the inner classical turning point of the B state. The transition from the B to the a state is forbidden by symmetry in the isolated molecule but becomes allowed when the molecule is placed in a solvent.

In terms of the gradient of the stationary-state locus, the condition expressed by eqn (7.55) locates a singular point as dx/drres - 0/0. Loosely, we can think of the locus simultaneously having turning points both horizontally and vertically, as indicated in Fig. 7.7. This corresponds to situations such as (a) an isolated point or (b) the transcritical touching of two branches. 

The turning point in the structural studies came after the work by Hiraoka et al. (1980) revealed that a-lactalbumin is a metalloprotein in which calcium is strongly bound (see also Sections VI and VII). Soon afterward three new crystal forms of bovine a-lactalbumin were isolated by Fenna (1982a), particularly trigonal Form II. Fenna (1982b) also isolated calcium-containing crystals of human a-lactalbumin suitable for X-ray structural analyses. The various crystalline forms of a-lactalbumin are summarized in Table III. 

The invention of syndiospecific Cj-symmetric metallocenes has marked the turning point in the understanding of the mechanism of stereocontrol with metallocene catalysts. Again, the presence of isolated insertion errors of the type rrrrmmrrr is consistent with site control (Scheme 27). In the case of the syndiospecific Me2C(Cp)(9-Flu)ZrCl2 catalyst, in which the two sites are enantiotopic, occasional skipped insertions produce a minor amount of insertion errors of the type rrrrmrrrr, which are identical to those produced by chain-end control. In the case of isospecific C2-symmetric metallocenes, skipped insertions would not be observable due to the presence of two homotopic sites. 

Amino acids were isolated from natural sources, asparagine from plants, cystine from urinary stones in the first decade of the last century and became known later to be constituents of proteins. The peptide bond, however, as the fundamental feature of protein structure was recognized only in this century (Hofmeister 1902 E. Fischer 1906). Several more decades elapsed until sequence elucidation reached the point of practicality. The turning point was signaled by the determination of the structure of insulin (Sanger et al. 1953). The subsequent development of automated amino acid analysis (Spackman, Stein and Moore 1958) and automatic sequencing (Edman and Begg 1967)... 

The successful isolation and characterization of Cp ZnZnCp not only filled a vacancy in the homobimetaUic-bonded group of 12 elements but also became a turning point in this field. It initiated increased interest in G12-G12 chemistry and led to the preparation of numerous new complexes, especially those with Zn-Zn bonds. The synthetic methodologies for the formation of G12-G12 bonds are summarized in Scheme 12.5. 

Small cocks have often been knocked open or have vibrated open. They should never be used as the sole isolation valve (and preferably not at all) on lines carrying hazardous materials, particularly flammable or toxic liquids, at pressures above their atmospheric boiling points (for example, liquefied flammable gases or most heat transfer oils v./hen hot). These liquids turn to vapor and spray when they leak and can spread long distances. 

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