Unsaturated system defined

The operation that interconverts stereoisomeric planes can be viewed as a ir rotation of the plane that breaks the bond(s) between the plane and the extraplanar atom(s) and then restores it (or them) in a sterically different way. The axes of this rotation pass through the center of the unsaturated system. As they are infinite in number (46) they cover the plane and in a sense define it. Although several of these axes coincide with bonds, the operation differs profoundly from that characteristic of a line of torsion. Rotation of the plane causes a change in bonding, whereas torsion about a line alters the torsional angles between the ligands that are attached to the terminal atoms of the bond that defines the line. 

The hydroxide anion catalyzed rate constants for the scries of alkyl tris-(2-methoxyethoxy)silanes obtained by Fohl were used to define a modified Taft equation, log(/cHO//cHO ) = 2.48cr +1.67 Es [42], A good correlation was obtained, except for vinyl and phenyl substituents. The anomalous behavior observed for phenyl and vinyl tris-(2-methoxyethoxy)silanes may have resulted because the steric parameter or the polar parameter may be influenced by the carbon-carbon double bond. The steric parameter for a,yS-unsaturated substituents may include an appreciable resonance effect. The polar parameter values may be influenced by the ability of silicon to back-bond through d orbitals with the a,/J-unsaturated system [37,49]. 

Behrens C, Cichon MK, Grolle F, Hennecke U, Carell T (2004) Excess Electron Transfer in Defined Donor-Nucleobase and Donor-DNA-Acceptor Systems. 236 187-204 Bertrand G, Bourissou D (2002) Diphosphorus-Containing Unsaturated Three-Menbered Rings Comparison of Carbon, Nitrogen, and Phosphorus Chemistry. 220 1-25 Betzemeier B, Knochel P (1999) Perfluorinated Solvents - a Novel Reaction Medium in Organic Chemistry. 206 61-78 Bibette J, see Schmitt V (2003) 227 195-215 Blais J-C, see Astruc D (2000) 210 229-259 Bogar F, see Pipek J (1999) 203 43-61 Bohme DK, see Petrie S (2003) 225 35-73 Bourissou D, see Bertrand G (2002) 220 1-25 Bowers MT, see Wyttenbach T (2003) 225 201-226 Brand SC, see Haley MM (1999) 201 81-129... 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

In aqueous systems, membrane lipids may exist in a gel-like solid state or as a two-dimensional liquid. In the case of pure phospholipids, these states interconvert at a well-defined transition temperature, Tc, that increases with alkyl chain length and decreases with introduction of alkyl chain unsaturation. In cell membranes, which have marked heterogeneity in both the polar and nonpolar domains of the bilayer, this state is described as liquid disordered . The presence of sufficient sphingolipids, with... 

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