Torsion stereoisomers

The arrangements of atoms or groups of atoms in space about a single bond of molecules of definite configuration are known as conformations or constellations when these spatial arrangements are not superimposable. Torsion stereoisomers produced by rotation about double bonds or partial double bonds, as, for example, with helicenes or amides, are also sometimes included in this classic definition of conformation. The concepts of conformation and configuration are partially merged by this extension. 

In the previous section, the adaptation of the RIS model was based on the distance between next-nearest neighbor beads. This approach is obviously inadequate for CH3-CHX-CH2-CHX-CH3, because it necessarily abandons the ability to attribute different conformational characteristics to the meso and racemo stereoisomers. Therefore a more robust adaption of the RIS model to the 2nnd lattice is necessary if one wants to investigate the influence of stereochemical composition and stereochemical sequence on vinyl polymers [156]. Here we describe a method that has this capability. Of course, this method retains the ability to treat chains such as PE in which the bonds are subject to symmetric torsion potential energy functions. 

It is evident that 11, 12, and 13 can all be converted into their enantiomers by conceptual torsions about the bonds that link the starred atoms. The success of the operation does not depend on the number of ligands that are attached to these atoms, which constitute the terminal atoms of the lines of torsion (Fig. 3d, e, henceforth types d and e). Similarly, there is no place in this scheme for any restriction on dihedral angles a conceptual torsion can interconvert the stereoisomers of hexahelicene (8) or of dioxepin (7) (both type f, Fig. 3/) as readily as those of ordinary biphenyls (type d) with their perpendicular orientation of the rings. No idealization is required. 

If we regard axial and equatorial cyclohexanol as stereoisomers, we find that they can be intercon verted either by a permutation at C( 1) or by the joint torsions... 

The claimed lack of ambiguity does not preclude a physical overlap between elements. If one chooses to regard as stereoisomers the three nonsuperposable staggered forms resulting from the torsion of a carbon-carbon bond with six different ligands, we have three separate elements of stereoisomerism, two centers and one line. This is the minimum number required to account for the existence of the 12 stereoisomers. 

It is investigated whether the stereochemical 13C NMR chemical shifts in the resonance peaks can be ascribed to differences in the conformations in the various stereoisomers. The authors follow Boyd and Breitling (A 022) in thair statistical treatment of the PP chain, with the exception that here conformational sequences are not excluded of the type XG/G Y for two adjacent diads unless XG or G Y imply another c (syn-axiall interaction within either diad. Therefore this treatment, which is more rigorous but consistent with Boyd and Breitling s energy calculations, requires statistical weights which are functions of three adjacent torsional angles. 

The orbital tilting argument implies that the size of the dienophile should have no influence on the diastereoselectivity, hence variations in stereoselectivity with different dienophiles could not be explained. More recently an explanation based on torsional and steric effects has been advanced that permits predictions of stereoselectivities for as yet unstudied cases. A computational model has also been developed to provide semiquantitative predictions of stereoisomer ratios in reactions of substituted species118. 

Spin-spin coupling in nmr, 242 Spontaneous reaction, 36 Stability, heat of combustion, 90 of radicals, 61 resonance, 24 Staggered conformation, 51 Stereoisomers, 69 Stereomers, conformational, 80 Stereoselective reactions, 91 Steric strain, 54, 126 Stilbene, 218 Strain, torsional, 54, 167 Structure of benzene, 189 cyclooctatetraene, 195 Substitution, ally lie, 105 in arenes, 205 in naphthalene, 213 nucleophilic aromatic, 215 ... 

Accordingly, while the calculations on ZsZsZa BV, ZsZsEa BV and ZaZsZa P4>B discussed above were concerned with both FC relaxation and photoisomerization paths, the calculations on the other stereoisomers/bilins hitherto not discussed— ZaZsEa BV, ZsZsZa PCB, ZsZsEa PCB, ZaZsEa PCB, ZaZsEa P4>B and ZsZsEa P B—focused exclusively on the FC relaxation. For all these systems, the calculations showed that the FC relaxation is strongly dominated by stretching and torsion at the 10-11 bond. This indicates that all bilin chromophores utilized by phytochromes are intrinsically most prone to isomerize at 10-11, irrespective of which configuration (Pr or Pfr) they adopt. Importantly, it should be stressed that this conclusion is not to be taken as support for an alternative mechanism where the primary event of phytochromes is achieved by a Z -> photoisomerization at 10-11 instead of 15-16, but should be viewed as evidence that the protein must substantially alter the photochemical reactivity of the bilin chromophores for the primary event to occur at 15-16. 

Conformational isomers (conformers) Stereoisomers at potential energy minima (local or global) having identical constitution and configuration, which differ only in torsion angles. 

Rotamers Stereoisomers of the same constitution and configuration, that differ only by torsion angles. 

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