Achiral point groups

The examples 6a and 6b are added as a test of spatial vision one is achiral (point group. S ), one is chiral (point group D2). This problem is best solved (6a ), 6b S4) by projecting the models on a plane through the spiro-carbon and perpendicularly arranged to the obvious two-fold axis. 

The focus of this chapter is on isotropic chiroptical properties. Therefore, we briefly mention here some of the few available studies of measurements of ECD and OR tensors, and relevant TDDFT computations. Experimental techniques for measurements of OR tensors have been reported in [66-68]. For an overview see Clabom et al. [69]. Measurements of anisotropic CD in liquid crystals have been reported and reviewed in [70-72]. A measurement of the OR tensor at X = 670 nm has been reported for pentaerythritol which crystallizes in the achiral point group S4 [73]. TDDFT computations were also performed on a single pentaerythritol molecule (B3LYP/aug-cc-pVDZ). The computations yielded good agreement with experiment regarding the orientation of the OR tensor. TDDFT computations of... 

By definition, a molecule is achiral if it is left invariant by some improper operation (reflection or rotary reflection) of the point group of the skeleton. Writing the permutation s corresponding to a given improper operation in cyclic form,... 

Noncentrosymmetric achiral crystals of monoclinic point symmetry m and of orthorhombic point symmetry mm2 are also appropriate for determination of the absolute configuration of chiral resolved additives. For the point group m, only the left or right halves of Schemes 13a are relevant. In such an arrangement,... 

Homotopic Groups Not distinguishable under any conditions, chiral or achiral. To have homotopic groups, a molecule must have a finite axis of rotation. Thus the only molecules which cannot have homotopic groups are those whose point groups are Ci, CS) Q, and CCXJV. 

Absolute configuration of complexes. 495-496 Absolute electronegativity, 351 Absolute hardness. 351 Absorption spectra of lanthanide and actinide ions. 604-607 Acceptor number CAN). 370 Achiral molecules, and point groups, 64 Acids. 2... 

The existence of a rigidly achiral presentation suffices as proof of the knot s amphicheirality because all chiral presentations can be isotoped to their mirror images by way of the achiral one. The only possible point group for rigidly achiral presentations of prime (but not composite) knots13 is S2 , n- 1,2,. Figure 23 depicts diagrams of such presentations for a selected number of amphicheiral prime... 

The time-averaged symmetry of the [2]catenane is Cs, which expresses the chemical achirality of the molecule, even though no individual conformation or presentation of the catenane belongs to this point group. It thus resembles (l/ )-menthyl (IS)-menthy 1 2,2, 6,6 -tetranitro-4,4 -diphenate, in which con-... 

There are six isomers of difluorocyclobutane (see below). The vicinal di-substituted isomers B and C (both with a twofold proper axis of symmetry, symmetry point group C2) are chiral and are enantiomers of each other. The cis-configured compound D (with a plane of symmetry, symmetry point group Cs) is achiral and is a meso compound. The compounds A and F (both with two planes of symmetry and on the line of intersection of both planes a twofold axis of symmetry, symmetry point group C2V) and E (with a plane of symmetry, a twofold axis of symmetry perpendicular to it and a centre of symmetry, symmetry point group C21O are all achiral. These results can be verified from the flow chart given in the appendix. 

The compound has two chirality centres and three pseudo chirality centres. There is however, only one (achiral) diastereomer of the compound shown in the question. The two isomers can be distinguished from one another solely on the relative position of the chlorine or bromine atoms which lie in a plane which also happens to be the plane of symmetry of the molecule (this is the only symmetry element present, therefore the symmetry point group is Cs). It is possible in this instance to specify the configuration unequivocally using the descriptors E and Z. However, in systematic nomenclature the complete configuration of all the stereogenic centres is specified. Thus the (so-called) Z isomer is (ls,3r,5 ,6r,7S)-l,6-dibromo-3,6-dichloroadamantane and the isomer is (ls,3r,5 ,6s,7S)-l,6-dibromo-3,6-dichloroadamantane, i.e. the two isomers can be distinguished simply by the descriptor used for position 6. 

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