Achiral structure

Only three not four stereoisomeric 2 3 butanediols are possible These three are shown m Eigure 7 10 The (2R 3R) and (2S 3S) forms are enantiomers of each other and have equal and opposite optical rotations A third combination of chirality centers (2R 3S) however gives an achiral structure that is superimposable on its (2S 3R) minor image Because it is achiral this third stereoisomer is optically inactive We call achiral mole cules that have chnahty centers meso forms The meso form m Eigure 7 10 is known as meso 2 3 butanediol... 

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

H. D. Flack, Chiral and achiral structures. Helv. Chim. Acta 86 (2003) 905. 

The planar C2h and C2V geometries of the 1,3-butadiene moiety are achiral structures and obviously they cannot show optical activity (i.e. ORD and CD). This has, of course, a spectroscopic origin. The optical activity of a transition Pq — Pi is determined by its Rotational Strength (R)1 defined as the scalar product... 

Mink, J., and Zou, X. (2008) A zeolite family with chiral and achiral structures built from the same building layer. Nat. Mater., 7, 381-385. 

Compelling evidence for equilibration from BN, with an attendant small activation barrier has been provided by Irie and coworkers (45). They claim that Ea = 1.9 kcal mol- -. However, at lower temperatures, a conformational relaxation process with a 9.3 kcal mol-l barrier is identified. It is ascribed to a motion which brings twisted BN to a nearly planar (achiral) structure... 

An analogous structure is produced when the helical tubuland diol 12 forms a 1 1 co-crystalline adduct with methanol (Figure 15). The lattice, as before, comprises stacked layers of alternating chirality, but overall is an achiral structure [34],... 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

The 2D chiral systems of NN and PVBA do not need a change in local adsorbate geometry in order to show a chiral phase transition [83,86]. Homochiral structures have been observed for PVBA (Fig. 7a) and SDA on Cu(100) [95]. A CW-rotated structure contains exclusively A.-PVBA, while the CCW-rotated structure contains only 5-PVBA (Fig. 29) [86]. Increasing the PVBA coverage above 0.05 molecules per copper atom induces a phase transition into a single achiral structure that possesses two mirror planes (Fig. 29c). Highly resolved STM shows that this structure is now comprised of a <5-/k-PVBA racemate and that the unit cell contains equal numbers of CCW-and CW-rotated units. 

Fig.B. Chiral and achiral structures (a) chiral (b) achiral (c) chiral.

Achiral structures are superimposable on their mirror images... 

We don t need to write ( ) next to the isomer that isn t formed, because it is an achiral structure—it has a plane of symmetry and is a meso compound. See p. 396. 

In 1993, the number of theoretically possible polychlorinated bornanes was calculated as 32,768 congeners (16,128 pairs of enantiomers and 512 achiral structures) [71]. This surprisingly high number is obtained by the product of the possibility to chlorinate nine positions on secondary or tertiary carbons (29) and three primary carbon positions (C8, C9, and CIO) with a variety of 4 giving... 

Enantiomerization of chiral tetraatomic molecules by unimolecular processes unavoidably requires the intermediacy of achiral structures, as, for example, in the racemization of NHDF by inversion at nitrogen or of H2O2 by rotation about the O — O bond. Abstractly considered, it is easily seen that any chiral set of four differently labeled points ABCD in... 

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