Molecules, properties chiral

The first line of the connection table, called the counts line (see Figure 2-21), specifies how many atoms constitute the molecule represented by this file, how many bonds arc within the molecule, whether this molecule is chiral (1 in the chiral flag entry) or not, etc. The last-but-onc entry (number of additional properties) is no longer supported and is always set to 999. The last entry specifics the version of the Ctab format used in the current file. In the ease analyzed it is V2000". There is also a newer V3000 format, called the Extended Connection Table, which uses a different syntax for describing atoms and bonds [50. Because it is still not widely used, it is not covered here. 

A chiral complex is one that is not identical to its mirror image. Thus, all optical isomers are chiral. The cis isomers of [CoCl2(en)2 + are chiral, and a chiral complex and its mirror image form a pair of enantiomers. The trans isomer is superimposable on its mirror image complexes with this property are called achiral. Enantiomers differ in one physical property chiral molecules display... 

Enantiomers differ in one physical property chiral molecules display optical activity, the ability to rotate the plane of polarization of light (Section 16.7 and Box 16.2). If a chiral molecule rotates the plane of polarization clockwise, then its mirror-image partner rotates it through the same angle in the opposite direction. 

In order to obtain asymmetric spiro compounds, there are two different possibilities. First, one can connect two different chromophores via a common spiro center. The thiophene compounds 39a and 39b are one example [84, 85]. Second, one can connect two equal but asymmetric chromophores. Based on this principle are Spiro-PBD (40), spiro-bridged bis(phenanthrolines) (41) [86], and the branched compounds Octol (42a) and Octo2 (42b) [87]. Because of their symmetry, these molecules are chiral. The glass transition temperatures of 40 and 42b are reported to be 163 and 236°C, respectively [88], Unfortunately, reports on the thermal properties of 39 and 41 are lacking. 

Molecules have important symmetry properties chirality... 

Starting with the semiempirical approach of Kauzmann et al. (16), Ruch and Schonhofer developed a theory of chirality functions (17,18). These amount to polynomials over a set of variables that correspond to the identity of substituents at various substitution positions on a particular achiral parent molecule. The values of the variables can be adjusted so that the polynomial evaluates to a good fit to the experimentally measured molar rotations of a homologous series of compounds (2). Thus, properties 1 and 2 are satisfied, but the variables are qualitatively distinct for the same substituent at different positions or different substituents at the same positions, violating property 3. Furthermore, there is a different polynomial for each symmetry class of base molecule. Thus, chirality functions are not continuous functions of atom properties and conformation (property 4). 

Molecular size, as basis of separations 100 Molecular weight 4 See also Relative molecular mass Molecules, properties of chiral 42... 

Because of die tetrahedral geometry of saturated carbon and the associated three-dimensional properties, molecules can have chirality as one stereochemical feature. Any object is chiral if it is different (nonsuperimposable) than its mirror image. Likewise a molecule is chiral if it is nonsuperimposable on its mirror image. This requirement does not consider conformational changes (rotations about single bonds) as valid conditions for nonsuperimposability. Thus, for the molecules below, the first is achiral (not chiral) because it is superimposable on its mirror image and the second is chiral because it is not superimpo sable on its mirror image. 

Interestingly, completely different types of organocatalyst have been found to have catalytic hydrocyanation properties. Among these molecules are chiral diketo-piperazine [4, 5], a bicydic guanidine [6], and imine-containing urea and thiourea derivatives [7-13]. All these molecules contain an imino bond which seems to be beneficial for catalyzing the hydrocyanation process. Chiral N-oxides also promote the cyanosilylation of aldimines, although stoichiometric amounts of the oxides are required [14]. 

Thiourea functions were used to attach chiral saccharide units to the molecule of calix[4]arene [72]. The complexation properties of these molecules toward chiral anions have not yet been examined. However, in the preliminary complexation studies (XH NMR titrations in DMSO-d6) the affinity toward acetate and AT-Ac-L-alaninate was observed. 

Properties which change concomitantly with diarylethene derivative photoisomerization are geometrical structures, electronic structures, refractive indices, and chiral properties (when the molecules have chiral substituents). Table 1 shows how the above property changes are applied to various photoswitching molecular systems. Details of these photoswitching functions are described in Sections 2.3 to 2.6. 

This result demonstrates the tendency of an optically active material to rotate the electric vector as it propagates through the sample. Materials possessing this property are normally composed of molecules having chiral symmetry. This effect leads to circular birefringence and circular dichroism, two optical properties that are frequently used in the characterization of biomaterials. 

The definition of chirality and its measurement are described in great detail in a number cf texts (3) however, a brief introduction to the key issues is given in this section. Specifically, chirality is a term referring to a property cf a molecule that is nonsuperimposableon its mirror image as shown in Fig. 18.1, where such a molecule is chiral. 

Many molecules are not superimposable on their mirror image. Such molecules, labeled chiral or dissymmetric, may have important chemical properties as a consequence of this nonsuperimposability. An example of a chiral organic molecule is CBrClFI, and many examples of chiral objects can also be found on the macroscopic scale, as in Figure 4-18. 

Because enantiomers have oppositely signed pseudoscalar properties, chiral zeroes are unavoidable at some stage in the conversion of a molecule into its enantiomer along a chiral pathway. This is true of chirally connected enantiomeric conformations in chemically achiral molecules, such as (lf )-menthyl (15)-menthyl 2,2, 6,6 -tetranitro-4,4 -diphenate, and of chirally connected enantiomers, such as ( + )- and (- )-isopro-pylmalonamic acids. More generally, as previously noted, any chiral molecule composed of five or more atoms is in principle always capable of conversion into its enantiomer by chiral as well as by achiral pathways, provided that this is energetically feasible. Hence, unless it can be demon-... 

Let us use as an example a property chiral molecule, such as a sugar or an amino acid. Take to be the decomposition of the thermal state into its eigenstates L,. and These eigenstates are unstable under external perturbations and could decay, for example, into the chiral states... 

Enantiomers differ in only one physical property chiral molecules display optical activity, the ability to rotate the plane of polarization of light. 

Thomson (Lord Kelvin) coined a word for this property. He defined an object as chiral if it is not superposable on its mirror image. Applying Thomson s term to chemistry, we say that a molecule is chiral if its two mirror-image forms are not superposable in three dimensions. The work chiral is derived from the Greek word cheir, meaning hand, and it is entirely appropriate to speak of the handedness of molecules. The opposite of chiral is achiral. A molecule that is superposable on its mirror image is achiral. 

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