Polar axis absolute, direction

I. Determination of the Absolute Direction of a Polar Axis in a Chiral or... 

This discussion brings out an important principle that will reappear time and again in this chapter. If we can assign the absolute direction of the hand vis-a-vis the polar axis b (Figure 2), that is, establish whether the wrist or the fingers of the hand point up, then the absolute chirality of the hand is fixed as well. This determination of absolute polarity can be achieved in several ways other than anomalous scattering of X-rays or neutrons, for example, by an external means such as a solvent, a chemical reagent, or a physical measurement (e.g.,... 

From the relative intensities of the two diffracted waves one can deduce the absolute direction of the vector W —> Y with respect to the b axis. This type of reasoning was exploited by Nishikawa and Matsukawa (14) in 1928 and independently by Coster, Knol, and Prins (15) in 1930 to determine the absolute polarity of successive layers of zinc and sulfur in a polar crystal of zinc sulfide. In the zinc sulfide crystal, planes of zinc and sulfur alternate parallel to the face (111), as shown in Scheme 3 the distance between the close pairs of zinc and sulfur planes is one-quarter of the whole 111 spacing. 

The morphological differences between crystals grown in the presence and absence of the additive would then indicate the direction of the substrate molecule W-Y with respect to the polar axis. Consequently, the absolute configuration of the crystal and of the chiral molecular constituents can be derived. The additive need not be chiral, and even if it is, the assignment of the absolute configuration... 

In principle, an anisotropic reaction performed on a crystal of polar symmetry may fix the absolute direction of the polar axis. In the case of an asymmetric reaction carried out in a centrosymmetric (enantiopolar) crystal, one may establish the absolute configuration of the chiral product. The degree of reliability of the assignment will depend on knowledge of the various states of the reaction pathway. Here we briefly describe some heterogeneous reactions in polar and enantiopolar crystals that illustrate this approach. 

In this study, an important piece of information was gleaned regarding the mechanism of the reaction from the assignment of the absolute sign of the polar axis. Of course, in transformations of this type, if the reaction mechanisms are well established, one may proceed in a reverse manner and assign the absolute polarity of the crystal (and therefore the absolute configuration of the chiral molecules) by determining the preferential direction of the attack. 

From Equation (1.33) it is seen that the transition moment is a vector quantity. The square of its absolute value determines the transition probability, whereas its direction is called the polarization direction. If one of the principal axes of a molecule is the long axis, the transition as well as the corresponding absorption band is often called parallel or long axis polarized if the direction of the transition moment vector is perpendicular to the long axis, the transition and the absorption band are called perpendicular or short-axis polarized. The longest wavelength transition of s-cis-buta-... 

The net polar effect is directed along the axis of the molecule in line with the nitrogen lone pair. Table 10 collects some up values. The Roberts and Moreland approach, while giving excellent values absolutely independently of resonance or steric effects requires the synthesis of some very challenging molecules and the same might be said of Grob s method. Taft and Lewis [51] defined a a, value which is very accessible substituted acetic acids are readily available as are their ionisation... 

The signs of the exciton-split Cotton effect reflect the absolute configuration of the molecule if the direction of the transition moment in the chromophore is established. For example, chiral 9,9 -spirobifluorene derivative 1 has R configuration as indicated by a strong positive CD couplet (A = +1111.7) due to the LBb band of the anthracene chromophores, the electric transition moment of which is polarized along the long axis of the chromophore (Figure 14)100... 

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