Left-handed crystal

In the first half of the nineteenth century, it was known that certain minerals, the prime example being quartz, formed chiral crystals. Often, it was seen that rocks could be composed of a physical mixture of small but macroscopic right-handed and left-handed crystals. This kind of mixture, composed of macroscopic chiral domains (crystals) occurring in both enantiomeric forms, was termed a conglomerate. 

Single crystals of Te are highly anisotropic and occur as either right-hand or left-hand crystals. It would be interesting to use the Te Mossbauer effect to measure the anisotropic Debye-Waller factor in such crystals. 

Aq. soln. of sodium chlorate are optically inactive no matter whether the soln. be made from the right- or left-handed crystals—presumably because for a state of stable equilibrium—dynamic isomerism—equal quantities of the two isomers are required, and when the aq. soln. of either form is allowed to recrystallize, H. Landolt7 found equal proportions of the two forms are obtained but D. Gernez showed that if a right- or left-handed crystal be added to the undercooled soln., only one... 

Fias. 13 and 14.-—Right- and Left-handed Crystals of Sodium Chlorate. 

In 1848, Louis Pasteur noticed that a salt of racemic ( )-tartaric acid crystallizes into mirror-image crystals. Using a microscope and a pair of tweezers, he physically separated the enantiomeric crystals. He found that solutions made from the left-handed crystals rotate polarized light in one direction and solutions made from the right-handed crystals rotate polarized light in the opposite direction. Pasteur had accomplished the first artificial resolution of enantiomers. Unfortunately, few racemic compounds crystallize as separate enantiomers, and other methods of separation are required. 

This kind of chiral crystallization of an achiral molecule always gives both right- and left-handed crystals. However, under ordinary crystallization conditions, enantiomorphous control is not possible. We have reported that a cocrystal is formed by the crystallization of a solution of a 1 1 mixture of 3-indolepropionic acid and phenanthridine in acetonitrile [28]. When the acetonitrile solution was divided into six parts and spontaneously evaporated in six containers, P-crystals appeared in four containers and M-crystals in two. If such chiral crystallization is carried out using a much larger number of flasks, the ratio of the formation of both enantiomeric crystals will approach 1 1. 

Louis Pasteur was the first scientist to study the effect of molecular chirality on the crystal structure of organic compoimds [23], finding that the resolved enantiomers of sodium ammonium tartrate could be obtained in a crystalline form that featured nonsuperimposable hemihedral facets (see Fig. 9.1). Pasteur was quite surprised to learn that when he conducted the crystallization of racemic sodium ammonium tartrate at temperatures below 28 °C, he also obtained crystals of that contained nonsuperimposable hemihedral facets. He was able to manually separate the left-handed crystals from the right-handed ones, and foimd that these separated forms were optically active upon dissolution. More surprising was the discovery that when the crystallization was conducted at temperatures exceeding 28 °C, he obtained crystals having different morphologies that did not contain the hemihedral crystal facets (also illustrated in Fig. 9.1). Later workers established that this was a case of crystal polymorphism. 

Working carefully with tweezers, Pasteur was able to separate the crystals into two piles, one of right-handed crystals and one of left-handed crystals like those shown in Figure 9.6. Although the original sample (a 50 50 mixture of right and left) was optically inactive, solutions of the crystals from each of the sorted piles were optically active, and their specific rotations were equal in amount but opposite in sign. 

Enantiomorphic Mirror-image habit and optical characteristics right- and left-handed crystals Quartz... 

Enantiomers cannot be separated by the usual separation techniques such as fractional distillation or crystallization because their identical boiling points and solubilities cause them to distill or crystallize simultaneously. Louis Pasteur was the first to separate a pair of enantiomers successfully. While working with crystals of sodium ammonium tartrate, he noted that the crystals were not identical—some of the crystals were right-handed and some were left-handed. He painstakingly separated the two kinds of crystals with a pair of tweezers. He found that a solution of the right-handed crystals rotated the plane of polarized light clockwise, whereas a solution of the left-handed crystals rotated the plane of polarized light counterclockwise. 

SU-32 enantiomorphs are built only from the large [4 5 8 10 ] cavity and are intersected at different heights by straight 8-ring channels, which are related by the 6i (for right-handed crystals) or 65 (for left-handed crystals) screw axes and thus run in three different directions. Under the synthesis conditions described in Tang et which lack any chiral... 

Pasteur made up a solution of the left-oriented crystals and found that it rotated polarized light in one direction. A solution of right-handed crystals gave the opposite rotation. He made up a mixture of equal amounts of both right-handed and left-handed crystals, and it had no effect on polarized light. (The term racemic mixture has come to indicate a mixture that contains equal portions of the two possible orientations of a chiral compound.) He told Biot, who by now was an elderly man, and Biot did not believe him. Pasteur went to Biot s laboratory, used Biot s chemicals, did the separation in front of him, and let Biot make up the solutions for testing. They had exactly the properties Pasteur claimed. Biot became an enthusiastic supporter of Pasteur s work. 

Louis Pasteur was the first to resolve a racemic mixture when he separated the sodium ammonium salts of ( + )- and ( —)-tartaric acid. In a sense, he was the chiral reagent, since he could distinguish between the right- and left-handed crystals. In Chapter 11, we will see a specific example of how this is done chemically. 

The two kinds of quartz crystals also differ in their face development, as shown in Figure 6-4. They are mirror images of one another—one can be described as a right-handed crystal and one as a left-handed crystal. 

Right-handed and left-handed crystals of sodium ammonium tartrate. 

Fig. 7.4 Temperature dependence of the piezoelectric coefficients of a-quartz (for left-handed crystal)...

Fig. 3. Natural quartz crystal (a) including Brazil-type twin of right and left hand crystals (b). m mirror plane.

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