Crystal picking

The crystal picking method, that is the separation by hands of crystals, bring to mind very old techniques (Pasteur s method, 1848) and seems an out of fashion approach. In addition, it can be applied only if the racemic molecule exists as a mechanical mixture of two types of crystals, the (+) and (—) forms (a racemic mixture or ( )-conglomerate). 

Even if it carmot be said that this method is general, practical, and easy, still it can be considered useful for special cases and in fact some enantiopure thiahelicenes have heen obtained by this technique. Of course, this method is employed only for optical resolution on a small scale (1968JA5339, 1970JA6664, 1971J02797). 

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Resolution of helicenes has also been performed by the very laborious method of picking single crystals 4,5 29,79,80). After recrystallization of the partially resolved mixture the procedure can be repeated, until no more variation in optical rotation occurs. Because some helicenes crystallize into racemic crystals by lamellar intergrowth of pure P and pure M forms (see Sect. 6) the crystal picking method is not always applicable, however. A [7]-heterohelicene was partially resolved by crystallization from the chiral solvent54) (—>x-pinene. 

If a quartz plate is subjected to an alternating electric field, the reverse piezoelectric effect causes it to expand and contract at the field frequency. If this field frequency is made to coincide with the natural elastic frequency of the crystal, the plate resonates the direct piezoelectric effect then augments the applied electric field. This is the basis of the crystal oscillator and the quartz clock. See also CRYSTAL MICROPHONE CRYSTAL PICK-UP. 

For aromatic hydrocarbon molecules, in particular, the main acceptor modes are strongly anharmonic C-H vibrations which pick up the main part of the electronic energy in ST conversion. Inactive modes are stretching and bending vibrations of the carbon skeleton. The value of Pf provided by these intramolecular vibrations is so large that they act practically as a continuous bath even without intermolecular vibrations. This is confirmed by the similarity of RLT rates for isolated molecules and the same molecules imbedded in crystals. 

The biologically important anti-/8-phenylalanine, /8-2-thienylalanine (39), has been resolved into antipodes through enzymatic resolution with carboxypeptidase/ The pharmacologically important 3-piperidino-l,i-bis-2-thienyl-l-butene (40), which crystallizes as. a conglomerate, has been resolved by hand picking. ... 

In order to resolve the mixture melting at 135°, it should be dissolved in cold acetic ether to saturation, and allowed to stand so that the solvent evaporates very slowly. Two distinctly different forms of crystals separate which can be picked out and recrystallised separately, when the two semi-carbazones are obtained in a state of purity. 

Similarly, we can pick another example in crystal growth in melt. In this case, the growth occurs at the interface between the melt and a substrate that is kept at a constant temperature that is lower than the critical temperature for crystallization. The morphology characteristic of the instability is formed by the coupling of the heat flux and the surface-form fluctuation. This problem was first theoretically analyzed by Mullins and Sekerka.57-62... 

The compounds of Mg with Ca, Sr and Ba are also simply prepared mixtures of different proportions are heated to above the liquidus temperature and cooled for examination under inert conditions. Single crystals can be picked out from those alloys that contain a distinct compound phase as detected by x-rays. ... 

If heating of amorphous selenium with absorbed ethyl radicals is conducted in presence of nitrogen or inert gas with pressure 100 Torr no signal is picked up from the sensor. This implies that in this case the signals of the sensor are controlled by adsorption of ethyl radicals on its surface. Consequently, the heating of amorphous selenium with adsorbed radicals and resulted crystallization lead to emission of radicals. 

A sample of melezitose was dissolved in an excess of water and filtered into a small beaker, which was covered with filter paper and allowed to stand at ordinary temperature for several days. The crystals which separated attained a diameter of a millimeter but were crowded together to such an extent as to make them unsuitable for crystallographic measurements. A few minute simple ones were picked out and the balance redissolved by addition of a little water and warming. The liquid was then allowed to cool and the crystals previously picked out were Tntroduced, the vessel being allowed to stand in a room kept at a constant temperature of 20° for several days. The same procedure was followed, using 50% alcohol as the solvent. 

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