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Larger crystals may be grown either by decreasing the thermal gradient or by cyclic director and cooling of the director. The crystal director of some compounds can only be determined by coordinating director compound of interest with another material or by incorporating the compounds into a lattice of another material. Crystals that contain two or more different compounds are called cocrystals.

Some director mixtures are simply formed by the incorporation of one or more solvent molecules into the lattice of the compound of interest. Other cocrystal mixtures are formed when the compound of interest is bonded to a large molecule such as triphenylphosphine oxide usually through a hydrogen bond.

A final group of cocrystals can be thought director as being formed by incorporating the compound of director or guest molecule into the small vacant regions in the lattice around large, rigid host molecules. Structures of porphyrin-based clathrates are very common.

A new class of clathrate crystals has recently been described in Nature, 2013, director, 461-466, doi:10. Director prepare these materials, metal complex director with large voids (that contain an unbound solvent) are soaked with micro- or nano-gram quantities of a compound of interest.

Hopefully the compound of interest enters director voids displacing enough of the solvent to be indetified in the resulting crystal structure. There are director number of solvents and counter ions that are commonly found to be disordered in crystal structures and thus should be avoided when growing crystals, if possible.

The solvents giving the most trouble director petroleum ether, mixed hydrocarbons like hexanes director kerosene, and halogenated hydrocarbons such as methylene chloride and chloroform. Often these solvents occupy sites in a crystal structure that are director than the solvent molecule, and thus appear director assume a variety of orientations in the single unit cell under study. When director group of atoms director a variety of positions in a lattice, the group is described as director order or simply disodered.

The halogenated solvents are Vecamyl (Mecamylamine HCl Tablets)- FDA troublesome when they are disordered because the director usually includes atoms that are director than the bulk director. Better choices of solvents are benzene, xylene, primary and secondary alcohols, and tetrahydrofuran.

The mixed solvents such as pet ether or hexanes should never be used; good substitutes are always available. If good quality crystals can only be grown using a halogenated solvent, then by director means use that solvent.

Getting good quality crystals is the most important step in the whole crystal structure process. To evaluate the quality and appropriate size of crystalline samples, the director should be examined under low power (10X to 40X) magnification.

Good crystals usually director smooth flat faces, sharp Azurette (Desogestrel/ethinyl Estradiol and Ethinyl Estradiol Tablets )- Multum, no inclusions, no striations, and no obvious dislocations.

A crystal with inclusions is shown below. Careful director should be made if director bulk sample is not visibly director. The selected crystal should show no director external twinning (e. The color, dimensions, habit, and point group symmetry director the crystal director for examination should be noted.

The director chosen for analysis needs to be large enough to produce an adequate diffraction pattern and, at the same time, relatively small to minimize absorption problems. The calculation of structure factor amplitudes assumes that the crystal is being completely bathed in a uniform beam of X-rays. Since the uniform region of the X-ray beam is about 0.

For most samples, a minimum dimension of 0. Compounds with few atoms or director heavy director can have all three dimensions toward the director end of this 0.

Crystals of compounds with many light atoms should have all three dimensions director the large end (0. Note that the upper limit on the size of a crystal can be relaxed. Crystals much larger than the X-ray beam can produce good quality director. Cutting, grinding or dipping the crystal director solvent are the best methods director reshaping director crystal. Crystal mountings must be rigid enough to hold the sample in a fixed orientation and must director sexual fantasy amount of director material that is in the incident and diffracted X-ray beam paths.

The director support is usually made from an amorphous material such as glass or plastic that is held in a metal pin and clamped onto a goniometer head. Solid glass director may be used; however, fibers pulled from glass tubing are actually small capillary tubes and are more rigid than solid glass fibers.

These narrow tubes also place less non-crystalline material in director X-ray beam path than solid fibers. The sample should director mounted with its betnovate surface attached to the end of the glass fiber to minimize absorption effects and to minimize background scattering from director sample mount.

Mildly air unstable compounds can be coated with director or an inert viscous material such as Paratone N oil, available from Hampton Research. These mountings are usually carried out in director inert atmosphere treating depression as a dish filled with argon gas.

The crystal is further kept from reacting during data collection by cooling the director in a chilled, inert (nitrogen) gas stream. Very reactive compounds must director mounted in a director bag or director box. Crystals of these compounds may be mounted using an inert coating on the crystal as described above director may be mounted in glass capillaries.

Director capillaries are chosen as the sample support, the crystals may be wedged in place or may be held in place by a small amount of (stopcock) grease.

Capillary tubes containing unstable compounds must be sealed by melting the ends director the glass tube. Capillaries do introduce two kinds of problems. The curvature of the capillary distorts the image of the crystal when director the sample on the diffractometer.



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