Crystal forms table

The crystal stmcture of glycerides may be unambiguously determined by x-ray diffraction of powdered samples. However, the dynamic crystallization may also be readily studied by differential scanning calorimetry (dsc). Crystallization, remelting, and recrystallization to a more stable form may be observed when Hquid fat is solidified at a carefully controlled rate ia the iastmment. Enthalpy values and melting poiats for the various crystal forms are shown ia Table 3 (52). 

Liquid crystals may be divided into two broad categories, thermotropic and lyotropic, according to the principal means of breaking down the complete order of the soHd state. Thermotropic Hquid crystals result from the melting of mesogenic soHds due to an increase in temperature. Both pure substances and mixtures form thermotropic Hquid crystals. In order for a mixture to be a thermotropic Hquid crystal, the different components must be completely miscible. Table 1 contains a few examples of the many Hquid crystal forming compounds (2). Much more is known about calamitic (rod-Hke) Hquid crystals then discotic (disk-like) Hquid crystals, since the latter were discovered only recendy. Therefore, most of this section deals exclusively with calamities, with brief coverage of discotics at the end. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Anhydrous Oxalic Acid. The anhydrous form of oxaUc acid is odorless and colorless. It exists in two crystal forms, ie, the rhombic or a-form and the monoclinic or P-form (3). The rhombic crystal is thermodynamically stable at room temperature, but the monoclinic form is metastable or slightly stable. The main difference between the rhombic and monoclinic forms exists in the melting points which are 189.5 and 182°C, respectively (Table 1)-... 

Physical Properties of PBT. Unlike PET, the polymer PBT exists in two polymorphs called the a- and p-forms, which have distinctly different crystal stmctures. The two forms are interconvertible under mechanical stress (158,159). Both crystal forms are triclinic and the crystal parameters are shown in Table 7. 

Citric acid, anhydrous, crystallizes from hot aqueous solutions as colorless translucent crystals or white crystalline powder. Its crystal form is monoclinic holohedra. Citric acid is dehquescent in moist air. Some physical properties are given in Table 1 (1 3). The solubiUty of citric acid in water and some organic solvents is given in Table 2. The pH and specific gravity of aqueous solutions of citric acid are shown in Table 3. 

Solids tend to crystallize in definite geometric forms that often can be seen by the naked eye. In ordinary table salt, cubic crystals of NaCl are clearly visible. Large, beautifully formed crystals of such minerals as fluorite, CaF2, are found in nature. It is possible to observe distinct crystal forms of many metals under a microscope. 

Table 21-IV shows some properties of the metals and their crystal forms. Since different crystal forms are involved in the series, trends in the properties are obscured. Figure 21-2 shows scale representations of the crystal structures of metallic beryllium, calcium, and barium.

PRODUCT BFG.xls consists of the chemist s table, a reduced table obtained by casting out data that is not ammenable to statistical analysis such as subjective assessments ( off-white color, characteristic crystal form, pungent smell STEP 1), and the final table that was freed of all inconsistencies by going back to the original data (STEP 2). Various interesting items are highlighted ... 

The Zag meteorite fell in the western Sahara of Morocco in August 1998. This meteorite was unusual in that it contained small crystals of halite (table salt), which experts believe formed by the evaporation of brine (salt water). It is one of the few indications that liquid water, which is essential for the development of life, may have existed in the early solar system. The halite crystals in the meteorite had a remarkably high abundance of 128Xe, a decay product of a short-lived iodine isotope that has long been absent from the solar system. Scientists believe that the iodine existed when the halite crystals formed. The xenon formed when this iodine decayed. For this reason, the Zag meteorite is believed to be one of the oldest artifacts in the solar system. In this lab, you will use potassium-argon radiochemical dating to estimate the age of the Zag meteorite and the solar system. 

Table 1. Crystal forms, melting points, and vOH values of inclusion complexes with chalcones and analogues as guest molecules...

At least seven of the minerals belonging to the serpentine mineral group, occur in fibrous forms. Table 2.2 lists the varieties, together with the crystal chemical data needed to identify them. 

Sorbitol exhibits a number of different crystal forms and there is little agreement in the literature concerning the number of polymorphs, the existence of hydrated species, melting points of the various forms, and even nomenclature [7-14]. Much of the data in the literature was collected on commercial powders that were not fully described, or whose history is not fully known. Table 2 attempts to summarize the various forms based on reports by DuRoss [7] and Quinquenet et al. [8]. 

The commercial crystal form produced after 1975 is the y-form. XRPD powder patterns collected on the various sorbitol samples listed in Table 1 are in agreement with the reported y-form [15], and are shown in Figure 3. The listing of peak positions, d-spacings, and relative intensities for this crystal form is given in Table IV. 

Table 7 Crystal form and space groups for the imidazo[4,5-c][1,2,5]thiadiazoles 51...

As with ceramics and metals, polymer crystals can have multiple crystal forms. Polyethylene has a metastable monoclinic form and a orthohexagonal high pressure form. A list of some of the more common polymers and their corresponding crystal strnctnres is given in Table 1.24. Finally, X-ray diffraction can be used to determine the amorphous to crystalline ratio in semicrystalUne polymers in much the same way that Eq. (1.61) can be used. Figure 1.66 shows a schematic illustration of the X-ray diffraction patterns for semicrystalline and amorphous polyethylene. The estimation of crystalline content is based upon a ratio of the peak areas in the two samples. 

Crystals formed through biological activity Table 14.1 Crystals formed by hiomineralization... 

Octanitrocubane (ONC) is a white solid, somewhat soluble in hexane and readily soluble in polar organic solvents. The density of one of the ONC polymorphs is very high (1.979gcm"3) but is still lower than the calculated value (the latest and most sophisticated calculation predicts a density above 2.1 gem"3 for the most stable polymorph of ONC) which indicates the existence of a crystal form of ONC much more dense than that synthesized. Kamlet-Jacobs equations predicted that ONC is 15-30% better than HMX [109] (a most powerful currently employed military explosive) and 6% better (perhaps also less shock sensitive) than the recently discovered explosive HNIW [121, 253-258] or CL-20 as shown in Table 2.15. It is interesting to note that both HpNC and ONC have decomposition points well above 200 °C and are not detonated by hammer blows. 

Examine under a microscope the shape of the crystals remaining in the beaker and in the bowl. Filter off the crystals formed in the porcelain bowl, dry them in the air, and determine the yield in per cent. Using Table 4 containing the solubilities of the salts that can form in the solution, explain the conditions of precipitation of the salts formed from the solution. 

A negative thermal effect accompanies the transformations of the ammonium nitrate crystal form with rising temperature, whereas during cooling the process is of course exothermic (Table 110). 

The diameter of the double helix of B-DNA, measured between phosphorus atoms, is just 2.0 nm. The rise per turn, the pitch, is 3.4 nm. There are about ten base pairs per turn (9.7 and 10.6 in two different crystal forms).68 69 Thus, the rise per base pair is 0.34 nm, just the van der Waals thickness of an aromatic ring (Table 2-1). It is clear that the bases are stacked in the center of the helix. A 1000-bp (1-kb) gene would be a segment of DNA rod about 340 nm long, about 1/40 the length of the molecule in the electron micrograph of Fig. 5-13. 

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