Recrystallization, metastable

Compound 6 crystallizes from cyclohexane as colorless needles which have no definite melting point there is a change of color to yellow at 128-134 C and the compound then melts sharply at 187-189 r C. When the colorless form is kept for a long time or recrystallized from pyridine or dimethyl sulfoxide it is changed into the yellow modification of mp 187-189 C recrystallization from cyclohexane reverses the process. It has been suggested that the yellow stable form has structure 6A and that the colorless metastable compound is the tautomer 2-methyl-l//-pyrido[2,3-6][l, 4]diazepin-4(5//)-one (6B). There is evidence from 1H NMR spectroscopy that the isomeric pyridodiazepin-2-one, yellow crystals, mp 195—197 " C, exists as an inseparable mixture of the tautomers 4-methyl-l//-pyrido[2,3-6][l,4]diazepin-2(3//)-one (7 A) and 4-methyl-l H-pyrido[2,3-6][l, 4Jdiazepin-2(5//)-one (7B) in the ratio 1 3. 

Polymorphism is also a common phenomenon in Pcs. Among them, CuPc has been the most studied compound because of its stability and its extensive use in the dye industry. Five polymorphs, namely the a-, fi-, y-, S- and s-forms of CuPc are known. /1-CuPc is the thermodynamically stable polymorph while the a-, y-, S- and g-CuPc modifications are metastable and can be converted to j6-CuPc upon heating or solvent recrystallization. 

Once the melt is completed, the sample is shock-cooled by removal from the calorimeter vessel and placement on the isothermal calorimeter block. This cools the sample sufficiently rapidly that most materials will be "trapped" in their metastable supercooled liquid state. A rescan of the sample over the same temperature span will show the cold recrystallization of the material followed by the melt. For maximum time efficiency and sensitivity, a fast scanning rate is recommended. 

As-polymerized PVDC is not in its most stable state annealing and recrystallization can raise the temperature at which it dissolves (78). Low crystallinity polymers dissolve at a lower temperature, forming metastable solutions. However, on standing at the dissolving temperature, they gel or become turbid, indicating recrystallization into a more stable form. 

This was the first proven example of the operation of the principle that free energy stored in the metastable amorphous alloy can be used to create a catalytically active species which is still metastable against phase separation and recrystallization, but which is low enough in residual free energy to maintain the catalytically active state for useful lifetimes. 

Gel dissolution Zeolite nucleation Crystal growth of the zeolite nuclei Dissolution/recrystallization of metastable phases (Oswald s Law of successive transformations)... 

The other comonomers for PTS given in Table 5 form mixed crystals only in a limited concentration range. In contrast to PBS the chloro and bromo derivatives PCS and PBS form completely unreactive triclinic structures (cf. Table 1). However, by crystallization of these monomers at very high supersaturations crystals of metastable reactive modifications are obtained which have been shown to be isomorphous to PTS Owing to the crystallization conditions these crystals are unfortunately not suited for detailed structural investigations. The metastable phases melt up to 50 degrees below the stable modifications which spontaneously recrystallize from the melt. 

A special type of aging occurs in calcium oxalate, which at room temperature is precipitated as a mixture of dihydrate and trihydrate. Upon digestion at higher temperatures these products become metastable with respect to the monohydrate. As a result of the drastic recrystallization, coprecipitated impurities are largely removed by digestion. 

Despite these precautions, a slow freezing process is not always sufficient to ensure a dimensionally stable pellet of good cosmetic appearance. In this respect, formulated solutions that produce interstitial metastable glasses on cooling are sometimes a source of problems. In such systems, devitrification followed by partial erratic recrystallization of the excipient may occur during lyophilization, thereby generating a pellet of poor powdery appearance. 

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