Sublimation crystals

The addition of solid Ba2Xe06 to cold cone H2SO4 produces the second known oxide of xenon, Xe04. This is an explosively unstable gas which may be condensed in a liquid nitrogen trap. The solid tends to detonate when melted but small sublimed crystals have been shown to melt sharply at —35.9°C. Xe04 has only been incompletely studied, but electron diffraction and infrared evidence show the molecule to be tetrahedral. 

Schmidbaur, H. et ai, Inorg. Chem., 1966, 5, 2069 Sublimed crystals decomposed explosively at 120°C. See related ALKYLMETALS See other GOLD COMPOUNDS... 

Typical methods for generation of polymorphs include sublimation, crystallization from different solvents, vapor diffusion, thermal treatment, melt crystallization, and rapid precipitation. High-throughput screening methods have been reported for polymorph screening.50... 

Figure 3.6. Schematic representation of an apparatus for production of sublimation crystals in a horizontal arrangement. The total length is typically 400 mm. Adapted from Laudise et al, 1998.

Physical vapour growth in horizontal and vertical systems has been successfully used to grow mm-cm sized single crystals of single-component compounds, e.g., a-4T, a-6T, pentacene, anthracene and CuPc (Kloc et al., 1997 Laudise et al., 1998). A scheme of the apparatus used in a horizontal arrangement for producing sublimation crystals is shown in Fig. 3.6. 

Bubbles, drops, and solid particles are of importance in many processes, such as boiling, condensation, sublimation, crystallization, cavitation, electrolysis, and effervescence, in which a change of phase occurs. A detailed review of these subjects is beyond the scope of this book, but a few basic points and useful references will be given. 

Kofler Micro Hot State Apparatus. Electrically heated apparatus which might be considered a modification and improvement of the Fisher-Johns apparatus. It permits microscopic studies of fusion, sublimation, crystallization and explosion phenomena Refs 1) L. Kofler, Mikrochemie 15, 242 (1934) 2) N.D. Cheronis J.P. Entrikin,... 

This very high compressibility is to be expected from the structure of the crystal which, as seen above, is molecular rather than ionic and moreover contains large open spaces between the atoms. The refractive indices at 17° C. are 1-733 for sodium light and 1-748 for lithium light.2 Klocke observed 3 that for yellow light the sublimed crystals exhibit double refraction, but this could not be confirmed by Brauns.4... 

The crystals have density 3-60 at 19° C.4 According to Krenner,5 the sublimed crystals have axial ratios a b c =0-58787 1 0-88258 the optic axial angle 2Ha = 108° 46 and the optical character is positive. 

Figure 7.4 5 Immersing sublimed crystals of the apparently guest-free po phase of p-t-butylca 1 ix [4] arene results in the immediate displacement of bubbles of air from the crystal lattice as the air molecules are replaced by nitrobenzene guest (reproduced by permission of The Society of Chemistry).

Me2SiCl2 forms with A-methylimidazole (NMI) a 1 2 adduct, but after sublimation crystals of a 1 4 adduct were obtained. The crystal structure analysis of the 1 4 adduct revealed a pentacoordinated silicon atom108 (equation 66). 

A verification of the interpretation of the surface of sodium bromide, when prepared under usual conditions, as a dual distribution is again provided by a new preparation under anhydrous conditions of a sample of the sublimed crystals, which when used as an adsorbent shows evidence of type 2 surface only. The... 

Preparation of the Menstruum begins with the sublimation of Sal Ammoniac (a commercial product is alright for this preparation). This is easily done using Corning Ware casseroles over electric or gas heat as mentioned earlier when we discussed sublimation. After the first sublimation, the sublimed crystals will take on a pale yellow color. Collect them and sublime again. 

It forms colorless sublimable crystals, and ir and Raman spectra suggest that the molecule is isostructural with Ti(N03)4 and Sn(N03)4 it is soluble in water but insoluble in toluene. Hafnium gives Hf(N03)4-N205. Nitrato anions such as M(N03) are also known. 

Tellurium tetrafluoride forms colorless sublimable crystals that exist as chains of cw-linked square-pyramidal TeFs groups (1). Its chemistry is similar to SeF4, but is less useful for fluorination. 

A recent development in the low-valent lanthanide area is the synthesis and X-ray structural determination of the unsolvated complex (C5Me5)2Sm 121). This species is the first structurally characterized bis(cyclopentadienyl)lanthanide species which has no other ligands in the metal coordination sphere. As such, it is the closest lanthanide analogue of the bis(ring)metallocene sandwich compounds like ferrocene. Samarocene was obtained by desolvation of (C5Me5)2Sm(thf)2 under high vacuum and sublimation of the product [Eq. (58)]. The sublimed crystals have a bent... 

In normal preparations the (4) formed is usually not isolated. The end of the condensation step is announced by the cleavage of nearly the theoretical amount of amine. The temperature of the mixture is then raised until a new evolution of di-methylamine sets in. During the thermolysis the color of (4) slowly fades and the second equivalent of amine is split off. Finally, the frequently dark reaction solution is worked up in a usual manner, e.g., by distilling off the quinoline in vacuo, or by dissolving the quinoline in dilute mineral acid when it is a small-scale preparation or when the product is too volatile for a separation by distillation. Further purification can be achieved by sublimation, crystalization, and column chromatography on alumina or silica gel. 

Our first use of crystal polarization was in hexamethylbenzene. It had all molecular planes parallel to one another and perpendicular to the main faces of a sublimed crystal. It was known that the lowest absorption system was polarized in the molecular plane, conforming with its assignment to a n-n transition. However in the next system of benzene at 200 nm, corresponding to the hexamethylbenzene system at 230 nm, the vapour phase spectrum had been assigned to a Rydberg transition polarized perpendicular to the molecular plane [210]. 

Figure 25.7 Depth dependent profile of the chemical composition of tetracene sublimation crystal. Specifically, the stable oxidation product 5,12-tetra-cene-quinone, indicated by the arrow, is concentrated to a much larger amount at the surface than in the bulk.

In addition to showing that solvent-free melt and sublimation crystallization conditions offer an attractive route to new polymorphs, a CSD survey of these methods of crystallization and the frequency of Z was performed (Table 3-5). There is a dramatic increase in the occurrence of Z > 3 crystal structures when melt or sublimation crystallization conditions are used [20]. The occurrence of high Z in melt crystallization and sublimation methods is ascribed to the rapid cooling of the hot liquid or vapor (100-300° C) in the open flask or on the cold finger (kinetic phase), conditions under which hydrogen-bonded clusters are likely to condense in a pseudo-symmetric crystalline arrangement. On the other hand, the slower nucleation process of solution crystallization gives the frequent situation of Z < 1 (88% hits). 

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