Vacuum sublimation technique

Amorphous Films. An amorphous film is generally prepared by solvent evaporation of a dry organic solution of Chi on a solid substrate surface. The vacuum sublimation technique, which is widely employed for most synthetic dyes, is not applicable to Chi due to possible thermal degradation of the pigment. 

Besides phthalocyanine pigments developed for office copiers, new polymorph materials for laser printer utilization have been prepared. These materials, whose absorption characteristics were extended to the near-infrared region, were prepared by vacuum sublimation techniques. Recently, for safety reasons and to reduce the cost of production, solvent-pigment interactions have received great attention [41]. 

Fazio et al. [927] have described a multideterminative procedure for various antioxidants (DLTDP, BHT, lonox 100, PG, BHA) in food by means of a (10 " mm Hg) vacuum sublimation technique (with GC and confirmed by UV, IR or MS) and achieved recoveries exceeding 85%. Although no indications were given that this isolation procedure would be equally successful for the quantitative determination of these indirect food additives when incorporated in a polymeric matrix, Shlyapnikov et al. [928] have reported the direct determination of AOs in PE by vacuum sublimation. 

Fig. 8. Summary of real and imaginary e2(tu) parts of the dielectric function for Cgo vacuum-sublimed solid films at room temperature over a wide frequency range, using a variety of experimental techniques. The arrow at the left axis points to i = 4.4, the observed low frequency value of ei obtained from optical data [81].

Vacuum sublimation of a Zn(III) complex solution produced a pink-colored solid, which was Zn(III) perfhiorotetraphenylporphyrin. NMR analysis of this product showed the absence of pyrrole hydrogen atoms, which is the evidence for complete replacement. Then bivalent iron chloride (FeCl2) was prepared according to the common technique [17]. This compound was then used for Zn(III) ion replacement in a porphyrin ring. For this purpose, the Zn(III) complex was dissolved in dimethylformamide (DMF) and then, with the addition of an FeCl2 solution, also in DMF. The reaction mixture was heated in a steam bath at water boiling point for 5 h. Finally, a dark-brown sediment was deposited, and the solution was colored blue, which is typical of iron(III) perfhiorotetraphenylporphyrin (perFTPhPFe(III)) dissolved in DMF. In this case, the active weight of the catalyst iron(III) perfhiorotetraphenylporphyrin equaled 47.05 mg. 

In this kind of separation, the target must be very thin to get good yields of the recoiled atoms. Thus the preparation technique for thin targets is very Important. For such purposes electrodeposition or vacuum sublimation methods can often be used to advantage. 

Both Fe(C0)5 and (C I Fe were introduced Into several zeolites by sublimation techniques employing a vacuum line. The zeolite and either of the two above-mentioned iron compounds were introduced into two separate ends of an inverted U-tube which was connected to a vacuum line. The iron compounds were frozen with... 

Doubly sublimed magnesium high dilution, vacuum line technique. Yield 14%... 

The catalytic ability of Iron(III) acetylacetonate seems to depend very much upon the method of preparation, and. Indeed, considerable batch-to-batch variations have been observed between materials made by nominally Identical procedures and from the same starting materials. Attempts to purify this compound by techniques such as recrystalllsatlon and vacuum sublimation have led to even greater variations In catalytic activity. It should be added that the many samples of the chelate which have been prepared have frequently differed In appearance from one another, and that a broad correlation has been observed between appearance and catalytic activity. 

A search for new efficient techniques of rare earth element separation and purification from calcium is a current problem, as production of high-purity rare earths is of great importance in advanced technology and material science. This problem may be solved by vacuum sublimation of volatile compounds when the difference in vapor pressure of the components present is used. This technique of purification was tested for Nd and Ca separation in vacuum. The well-known volatile and thermally stable dipivaloylmethanates were taken as starting substances. It was found that the addition of pivalic acid to the Nd(dpm)3 + Ca(dpm)2 mixture caused an increase in the separation efficiency and led to pure Nd(dpm)3 in the sublimate . ... 

Since these compounds are extremely reactive toward moisture and oxygen, the equipment used to handle them will be discussed briefly. (A high degree of experimental skill and extreme care are required for safe manipulation of these compounds ) A simple vacuum line, consisting of a manifold equipped with a mercury blowout manometer, a gas inlet tube, and several standard-taper joints, is employed. A worthwhile introduction to the use of vacuum-line techniques in synthetic chemistry is given by Jolly and by Sanderson. The sublimation apparatus shown in Fig. 2 and the vacuum filtration apparatus and... 

A more direct study of the reaction between CO and NO2 was made by Brown and Crist [485], who used a KCl coated Pyrex reaction vessel fitted with a greaseless valve to avoid decomposition of the NO2. In order also to avoid complications due to gas phase dissociation of the NO2, its pressure was kept very low (<0.5 torr), and the reaction times were kept comparatively short. Amounts of reaction were measured by freezing and then analyzing for the product CO2 by vacuum sublimation from the nitrogen oxides. In order to obtain measurable amounts of reaction under the conditions stated, it was necessary to employ high concentrations of CO. Even then the partial pressures of CO2 in the products were less than 30 microns, and often as little as 5 microns, so that good experimental technique was required. It was confirmed that the reaction was second order over some two- to three-fold variation of the partial pressures of CO and NO2. Mean rate coefficients between 500 and 563 K are given in Table 57. 

A detailed study of the SHG from both spin-coated and evaporated films of trinitro-(l) [73,74], trhodo-(4) [74] and trioctylsulfonyl-(7) [74] subphthalocyanines has been carried out. In order to induce molecular ordering in hie spin coated film, a corona poling technique was used. Films were prepared by spin-coating from a solution of the corresponding SubPc in poly(methylmethacrylate) (PMMA). For comparison purposes, evaporated films were also prepared by vacuum sublimation onto amorphous silica plates. 

Vacuum sublimation is a convenient purification technique, frequently used in synthetic organometallic chemistry. There are many types of sublimation flasks, varying chiefly in degree of sophistication of design. The main practical problem with vacuum sublimation is the inefficient transfer of heat from heat source to the sample being sublimed. Virtually all set ups use an oil- or metal bath and rely on conduction as the main mechanism of heat transfer. Under the vacuum conditions of the usual sublimation (10 - 10 mm Hg) both... 

In the case of the solids,initial experiments showed that the conventional purification techniques such as recrystallization from solutions do not offer sufficiently pure materials. For example,PMDA samples, after several recrystallizations and vacuum sublimations appear colorless,but show high levels of ionic contaminations when analyzed for inorganic ions. To eliminate this problem,the technique of zone refining as used in semiconductor materials purifications has been used to purify the starting materials. This technique has been applied for a number of organic materials in our laboratory. PMDA for example,when subjected to a simple zone refining process ( as few as 25 zones), shows removal of impurities as high as 3% even if preceded by recrystallization and sublimation. In syntheses of polyamic acids,a variation as small as 3% in stoichiometry can cause considerable variation in the final batch-to-batch synthesis. 

More recently. Fair and Downs [119] investigated TIN3 thin films a few hundred angstroms thick prepared by both vacuum sublimation and by reaction of gaseous HN3 with vacuum-deposited thallium metal. The spectra from the two methods of preparation were identical, tending to confirm the reliability of the vapor-solid reaction technique. In addition to the low-energy peaks observed by Deb and Yoffe [84], the investigators found a peak at 275.0 nm (4.5 eV) and one at 230.0 nm (5.4 eV). The latter was taken to be the intra-anionic ex-... 

C, gave a very dark, powdery crystalline product—crude Mo(NMe2)3. This was collected by filtration (Schlenk technique) and dried in vacuo. Analytically pure Mo(NMe2)3 is pale yellow it was obtained from the crude product by vacuum sublimation (100°C, cm Hg). 

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