Shortening Crystallization

A good compilation of the functions of fats in various food products is available (26). Some functions are quite subtle, eg, fats lend sheen, color, color development, and crystallinity. One of the principal roles is that of texture modification which includes viscosity, tenderness (shortening), control of ice crystals, elasticity, and flakiness, as in puff pastry. Fats also contribute to moisture retention, flavor in cultured dairy products, and heat transfer in deep fried foods. For the new technology of microwave cooking, fats assist in the distribution of the heating patterns of microwave cooking. 

Too many gland adjustments will cause the packing to crystallize and burn, which will shorten the packings useful life and damage the shaft or pump sleeve. 

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

The molecular and crystal stmctures of 2-phenylbenzotellurazole were determined by X-ray (89KGS1690). The dihedral angle between the planes of the five-membered heterocycle and 2-phenyl ring is 31.2°. In contrast to benzoisotel-lurazole, no shortened intermolecular Te- N contacts were found in the crystal stmcture of 2-phenylbenzotellurazole. Consequently, no anomalies in solubility and melting point were revealed for this compound as compared with its sulfur and selenium congeners. 

PA-6,6 is made from the relatively expensive materials hexamethylene diamine and adipic acid. An alternative synthesis of PA-6,6 from adiponitrile and hexamethylene diamine utilizing water is under investigation.16 PA-6 can be synthesized in a continuous process at atmospheric pressure, but reaction times are very long as the ring-opening initiation step is particularly slow. The reaction time can be shortened considerably by carrying out prepolymerization in the presence of excess water at pressure however, this makes the continuous polymerization process more complex. Copolymers with amide units of uniform length (diamides) are relatively new the diamide units are able to crystallize easily and have a thermally stable crystalline structure. 

Our group has recently observed (ref. 10) that often synthesis time of zeolites can be substantially shortened when microwave radiation is applied instead of conventional heating. Table 3 gives some examples. Also a narrow crystal size distribution is obtained in this way. The crystallization temperature is rapidly (1 min) reached by microwave heating, this may be a factor in homogeneous and essentially simultaneous nucleation. 

The presence of isotopic impurities causes clear effects in the vibrational spectra. Almost all modes studied so far show frequency shifts on S/ S substitution [81, 107]. The average shift of the internal modes is ca. 0.6 cm , and of the external modes it is 0.1-0.3 cm (Tables 3, 4 and 5). Furthermore, the isotopomers which are statistically distributed in crystals of natural composition can act as additional scattering centers for the phonon propagation. Therefore, in such crystals the lifetime of the phonons is shortened in comparison with isotopically pure crystals and, as a conse-... 

In a comparison of fluorescence spectra between the ester and thioester derivative crystals of PDA, the ester crystal shows a strong emission whereas the thioester crystal fluoresces much more weakly. For example, the intensity of a PDA methyl thioester crystal is about one-thousandth of that of a PDA methyl ester crystal. Furthermore, the fluorescence lifetime of mixed crystals which consist of a large amount of PDA methyl ester and a small amount of the corresponding thioester moiety is much shortened, compared to the lifetime of pure PDA methyl ester crystals. In quenching experiments in solutions of PDA ester, the fluorescence of the PDA ester is dramatically quenched by thioacetate. Similar behaviour has been obtained with several types of diolefin derivatives having a thioester moiety, where crystal structures are isomorphous with the corresponding ester derivatives. 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

In a similar manner, the diffusion of hexane into dichloromethane solutions containing mixtures of the alkylammonium salts of bromide and the olefinic acceptors o-CA and TCNE result in the formation of brown-red crystals [23]. X-ray analysis reveals the (1 1) complex of bromide with o-CA, in which the anion is located over the center of the C - C bond of the acceptor moiety (Fig. 15b) and Br - C contacts are shortened by as much as 0.6 A relative to the sum of van der Waals radii (Table 3). In bromide complexes with TCNE, the location of the anion relative to the acceptor is variable. In fact, a 2 1 complex [(Br )2,TCNE] is isolated in which both anions reside over the olefinic bond when the tetraethylammonium salt of bromide is used. In comparison, if the tetrapropyl- or tetrabutylammonium salts of the same anion are employed, the (1 1) complexes [Br ,TCNE] are formed in which the bromide donors are shifted toward the cyano substituents (Fig. 15a). In both cases however, the short intermolecular separations that are characteris-... 

Tellurium crystallizes isotypic to a-selenium. As expected, the Te-Te bonds in the chain (283 pm) are longer than in selenium, but the contact distances to the atoms of the adjacent chains are nearly the same (Te- Te 349 pm). The shortening, as compared to the van der Waals distance, is more marked and the deviation from a regular octahedral coordination of the atoms is reduced (cf. Table 11.1, p. 111). By exerting pressure all six distances can be made to be equal (cf. Section 11.4). 

The resulting crystal proved amenable to a conclusive XRD analysis. As shown in Eq. (26), the C—C ring bond is lengthened over what it is in the structurally similar cyclopropene (1.304 A) and the B—C bonds shortened relative to the electronically analogous bond in trivinylborane (1.558 A). Thus, one can safely conclude that there is extensive 7r-electron delocalization and Hiickel aromatic character in the borirene ring. 

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