Molecular compounds crystals

Other Peroxohydrates. Potassium, mbidium, and cesium carbonates all form peroxohydrates having the general formula M2CO2 3H20. Crystal stmctures have not been estabflshed Raman spectra (31) confirm the presence of molecular hydrogen peroxide in the crystal. These compounds are unstable and have no commercial appHcation. 

Sodium Salicylate.—When an aldehyde is shaken with a saturated solution of sodium salicylate, there seems to be evidence of the formation of a weak molecular compound, and with cinnamic aldehyde well-defined crystals have been obtained which give on analysis —... 

The chemistry of silicon in very low oxidation states is one of the most fascinating research areas, which can be located between molecular compounds of silicon and elemental (perhaps amorphous) silicon [190-194]. Most interesting results have recently been obtained by structural investigations of siliddes in Zintl phases. However, compounds of silicon with negative oxidation states and very low coordination numbers of 1, 2, and 3 are so far only known in the composite of a crystal lattice. 

The formulas of ionic compounds have a different meaning from those of molecular compounds. Each crystal of sodium chloride has a different total number of cations and anions. We cannot simply specify the numbers of ions present as the formula of this ionic compound, because each crystal would have a different formula and the subscripts would be enormous numbers. However, the ratio of the number of cations to the number of anions is the same in all the crystals, and the chemical formula shows this ratio. In sodium chloride, there is one Na+ ion for each Cl ion so its formula is NaCl. Sodium chloride is an example of a binary ionic compound, a compound formed from the ions of two elements. Another binary compound, CaCl2, is formed from Ca2+ and Cl- ions in the ratio 1 2, which is required for electrical neutrality. 

In literature, SOHNCKE space-group types are often termed chiral space groups , which is not correct. Most chiral molecular compounds do not crystallize in a chiral (enantiomorphic) space group. For details see [86]. 

The molecules in crystalline chlorine, bromine and iodine are packed in a different manner, as shown in Fig. 11.1. The rather different distances between atoms of adjacent molecules are remarkable. If we take the van der Waals distance, such as observed in organic and inorganic molecular compounds, as reference, then some of the intermolecular contacts in the b-c plane are shorter, whereas they are longer to the molecules of the next plane. We thus observe a certain degree of association of the halogen molecules within the b-c plane (dotted in Fig. 11.1, top left). This association increases from chlorine to iodine. The weaker attractive forces between the planes show up in the plate-like habit of the crystals and in their easy cleavage parallel to the layers. Similar association tendencies are also observed for the heavier elements of the fifth and sixth main groups. 

The crystal data compared to expected values assuming no distortions are summarized in Table 18.1. Inspection of the atomic coordinates reveals that the distortions of the packing of spheres are only marginal. As expected, the greatest deviations are observed for the molecular compounds PI3 and NMe3. 

Thus Pasteur noted that the amide of (-) malic acid forms molecular compounds of different properties with the enantiomeric amides of tartaric acid. With amide of (+) tartaric acid large transparent crystals are formed whose solubility is 18% at 20°C, while with the amide of (-) tartaric acid, thin needles are formed with solubility almost two times higher. Free malic and tartaric acids also form diastereomeric molecular compounds. 

An interesting molecule of the type just discussed is 8-dimethylamino-1 -naphthoic acid, in whose crystal there are two symmetry-independent molecules in the asymmetric unit. One of these shows distortions in agreement with the above generalizations, whereas there are qualitatively different distortions in the second molecule. The explanation is that the crystal is actually a 1 1 molecular compound of the amino acid, which shows the N 0=0 interaction, and the corresponding zwitterion, which does not. 

TABLE A2 Thermochemical Data of Selected Chemical Compounds Aluminum nitride (AIN), crystal, molecular weight = 40.98824... 

The importance of high density as a feature of potential fuel systems that seek to maximize net volumetric heat of combustion is well documented. The prediction of the crystal density of an unknown compound typically has been approached through the use of volume additivity procedures [29-32]. Here, the crystal-molecular volume ycm) is calculated by summing appropriate crystal-atomic or group volumes Vca Vcm = ca) and the corresponding crystal density is ob-... 

Chemical reactions in the sohd state have intrinsic features different from those for reactions performed in solution or in the gaseous state. For example, sohd-state organic reactions often provide a high regio- or stereoselectivity because the reactions and the structiue of a product are determined by the crystal structure of the reactant, i.e., the reaction proceeds under crystaUine lattice control [1-8]. When the reactant molecules are themselves crystalhne (molecular crystals) or are included in host crystals (inclusion compounds), the rate and selectivity of the reaction are different from those obtained in an isotropic reaction medium. 

In that commonest of space groups, Flja, the general position has a multiplicity of 4, but there are also four, twofold special positions, each on a center of inversion. If a molecular compound crystallizes in this space group... 

Thus the addition of n-pentane to mixtures of p-xylene and m-xyiene permits complete separation of the xylenes which form a binary eutectic with 11.8% para. Without the n-pentane, much para is lost in the eutectic, and none of the meta is recoverable in pure form. A detailed description of this process is given by Dale (1981), who calls it extractive crystallization. Other separation processes depend on the formation of high melting molecular compounds or clathrates with one of the constituents of the mixture. One example is carbon tetrachloride that forms a compound with p-xylene and alters the equilibrium so that its separation from m-xylene is... 

Even so great an admirer of Werner as Paul Pfeiffer (1875-1951),93 Werner s former student and one-time chief of staff at the University of Zurich and the man who first applied Werner s theory to crystal structures (see Section 1.1.5.4), proposed modifications of the coordination theory. He applied what he called the principle of affinity adjustment of the valencies to overcome certain shortcomings of Werner s theory.94 He considered the ionizable radicals or atoms in the outer sphere to be combined with the complex radical as a whole and not attached definitely to the central atom or to any of its associated molecules. He also applied this idea to complex organic molecular compounds. However, Pfeiffer s modifications should not be interpreted as attacks on Werner s ideas. 

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