Molecular crystals species

In a solid-gas reaction involving a molecular crystal, the reactants are respectively the molecules in the crystalUne solid and the molecules in the gas phase and the product is the product crystal, which can be crystalUne or amorphous. Vapour uptake to generate a solvate crystal (e.g. hydration) is a related process. In fact the difference between a crystal solvation process and a solid-gas reaction leading to new molecular/ionic species is mainly in the energetic scale of the processes and in the fact that in solvation processes, molecules retain their chemical identity. On this premise there is a relevant analogy between the uptake of smaU molecules by a nanoporous material [16] and the reaction between a molecular crystal and molecules to yield a co-crystal or a salt (e.g. acid-base... 

Two later sections (1.6.5 and 1.6.6) look at the crystalline structures of covalently bonded species. First, extended covalent arrays are investigated, such as the structure of diamond—one of the forms of elemental carbon—where each atom forms strong covalent bonds to the surrounding atoms, forming an infinite three-dimensional network of localized bonds throughout the crystal. Second, we look at molecular crystals, which are formed from small, individual, covalently-bonded molecules. These molecules are held together in the crystal by weak forces known collectively as van der Waals forces. These forces arise due to interactions between dipole moments in the molecules. Molecules that possess a permanent dipole can interact with one another (dipole-dipole interaction) and with ions (charge-dipole interaction). Molecules that do not possess a dipole also interact with each other because transient dipoles arise due to the movement of electrons, and these in turn induce dipoles in adjacent molecules. The net result is a weak attractive force known as the London dispersion force, which falls off very quickly with distance. 

Urease activity in soils has been found to reflect the bacterial count and content of organic matter. The urease isolated from an Australian forest soil (87) was crystallized and found to have a specific activity of 75 Sumner units (S.U.) per mg. The molecular weight species were estimated (sedimentation velocity) to be 42, 131, and 217 X 103. That urease activity persists in soils is shown by the finding that enzymic activities, including urease, could be demonstrated in soil samples over 8000 years old (88). 

Lattice substitution requires that the incorporated impurity be of similar size and function to the primary crystallizing species. In other words, the impurity must fit into the lattice without causing significant dislocations. An example of such a system is found in the crystallization of L-isoleucine in the presence of trace quantities of L-leucine. The two species have similar molecular structures, differing only by one carbon atom in the position of a methyl side group. In this system, the incorporation of L-leucine in L-isoleucine crystals is proportional to the concentration of L-leucine in the mother liquor. Moreover, the shape of the recovered crystals changes as the content of L-leucine in recovered crystal increases. 

Since the Braggs first determination, thousands of structures, most of them far more complicated than that of sodium chloride, have been determined by x-ray diffraction. For covalently bonded low molecular weight species (such as benzene, iodine, or stannic chloride), it is often of interest to see just how the discrete molecules are packed together in the crystalline state, but the crystal structures affect the chemistry of such substances only to a minor degree. However, for most predominantly ionic compounds, for metals, and for a large number of substances in which atoms are covalently bound into chains, sheets, or three dimensional networks, their chemistry is very largely determined by the structure of the solid. 

On this conceptual premise, uptake and release of solvent can be paralleled to a solid-gas reaction, whereby the reactants are the molecules in the crystalline solid and the molecules in the gas phase and the product is the solvated crystal. Clearly, the same reasoning applies to the reverse process, i.e. generation of a new crystalline form by means of gas release. In gas-solid reactions, gases are reacted directly with crystals or amorphous phases to give complete conversion and usually quantitative yields. What would then be the difference between a solvation reaction and a reaction leading to new molecular/ionic species if not the energetic scale of the processes and the fact that in solvation processes molecules retain their chemical identity ... 

In spite of the great interest in the phenomenon of polymorphism and of the increased research activity beyond the boundaries of organic solid-state chemistry, it is a fact that only a few molecular compounds possess several crystalline forms, whereas for many other tens of thousands of molecular compounds only one crystalline form is known. In other words, why are there so few molecular crystals polymorphs The often quoted association between number of known forms and the time and energy spent in searching for them put forward by McCrone probably contains the answer to this question. It is probable that if thorough (combinatorial ) crystallization experiments were carried out on any given molecular species or molecular salt, alternative crystalline forms would be found. It is probable but not certain. 

The first step for crystal growth by molecular deposition is that the molecules of the crystallizing species must diffuse through the matrix to reach the surface of the growing crystal. Counter-diffusion of noncrystallizing molecules away from the growing surface must also be considered. 

Here, Cr is the saturation concentration for a crystal of radius r Cs is the saturation concentration for a large, flat crystal MW is molecular weight crystallizing species y is interfacial tension at the crystal surface p is crystal density R is the ideal gas constant is the equilibrium melting temperature (for a large, flat crystal) and Tr is melting temperature for crystal of radius r. 

In order to analyze the motions within a molecular crystal, it is of great value to have both the Raman and the IR spectra of single crystals. If the crystal is oriented properly, it is thus possible to assign the observed spectra to vibrational species of the unit cell. If, as... 

Polymorphism is an ability of the drug substance to form crystals with different molecular arrangements giving distinct crystal species with different physical properties such as solubility, hygroscopicity, compressibility, and others. This phenomenon is well known within pharmaceutical companies. The reader can find additional information in references 47 and 48. The determination of possible polymorphic transition and existence of thermodynamically unstable forms during preformulation stage of drug development is important. Typical methods used for solid-state characterization of polymorphism are DSC,... 

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