Crystal types atomic-molecular

WebLab Viewer gives a very-high-quality display suitable for publication and presentation. Molecules can be displayed as lines, sticks, ball and stick, CPK, and polyhedrons. In addition, different atoms within the same structure may be displayed in different ways. Text can be added to the display as well as labeling parts of the structure in a variety of ways. The user has control over colors, radii, and display quality. The program can also replicate a unit cell to display a crystal structure. Several types of molecular surfaces can be displayed. 

The initial configuration is set up by building the field 0(r) for a unit cell first on a small cubic lattice, A = 3 or 5, analogously to a two-component, AB, molecular crystal. The value of the field 0(r) = at the point r = (f, 7, k)h on the lattice is set to 1 if, in the molecular crystal, an atom A is in this place if there is an atom B, 0, is set to —1 if there is an empty place, j is set to 0. Fig. 2 shows the initial configuration used to build the field 0(r) for the simple cubic-phase unit cell. Filled black circles represent atoms of type A and hollow circles represent atoms of type B. In this case all sites are occupied by atoms A or B. 

Force fields split naturally into two main classes all-atom force fields and united atom force fields. In the former, each atom in the system is represented explicitly by potential functions. In the latter, hydrogens attached to heavy atoms (such as carbon) are removed. In their place single united (or extended) atom potentials are used. In this type of force field a CH2 group would appear as a single spherical atom. United atom sites have the advantage of greatly reducing the number of interaction sites in the molecule, but in certain cases can seriously limit the accuracy of the force field. United atom force fields are most usually required for the most computationally expensive tasks, such as the simulation of bulk liquid crystal phases via molecular dynamics or Monte Carlo methods (see Sect. 5.1). 

The variation of the Chin-Gilman parameter with bonding type means that the mechanism underlying hardness numbers varies. As a result, this author has found that it is necessary to consider the work done by an applied shear stress during the shearing of a bond. This depends on the crystal structure, the direction of shear, and the chemical bond type. At constant crystal structure, it depends on the atomic (molecular volume). In the case of glasses, it depends on the average size of the disorder mesh. 

In the crystal the atoms of this molecule are found to be distributed in three planes Hie two tolyl rings are rotated by 24° (trans to Br) and 68° (cis to Br) with respect to the ethylene plane. Semiempirical Huckel molecular orbital (MO)-type calculations give essentially the same angles in the crystal, but predict 35° and 45° for these angles in the Bee molecule. 

Molecular rotation In a normal crystal every atom occupies a precise mean position, about which it vibrates to a degree depending on the temperature molecules or polyatomic ions have precisely defined orientations as well as precise mean positions. When such a crystal is heated, the amplitude of the thermal vibrations of the atoms increases with the temperature until a point is reached at which the regular structure breaks down, that is, the crystal melts. But in a few types of crystal it appears that notation of molecules or polyatomic... 

A special type of pseudo-polymorphism is that related to the proton transfer along an X-H Y interaction. The motion may not be associated with a phase transition, but may well imply the transformation of a molecular crystal into a molecular salt. Wilson [34] has discussed, on the basis of an elegant neutron diffraction study, the migration of the proton along an O-H O bond in a co-crystal urea-phosphoric acid (1 1) whereby the proton migrates towards the mid-point of the hydrogen bond as the temperature is increased, becoming essentially centred at T > 300 K. Wiechert and Mootz [35] isolated two crystalline materials composed of pyridine and formic acid of different composition. In the 1 1 co-crystal the formic acid molecule retains its proton and transfer to the basic N-atom on the... 

A single-particle effect that adds features in the X-ray absorption spectrum of molecules not present in that of atoms is the shape resonance (74, 75). (In the case of solids this effect, caused by a modification of the density of states due to the presence of the other atoms in the molecule, is automatically accounted for in band calculations.) Localization of the excited electron inside the molecule in states resulting from an effective potential barrier located near the electronegative atoms in the molecule causes strong absorption bands in free molecules and near the inner-shell ionization limits of positive ions in ionic crystals (74). Consequently, molecular inner-shell spectra depart markedly from the corresponding atomic spectra. The type of structure of an inner-shell photoabsorption spectrum depends on the geometry of the molecule, the nature of its ligands, etc., and can sometimes be used to determine the structure of the molecule. 

Evidently many crystals contain bonds of two or more quite distinct types. In molecular crystals consisting of non-polar molecules the bonds within the molecule may be essentially covalent (e.g. 85 or Sg) or of some intermediate ionic-covalent nature (e.g. Sip4), and those between the molecules are van der Waals bonds. In a crystal containing complex ions the bonds within the complex ion may approximate to covalent bonds while those between the complex ion and the cations (or anions) are essentially ionic in character, as in the case of NaNOg already quoted. In other crystals there are additional interactions between certain of the atoms which are not so obviously essential as in these cases to the cohesion of the crystal. An example is the metal-metal bonding in dioxides with the rutile structure, a structure which in many cases is stable in the absence of such bonding. 

Konno H, Taylor LS (2006) Influence of different polymers on the crystallization tendency of molecularly dispersed amorphous felodipine. J Pharm Sci 95 2692-2705 Konno H, Handa T, Alonzo DE, Taylor LS (2008) Effect of polymer type on the dissolution profile of amorphous solid dispersions containing felodipine. Eur J Pharm Biopharm 70 493 99 Lauer ME, Grassmann O, Siam M, Tardio J, Jacob L, Page S et al (2011) Atomic force microscopy-based screening of drug-excipient miscibility and stability of solid dispersions. Pharm Res 28 572-584... 

The properties of five types of crystalline solids—atomic, molecular, ionic, metallic, and network cova/ent—depend on the type(s) of particles in the crystal and the resultant interparticle forces. (Section 12.6)... 

Relative molecular (atomic) weight Crystal type Lattice constant/nm Molar volume ... 

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