Properties Associated with the Molecular Level

Molecular properties may be defined as those material characteristics that theoretically can be measured for a small ensemble of individual molecules. Due to the minimal sample requirements, molecular properties are often determined at the earliest stages of drug development. Most of the molecular level techniques are spectroscopic in nature, but insofar as they are influenced by the physical state of the substance, substantial information of great use to formula-tors can be obtained from appropriately designed experiments. For example, a screening of stressed materials can be carried out on the microgram level using infrared microscopy, and the results of such work aid the preformulation characterization of a new chemical entity. 

With the exception of single-crystal transmission work, most solids are too opaque to permit the conventional use of ultraviolet/visible (UVWIS) electronic spectroscopy. As a result, such work must be performed using diffuse reflection techniques.Studies have been conducted where UVWIS spectroscopy was used to study the reaction pathways of various solid-state reactions. Other applications have been made in the fields of color measurement and color matching, areas that can be of considerable importance when applied to the coloring agents used in formulations. 

It was recognized some time ago that diffuse reflectance spectroscopy is a very useful tool for the study of interactions among various formulation components, and the technique has been successfully used in the characterization 

Jozwiakowski and (Jotmors used diffuse reflectance spectroscopy to study the adsorption of spiropyrans onto pharmaceutically relevant solids. The particular adsorbants studied were interesting in that the spectral characteristics of the binary system depended strongly on the amount of material bound. At low concentrations, the pyran sorbant exhibited its main absorption band around 550 nm. As the degree of coverage was increased, the 550-nm band was still observed, but a much more intense absorption band at 470 nm became prominent. This secondary effect was attributed to the presence of pyran-pyran interactions, which became more important as the concentration of sorbant increased. 

The perception of color is subjectively developed in the mind of an individual and, consequently, different people can perceive a given color in various ways. Such variability in interpretation causes great difficulty in the evaluation of color-related phenomena, leading to problems in making objective judgments. The development of quantitative methods for color determination was undertaken to eliminate the subjectivity associated with visual interpretative measurements. 

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II. PROPERTIES ASSOCIATED WITH THE MOLECULAR LEVEL A UV/Vis Diffuse Reflectance Spectroscopy... 

A systematic approach to the physical characterization of pharmaceutical solids has been outlined [6], and it will be filled out in significantly more depth in the chapters of the present work. Within this system, physical properties are classified as being associated with the molecular level (those associated with individual molecules), the particulate level (those pertaining to individual solid particles), or the bulk level (those associated with an assembly of particulate species). 

Polymers for second-order NLO applications have been extensively investigated during the last ten years. However, significant progress in material development efforts toward practical applications has been made only in recent years. This chapter discusses the current state of the art in the development of second-order NLO polymers. We start with a brief description of the NLO phenomena at both the molecular and bulk levels. The design considerations and property optimization of chromophores are associated with the molecular level optical nonlinearity. The induced noncentrosymmetric dipolar orientation of chromophores leads to the bulk second-order NLO properties, which are most commonly char-... 

Continuum models remove the difficulties associated with the statistical sampling of phase space, but they do so at the cost of losing molecular-level detail. In most continuum models, dynamical properties associated with the solvent and with solute-solvent interactions are replaced by equilibrium averages. Furthermore, the choice of where the primary subsystem ends and the dielectric continuum begins , i.e., the boundary and the shape of the cavity containing the primary subsystem, is ambiguous (since such a boundary is intrinsically nonphysical). Typically this boundary is placed on some sort of van der Waals envelope of either the solute or the solute plus a few key solvent molecules. 

At the molecular level, electric dipole moments are important because they give information about the charge distribution in a molecule. Examination of the experimental data for a few simple compounds reveals that the electric dipole moment is also a property associated with chemical bonds and their polarity. The... 

From the comparison of the results, it can be inferred that copper ions exchanged in the ZSM-5 zeolites assumes a bidentate (sites 12 and II) or tridentate coordination (sites M5, Z6, and M7). These two groups differ also in the molecular properties (Table 2.2). The I-centers are characterized by lower values of the valence index and greater partial charges, QCu, in comparison to the M and Z centers, which is associated with the deeper laying HOMO and LUMO levels. In the M5, Z6, and M7 sites Cu1 ions exhibit more covalent character, and the frontier orbitals have less negative energies. As a result, the chemical hardness of the I-centers, located at the channel intersections, is smaller than those located on the walls of the ZSM-5 zeolite. 

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