Molecular mechanics INDEX

Molecular descriptors must then be computed. Any numerical value that describes the molecule could be used. Many descriptors are obtained from molecular mechanics or semiempirical calculations. Energies, population analysis, and vibrational frequency analysis with its associated thermodynamic quantities are often obtained this way. Ah initio results can be used reliably, but are often avoided due to the large amount of computation necessary. The largest percentage of descriptors are easily determined values, such as molecular weights, topological indexes, moments of inertia, and so on. Table 30.1 lists some of the descriptors that have been found to be useful in previous studies. These are discussed in more detail in the review articles listed in the bibliography. 

Computed optical properties tend not to be extremely accurate for polymers. The optical absorption spectra (UV/VIS) must be computed from semiempiri-cal or ah initio calculations. Vibrational spectra (IR) can be computed with some molecular mechanics or orbital-based methods. The refractive index is most often calculated from a group additivity technique, with a correction for density. 

Molecular Connectivity Indexes and Graph Theory. Perhaps the chief obstacle to developing a general theory for quantification of physical properties is not so much in the understanding of the underlying physical laws, but rather the inabiUty to solve the requisite equations. The plethora of assumptions and simplifications in the statistical mechanics and group contribution sections of this article provide examples of this. Computational procedures are simplified when the number of parameters used to describe the saUent features of a problem is reduced. Because many properties of molecules correlate well with stmctures, parameters have been developed which grossly quantify molecular stmctural characteristics. These parameters, or coimectivity indexes, are usually based on the numbers and orientations of atoms and bonds in the molecule. 

Calculations of connectivity indexes and subsequent dielectric constant predictions were accomplished by using Molecular Simulations Inc. Synthia polymer module running under the Insight II interface on a Silicon Graphics Crimson workstation. All calculations were performed on the polymer s repeat unit, which was first energy-minimized through a molecular-mechanics-based algorithm. 

The ratio between both fluxes shows that the Prandtl number is an index giving the relative quantity of the momentum transported by the molecular mechanism and of the heat transported by the same mechanism at the interface ... 

The ratio between both fluxes shows that the Sherwood number can be considered as an index of the relative participation of the convective and molecular mechanisms to the transport process. 

Mendiratta, S. and Madan, A.K. (1994). Structure-Activity Study on Antiviral 5-Vinylpyrimidine Nucleoside Analogs Using Wiener s Topological Index. J.Chem.Inf.Comput.ScL, 34,867-871. Menziani, M.C. and De Benedetti, P.G. (1992). Molecular Mechanics and Quantum Chemical QSAR Analysis in Carbonic Anhydrase Heterocyclic Sulfonamide Interactions. Struct. CherrL,... 

Substantial improvement of therapeutic index is often possible either by enhancement of therapeutic potency, reduction of intrinsic toxicity, or both. If the molecular mechanisms of toxicity and activity are the same the prospect for improvement in therapeutic index is poor. An understanding of these mechanisms is thus important. 

An overview of the physics of glassy polymers and the relationships between molecular mechanisms and macroscopic physical, mechanical and transport properties of polymer glasses is presented. The importance of local translational and/or rotational motions of molecular segments in the glass is discussed in terms of the implications for thermodynamic descriptions of the glass (configurational states and energy surfaces) as well as history dependent properties such as expansivity, refractive index, gas permeability, and viscoelastic mechanical behaviour. 

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