Molecular structure determines properties principle

The electronic structure and physical properties of any molecule can in principle be determined by quantum-mechanical calculations. However, only in the last 20 years, with the availability and aid of computers, has it become possible to solve the necessary equations without recourse to rough approximations and dubious simplifications2. Computational chemistry is now an established part of the chemist s armoury. It can be used as an analytical tool in the same sense that an NMR spectrometer or X-ray diffractometer can be used to rationalize the structure of a known molecule. Its true place, however, is a predictive one. Therefore, it is of special interest to predict molecular structures and physical properties and compare these values with experimentally obtained data. Moreover, quantum-mechanical computations are a very powerful tool in order to elucidate and understand intrinsic bond properties of individual species. 

The basic concept of organic chemistry is that of Molecular Structure. It includes the idea that molecules can be regarded as isolated objects, i.e. as separable from their environment, that they possess a structure that determines their physical and chemical properties, and finally that this molecular structure can be adequately described by structural formulae. In terms of physics the notion of structure is directly related to the Bom-Oppenheimer description of molecules. Although Molecular Structure makes no appearance in a quantum treatment of molecules starting from first principles [1, 2] the concept is clearly justified by its overwhelming success in organic chemistry. 

These variations between woods reflect differences in microscopic structure and chemical organization of the material, for phase geometry can be as important as molecular structure in determining the properties of both natural and synthetic multiphase systems (31). Therefore, it is clear that the mechanical behavior of the wood-water system cannot be explained entirely at the molecular level or as interaction of macromolecules with solvent. Nevertheless, the general trends observed do follow general principles of solvent-polymer interaction and can be so explained. 

Somewhat closer to the designation of a microscopic model are those diffusion theories which model the transport processes by stochastic rate equations. In the most simple of these models an unique transition rate of penetrant molecules between smaller cells of the same energy is determined as function of gross thermodynamic properties and molecular structure characteristics of the penetrant polymer system. Unfortunately, until now the diffusion models developed on this basis also require a number of adjustable parameters without precise physical meaning. Moreover, the problem of these later models is that in order to predict the absolute value of the diffusion coefficient at least a most probable average length of the elementary diffusion jump must be known. But in the framework of this type of microscopic model, it is not possible to determine this parameter from first principles . 

The conceptual basis for similarity analysis is provided by the similarity-property principle that states that similar molecules have similar biological activity.This rather intuitive principle has been widely accepted and substantiated by a wealth of observations. The success of many similarity-based virtual screening calculations can only be rationalized on the basis of this principle. However, minor modifications in molecular structure can dramatically alter the biological activity of a small molecule. This situation is exploited in lead optimization elforts, but limits the potential of similarity methods. These considerations also suggest that there must be fundamental dilferences between the structure-activity relationships (SARs). Thus, difierent types of SARs are expected to critically determine the success of similarity methods and systematic SAR analysis helps to better understand on a case-by-case basis why similarity methods might succeed or fail. 

There are two main classes of isomers. Figure 22-10 shows compounds that are examples of structural isomers. The atoms of structural isomers are bonded in different orders. The members of a group of structural isomers have different chemical and physical properties despite having the same formula. This observation supports one of the main principles of chemistry The structure of a substance determines its properties. How does the trend in boiling points of C5H12 isomers relate to their molecular structures ... 

The underlying principle behind any S AR is that the molecular structure of an organic compound determines the properties of that compound... 

Hence, formation of the structure and properties of epoxy polymers during curing is determined by fundamental physical principles. This is accompanied by the change in the characteristic ratio C (molecular characteristics), although the structure of the macromolecule remains invariant. The use of the above physical principles even in the simplest version provides a correct description of the structure and properties of network polymers. 

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