Molecular configuration/conformation/constitution

Thus, a brief survey of the current understanding of the molecular and super-molecular structures of common thermoplastics is presented first. This review starts with a brief description of the current state-of-the-art knowledge of the constitution, configuration, conformation and supermolecular structure of common glassy and semicrystalline thermoplastics. Later in this chapter, specific features of the structure-property relationships are discussed in greater detail for the most frequently filled thermoplastics. Effects of fillers on the structural variables in polypropylene, considered the most commercially important matrix, are especially emphasized. 

On the other hand, the mechanical properties also depend on the materials molecular composition and structure, i.e., intrinsic parameters. Intrinsic parameters are, for instance, chemical composition or constitution, configuration, conformation, chain cross section, entanglement molecular weight, free volume, chain stiffness, macromolecular mobility, crystallinity, and others [4, 16, 17]. Chain length and chain length distribution (or molecular weight M ) have a basic influence on mechanical properties, which is illustrated in Fig. 1.17. Three regions can be identified ... 

Characterization methods for analyzing blends may be divided into two types single-phase and multi-phase. Evidently, the number of single-phase systems is limited to amorphous polymers with miscible additives. The semi-crys-taUine polymers (such as PA-6 or PET) are suspension of the crystalline phase in a vitreous or molten phase, thus subjected to the same analysis of phase formation and evolution with stress and temperature as that of an immiscible blend. The characterization of single-phase systems focuses on the individual macromolecules, their configuration, conformation, molecular weight and its distribution, as well as on properties directly related to the molecular mass and constitution, namely, stability, thermodynamic interactions, rheology, etc. 

A.s mentioned above, most molecules ean adopt more than one conformation, or molecular geometry, simply by rotation around rotatable bonds. Thus, the different conformations of a molecule can be regarded as different spatial arrangements of the atoms, but with an identical constitution and configuration, They are interconvertible and mo.stly they cannot be i.solatcd separately. Figure 2-101 show-s a. super-imposition of a set of conformations of 2R-benzylsuccinatc (cf. Figure 2-89). 

In addition to constitution and configuration, there is a third important level of structure, that of conformation. Conformations are discrete molecular arrangements that differ in spatial arrangement as a result of facile rotations about single bonds. Usually, conformers are in thermal equilibrium and cannot be separated. The subject of conformational interconversion will be discussed in detail in Chapter 3. A special case of stereoisomerism arises when rotation about single bonds is sufficiently restricted by steric or other factors that- the different conformations can be separated. The term atropisomer is applied to stereoisomers that result fk m restricted bond rotation. ... 

Nuclear magnetic resonance (NMR) spectroscopy is a very powerful tool for analyzing the conformation and molecular architecture of carbohydrate molecules. Both one- and two-dimensional (ID and 2D) methodologies have provided valuable information about small and large molecules, ranging from the anomeric configuration of a monosaccharide to the sequence of monosaccharide residues that constitute an oligo- or polysaccharide. 

Generation of Constitutional, Configurational and Conformational Molecular Diversity... 

Processes where the constitution of intermediate (if any) and final states (molecules) of the device does not differ from that of the initial molecule. The initial molecule is converted into a new conformation or configuration and this results in a molecular motion. There is no constitutional change and no addition or elimination of any chemical species. There are included, for example, configurational changes induced by photochemical or thermal stimuli (see, for example [77]), or conformational changes induced by external fields (see, for example [27,29]) ... 

Isomers Compounds that have the same molecular formula but which have different constitutions (constitutional isomers), configurations (enantiomers, diastereomers), or conformations (conformational isomers), and therefore have different chemical and/or physical properties. 

Figure 1 The relationship between composition, constitution, configuration, and conformation in the mathematical and chemical sense. Molecular structure may be modeled at any of these levels. Cyclohexane is given as an example. This figure is taken from Turro. °...

In addition to constitution and configuration, there is a third significant level of structure, that of conformation. Conformational isomerism is generally taken to refer to discrete molecular arrangements generated by rotation about formal single bonds. This aspect of stereochemistry will be dealt with more fully in Chapter 3. 

A major part of work in a chemical laboratory, both in research and industry, deals with analytical challenges. Important problems are separation of mixtures into pure compounds, and elucidation of their structures. Often, determination of a compound s constitution is the foremost aim, though a true identification of a compound should also include determination of its stereochemistry, i.e. its configuration and possibly conformation. We group this together under molecular structure elucidation. 

---