VIII. MOLECULAR STRUCTURE AND INTERMOLECULAR FORCES

1951 ◽  
Vol 24 (2) ◽  
pp. 336-343
Author(s):  
B. A. Dogadkin ◽  
G. M. Bartenev ◽  
M. M. Reznikovskii˘

Abstract 1. The molecular mechanism of the relaxation of deformation of high-elastic polymers has been studied. 2. It is shown that the slow relaxation, which is typical of high-elastic polymers, may be best explained as a restoration process, which either partial or complete (depending on the degree of development of side chains in the molecular structure formed by the main valence chains) of the balanced configurations of the molecular chains. 3. It is shown that the rate of the relaxation process in this case is determined by the molecular activity of the particular polymer. 4. An approximate equation for the kinetics of high-elastic deformation which expresses qualitatively the mechanical properties of high-elastic polymers is proposed. 5. Hypotheses concerning the relation between the time of relaxation and the unbalanced stress are advanced. Equation (2) is derived as characteristic of this relation. 6. It is shown that the joint application of Equations (1) and (2) makes it possible to describe qualitatively the relaxation of stress at constant deformation.


1997 ◽  
Vol 53 (4) ◽  
pp. 569-586 ◽  
Author(s):  
C. B. Aakeröy

The area broadly described as crystal engineering is currently expanding at a brisk pace. Imaginative schemes for supramolecular synthesis, and correlations between molecular structure, crystal packing and physical properties are presented in the literature with increasing regularity. In practice, crystal engineering can be many different things; synthesis, statistical analysis of structural data, ab initio calculations etc. Consequently, we have been provided with a new playing field where chemists from traditionally unconnected parts of the spectrum have exchanged ideas, defined goals and made creative contributions to further progress not only in crystal engineering, but also in other disciplines of chemistry. Crystal engineering is delineated by the nature and structural consequences of intermolecular forces, and the way in which such interactions are utilized for controlling the assembly of molecular building blocks into infinite architectures. Although it is important to acknowledge that a crystal structure is the result of a subtle balance between a multitude of non-covalent forces, this article will focus on design strategies based upon the hydrogen bond and will present a range of approaches that have relied on the directionality and selectivity of such interactions in the synthesis of predictable one-, two- and three-dimensional motifs.


The molecular structure of dibenzyl has been determined to an accuracy of about 0.01A by using extensive X-ray data in three-dimensional Fourier syntheses to refine the approximate atomic parameters originally derived by Robertson. The three formally single-carbon bonds joining the benzene rings have lengths of 1.50, 1.48 and 1.50A and make angles of 115° with each other. The departure from the standard length of 1.54A is discussed in relation to the analogous systems of polyisoprenes and 1.5 dienes where related effects have been observed. The dimensions of the benzene rings also reflect the unusual character of the acyclic carbon bonds, two of the aromatic bond lengths being 1.39A and the other four 1.37A. The molecule, although possessing a centre of symmetry, is not absolutely symmetrical, since the central CH 2 — CH 2 bond is inclined at 70.5° to the plane of the benzene rings. This is ascribed to the influence of intermolecular forces.


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