Without Chemical Bonds

Let us consider another way of connecting molecules without chemical bond formation. If through a macrocy-clic molecule passes a linear one with bulky terminal groups, the system thus obtained represents a topological isomer of the type dumbbell in a ring (Fig. 31 ). Such 

Still another type of topological isomerism is molecule in a cage . Formally, here belong the inclusion compounds 

Also known are other types of topological isomers which draw the researchers attention, but they exist only on paper so far. Catenanes and rotaxanes are the only topological structure classes which have been investigated experimentally. 

Statistical synthesis. As far back as in 1952 an idea was ventured concerning an accidental formation of a catenane during the polymerization reaction, e.g. in polyphospho- 

The probability of free intertwining grows with the length of molecules, but such molecules are very bothersome to work with because their solubility decreases drastically as the chain gets longer. Wasserman subjected diether ROOC —(CH2)32—COOR to acyloin [condensation in the presence of partially deuterated cyclotetra-triacontane (Gg4He3D5). The latter (not deuterated, of course) was obtained back in 1926 by the famous Swiss chemist L. Ruzicka during the analysis of natural aromatic musk substances. 

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Lundstrdm, 1981 Lundstrom and Soderberg, 1981 Lundstrdm et al., 1975 Shivaraman et al., 1976), (2) absorbed H reaction with Pd to form a hydride with lower work function (Steele et al., 1976), and (3) formation of a Pd-H system without chemical bonding of H atoms, with a lower work function (Lewis, 1967). The second one concerns the penetration of H atoms through the oxide layer and the formation at the SiO /Si interface of trapping states that determine tunneling of electrons between the metal and the semiconductor (Keramati and Zemel, 1982 Zemel et al., 1980 Kera-mati, 1980). 

A blend indicates a mixture of two or more molecular species without chemical bonding between them and can be induced by mechanical blending, physical means such as coprecipitation, or chemical means such as degrafting. 

Some ten years ago only the few were aware of the existence of molecules without chemical bonds. Nowadays a new scientific discipline, chemical topology, appeared to study them. This science has succeeded in predicting and producing many remarkable chemical compounds (e.g. catenanes whose molecules are interconnected like links of a chain), in defining the electronic structure of molecules more precisely, and so on. 

I. S. Dmitriev, Molecules Without Chemical Bonds, Mir Publishers, Moscow (1981). 

LS. Dmitriev, Molekuly bez khimicheskih svyazei Khimia, Leningrad 1980. English translation I.S. Dmitriev, Molecxtles Without Chemical Bonds, Mir, Moscow 1981. 

We have not discussed topolo cal isomers such as rings versus knotted rings [162], or molecules without chemical bonds such as cat aaes or rotaxanes [163,164]. Such spedes are known, and they ve rise to interesting enumeration problems which have not yet caught the attention of graph-theorists. 

A mixture of elements, without chemical bonding between the different types of atoms, is not a compound. Mixtures of elements can often be separated by simple physical means, e.g. a magnet removes iron from a mixture of iron filings and sulphur. But the main distinction between mixtures and compounds is that the proportions of the elements in a mixture may vary considerably from sample to sample in a given compound the proportions are always the same. 

Furthermore, care has to be taken to ensure a smooth surface of the fibre. Even without chemical bonding between fibre and matrix, a rough surface may impede the pull-out of the fibre by mechanical clamping in the matrix [28]. 

Clathrates (cage or inclusion compounds) are compounds that consist of a lattice of one type of molecule (a host molecule) trapping another type of molecule (a guest molecule) by intermolecular interactions. A clathrate is therefore a mechanical structure without chemical bonds. Such compounds include protein complexes containing -carotene or lipids (fatty acids) bound in the starch macromolecules (see Section 4.5.1.1). In some technological processes, such as extrusion, part of the free lipids is bound in this way. 

Dmitriev IS (1981) Molecules without chemical bonds (English Translation). Mir Publishers,... 

Farmgia LJ, Camtaxrn E, Tegel M (2006) Chemical bonds without chemical bonding a combined expraimental and theoretical charge density study on an iron trimethylenemethane complex. J Phys Chem A 110 7952—7961... 

The interfacial term of friction is the main cause of adhesive wear and may involve processes such as stick-slip motion, transfer film formation with or without chemical bonding, chemical wear, fatigue wear, and thermal softening of the interface (2,24). The adhesive wear is promoted only when the roughness of the counterface is very low, a value where cohesive wear or ploughing due to counterface asperity interaction may be safely neglected, and the adhesive interaction between the polymer and the counterface as a result of junction growth becomes an important factor. 

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