Linkage types proportions

The integral intensities of signals of polysaccharides, obtained with a modem spectrometer under the usual operating conditions, are proportional or quasi-proportional, to the number of 13C nuclei present. This has been observed, in particular, in the case of linear hexo-pyranan structures containing one type,11,47,51,55,56 58-61 or two equal types, of linkage,62 or branched-chain polymers having two linkage types,53,58 where the resonances are readily resolved. In such cases, the T values would be low, 0.2 s or less, and the n.O.e. values would be approximately equal. However, few actual determinations of these values have been made, and extrapolation of such assumptions to more-complex polysaccharide structures is not recommended, as outlined in Section V,2,... 

Carbohydrates in nature are optically active and polarimetry is widely used in establishing their structure. Measurement of the specific rotation gives information about the linkage type (a or (3 form) and is also used to follow mutarotation. Nuclear magnetic resonance spectroscopy (NMR) can be used to differentiate between the anomeric protons in the a- or /3-pyranose and furanose anomers and their proportions can be measured from the respective peak areas. 

Both Adler and Freudenberg have constructed structural schemes for spruce lignin and Nimz for beech lignin in which the main units and linkage types are represented in their approximate proportions. The scheme by Adler is depicted in Fig. 4-9. 

Another type of network imperfection, resulting from cross-linking of two units not distantly related structurally, is indicated in Fig. 94. Cross-linkages such as B are wasted (except insofar as the loop may be involved in entanglements not otherwise operative). The proportion of these short path cross-linkages should be small ordinarily but could become very large if the cross-linking process were carried out in a dilute solution of the polymer. 

Tables 3.1, 3.2 and 3.3 compiled by Povarennykh (1963) specify the initial data accepted for the calculation of hardness from formulae (3.5) and (3.6). As the ratio WJWa increases, the coefficient a decreases (Table 3.1). For compounds with ratios inverse to those given in the table, i.e., for compounds having a so-called antistructure, the coefficient a will be exactly the same, e.g., 1/2 and 2/1. In both cases, x — 80. The link attenuation coefficient / varies over a relatively narrow range, usually between 0.7 and 1.0 (Table 3.2). This coefficient requires the state of lattice linkage to be considered in each case, and like coefficient a it depends on the type of compound involved. For various types of compounds, the values of the coefficient / may be lower taking as an example minerals in the pyrite and skutterudite group, they are as follows for compounds 2/2—0.60, for 3/3—0.48 and for 4/4—0.39. The values of the coefficient y grow proportionally with coordination number (Table 3.3). The constancy of the coefficient y depends on the constancy of the coordination number which is influenced by the valence ratio of electropositive and electronegative atoms. Lattice spacings, state of chemical bonds and electron-shell structure, and for complex compounds, also the degree of action of the remain-...

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