Bond-Type Assignments

A special type of bond-type assignment problem concerns tautomerism. Some chemical databases provide specific tautomeric bond types, or analyse a query for either the keto or enol constructions and add the missing one (as an. OR. option) automatically. In the CSD, only the single tautomeric form identified by the crystal structure determination is encoded in the connectivity tables. The software has not yet been upgraded to accommodate searches for both forms, hence the onus is on the user to encode both representations if that is the desired goal of a query. 

Finally in this section, we must mention the problem of polymeric bonding, primarily in metal-organic systems, through which chains, sheets and nets are formed in the extended crystal structure. In the CSD, the principal links emanating from the monomeric unit that fall within the covalent distance criteria, are encoded as polymeric bonds (bt = 6). 

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Add lone pair electrons and assign formal atomic charges in each (do not change bond types). Compare to calculatec charges for formaldehyde BF3 complex. Which structure, if either, is more reasonable ... 

In the discussion of metallic radii we may make a choice between two immediate alternative procedures. The first, which I shall adopt, is to consider the dependence of the radius on the type of the bond, defined as the number (which may be fractional) of shared electron pairs involved (corresponding to the single, double, and triple bonds in ordinary covalent molecules and crystals), and then to consider separately the effect of resonance in stabilizing the crystal and decreasing the interatomic distance. This procedure is similar to that which we have used in the discussion of interatomic distances in resonating molecules.7 The alternative procedure would be to assign to each bond a number, the bond order, to represent the strength of the bond with inclusion of the resonance effect as well as of the bond type.8... 

Is there a corresponding decomposition for nonelectrolytes One might suppose that the analogous building blocks of a nonelectrolyte compound are its chemical bonds. Based on the known similarity and transferability of particular bond types from one molecule to another, one could then attempt to assign each bond a specific bond enthalpy (denoted 7)//°[bond]) such that the overall sum of DH°s is related to (the negative of) AHf by... 

With the characteristic of each bonding type identified, we shall now show how all the material in this universe can be assigned to a particular type of bonding or a combination of bonding type as summarized in Fig. 3. 

In the model of bond percolation on the square lattice, the elements are the bonds formed between the monomers and not the sites, i.e., the elements of the clusters are the connected bonds. The extent of a polymerization reaction corresponds to the fraction of reacted bonds. Mathematically, this is expressed by the probability p for the presence of bonds. These concepts can allow someone to create randomly connected bonds (clusters) assigning different values for the probability p. Accordingly, the size of the clusters of connected bonds increases as the probability p increases. It has been found that above a critical value of pc = 0.5 the various bond configurations that can be formed randomly share a common characteristic a cluster percolates through the lattice. A more realistic case of a percolating cluster can be obtained if the site model of a square lattice is used with probability p = 0.6, Figure 1.5. Notice that the critical value of pc is 0.593 for the 2-dimensional site model. Also, the percolation thresholds vary according to the type of model (site or bond) as well as with the dimensionality of the lattice (2 or 3). 

Vibrations of thiophene, selenophene, and symmetrically deuteri-ated derivatives of the rings were assigned to the completely symmetric in-plane type Au mainly on Raman evidence (polarized and, as a rule, intense bands). Stretching vibrations of the CH or CD bonds were assigned using empirical regularities in the Raman frequencies and intensities of the CH bands in thiophene, selenophene, and their derivatives. The assignment to vibrations of type B1 was also made (depolarized bands in the Raman spectra, type B in IR). 

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