Atoms classes

The three fluorides CIF5, Brp5 and IF5 are the only known hexa-atomic interhalogens, and IF7 is the sole representative of the octa-atomic class. The first to be made (1871) was IF5 which is the most readily formed of the iodine fluorides, whereas the more vigorous conditions required for the others delayed the synthesis of BrFs and IF7 until 1930/1 and CIF5 until 1962. The preferred method of preparing all four compounds on a large scale is by direct fluorination of the element or a lower fluoride ... 

Many observations concerning these trends had been made over the years, and in the 1950s S. Ahrland, J. J. Chatt, and M. Davies presented a classification of metals based on their preferred interaction with donor atoms. Class A metals are those that interact preferentially when the donor atom is in the first row of the periodic table. For example, they prefer to bond to N rather than P donor atoms. Class B metals are those which interact better when the donor atom is in the second row of the periodic table. For example, a class B metal would bond better to P than to N. The following table summarizes the behavior of metal atoms according to this classification. 

Now we turn to the scoring function. The function for evaluating the closeness of analogy involves a sensitivity parameter for the atom class, times the dissimilarity parameter, d ... 

This surface area classification notion naturally can be extended to other properties. For example, a collection of pharmacophore-type VS A descriptors can be calculated by summing the V, contribution of each in a molecule of a specific type. For example, if the atom classes are donor, acceptor, polar, hydrophobe, anion, and cation, then six VSA descriptors can be calculated such that for any given molecule the sum of the six descriptors is the VSA of the entire molecule and each descriptor is the VSA of all atoms one of the six classes. Such descriptors can be used for rough pharmacophore-based similarity measures. 

Isolated macrolides typically have 12, 14, or atom aglycones, with the 14-atom class serv-asthe basis for most clinically relevant mac-... 

Two schemes have been proposed which systematize the available stability constant data. In addition to data contained in the compilations given at the end of this chapter, more qualitative evidence, based for example on the results of displacement reactions, has been included in arriving at the generalizations. Historically, the first scheme is that due to Chatt and Ahrland who pointed out that electron acceptors may be placed in one of three classes. Class-a metals, the most numerous, form more stable complexes with ligands in which the coordinating atom is a first-row element (N, O, F) than with those of an analogous ligand in which the donor is a second-row element (P, S, Cl). Class-b has the relative stabilities reversed. It is not difficult to extend the stability relationships to include heavier donor atoms. Class-a behaviour is, then, typified by a stability order... 

We need not duplicate in the atom class those attributes which are already present in the element class, since each atom object is connected to the corresponding element object. If it is important that an atom object behaves as if it actually contain all the attributes of an element object, we can define methods which merely look up the corresponding attributes of the element. For example, a method called symbol applicable to atoms would retrieve the element of the atom and look up the element s symbol. In effect, the method would be delegated from one object to another, closely related, object. 

Figure 4 Definition of atom classes 4.3 Molecular Structures...

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