Structure Increment

Let us consider one homologous series—generally indicated as /-series—and characterized by a given structural increment, for example, a CH2 group. At the same time, let us separate this series through a ID system where the separation coordinate is labeled as x. The partition free energy of the 11, component of the /-series is... 

The above relationship can be related to the change of partition free energies of members of homologous series due to the addition of a given structural increment or to any physical-chemical property that affects the retention. 

The additive structural increments in either atomic constants or in binding constants can be determined purely by calculation. Secondly, additional values or structural increments can be added for certain structural elements e.g. ring systems, branching and others. 

The structural increments are considered to be purely calculated quantities for which no simple physical meaning can be established. 

There already exists a substantial literature devoted to the estimation of various material properties with the help of additive structual increments (Reid et. al, 1987, Van Krevelen, 1990). The regular solution theory in combination with additive structural increments has a wide application for estimating the relative solubilities of organic substances in polymers and the solubility of polymers in various solvents (Barton, 1983) and will be described later in this chapter. When estimating partition coefficient values, one is quickly confronted with this method s application limits, particularly with polar and non-polar structures, for example the partitioning of substances between polyolefins and alcohol (Baner and Piringer, 1991). 

Because of the lack of quantitativeness of the Regular Solution Theory and large amount of effort and computing power required for the UNIFAC method, yet another way will be taken here. This way leads to values using simple means which can adequately estimate values for the most important practical cases. The method described in this section is based on the potential already recognised in gas chromatography that the partition of a substance between a gas and a polymer liquid can be estimated based on its structural increments and these can be used as characteristic quantities for identification. 

A dimensionless molecular retention index Me parameter can be defined as the sum of Mr (relative molecular weight) and a structural increment W. Contained in W are all the additive contributions of the functional groups (see Eq. 4-52) which differ from a hypothetical n-alkane with the same Mr value. According to definition, the W values of the n-alkanes are always equal to zero. In this manner it is possible to estimate the partition coefficients of any given organic compound between a gas and any given liquid or polymer with help of additive structural increments. 

For the structural increment w[ P the values given in Table 4-6 for the nonpolar phases can be used, i.e. Wp p = Wpv-I,M. The estimated values using Eq. (4-98) for several partition coefficients are given in the first column of Table 4-7. 

The Vflp/L) values for several structural increments in polyolefin/ethanol and poly-olefin/methanol are given in Table 4-8. 

The molecular structure of the polymer additive IRGANOX 1076 with Mr i = 531 is shown in Appendix III. To calculate the IRGANOX/ethanol partition coefficient use Eq. (4-103) and Table 4-8. Two structural increments are considered Wj(p/L> = 180 for ester and Wj(P/L) = 140 for the 2,6-di-tert-butyl-phenol ... 

Bondi, A. (1963). Heat of sublimation of molecular crystals. A catalog of molecular structure increments. J. Chem. Eng. Data, 8, 371-81. [168]... 

An alternative to sequential docking can be followed if combinatorial libraries are evaluated. Quite a few programs have been specifically designed for speed-up by so-called combinatorial docking. They profit from the structured, incremental nature of combinatorial libraries and the fact that molecules of a combinatorial library consist of a common core. This core is assumed to form common specific interactions with the receptor (possibly supported by experimental evidence) and can thus be prepositioned in the binding pocket in one or a few similar orientations. It then serves as skeleton for the addition of substituents. Obviously, this step is ideally suited for incremental construction algorithms (361)... 

Input data Molecular structure, atom and structural increments. Ay in Table... 

Description An atom contribution method with structural increments for V, given by... 

By calculating the constant a additively for atomic and structural increments a good agreement with experimental results is obtained. In about half the cases investigated the difference between calculated and measured values of a amounted to less than 1 )er cent, and the variations in a due to temperature were small. B "... 

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