Group contribution techniques

An area that has used chemical stmctures for predictive purposes quite successfully is the estimation of thermophysical properties of compounds. There has been an extensive compilation of estimation methods (81), and prediction of physical properties has been automated using these techniques (82). More recendy, the use of group contribution techniques to design new molecules that have specified properties has been described (83). This approach to compound design is being used to develop replacement materials for chloroduorocarbons. 

A relatively simple example of a group contribution technique is the method for estimating Hquid and soHd heat capacities (159). This method is a modification of Kopp s rule (160,161) which was originally proposed in 1864. Kopp s rule states that, at room temperature, the heat capacity of a soHd compound is approximately equal to a stoichiometric summation of the heat capacities of its atoms (elements). The Hurst-Harrison modified equation is as follows ... 

Group contribution techniques are based on the concept that a particular physical property of a compound can be considered to be made up of contributions from the constituent atoms, groups, and bonds the contributions being determined from experimental data. They provide the designer with simple, convenient, methods for physical property estimation requiring only a knowledge of the structural formula of the compound. 

Van Krevelen DW, "Group Contribution Techniques for Correlating Polymer Properties with Chemical Structure", In Bicerano J (Ed), "Computational Modelling of Polymers", Marcel Dekker, New York, 1992, Chap. 1. Van Krevelen DW and Hoftyzer PJ, J Appl Polym Sci 10 (1966) 1331 11 (1967) 1409 11(1967) 2189. 

The mechanical properties of polymers are controlled by the elastic parameters the three moduli and the Poisson ratio these four parameters are theoretically interrelated. If two of them are known, the other two can be calculated. The moduli are also related to the different sound velocities. Since the latter are again correlated with additive molar functions (the molar elastic wave velocity functions, to be treated in Chap. 14), the elastic part of the mechanical properties can be estimated or predicted by means of the additive group contribution technique. 

Starting with a compound s molecular structure, the T, P, and are estimated first, using the three group contribution techniques indicated above. These values are then used in an equation-oriented technique to yield the final estimate for P. ... 

Chemical constraints are applied once the satisfactory candidate molecules have been enumerated. Typically, chemical constraints prevent the generation of unstable substructures within the structure of generated molecules. For example, if the substructure —O—O— occurs in a compound desired to be stable, that compound is pruned. For design procedures based on group-contribution techniques, the application of chemical constraints must occur after enumeration, since the relative locations of the various groups within a molecule remain unspecified at earlier stages. 

We need to employ an estimation procedure for and the associated independent physical properties, such as those described in Section I.B [see Eqs. (2) and (3a-c)], in order to determine each candidate molecule s vapor pressure at 273 K. Starting with a compound s molecular structure, and P are first estimated using the three group-contribution techniques [Eqs. (3a), (3b), and (3c), respectively]. These values are then used in an equation-oriented technique to yield the final estimate for P [Eq. (2)1. 

Cp (groups)—specific heat by the van Krevelen group-contribution technique... 

This estimation procedure requires two properties to be estimated by group-contribution techniques / ll and V. 

N, = N[,( groups)—number of backbone groups by the Salame group-contribution technique. 

This estimation procedure requires two properties to be estimated by group-contribution techniques it, and N, . These procedures result in seven physical properties that are estimated by group-contribution techniques c , V, U, Pll. and it. 

The framework of interactive design is entirely built on the premise of additivity of group contributions for the estimation of physical properties. Let us look at some typical illustrations. Table XV shows the estimation of normal boiling points T, and normal melting points T , using Joback s group contribution techniques (Joback and Reid, 1987), along with the... 

Physical property estimation procedures are at the heart of the design procedures. The group-contribution section provides facilities for entering group contribution and equation oriented correlations. Models in the form of LISP code are entered for both types of estimation techniques. Additionally, groups and their contributions are specified for group contribution techniques. 

P (Fi Fj Fj) - JobacW Pc Factor - Equation Oriented Technique P () - Joback Pc - Group Contribution Technique... 

---