Group contribution techniques, physical properties

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. 

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. 

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. 

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. 

As the limitations of the additive group-contribution techniques become more apparent, new representational models will be required to solve the product design problem. These models must maintain some simplicity in the structure-property relationships, which can be inverted in an efficient and explicit manner to yield the structure(s) of the feasible molecule(s). Such models have yet to be invented, but it is important to keep in mind the needs of the product design, as new theories and techniques are being written for the estimation of physical properties. 

Many chemical structure/physical property relations have been reported in literature but the most important contributions in this field have been made, we think, by Van Krevelen [1] and Bicerano [2]. Both authors present a total concept for polymer properties/molecular structure correlations based on the group contribution technique (Van Krevelen) and on connectivity indices calculations (Bicerano) covering all the properties mentioned in group A and B and some of the properties of group C. Seitz published a concept to estimate the mechanical properties (from group A and C) of polymers from their molecular structure [3]. 

The group contribution technique employs a special type of mathematical function for each property analyzed [14-20], whose parameters are calculated with the contributions of defined fragments in the molecule. The choice of the functions is made on the basis of physical meaning and/or previous practices. For example, Iwai et al. [ 14] estabHshed a model for the vapor pressure of alkane isomers by resorting to the following formula... 

The development of thermal analysis methods in materials research has led to a plethora of new methodologies since the elaboration of the first thermal method by by Le Chatelier and Robert-Austen [16,86], Thermal analysis consists of a group of techniques in which a physical property of a material is measured as a function of temperature at the same time when the substance is subjected to a controlled increase, or in some cases, decrease of temperature. Temperature-programmed techniques, such as DTA [87-89], TGA [87], DSC [53,90], TPR [91,92], and TPD [93-96], contribute to perform a more complete characterization of materials. 

High correlations between two properties indicate the possibility of replacing one with a function of the other. This would enable us to reduce the dimensionality of a design space. One study (Joback, 1984) found that nine physical properties were well approximated by three new properties called factors. Table XVI shows several of the derived physical property estimation techniques, using essentially two of these factors, F, and F. Group contribution estimation techniques were developed for both factors, f, and Fy... 

Further, molecular simulation and computational chemistry have evolved, and are evolving, into important tools for developing better characterization techniques where it is not possible to measure all data. Even so, it is precisely the molecular complexity of petroleum fluids that seems to be an inhibiting factor in the use of these methods for developing better characterization methods. However, identification of important functional groups in petroleum fluids applying molecular simulation and/or computational chemistry for use with group contribution methods to predict thermo-physical properties may be an area for further research. 

The extended method was then used as the foundation of a scheme of predictive techniques and correlations for the calculation of the physical properties of polymers. Connectivity indices were correlated either with experimental data or with tables of group contributions. For example, there is a sufficient amount of reliable experimental data for the molar volume or the... 

About one third of all localized tumors, which contribute to about 60% of all cancers, are cured by radiotherapy alone or by radiotherapy in combination with surgery [2]. For another one-third of the localized tumors, however, a failure of local control is observed, and this group is expected to benefit from the development of techniques allowing a better conformation of dose to the tumor. The compromise between tumor cure and minimal side effects is largely determined by the physical properties of the radiation type under consideration, namely the depth dose profile. The depth dose profile defines the ratio of the dose delivered to the tumor and the dose delivered to the healthy tissue. 

There are several problems in comparing results obtained by different groups using the multifarious experimental techniques that are available to investigate the NLO properties of molecules the most important of these include dispersion, the measurement of different tensorial components, different physical processes contributing to nonlinearity, and solubility problems. 

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