Chemical property estimation systems

CHEMEST Chemical Property Estimation System Camp, Dresser, Sl McKee, Inc. Dr. Warren Lyman 1 Center Plaza Boston, MA 02108 (617) 742-5151 x5711 Designed to predict environmentally important properties of organic chemicals. Requites DEC VAX and IBM PC. 

General Properties of Computerized Physical Property System. Flow-sheeting calculations tend to have voracious appetites for physical property estimations. To model a distillation column one may request estimates for chemical potential (or fugacity) and for enthalpies 10,000 or more times. Depending on the complexity of the property methods used, these calculations could represent 80% or more of the computer time requited to do a simulation. The design of the physical property estimation system must therefore be done with extreme care. 

PCGEMS. 1988. Personal Computer Conversion Graphical Exposme Model System. Chemical properties estimation (CHEMEST). U. S. Environmental Protection Agency, Washington DC. 

On-line system. Provides support for exposure assessments of toxic substances. Includes chemical property estimation techniques, statistical analysis, multimedia modeling, and graphics display (including models)... 

S Physical Property Estimation System , American Institute of Chemical Engineers, New York, 1965. 

Data Structures. Inspection of the unit simulation equation (Equation 7) indicates the kinds of input data required by aquatic fate codes. These data can be classified as chemical, environmental, and loading data sets. The chemical data set , which are composed of the chemical reactivity and speciation data, can be developed from laboratory investigations. The environmental data, representing the driving forces that constrain the expression of chemical properties in real systems, can be obtained from site-specific limnological field investigations or as summary data sets developed from literature surveys. Allochthonous chemical loadings can be developed as worst-case estimates, via the outputs of terrestrial models, or, when appropriate, via direct field measurement. 

Burkhard, L.P. (1984) Physical-Chemical Properties of the Polychlorinated Biphenyls Measurement, Estimation, and Application to Environmental Systems. Ph.D. Thesis, University of Wisconsin-Madison, Wisconsin. 

Methods have been presented, with examples, for obtaining quantitative structure-property relationships for alternating conjugated and cross-conjugated dienes and polyenes, and for adjacent dienes and polyenes. The examples include chemical reactivities, chemical properties and physical properties. A method of estimating electrical effect substituent constants for dienyl and polyenyl substituents has been described. The nature of these substituents has been discussed, but unfortunately the discussion is very largely based on estimated values. A full understanding of structural effects on dienyl and polyenyl systems awaits much further experimental study. It would be particularly useful to have more chemical reactivity studies on their substituent effects, and it would be especially helpful if chemical reactivity studies on the transmission of electrical effects in adjacent multiply doubly bonded systems were available. Only further experimental work will show how valid our estimates and predictions are. 

A correlation analysis is a powerful tool used widely in various fields of theoretical and experimental chemistry. Generally, such an analysis, based on a statistically representative mass of data, can lead to reliable relationships that allow us to predict or to estimate important characteristics of still unknown molecular systems or systems unstable for direct experimental measurements. First, this statement concerns structural, thermodynamic, kinetic, and spectroscopic properties. For example, despite the very complex nature of chemical screening in NMR, particularly for heavy nuclei, various incremental schemes accurately predict their chemical shifts, thus providing a structural analysis of new molecular systems. Relationships for the prediction of physical or chemical properties of compounds or even their physiological activity are also well known. 

Computer-Aided Property Estimation Computer-aided structure estimation requires the structure of the chemical compounds to be encoded in a computer-readable language. Computers most efficiently process linear strings of data, and hence linear notation systems were developed for chemical structure representation. Several such systems have been described in the literature. SMILES, the Simplified Molecular Input Line Entry System, by Weininger and collaborators [2-4], has found wide acceptance and is being used in the Toolkit. Here, only a brief summary of SMILES rules is given. A more detailed description, together with a tutorial and examples, is given in Appendix A. 

The ability to predict the behavior of a chemical substance in a biological or environmental system largely depends on knowledge of the physical-chemical properties and reactivity of that compound or closely related compounds. Chemical properties frequently used in environmental assessment include melting/boiling temperature, vapor pressure, various partition coefficients, water solubility, Henry s Law constant, sorption coefficient, bioconcentration factor, and diffusion properties. Reactivities by processes such as biodegradation, hydrolysis, photolysis, and oxidation/reduction are also critical determinants of environmental fate and such information may be needed for modeling. Unfortunately, measured values often are not available and, even if they are, the reported values may be inconsistent or of doubtful validity. In this situation it may be appropriate or even essential to use estimation methods. 

It should be noted that the value of each of the chemical constants kom and k0A2 depends on the physical-chemical properties of two reacting phases. The value of kom depends on the nature of substance A and the compound ApBq, while the value of k0A2 depends on the nature of substance B and the compound ApBq. Both physical (diffusional) constants depend only on the nature of the chemical compound ApBq and are therefore characteristic of this compound layer wherever it grows. However, as will be demostrated in the next chapters, the stoichiometry of adjacent phases must also be taken into account when estimating the growth rate of the ApBq layer in various reaction couples of the A-B binary system. 

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