Group 12 thermochemical properties

Figure 7-1 shows the groups that are obtained for alkanes, and the corresponding notation of these groups as introduced by Benson [Ij. Table 7-2 contains the group contributions to important thermochemical properties of alkanes. Results obtained with these increments and more extensive tables can be obtained from Refs. [1] and [2]. 

Two standard estimation methods for heat of reaction and CART are Chetah 7.2 and NASA CET 89. Chetah Version 7.2 is a computer program capable of predicting both thermochemical properties and certain reactive chemical hazards of pure chemicals, mixtures or reactions. Available from ASTM, Chetah 7.2 uses Benson s method of group additivity to estimate ideal gas heat of formation and heat of decomposition. NASA CET 89 is a computer program that calculates the adiabatic decomposition temperature (maximum attainable temperature in a chemical system) and the equilibrium decomposition products formed at that temperature. It is capable of calculating CART values for any combination of materials, including reactants, products, solvents, etc. Melhem and Shanley (1997) describe the use of CART values in thermal hazard analysis. 

A powerful and readily applied method for the estimation of thermochemical properties for gas phase species is that of group additivity developed by Benson and his coworkers15 21-22. The method is based on the observation that the thermochemical properties of a molecule can be represented as a sum of contributions from the individual groups which make up the molecule. The method of defining groups and arriving at group... 

These results thus show that whereas the flashpoint was only moderately influenced by the compound structure (their chemical functionality but especially their atomic composition and vapour), autoignition temperatures seem to be closely linked to the structural factors that affect the chain. So additivity rules for estimation of AIT should be sought. Every time a chemical or physical property is highly influenced by the structure, chemists tried to establish rules that enable one to reduce a molecule to characteristic groups for which the contribution to the value of this property is known. This was done for instance by Kinney for boiling points and Benson2 for thermochemical properties. 

The application of group additivity is also straightforward. For example. Table V illustrates the application of this method for the estimation of the thermochemical properties of I-C4H8. Computerized versions of this approach are also available (Martinez, 1973 Seaton et al., 1974). 

In summary, bond and group additivity rules, as well as the model compound approach, in conjunction with statistical mechanics, represent useful tools for the estimation of thermochemical properties. However, their utility for the determination of thermochemistry of new classes of compounds is limited, especially with regard to the determination of Aiff. For new classes of compounds, we must resort to experiments, as well as to computational quantum mechanical methods. 

All these fuels belong to a group of high-energy-density fuels with compact molecular structure rendered by the presence of pentacyclic cages. They are stable and nonvolatile at room temperature and pressure. Three formulations are solid and the fourth is a viscous liquid. Their S3mthesis and molecular structure analysis that uses X-ray crystallographic methods have been described by Marchand [5, 6]. Their molecular structure and physical properties are presented briefly below. Measured thermophysical and thermochemical properties follow. 

Values based on rule of group additivity of thermochemical properties, using an assumed value, Hf0, for O—(6)2 group of 19 2 kcal., and further assuming that in the molecular series H—OnH, DH°(H—OnH) = 90 2kcal. for n > 2. 

This is reasonable because the conjugate acids, R3NH, are likely to be stabilized by electron-donating and polarizable alkyl groups, thereby making R3N a stronger base. That the same trend is not evident in aqueous solution again shows the influence of the solvent on thermochemical properties (see Section 11-8A). 

A natural extension of the bond energy approach is to account for interactions close to the chemical bond in question (which certainly affect the stability, and hence the thermodynamic properties). Based on this concept, a number of group contribution methods have been developed over the years, and many of these methods have been reviewed in Ref. 171. Benson s second-order group contribution method, probably the most successful and widely embraced method, was developed some 30 years ago as an improvement to bond energy (or bond contribution) methods for the prediction of thermochemical properties.167 This improvement was accomplished by accounting for ... 

Thermodynamic properties of molecular species that are used in reactor design problems can be readily estimated from thermodynamic data tabulated in standard reference sources such as Perry s Handbook or the JANAF Tables. Thermochemical properties of molecular species not tabulated can usually be estimated using group contribution methods. Estimation of activation energies is, however, much more difficult due to the lack of reliable information on transition state structures, and the data required to cany out these calculations is not readily available. 

Balducci et al. [383-387] identified numerous gaseous ternary europium-containing high-temperature species of the systems Eu-X-O (cf Table 17). The thermochemical properties of these species and of further molecules of the composition EuXO (g) (n = 1 to 4 X = IVa, Va, Via, group metals) are summarized and discussed by Balducci et al. [385]. Some are given in Table 16. 

On the other hand, the last two entries in Table 3.2 hint at the weakness of empirical methods. They show that the environment of a group may lead to many parameters other than simple group energies when we are talking about a molecule of, say, biochemical or medical complexity. For every new group and every new environmental factor characterized, one or more new parameters must be determined, perhaps causing the parameter base to grow beyond reasonable bounds. Of course, any environmental factor that is real but is not included in the parameter set, causes an error in the calculated thermochemical properties. 

Consequently, our first goal is to determine thermochemical properties of a number of species involving peroxy and peroxide groups (radicals and molecules) which are essential and required in this work. Because of the drastic lack of available data in the literature, we have extended our estimation to a wide range of peroxide species in order to gain in understanding... 

There is little or no thermochemical property data available for unsaturated alkylperoxy and peroxide species. Peroxides are often impure and/or instable, and therefore difficult to isolate and characterize by experimental methods. There is no experimental data on vinyl, phenyl, ethynyl, or allyl peroxides that we are aware of. Experimental studies on the reaction of vinyl radical and allyl radical with O2 to form the corresponding peroxy radical have been reported by Gutman et al. [11, 12]. The phenyl-peroxy radical was reported by Lin s group as a major product in the phenyl radical reaction with O2 at ambient temperatures [17]. 

This study reports bond energies, enthalpy, entropy, heat capacity, internal rotation potential, and structure data for a series of unsaturated peroxides. Thermochemical property groups are developed as well for future use in group additivity estimation methods. 

The thermochemical properties of all groups are estimated the way described above and listed below in Table 4.7. Each groups is estimated from only one molecule system except for the calculation of O/CD/0, O/CT/0, CT/0 and 0/C/CT groups which are based on 2 to 5 molecules. The detailed calculations for these four groups are given in Table 4.6. All groups are derived from thermodynamic data determined by DFT calculations and isodesmic reactions. 

A method to estimate thermochemical properties for radicals from the corresponding properties of the parent and of derivation of hydrogen bond increment (HBI) groups, is described by Lay et al. [25] and Sun and Bozzelli [133]. The method uses the bond energy (298.K) for loss of a hydrogen on the central atom for the enthalpy term, the difference between the radical and the parent for the heat capacity (Cp(T)) term and the intrinsic entropy difference for the term. 

Thermochemical properties for the HBI groups are listed in Table 4.8. An example is loss of a hydrogen atom from the secondary vinylic carbon in CH2=CHCH=0, represented by C CJC 0 (CH2=C CH=0). The calculated BDE (Table 4.8) are given in Table 4.4 with the corresponding parent and radical. 

Table 6.5 Thermochemical Properties of Y(C3H40)=0 and Y(C6H80) to use for Group Additivity...

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