Benson group values

Where no data exist, one wishes to be able to estimate thermochemical quantities. A simple and convenient method to do that is through the use of the method of group additivity developed by Benson and coworkers15,21 22. The earlier group values are revised here, and new group values calculated to allow extension of the method to sulfites and sulfates. In addition, a method based on the constancy of S—O bond dissociation energies is applied. 

As stated above, the thermochemistry of free radicals can also be estimated by the group additivity method, if group values are available. With the exception of a few cases reported in Benson (1976), however, such information presently does not exist. Therefore, we rely on the model compound approach (for S and Cp) and bond dissociation energy (BDE) considerations and computational quantum mechanics for the determination of the heats of formation of radicals. 

The group value of 0-(0)(C) calculated from the dialkyl peroxides gives a value of —57.1 kcal. per mole for the heat of formation of tert-BuOOH, compared with the measured value (11) of —52.3 kcal. per mole. Since the calculated value is more negative, one cannot account for the difference by the otherwise reasonable assumption that the hydroperoxide decomposed a little prior to combustion. An alternative would be that group additivity did not apply to the hydroperoxides, but Benson s... 

This example shows how to determine the correct group value for AHf and S for C-(H)(Br)2(C) by considering known values in the sequence C-(H)3(C) to C-(Br)4. This example represents changes in the main/central group. Note that the entropy values for Benson groups such as C-(Br)4 have the symmetry contribution removed. The user must add in any symmetry contribution after the molecule is built with the complete set of Benson groups. For consistency in any interpolation scheme, one must remove the symmetry contribution from the entropy for the whole molecule. In this case, owing to the tetrahedral symmetry, an amount R In 12 (where R is the gas constant) was subtracted from the literature entropy value for C-(Br)4. 

The data necessary for thermodynamic estimates are available from experimental as well as computational methods. In many systems AGh can be approximated by experimentally accessible AGJ. The approximation is valid (to within 0.05-0.15 eV) if the radical coupling has no barrier (is diffusion limited) and the thermolysis is carried out under conditions selected to minimize the cage recombination [79]. The homolytic bond strengths can also be obtained in many cases from the Benson group-additivity tables [80] or semiempirical quantum or molecular mechanics calculations [81]. With appropriate entropy corrections [75f], relatively accurate AGh values can be obtained in that way. 

Another problem with the Benson data (Table 10.3) is that it does not give a group value for —(NO2). It does give a value for Cg—(N), but if we use this... 

As O Neal and Benson have pointed out, only six of the group values are linearly independent one can be assigned arbitrarily. As they did, we choose to set / = P. The other six terms can be calculated from experimentally determined enthalpies of formation of the radicals ethyl, n-propyl, i-propyl, t-butyl, i-butyl and neo-pentyl. The enthalpy of 5-butyl can be substituted for that of either n-propyl or i-propyl. The relationships are as follows ... 

In fact, there is a hierarchy in calculating molecular properties by additivity of atomic, bond, or group properties, as was pointed out some time ago by Benson [1, 2]. The larger the substructures that have to be considered, the larger the number of inaements that can be derived and the higher the accuracy in the values obtained for a molecular property. 

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. 

Benson and coworkers15, and the values differ somewhat. The differences probably reflect the small differences in the enthalpies of formation of the parent compounds used in the two cases (i.e., the differences between enthalpies of formation compiled by Benson and coworkers15 and those compiled by Pedley, Naylor and Kirby17) as well as different approaches to deriving the group contributions. The major differences probably arise from the different values used for the relevant enthalpies of fusion and vaporization. It should be emphasized that the derivation of group contributions is not purely quantitative in nature, and that subjective elements enter into the selection of molecules to be used to... 

The Benson tables lack some information about the nitro group bonded with an aromatic cycle, which is one of the most frequent groups amongst unstable compounds. An attempt was made to estimate the Cat-N02 value using the experimental data from Part Three. The results are incoherent and this explains why Benson did not mention anything. As a result, only the approach of the second table could be used, which gives results with a high level of error. 

This useful and simple-to-use software package relies on Benson s group additivity scheme [47] to estimate thermochemical data for organic compounds in the gas phase. It also contains values from several NIST databases, including NIST Positive Ion Energetics [32] and JANAF Tables [22]. The first version of... 

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