Benson’s groups

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... 

THERM Thermodynamic Property Estimation for Radicals and Molecules, Edward R. Ritter and Joseph Bozzelli, Int. J. Chem. Kinet. 23, 767-778, (1991). A computer program for IBM PC and compatibles for estimating, editing, and entering thermodynamic property data for gas-phase radicals and molecules using Benson s group additivity method. 

Estimated by Benson s group additivity scheme as the difference between the heats of formation of ethylene and of two —CH2—groups. See Table 2.6 p. 75. 

The ring strain energies are either determined experimentally or by calculation. Engel has calculated the AHf of a strain-free reference compound (based on Benson s group contribution method) and has substracted this from the AHf of the compound of interest <1976JA1972>. The difference between these two values is... 

Related Calculations. AG 29X may also be calculated via the relationship AG/gg = A //2°98 — T AS 9g, where S is entropy. Values for S can be found in the literature, or AS 98 can be estimated by Benson s group-contribution method (see example for estimating A f//298 by that method above). However, the deviations for entropy values calculated by Benson s method are high compared with those for A77(29g, so when such group methods are used in the course of calculating AG, only qualitative feasibihty studies are possible, based on these rules-of-thumb (1) if AGj. is less than zero, the reaction is clearly feasible (2) if the value is less than 4.184 x 107 J/(kg mol) [or 1.8 x 104 Btu/(lb mol)], the reaction should be studied further and (3) if the value is greater than 4.184 x 107 J/(kg mol), the reaction is feasible only under exceptional circumstances. (Thus, in the present example, the reaction appears unfavorable at 298 K, but much more promising at 443 K.)... 

Thermodynamic calculations by BENSON s group contributions (10) show that ( clizations involving a six membered ring formation are favoured. Then, from the easiest to the more difficult, come the 5-membered ring, the 8, 4 and 3-membered ring. These calculations are in agreement with the well-known ring strains and eclipsed bonds. 

Table 11 lists the AH s for the C H2n-6 benzenoid isomers. The values for C8-C12 were obtained utilizing Benson s group increments (25a) and agree well with experimental values. Empirical force field calculations can be used to obtain a strain energy for aromatic molecules but this is not necessary when group increment values are available. The best aromatic for each C was selected based on group increment estimatimi of AH from a list of candidates generated... 

Cohen and Benson s) group additivity values polysubstituted benzene derivatives density functional theory strain-induced bond localization... 

If a reaction is sufficiently well balanced, its enthalpy change will be close to zero. For reactions (7)-(10), the experimental enthalpies of formation imply gas-phase reaction enthalpies (298.15 K) of 5674 + 7, —518 + 7, 52 + 7, and 30 + 7kJmol respectively. All quantum calculations can be avoided by assuming the reaction enthalpy to be zero for a well-balanced reaction. This is the basis of empirical methods such as Benson s group-additivity scheme [5,6]. Where adequate auxiliary data are available, a group-balanced reaction is an excellent choice for quantum chemistry calculations [121]. 

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