BENSONs molecular groups

The most generally apphcable method for prediction of the property is the method or Seaton, which depends only on the molecular structure of the molecule and utilizes second order (Benson-type) groups to construct the molecule. Equation (2-175) sums the groups number of each type group (/id to get both the upper and lower limits. 

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. 

What is necessary to compute a heat of formation, then, is to define the heat of formation of each hypothetical, unstrained atom type. The molecular heat of formation can then be computed as the sum of the heats of formation of all of the atom types plus the strain energy. Assigning atom-type heats of formation can be accomplished using additivity methods originally developed for organic functional groups (Cohen and Benson 1993). The process is typically iterative in conjunction with parameter determination. 

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. 

The molecular geometry of the whole molecule of seven linear and five angular polyacenes was used to calculate the MFC values, which are compared with the aromatic stabilization energy (ASE) calculated from the Benson and Cohen group additivity values (CAV) within the thermochemical procedure.Table 14 presents normalized ASE values (divided per one 7r-electron) depicted as E, MEC/n n... 

S.W. Benson, N. Cohen, Current status of group additivity, in K.K. Irikura, D.J. Frurip (eds.). Computational Thermochemistry Prediction and Estimation of Molecular Thermodynamics, ACS Symposium Series 677. American Chemical Society, Washington, DC, 1998, p. 20. 

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