2.3.2 Energetic Corrections

Another set of energetic corrections has to do with ring strain and rotational hindrances. In both cases, the correction is recognized through representative substructures in the database. Associated with each structure is a set of thermodynamic corrections similar to the Benson rules.

Ring Strain Corrections

Ring strain energies are recognized through representative ring substructures. These substructures are usually represented by the heavy atom (meaning no hydrogen) skeletal structure of the ring. For multiple ring structures, the largest matching ring is chosen. Currently there are 65 single ringed cyclic corrections corresponding to the cyclic structures in Benson's tables[18]: Hydrocarbons (Table A.1), Oxygen-containing rings (Table A.2), nitrogen-containing rings (Table A.3) and organosulfur ring compounds (Table A.5). Data have been up dated and supplemented from Domalski and Hearing[48] and Cohen[27]. Once again, the exact energetic contribution of a particular ring is a design decision as to which is the prominent or average configuration.

Steric Hindrances

Steric hindrances are caused by rotation around single bonds. The corrections for cis-alkene are not included because this is not represented in the current molecular form. The 2D-graphical representation has no spatial component cis-trans information would have to be given as supplementary flag to the double bond. Steric hindrances are determined also by substructure matching. In these cases, the spatial information is implied by the structure. When a substructure is found in the molecule, then the thermodynamic quantities associated with that structure are applied.