Force field methods bending energies

Lindner 1040 used a rc-SCF-force field method to calculate the energies of racemiza-tion of penta-, hexa- and heptahelicene. Two different pathways for the thermal racemization were compared. In one of them the terminal rings are brought in an almost parallel position to reach the intermediate state, as suggested by Martin (see Scheme 21). In the other way the terminal rings are in one plane whereas the remainder of the molecule is strongly bended. 

Other important and effective approaches to computational chemistry are those called Empirical Force-Field methods (EFF methods), based on a mechanistic view of the molecule in terms of force constants of bonds, bending, torsion and other special interaction terms. The set of force constants constitutes a field of empirical parameters used for the calculation of molecular geometries and energies. 

The utility of these methods can be illustrated by investigating a generic molecular mechanics force field. The potential energy function for such a force field is usually divided into 4 parts bonds, bends, torsions, and non-bonded (electronic and van der waals) interactions. 

This type of potential field is called a generalized valence force (GVF) field. It consists of stretching and bending force constants, as well as the interaction force constants between them. When using such a potential field, four force constants are needed to describe the potential energy of a bent XY2 molecule. Since only three vibrations are observed in practice, it is impossible to determine all four force constants simultaneously. One method used to circumvent this difficulty is to calculate the vibrational frequencies of isotopic molecules (e.g., D2O and HDO for H2O), assuming the same set of force constants.+ This method is satisfactory, however, only for simple molecules. As molecules become more complex, the number of interaction force constants in the GVF field becomes too large to allow any reliable evaluation. 

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