Molecular geometry strain

Alkenes exhibit large strain energy when molecular geometry does not permit all the bonds to the two sp -hybridized carbons to be coplanar. An example that illustrates this point is -cycloheptene ... 

A force field describes the restoring forces which occur in a molecule when the geometry of minima potential energy is disturbed. Force fields of the kind dealt with in this article describe the occurring forces with respect to a so-called strain-free molecular geometry or, more generally, with respect to a molecular reference geometry . 

Terms representing these interactions essentially make up the difference between the traditional force fields of vibrational spectroscopy and those described here. They are therefore responsible for the fact that in many cases spectroscopic force constants cannot be transferred to the calculation of geometries and enthalpies (Section 2.3.). As an example, angle deformation potential constants derived for force fields which involve nonbonded interactions often deviate considerably from the respective spectroscopic constants (7, 7 9, 21, 22). Nonbonded interactions strongly influence molecular geometries, vibrational frequencies, and enthalpies. They are a decisive factor for the transferability of force fields between systems of different strain (Section 2.3.). 

Two different kinds of force fields have appeared in the hterature. In the first type a set of internal coordinates, sufficient to describe the molecular geometry, is choosen. The total strain energy is then calculated from the deviation of these internal coordinates (plus nonbonded interactions) from their strain free values. The redundant coordinates are given zero force constants and do not contribute to the potential energy. 

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