Strain energy minimization

Figure 6. Two Steroids—19-nor androstenediol (a) and 7-ame-19-nor androstenediol (b)—as found in crystal (viewed normal to C ring). Dotted lines in (c) show a possible placement of extra potentials for linking the molecules during simultaneous strain energy minimization.

It is important to realize that for any arrangement of more than two atoms the strain energy minimized structure does not have ideal (zero strain) distances and angles. This is demonstrated in the case of ethane (Fig. 2.2), where, due to the repulsion of the protons, the experimentally determined C-C distance in ethane of 1.532 A, which is well reproduced by empirical force field calculations, is slightly longer than the ideal C-C separation of 1.523 A used in the MM2 force field1. Further examples are presented in Table 2.1. With increasing substitution of the carbon atoms the C-C separation increases up to 1.611 A in tris(t-butyl)methane. 

Comba, P Hambley, T. W. Okon, N. MOMEC, A Strain Energy Minimization Package Adapted to HyperChem , Altenhoff Schmitz, Dortmund Germany (1995). 

Section IV correlates intramolecular and intermolecular steric effects where it is important to consider the ease of strain energy minimization in the order rotation > bending streching, and also the three major structural situations discussed below in Sections IVA-C. 

Some examples (different electronic ground states) of experimentally determined structures and geometries obtained by strain energy minimization with the two models, i. e., with and without the harmonic sine term for the metal-ligand-metal interaction, are presented in Table 12.3. 

Wiberg KB, Boyd RH (1972) Application of strain energy minimization to the dynamics of conformational change. J Am Chem Soc 94 8426- 8430... 

T. W. Hambley, MOMEC85, A Program for Strain Energy Minimization, University of Sydney, Australia, 1987b. 

For cobalt(III) compounds, P. Comba calculated five of the six conformations possible for sarcophaginate complexes (Fig. 3), using strain energy minimization [230]. The sixth Dsleh conformation does not exist in cobalt(III) cages. Some structural parameters of the strain energy-minimized structures of the cobalt (III) complex conformers are presented in Table 4. 

Thus, the application of the ligand field model [178] in combination with the strain energy minimization model [230] makes it possible to calculate with high accuracy the geometrical parameters of sepulchrate and sarcophaginate frameworks. 

Wiberg, K. B. (1965) A Scheme for Strain Energy Minimization. Application to the Cyclo-alkanes, J. Am. Chem. Soc. 87, 1070-1078. 

The first four terms of the function are commonly found in molecular mechanics strain energy functions, and they are modified Hooke s law functions. The last term has been added to insure the proper stereochemistry about asymmetric atoms. A model is refined by minimizing the highly nonlinear strain energy function with respect to the atomic coordinates. An adaptive pattern search routine is used for the strain energy minimization because it does not require analytical derivatives. The time necessary to obtain good molecular models depends on the number of atoms in the molecule, the flexibility of the structure, and the quality of the starting model. 

---