Molecular mechanics Westheimer method

The molecular mechanics technique has been called by many different names, including Westheimer method, strain-energy method, conformational energy calculations, empirical potential energy calculations, atom-atom pair potential method, and force field calculations. Empirical force field is widely used, but somewhat long, and many authors omit empirical, leading to confusion with spectroscopic force field calculations. Molecular mechanics (11) now appears to be favored (10a) and is used (abbreviated as MM) throu out this chapter. 

History and Concepts. A complementary approach for molecular structure calculations is available, and it is referred to as the molecular methanics or force field method it is also known as the Westheimer method. In 1946, twenty years after the impressive development of quantum theory, three papers appeared in the literature which applied classifical mechanical concepts to problems of chemical interest. Westheimer investigated the racemization of some optically active biphenyl derivatives. His work demonstrated the potential usefulness of molecular mechanics. The other two papers were attempts to tackle more complex problems. 

Theoretical approaches to the study of protein structure and function draw upon concepts and techniques from theoretical chemistry, including molecular mechanics, reaction dynamics, quantum and statistical mechanics (Brooks et al. 1988). Molecular mechanics methods were first developed for the study of small organic molecules (Barton 1948 Westheimer 1956 Hendrickson 1961 Wiberg 1965 Allinger et al. 1967 ... 

Finally, we come to the force-field method, which is also known as the Westheimer method or the molecular mechanics method. An excellent review of this method was published earlier (Williams et al.,... 

One important area of research in organic chemistry in recent years has been concerned with rates of chemical reactions. In order to predict and understand reaction rates, one must understand both the starting and transition states of the reaction. The calculations we have discussed so far have applied to starting states. If we can apply similar calculations, modified as appropriate, to transition states, presumably we can learn something about chemical reactions and in particular about reaction rates. Indeed, Westheimer s initial application of the molecular mechanics method was to such a problem the rates of racemization of some ort/ -substituted biphenyls. 

Still another method designed for the same purpose, namely equilibrium ground state geometries, must be mentioned. Starting with Westheimer s early calculations [141], the molecular mechanics or force field methods became a very important tool, especially for very large molecular systems, as encountered in biochemical applications. Here the contributions by Lifson/Warshel [142] and Allinger [143] deserve attention. 

Molecular mechanics methods, based on Westheimer s approach (see chapter 6), were refined into software packages by a number of groups. Molecular mechanics calculations depend heavily on input data from known... 

A system of analyzing the energy differences among molecules and among various geometries of a particular molecule has been developed, based on some fundamental notations formalized by Westheimer. The method is now known by the term molecular mechanics, although the expressions empirical force field calculations or the Westheimer method are sometimes applied. 

---