History of Molecular Mechanics

The expression molecular mechanics is currently used to define a computational method designed to give accurate structures and energies of molecules. The method is a natural outgrowth from older ideas of bonds between atoms in molecules and of van der Waals and electrostatic forces between non- 

Extensive documentation of the accuracy of molecular mechanics calculations has been reported.For the most part, the discrepancies between experimental and calculated molecular geometries are within experimental error. Many of the systematic discrepancies in MM2, for example, have been documented. Some of the apparent errors can be associated with incorrect comparisons of bond lengths that are defined differently in various experimental and computational methods. The bond lengths have different numerical values because they are different physical quantities, rather than being real errors. Other problems have been attributed to a lack of accurate experimental data when the force field equations and parameters were formulated. 

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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. 

Kipnis, A. Ya. (1990) From the history of molecular physics A.I. Nadezhdin 1858-1886, in Studies on history of physics and mechanics. 1990, Nauka, Moscow, 5-36 (in Russian). 

At one point in the history of structural chemistry molecular mechanics calculations dominated the computational work for relatively large molecules. The origins of these calculations were intimately connected to another modeling approach that one of its initiators vividly described. Frank Westheimer (Fig. 1.2a) had participated in the American defense efforts during WWII and when the war had ended, he returned to the University of Chicago to resume his teaching and research. He had to start anew and had time to think about basic problems. This is how half a century later he remembered the birth of molecular mechanics [5] ... 

There has been considerable progress over recent years in the study of molecular mechanisms involved in crazing and shear yielding [1,2]. When sample preparation and test conditions (temperature, stress state, strain, strain rate, and thermal history) are chosen to be the same, the effect of molecular variables can be determined. It is now well established that (network) strand density plays an important role in determining the deformation mechanism and in affecting the craze-shear deformation transition. Polymers are... 

All life forms we know work on the same principle cells surrounded by lipid membranes, with DNA as the storage medium of genetic information, RNA for diverse information transfer purposes, and proteins to carry out mechanical and chemical functions. This uniformity is convenient for students who read biochemistry, because most knowledge can readily be generalized. However, as only the analysis of diversity patterns allows us to trace back the family history of molecular biology, this also means that we don t know anything about the time before evolution invented the cell and the DNA/protein machinery. 

Later reconstructing the history of this period, Coulson claimed that the development of wave mechanics from Schrodinger to Dirac came to a "full stop" about 1929 insofar as chemistry was concerned. It was one thing to deal with the simplest cases of the H2+ ion and the H2 molecule. These problems hardly were comparable to methane or benzene. "Despondency set in."46 Joseph Hirschfelder later recalled that there were many new techniques to be learned in the 1920s, but then the realization set in that although "nature might be simple and elegant, molecular problems were definitely more complicated.. .. At this point, the theoretical physicists left the chemist to wallow around with their messy molecules while they resumed their search for new fundamental laws of nature."47... 

The glass transition temperature can be measured in a variety of ways (DSC, dynamic mechanical analysis, thermal mechanical analysis), not all of which yield the same value [3,8,9,24,29], This results from the kinetic, rather than thermodynamic, nature of the transition [40,41], Tg depends on the heating rate of the experiment and the thermal history of the specimen [3,8,9], Also, any molecular parameter affecting chain mobility effects the T% [3,8], Table 16.2 provides a summary of molecular parameters that influence the T. From the point of view of DSC measurements, an increase in heat capacity occurs at Tg due to the onset of these additional molecular motions, which shows up as an endothermic response with a shift in the baseline [9,24]. 

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