Strain and Stability

The ability of the MM method to calculate accurately potential energies and geometries of molecules is best suited for quantitative conformational analysis (44). Numerical values can readily be given to such terms as strain and stability (45). The topic is extensively covered in Allinger s review (lOd), therefore only some recent developments are mentioned. 

Electrostatic forces 179-181, 325-327 Electrostatic strain and stability 74 Enamines 76, 77 Enantiomer 245 Encounter complex 159 Encounter frequency 164,166 Enediols 251, 252... 

Each of these approaches leads to a different description of strain and stability of 1 and therefore the question is whether the various rationalizations of strain and stability are compatible. 

While the three approaches discussed above lead to a quantitative reproduction of the SE of 1, a dissection of the HF wave function suggested by Inagaki and coworkers47 80 provides qualitative arguments for a rationalization of strain and stability of 1. The method is based on a configuration analysis of the HF wave function, PHF according to equation 15 ... 

Quantitative Correlations Between Strain and Stability Differences 115... 

The structures of cycloproparenes and the chemistry and properties of alkylidenecy-cloproparenes have been reviewed. Much experimental and theoretical work has been devoted to strain and stability and to bond fixation in such fused aromatic systems resulting in much controversy. The anellation of small rings to a benzene ring gives rise to substantial angular deformation and bond distance change. 

Heats of hj drogenation have been measured for a wide variety of unsaturated organic compounds, to give information about strain and stabilization (resonance) energies. However, this is only one type of addition reaction for which thermochemical data have been accumulated. Lacher and his co-workers have measured heats of halogenation of ethylenic compounds, for example the chlorination of the homologous series of perfluoroalkenes, viz.,... 

The heats of a large number of chemical reactions have been measured with great accuracy in recent years. These data have been used to calculate the strengths of chemical bonds and also strain and stabilization energies in various molecules. Although a number of review papers have been published on these topics, it seemed that there was a need to gather together some of this information under one title. 

As we did with electronic structure in Chapter 1, in this chapter on molecular structure we will consider reactive intermediates alongside stable molecules. There are many parallels and interconnections, but also some unique features for reactive intermediates. The same notions of strain and stability can be applied to molecules even if they are reactive, because structural issues are fundamentally the same regardless of whether an atom has an octet or not. Before looking at thermochemistry, however, we must consider a general issue for reactive intermediates. We consider a reactive intermediate to be "unstable", although for all types of reactive intermediates there are special cases that are considered "stable". These are imprecise terms, and in the next section we will seek to clarify their meaning. 

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