The conformation of linear systems

Conformational isomerism, as already defined (Section 3.b), is a property of stereoisomers separated by a low barrier of energy. The separation of isomers at room temperature requires half-lives of several hours, which correspond approximately to a free energy of activation of AG 20 kcal/mol [56]. An operational and convenient definition of conformational isomerism is thus to consider as conformers those stereoisomers which are not physically separable under ordinary conditions, in other words, which are separated by an energy barrier lower than 20 kcal/mol. Such a definition is further useful in that it sets no conditions as to the chemical process by which conformer interconversion occurs while bond rotation is the most frequently encountered interconversion process, inversion processes are also important. 

Stereoisomers in general, and conformational isomers in particular, are characterized not only by the energy barrier separating them, but also by their free energy difference AG°, which is related to the conformational equilibrium constant K (or conformational ratio) by the equation  

For example, energy differences of 1 and 3 kcal/mol correspond at 290°K to isomeric compositions of 85/15 and 99.5/0.5, respectively. Comprehensive tabulations covering a wide range of temperatures have been published [69]. Two classical books stand as important milestones in the development and evolution of the conformation concept [70,71]. 

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