Knowledge Bank

The conformations of cyclohexane, s-trioxane, p-dioxane, m-dioxane, and tetrahydropyran are defined in terms of a new coordinate system which allows the structure and energy of all the significant conformations to be specified by only two coordinates. Using a potential derived from vibrational and geometrical data, detailed maps of conformational energy are constructed. These maps show that the more symmetric molecules are almost freely pseudo--rotating in the transition state between the chair and boat-twist forms. From calculated structural and potential data, the kinetic parameters for chair inversion are determined. A detailed picture of the chair-chair inversion process is given and the calculated rates are compared to experiment. As a result of the pseudo-rotation, values of the entropy of activation are found which are large and positive.