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Conformations
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
No! When a C–C bond is confined to a ring, rotation about 360° is impossible without breaking a bond and disrupting the ring. A twisting motion about the bond occurs, called pseudorotation, in order to dissipate excess energy. Pseudorotation of the bonds in rings leads to changes in the shape of the molecule – different conformations. Is “free rotation” possible around carbon–carbon bonds in cyclic molecules? Why or why not?
A VSEPR-inspired force field for determining molecular properties of PF5
Published in Molecular Physics, 2019
Laura M. McCaslin, John F. Stanton
In 1973, Bartell and Plato published work applying VSEPR theory to molecules of the form XY, where atom X is bonded to five Y atoms [2]. They were interested in applying VSEPR theory to these systems in order to understand the mechanism of Berry pseudorotation, a process for axial and equatorial ligand switching, moving from its minimum D structure through a C transition state structure [3]. Bartell and Plato used a very simple points-on-a-sphere (POS) model to calculate the five unique relative bending force constants of PF, as well as the barrier to pseudorotation. In this model, each of the five ‘Y’ atoms repels the others through the following two-parameter potential energy function: where K and n are parameters and r is the distance between atoms Y and Y. In the model used, the bond lengths between the central X and surrounding Y atoms were taken to be equal for all five ligands, creating a model where the Y atoms can be thought of as moving on a sphere with a radius equal to the X-Y bond distance. This simple model was used to calculate a (K and n dependent) bending force field, which was compared to that calculated with Extended Hückel Molecular Orbital (EHMO) theory [4]. The comparison revealed that the simple POS model was qualitatively consistent with the quantum-mechanically calculated force constants over a fairly wide range of reasonable choices of K and n.