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Effect of molecular shape on electric forces

Page history last edited by Mark Eichenlaub 2 years, 5 months ago



The electric attractions and repulsions of the ions on the surface of complex molecules play a critical role in many biochemical reactions. An interesting component of some of these processes is that chemical reactions or electron transfers result in a change of the shape of a molecule, exchanging chemical energy for electrical energy, and resulting in a different surface configuration and therefore different interactions. In this problem we will explore the effect of a change of configuration of a molecule on the force between it and an ion in a neighboring molecule.

Consider the electric force between four ions on the surface of two large molecules as shown in the figure at the right. The heavy black lines running out of the frame indicate bonds to other atoms in the molecule that we will not consider here. The purple ion in the molecule on the right has a charge +e, the red ion in the molecule on the left has a charge -2e, and the two green molecules in the molecule on the left each have charges +e. A schematic diagram of these ions is shown on the left below indicating relative distances.


A.Qualitatively (without doing a calculation), would you expect the force of the ions (ABC) on ion D to be attractive or repulsive? Explain your reasoning.


B. Now do the calculation to find the net force between (ABC) and D. Express your result as a multiple of the combination F0 = kce2/d2.

C. As a result of an electron being added to another part of the molecule on the left, the configuration of the three ions (ABC) changes so that it looks like the figure on the right, drawing ion D in a bit closer as shown. Without doing the calculation, will the magnitude of the force that (ABC) exerts on D increase, decrease, or remain the same as it was in part B? Explain your reasoning. 


Joe Redish 12/29/11


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