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12-Crown-4: The four oxygen atoms of 12-crown-4 each
have a lone pair that points inward, toward the cavity. This suggests
the cavity should attract cations, and if the cation is small enough, it
will fit within the cavity. The outside of the crown ether consists
mostly of C-H bonds, and is therefore nonpolar. This region of the
crown ether should associate with other nonpolar molecules.
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| 12-Crown-4 Electric Potential Surface: The electric potential surface is a measure of charge distribution. Red indicates regions of negative charge, green corresponds to neutral areas, and blue indicates regions of positive charge. From this map we can see that the positive charge is distributed over a very large area, and that the center of the crown ether is negatively charged. |  |
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12-Crown-4 Lithium Ion Complex: We can model this on the computer by calculating the electric potential surface. This is done by probing the surface of the molecule with a positive charge (a proton) and calculating the attractive or repulsive forces on the proton. In essence, this maps the regions of positive and negative charge on the molecular surface. The result of a such a calculation for 12-crown-4 is shown below. In this electric potential surface map, blue indicates areas of positive charge and and red indicates areas of negative charge. In 12-crown-4, the cavity is negatively charged (the oxygen lone pairs), so the surface must bear some positive charge to keep the molecule neutral as a whole. |