A note to the viewer:
    Some of the parts of this web page are part of an animation in which various sites of a molecule are highlighted and rotated etc. The site works best when your web browser is maximized on your screen and when you click the buttons in order.  If the buttons are not clicked in the correct order, then the animation will not make much sense.  If you do examine the molecule before the animation sequence is complete, then you can start over by using the reload button on your browser.