We have a national bird, a national flower, a national tree, a national motto and a national march. It's the 21st Century, and the U.S. STILL does not have an official national molecule. It is high time the U.S. establish a national molecule. I nominate the Buckminsterfullerene, also known as C60 or the Buckyball.
It's one of the most important molecules discovered by Americans. It has applications in medicine, alternative energy, geology and planetary science, and they are used as building blocks for other important carbon based molecules. Hundreds of patents have been awarded based on Buckyballs, and thousands of important new compounds have been created.