AutoCorrect does not support Superscript and/or Subscript that I am aware
of. However what I use is the toolbar icon to minimize the time to create
this. Right-click your toolbar area and click "Customize". Click the
"Format" Category under the "Commands" TAB. Scroll down on the right and
find the "Superscript" and "Subscript" commands. Click and drag each one to
an existing toolbar (you will see "x" symbol change to a "+" when you in the
right place), then release your mouse.
Now all you have to do is to type the "H", click the icon, type your "2",
click the icon again to turn it off, then type the "O".