[kanonsviel]

About the structure of this drug
http://de.wikipedia.org/wiki/Aciclovir

It is named "2-Amino-9-(2-hydroxyethoxymethyl)- 3H-purin-6-one"
Why it is called a "3H-purin", which position does this "3H" refer to (according to the normal numbering of a purine ring, it should be the left bottom N, but it's apparently it has not a hydrogen attached)
And why it's not a "purin-6(1H)-one"