Molecular Structure of Caffeine I'm currently looking at a problem that asks about the structural relationship between the 4 nitrogens the caffeine molecule. I am asked to identify whether they are in perpendicular planar, coplanar, or inverted planar formations. 

I'm am a bit lost as to how I should approach this problem. Should I look at hybridization (3 Ns are sp3, 1 N is sp2)? Or resonance (seems that all Ns are involved in resonance)? Or should I be concerned with sterics? 

Lastly, what are some examples that might assume a perpendicular planar, coplanar, and inverted planar formation? I don't think I've ever seen such shapes before...

Would someone please point me to the right direction?
At some point in studying organic chemistry, you might have been taught that a nitrogen in an amide group is planar sp2, due to delocalization of its electrons into the carbonyl group. You also might have learned that nitrogen atoms in a pyrrole or imidazole ring are planar sp2 since they participate in the 6 electron aromatic pi bonding system. You would then conclude that all the nitrogen atoms in caffeine are coplanar. But without that essential background information, you might not have been expected to figure that out.