Neither one of
returns a structure diagram for the molecule. It's consistent since StructureDiagram is not in the list of PodIDs, but a structure diagram is returned when querying caffeine on wolframalpha.com, and when calling:
Also, page 394 of the book Hands-on Start to Wolfram Mathematica (2nd edition) shows a call to WolframAlpha["caffeine"] returning a structure diagram and a 3D structure.
Anyone know why the structure diagram is missing?