Determining aromaticity of compounds Which of the following planar structures would be aromatic? 




Could someone help me determining the aromaticity of the compounds?

My Attempt:

(i) Not an aromatic compound since it only has 4 pi electrons that does not satisfy Hückel's (4n+2) rule.

(ii) Not aromatic because it only has 4 pi electrons, however I'm not sure if the oxygen contributes a p-orbital, but it shouldn't matter since 5 pi electrons still does not satisfy Hückel's rule.

(iii) I'm very confused because I'm not sure if the ketones are considered "part of the ring" therefore do they also contribute p orbitals? However if we ignore the ketones, then I believe the Nitrogens contribute 2 e- since they are sp2 hybridized? Therefore the 2 N atoms and 2 C atoms contribute a total of 4-not aromatic either?
(i) that's correct

(ii) is aromatic, it fit's the 4n+2 rule with n=1.  Here is a drawing of furan showing 2 lone pair electrons in a p-orbital that completes the cyclic aromatic ring. Furan's other lone pair of electrons is perpendicular to the pi system and does not play a role in furan's aromaticity.



(image source)

(iii) Edit to incorporate @jerepierre's comment:

Just like phenol can exist in equilibrium with its non-aromatic tautomeric keto form



so can derivatives of pyrimidinedione (or uracil).  However, unlike phenol were the equilibrium lies far to the aromatic phenol side, these compounds usually (it depends on the substituents) exist in the keto form.  The strength of the keto and amide bonds outweigh the weaker aromaticity of the pyrimidine skeleton and shift the equilibrium to the keto side.



(image source)

The question remains, is the keto form aromatic?  As jerepierre points out, you can draw the following resonance structure for the keto form.  In this double-dipolar resonance structure there are 6 pi electrons 2 in the double bond and 2 [a lone pair] on each nitrogen) suggesting that this resonance structure would be aromatic.  Since this resonance structure contributes to the overall description of the molecule, it would be fair to say that this molecule has some aromatic character; certainly not as much as benzene or furan - but some.