$\pi$ Molecular orbital diagram of compounds like urea, guanidine, acetone etc

In my organic chemistry course, we've only been taught about how the $\pi$ MO diagrams of compounds like butan-1,3 diene, hexan-1,3,5 triene and similar single chain compounds look like. I was wondering how the $\pi$ MO diagrams of heteroatomic compounds like urea, guanidine etc. look like, but unfortunately I was unable to find any relevant stuff in google as well as Organic textbooks like Clayden, Solomons etc.


