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What is the ring count for the structure Cubane?

Following the RDBE blogs, I decided to run a little test pointed out to me by Robin Martin, Ph.D. Without using the RDBE formula, what is the RDBE (number of rings) for cubane (structure shown below)?

Rdbe_cubane1_3252008

Starting with the 2D representation of cubane, it comes down to counting how many bonds are removed to displace a ring.

Rdbe_cubane2_3252008

According to the diagram above, the RDBE of cubane is 5 and not 6 according to the 3D representation.

TIP: The number of faces on a 3D structure does not dictate the RDBE value.