Generally, in the case of a corporation, the underpayment rate is the federal short-term rate plus 3 percentage points and the overpayment rate is the federal short-term rate plus 2 percentage points. The rate for large corporate underpayments is the federal short-term rate plus 5 percentage points.

https://www.irs.gov/newsroom/interest-rates-increase-for-the...

Back taxes have a baseline rate that grows at certain increments of lateness but is ultimately capped. I don't think it's in any way tied to the base or prime rate.