A detailed ab initio study on the torsional potential of two heterobutadienes (nitrosoformaldehyde and N-nitrosomethanimine) it is performed by using state-of-the-art coupled-cluster methodologies. Special emphasis is given to basis set incompleteness with a profuse comparison of several cc-pVnZ basis sets and to the performance of common extrapolation formulas for estimating the complete basis set limit. We give high quality torsional functions from a fit to our best calculations for its use in current force field methods. We analyze similarities and differences between the present heterobutadienes and similar compounds such as 1,3-butadiene and glyoxal. Finally, we provide accurate estimates to the proton affinities of nitrosoformaldehyde and N-nitrosomethanimine.