Rotational barriers of a nitrophenylene ethynylene trimer

The rotational barriers for a nitrophenylene ethynylene trimer have been computed using density-functional theory. The local minima previously reported and used to explain the negative differential resistance, (NDR) for this class of molecules appear to be an artifact due to basis set superposition error. While there is a local minimum for highly tilted molecules, the tilt angle required to produce the correct barrier height is at the upper limit of the reported experimental values. We believe that it is premature to explain the experimentally observed NDR in terms of a rotational barrier of a monolayer with a parallel slipped commensurate structure.