Asymmetric Energy Barriers in Unidirectional Molecule-Rotors

By applying the nudged elastic band method in the framework of ab initio density functional theory we calculate the energy barriers of unidirectional molecule-rotors. For the rotation to be unidirectional, these barriers must be asymmetric. We study two systems. The first is an acetylene molecule which rotates on a basin formed by a Pd trimer on a chiral PdGa surface. The second are DMBI derivatives on the (111) surface of gold, which rotate after the formation of a strong chemical bond which provides an axle for the rotation. We show that the applied theoretical method is able to produce asymmetric barriers for both systems, although the barriers are too high compared to the experimental values. In addition, different details of the systems are discussed in connection with the experimental results.