In this contribution we have briefly analyzed the approach lying at the basis of semi-empirical calculation of the potential barrier for molecule internal rotations. In opposite to it, we have suggested the way how to find this barrier on purely theoretical grounds. For this purpose, we have used the bond-charge model with clamped charges, and in this contribution restricted ourselves by the simplest case of such molecules as ethane. It was found that this approach leads to theoretical results which are in a good agreement with experimental data. It means that the electronic theory of molecule vibrations developed earlier has a large degree of generality and can be used to study not only vibrations but also rotations of molecules and macromolecules on the basis of one and the same approach, excluding a lot of empirical parameters which are often do not clarify a subject but immerse it into darkness.