Knowledge Bank

Ab initio studies predict that protonated acetylene has two possible equilibrium structures separated by a low barrier. The potential function can be expressed in terms of an angle that describes the rotation of the three H atoms around the C-C core, and it features two alternating sets of minima corresponding to either possible structure, with a $\pi /6$ periodicity. We have applied the semirigid bender model, with the necessary modifications for a periodic potential, to study the rotation and `internal rotation' energy levels of this ion, and the variation with the energy difference and barrier height between the two structures.