Potential energy surface and rovibrational bound states of the H-CN van der Waals complex

Since their recent detection in the interstellar medium, anions have raised the question of their possible mechanisms of formation, destruction and excitation. This requires knowledge of their interaction with the most abundant interstellar species. In the present work, a four dimensional rigid rotor model of the potential energy surface is developed for the collision of CN with H. Ab initio calculations are performed with explicitly-correlated coupled-cluster theory via CCSD(T)-F12/aug-cc-pVTZ. Two linear equilibrium structures are found, different in the orientation of CN. Two more equilibrium structures, symmetrically equivalent, are obtained by the permutation of H atoms. The vibrational dynamics is mainly controlled by the considerable difference between the two bending frequencies that correspond to the hindered rotations of CN and H. This arises from the potential energy surface which is soft for rotation of CN and stiff for rotation of H, and also from the large difference in mass between both monomers. Although a high potential barrier prevents the rotation of H, a significant tunneling effect is observed which causes a splitting in the degenerate energy levels. On the contrary, the rotation of CN is allowed since the energy of the saddle points is lower than the energy of the bound states, but the wavefunctions remain localized around each linear structure unless a large excitation energy is available.