Molecules MH₃ (M = Cr, Mo, W) Are Pyramidal:  CCSD(T) Ab Initio Study of Structure and Vibrational Dynamics

Ab initio calculations were carried out on chromium, molybdenum, and tungsten trihydrides. The equilibrium parameters of the ground states, vibrational frequencies, IR intensities and the inversion barriers of the molecules were calculated at the CCSD(T) level with large basis sets. The relative energies to the excited states were evaluated by the EOM−CCSD method. All molecules were found to have pyramidal ( C 3 v ) equilibrium structures. The calculated inversion barriers of CrH 3, MoH 3, and WH 3 are 1353, 1086, and 472 cm -1, respectively. The vibrational levels corresponding to the totally symmetric H−M−H bending mode were predicted using the discrete variable representation approach. The tunneling splitting and rates were also evaluated within the Wentzel−Kramers−Brillouin approximation. The available experimental data are discussed in light of results from the ab initio calculations.