An effective Hamiltonian for calculating rotational energy levels of an open-shell diatomic molecule, in a 2Sϩ1 ⌺ electronic state, weakly bonded to a closed-shell partner was presented (W. M. Fawzy, J. Mol. Spectrosc. 191, 68 -80 (1998)). The Hamiltonian was given as H ϭ H ev ϩ H rot ϩ H sr ϩ H ss ϩ H cd ϩ H srcd ϩ H sscd , where all the quartic centrifugal distortion correction terms were included in the Hamiltonian term H cd but the sextic centrifugal distortion terms were ignored. This Hamiltonian is useful in cases where the complex has a well-defined equilibrium geometry and if the barrier to large-amplitude motion is large compared to the rotational constant of both the closed-shell molecule and its paramagnetic partner; if the barrier to large-amplitude motion is small compared to the rotational constant of one or both of the fragments, then a different treatment is required. In this paper, we introduce the new Hamiltonian terms H cd sex( A) and H cd sex (S) , which represent the sextic centrifugal distortion correction terms for an asymmetric rotor. We also introduce all the nonvanishing matrix elements of each of the H cd sex( A) and H cd sex(S) operators. These operators and their matrix elements are required for calculating the rotational energy levels of relatively high J values in the described type of weakly bonded open-shell complexes. The terms H cd sex( A) and H cd sex(S)
and their matrix elements are also valid for any stable asymmetric rotor in a nondegenerate electronic state. A brief discussion of the new Hamiltonian terms and their matrix elements is given.