Bound states of the OH(2Pi)-HCl complex on ab initio diabatic potentials

The bound states of the open-shell OH((2)Pi)-HCl complex were calculated in four dimensions with a diabatic model using electronic states that correlate asymptotically with the ground and excited spin-orbit states of the OH((2)Pi) fragment and the ground state of the HCl fragment. The ab initio diabatic potentials and their analytic expansion applied in these calculations were obtained earlier by Wormer et al. [J. Chem. Phys. 122, 244325 (2005)]. In addition to the four-dimensional calculations, we considered a (3+1)-dimensional model in which the intermolecular distance coordinate R is adiabatically separated from the remaining coordinates. Both models include the important spin-orbit coupling in the OH fragment. Energy levels and parity splittings were computed for a total angular momentum of J=1/2 and 3/2; rotational constants and other spectroscopic parameters were extracted from these calculations. The vibrationally averaged geometry in the ground state of the complex is planar and this state is more or less localized near the minimum in the lowest adiabatic potential with binding energy D(e)=1123 cm(-1); the dissociation energy D(0) with respect to OH((2)Pi(3/2)) and HCl is found to be 685 cm(-1). The splitting between the (2)Pi(3/2) and (2)Pi(1/2) spin-orbit states of free OH is largely reduced by the anisotropic interaction with HCl through the off-diagonal diabatic coupling potential and these states are strongly mixed. Low lying rovibronic states that correlate with the OH((2)Pi(3/2)) ground state were found at 14 cm(-1) for total angular momentum projection quantum number |Omega|=3/2 and 26 cm(-1) for |Omega|=1/2, relative to the ground state with |Omega|=1/2. The OH-HCl stretch fundamental frequency equals to 93.6 cm(-1), the lowest bend excited states (involving a coupled bend motion of both fragments) were found in the region of 150-160 cm(-1) above the ground state. Especially in the excited states important nonadiabatic effects are observed that involve both of the asymptotically degenerate adiabatic electronic states. In some of these excited states the vibrationally averaged geometry is nonplanar.