Controlling the relative orientation of reactants with intermolecular forces:: Intermolecular state-dependent structure in prereactive H2-OH complexes

A theoretical study has been carried out to examine the orientation distribution functions of the OH and H-2 diatoms in various intermolecular states of the prereactive H-2-OH complex. Multidimensional quantum calculations have been conducted on a high-quality ab initio intermolecular potential energy surface to obtain the energies and body-fixed wave functions for the rovibrational states of H-2-OH. These calculations show that the H-2 and OH diatoms undergo nearly free internal rotation within the complex. However, the angular anisotropy of the intermolecular potential orients the OH and aligns the ortho-H-2 internal rotational motions within the complex. The relative orientation of the reactants is found to be well-defined and strongly intermolecular-state-dependent. Thus, by accessing different intermolecular states, the relative orientations of the reactants can be systematically manipulated. The degree of body-fixed orientation of OH in some bound states of H-2-OH, including the ground state of ortho-H-2-OH, approaches the highest degree of space-fixed orientation that has been achieved in hexapole orientation studies of OH. The experimental implications of the results are discussed.