Probing the dependence of long-range, four-atom interactions on intermolecular orientation. 1. Molecular hydrogen and iodine monochloride.

The dependence of the long-range interactions between molecular hydrogen and iodine monochloride on the geometry between the molecules is investigated. Laser-induced fluorescence and action spectroscopy experiments have identified multiple conformers of the o,p-H2...I35,37Cl(X,v' '=0) van der Waals complexes. A conformer with the hydrogen molecule localized at the iodine end of the dihalogen, most likely with C2v symmetry, is significantly more stable than an asymmetric conformer with the hydrogen localized in the well oriented orthogonally to the I-Cl bond axis, D0' ' = 186.4(3) cm-1 versus 82.8(3) <or= D0' ' <or= 89.6(3) cm-1. Complexes containing the o-H2(j=1) species are more strongly bound than those with p-H2(j=0). The electronically excited o,p-H2...I35Cl(A,v') and o,p-H2...I35,37Cl(B,v') complexes are found to have preferred asymmetric structures with binding energies bracketed between 73.7-80.5 and 69.5-76.3 cm(-1) for o-H2... I35Cl(A,v'=23) and o-H2...I35Cl(B,v'=3), respectively. Calculations of the H2...I35Cl(B,v'=3) intermolecular vibrational energies and probability amplitudes undertaken using a scaled He + ICl(B,v'=3) potential enable us to make tentative assignments of the excited-state levels experimentally accessed.