Probing the dependence of long-range, four-atom interactions on intermolecular orientation: 3. hydrogen and iodine

Two-laser, action spectroscopy experiments have been performed in the I₂B-X, ν′-0 spectral region on H ₂•••I₂ and D₂••• I₂ complexes to investigate the dependence of the H ₂/D₂ + I₂ intermolecular interactions on orientation. The spectra contain features associated with at least two different conformers of the ground-state H₂/D₂••• I₂(X,ν′′ = 0) complexes; one conformer has a preferred T-shaped geometry with the H₂/D₂ moiety localized in a potential minimum that is orthogonal to the I-I bond axis, and the second conformer has a linear geometry with the H₂/D₂ moiety positioned in minima at either end of the I₂ molecule, along the bond axis. Those features associated with complexes containing para-H₂(j = 0), ortho-H₂(j = 1), ortho-D₂(j = 0), and para-D ₂(j = 1) are also assigned. The linear conformers are found to be more strongly bound than the T-shaped conformers with binding energies of 118.9(1.9) cm^-1 versus 91.3-93.3 cm^-1 for the ortho-H ₂•••I₂ complexes and 144.2(2.1) cm ^-1 versus 107.9 cm^-1 for the para-D₂• ••I₂ complexes, respectively. Electronic structure calculations of the complexes containing ICl and I₂ with H ₂, He, Ne, and Ar were performed to reveal the nature of the interactions and to shed insight into the origins of the different binding energies. The most stable minima in the H₂/D₂ + I ₂(B,ν′) excited-state potentials have T-shaped geometries. Calculated energies and probability amplitudes of the excited-state levels provide insight into the different excited-state intermolecular vibrational levels accessed by transitions of the two ground-state conformers.