Interplay of vibrational and van der Waals type bonding

The collinear IHI molecule and the I+HI reaction are studied using exact and approximate quantum mechanical techniques and employing two different potential energy surfaces: a minimum-free LEPS potential with a symmetrical IHI barrier and a semiempirical DIM-3C potential with a symmetrical barrier and two equivalent shallow wells for the I⋅⋅⋅HI and IH⋅⋅⋅I configurations. Comparison of the results for the two potentials yields a number of phenomena which are characteristic for vibrational bonding and its interplay with weak attractive forces: (i) The IHI→I+HI dissociation energy contains a contribution of approximately 9 kJ mol−1 due to vibrational bonding irrespective of the details of the potential. (ii) The symmetrical stretching frequency has a minimum value of ≳100 cm−1 due to vibrational bonding. (iii) Vibrational bonding reduces the average ground state interiodine distance from the large value implied by the van der Waals type minimum towards the saddle point configuration. (iv) Pure vibrational bonding allows only four collinear bound states with gerade parity, whereas the interplay of van der Waals type bonding enhances the number of bound states, including also levels with ungerade symmetry. The highest vibrationally excited levels describe very nonsymmetric I⋅⋅⋅HI or IH⋅⋅⋅I configurations. (v) Simultaneously, the interplay of both bonding mechanisms increases the number of scattering resonance energies.