A velocity map ion imaging study of difluorobenzene-water complexes: binding energies and recoil distributions
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The binding energies of the p-, m-, and o-difluorobenzene-H2O complexes have been measured by velocity map ion imaging to be 922±10, 945±10, and 891±4 cm−1, respectively. The lack of variation provides circumstantial evidence for water binding to the three isomers via the same interaction, viz. an in-plane O–H⋯F hydrogen bond to one of the fluorine atoms on the ring, with a second, weaker interaction of the water O atom with an ortho hydrogen, as determined previously for the p-difluorobenzene-H2O complex. The ground state binding energies for the difluorobenzene-H2O complexes are ∼ 5%–11% larger than that for benzene-H2O, where binding occurs to the π electrons out-of-plane. However, in the S1 state the binding energies of the o- and p-difluorobenzene-H2O complexes are smaller than the benzene-H2O value, raising an interesting question about whether the geometry at the global energy minimum remains in-plane in the excited electronic states of these two complexes. Recoil energy distributions for dissociation of p-difluorobenzene-H2O have been measured from the 3 1, 5 2, and 3 1 5 1 levels of the excited electronic state. These levels are 490, 880, and 1304 cm−1, respectively, above the dissociation threshold. Within the experimental uncertainty, the recoil energy distributions are the same for dissociation from these three states, with average recoil energies of ∼ 100 cm−1. These recoil energies are 60% larger than was observed for the dissociation of p-difluorobenzene-Ar, which is a substantially smaller increase than the 400% seen in a comparable study of dissociation within the triplet state for pyrazine-Ar, -H2O complexes. The majority of the available energy is partitioned into vibration and rotation of the fragments.