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Abstract

The strong temperature dependence of the I⁻·(H₂O)₂
vibrational predissociation spectrum is traced to the intracluster dissociation
of the ion-bound water dimer into independent water monomers that remain
tethered to the ion. The thermodynamics of this process is determined using
van’t Hoff analysis of key features that quantify the relative populations of Hbonded
and independent water molecules. The dissociation enthalpy of the
isolated water dimer is thus observed to be reduced by roughly a factor of
three upon attachment to the ion. The cause of this reduction is explored with
electronic structure calculations of the potential energy profile for dissociation
of the dimer, which suggest that both reduction of the intrinsic binding energy
and vibrational zero-point effects act to weaken the intermolecular interaction
between the water molecules in the first hydration shell. Additional insights
are obtained by analyzing how classical trajectories of the I⁻·(H₂O)₂ system
sample the extended potential energy surface with increasing temperature.