We
describe a two-color, isotopomer-selective infrared–infrared
population-labeling method that can monitor very slow spectral
diffusion of OH oscillators in H‑bonded networks and apply
it to the I<sup>–</sup>·(HDO)·(D<sub>2</sub>O) and I<sup>–</sup>·(H<sub>2</sub>O)·(D<sub>2</sub>O) systems, which are cryogenically cooled
and D<sub>2</sub>-tagged at an ion trap temperature of 15 K. These
measurements reveal
very large (>400 cm<sup>–1</sup>), spontaneous spectral
shifts
despite the fact that the predissociation spectra in the OH stretching
region of both isotopologues are sharp and readily assigned to four
fundamentals of largely decoupled OH oscillators held in a cyclic
H-bonded network. This spectral diffusion is not observed in the untagged
isotopologues of the dihydrate clusters that are generated under the
same source conditions but does become apparent at about 75 K.
These results are discussed in the context of the large-amplitude “jump”
mechanism for H-bond relaxation dynamics advanced by Laage and Hynes
in an experimental scenario where rare events can be captured by following
the migration of OH groups among the four available positions in the
quasi-rigid equilibrium structure
