The Grotthuss mechanism explains the anomalously high proton mobility in water as a sequence of proton transfers along a hydrogen-bonded (H-bonded) network. However, the vibrational spectroscopic signatures of this process are masked by the diffuse nature of the key bands in bulk water. Here we report how the much simpler vibrational spectra of cold, composition-selected heavy water clusters, D⁺(D₂O)_n, can be exploited to capture clear markers that encode the collective reaction coordinate along the proton-transfer event. By complexing the solvated hydronium “Eigen” cluster [D₃O⁺(D₂O)₃] with increasingly strong H-bond acceptor molecules (D₂, N₂, CO, and D₂O), we are able to track the frequency of every O-D stretch vibration in the complex as the transferring hydron is incrementally pulled from the central hydronium to a neighboring water molecule.

Wet surface sightings in clusters: In principle, the surface structure of water (H ₂ O) should be discernable from the O–H vibrations. In practice, however, so many configurations rapidly interconvert that the bands are bewilderingly broad. Yang et al. studied a cluster of 20 H ₂ O surrounding a cesium ion, using isotopomers that vary in the position of one H ₂ O amid 19 heavy water (D ₂ O) molecules. Precisely assigned spectral features from contributing configurations mapped well onto a bulk surface spectrum. Science , this issue p. 275

Unimolecular decay of Criegee intermediates produced in alkene ozonolysis is known to be a significant source of OH radicals in the troposphere. In this work, unimolecular decay of the methyl-substituted Criegee intermediate, syn-CH₃CHOO, to OH products is shown to occur at energies significantly below the transition state barrier for a 1,4 hydrogen transfer that leads to these products [Y. Fang et al., J. Chem. Phys. 144, 061102 (2016)]. The rate of appearance of OH products arising from tunneling through the barrier is obtained through direct time-domain measurements following the vibrational activation of syn-CH₃CHOO. IR excitation of syn-CH₃CHOO at energies nearly 2000 cm⁻¹ below the barrier is achieved through combination bands involving CH stretch and another lower frequency mode, and the resultant OH products are detected by UV laser-induced fluorescence. The observed syn-CH₃CHOO combination bands in the 4100–4350 cm⁻¹ region are identified by comparison with the computed IR absorption spectrum. The experimental decay rates are found to be ca. 10⁶ s⁻¹ in this deep tunneling regime, which is approximately 100-times slower than that in the vicinity of the barrier.The experimental results are consistent with statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the microcanonical decay rates with tunneling through the barrier, and notable deviations may originate from the sparsity in the density of states for syn-CH₃CHOO at lower energies. Thermal unimolecular decay of syn-CH₃CHOO is predicted to have significant contribution from microcanonical rates at energies that are much below the barrier.

Unimolecular decay of the dimethyl substituted Criegee intermediate (CH₃)₂COO is observed at energies significantly below the transition state barrier associated with hydrogen atom transfer [Y. Fang et al., J. Chem. Phys. 144, 061102 (2016)] with time-resolved detection of the resultant OH radical products. (CH₃)₂COO is prepared at specific energies in the 3900-4600 cm⁻¹ region through IR excitation of combination bands involving CH stretch and another lower frequency mode, and the OH products are detected by UV laser-induced fluorescence. OH appearance times on the order of microseconds are observed in this deep tunneling regime, which are about 100 times slower than that in the vicinity of the barrier. The experimental rates are in good accord with Rice–Ramsperger–Kassel–Marcus (RRKM) calculations of the microcanonical dissociation rates for (CH₃)₂COO that include tunneling. Master equation modeling based on these microcanonical rates is used to predict the thermal decay rate of (CH₃)₂COO to OH products under atmospheric conditions of 276 s⁻¹ at 298 K (high pressure limit). Thermal unimolecular decay of (CH₃)₂COO to OH products is shown to have significant contributions from tunneling at energies much below the barrier to H-atom transfer.