Unmasking Rare, Large-Amplitude Motions in D₂-Tagged I^–·(H₂O)₂ Isotopomers with Two-Color, Infrared–Infrared Vibrational Predissociation Spectroscopy

Abstract

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·(HDO)·(DO) and I·(HO)·(DO) systems, which are cryogenically cooled and D-tagged at an ion trap temperature of 15 K. These measurements reveal very large (>400 cm), 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.

Water Network Shape-Dependence of Local Interactions with the Microhydrated −NO2– and −CO2– Anionic Head Groups by Cold Ion Vibrational Spectroscopy

“…It is also possible that because many isomers are close in energy, the Ar tag is playing a role in fractionating the population among them. This is an interesting point and can be addressed using tag-free methods based on two-color, IR–IR dissociation of the bare ions, − thus representing a fruitful direction for further work on the small systems.…”

Gas-Phase Infrared Spectroscopy of Neutral Peptides: Insights from the Far-IR and THz Domain

“…Gas-phase infrared spectroscopy is a continuously developing method for the structural determination of biological molecules under isolated conditions. Vibrational or infrared spectroscopy brings detailed insights into the 3-dimensional structure of molecules and the intra- and intermolecular interactions via diagnostic vibrational modes. , Due to the close relationship between structure and structural changes with biomolecular function, IR action spectroscopy has been extensively applied to probe gas-phase structural details of a large variety of molecules and clusters such as peptides and proteins, − DNA bases, − glycans, − molecular motors, − metal complexes, − solvent clusters, − and astrochemical molecules. − …”

Specific Ion Effects on Hydrogen-Bond Rearrangements in the Halide–Dihydrate Complexes

“…Ion–water clusters are ideal systems to validate the ability of molecular models to characterize the molecular mechanisms associated with hydrogen-bonding (H-bonding) rearrangements, since, due to their relatively small sizes, they are amenable to high-level molecular modeling ,− and, at the same time, can be studied experimentally using high-resolution vibrational spectroscopy. − In this context, halide dihydrates hold a special place since they are the smallest complexes in which two water molecules can be simultaneously H-bonded to each other and to the ion, thus allowing for directly probing the interplay between ion–water and water–water interactions.…”