Infrared Spectroscopy of Small Protonated Water Clusters, H⁺(H₂O)_n (n = 2−5): Isomers, Argon Tagging, and Deuteration

Abstract

Infrared photodissociation spectroscopy is reported for mass-selected H(+)(H(2)O)(n) complexes and their deuterated analogues with and without argon "tagging." H(+)(H(2)O)(n)Ar(m) and D(+)(D(2)O)(n)Ar(m) complexes are studied in the O-H (O-D) stretching region for clusters in the small size range (n = 2-5). Upon infrared excitation, these clusters fragment by the loss of either argon atoms or one or more intact water molecules. Their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water and in the hydrogen bonding region. Experimental studies are complemented by computational work that explores the isomeric structures, their energetics and vibrational spectra. The addition of an argon atom is essential to obtain photodissociation for the n = 2-3 complexes, and specific inclusion of the argon in calculations is necessary to reproduce the measured spectra. For n = 3-5, spectra are obtained both with and without argon. The added argon atom allows selection of a subset of colder clusters and it increases the photodissociation yield. Although most of these clusters have more than one possible isomeric structure, the spectra measured correspond to a single isomer that is computed to be the most stable. Deuteration in these small cluster sizes leads to expected lowering of frequencies, but the spectra indicate the presence of the same single most-stable isomer for each cluster size.

Detection of an intermediary, protonated water cluster in photosynthetic oxygen evolution

“…The 2,880-cm −1 frequency observed here is most consistent with a cluster of five hydrogen-bonded water molecules, termed here W 5 , which comprises a hydronium ion core (Fig. 1C) and which occupies a number of low-energy conformations (44). The 1.9-Å PSII structure shows that a network of bound water molecules connects the OEC to peptide carbonyl groups ( Fig.…”

“…Theoretical (34,35) and experimental (44) investigations demonstrate a correlation between O-O bond distance, number of water molecules, and frequency. The 2,880-cm −1 frequency observed here is most consistent with a cluster of five hydrogen-bonded water molecules, termed here W 5 , which comprises a hydronium ion core (Fig.…”

“…Previously, experimental (44) and theoretical (34, 35) methods have assigned broad infrared features in the 2,900-cm −1 region to the protonation of small clusters of four or five hydrogen-bonded water molecules. To test whether the 2,880-cm −1 band in Fig PSII failed to exhibit the 2,880-cm −1 band.…”

“…Therefore, failure to observe the 2,880-cm −1 band in strontium PSII (this work) may be due to a shift in the size of the water cluster in the strontium-containing OEC. In gas phase studies, changes in the size and amount of proton delocalization dramatically altered the frequency and intensity of such proton continuum bands (44).…”

Perspective: Water cluster mediated atmospheric chemistry

“…145,146 Elegant experiments made it possible to follow the evolution of vibrational spectra and of reactivity with cluster size, allowing for molecular level descriptions of hydrated clusters. [147][148][149][150] Important and general messages emerged from these studies about the role of the hydration structure around the ionic core in determining spectroscopic and chemical properties of these species. 151 Computational studies predict red shifts in the vibrational spectra of hydrates and emphasize the importance and necessity for anharmonic effects to be treated appropriately to describe their properties.…”