Infrared photodissociation spectroscopy of H+(H2O)6·Mm (M = Ne, Ar, Kr, Xe, H2, N2, and CH4): messenger-dependent balance between H3O+ and H5O2+ core isomers

Abstract

Although messenger mediated spectroscopy is a widely-used technique to study gas phase ionic species, effects of messengers themselves are not necessarily clear. In this study, we report infrared photodissociation spectroscopy of H(+)(H(2)O)(6)·M(m) (M = Ne, Ar, Kr, Xe, H(2), N(2), and CH(4)) in the OH stretch region to investigate messenger(M)-dependent cluster structures of the H(+)(H(2)O)(6) moiety. The H(+)(H(2)O)(6), the protonated water hexamer, is the smallest system in which both the H(3)O(+) (Eigen) and H(5)O(2)(+) (Zundel) hydrated proton motifs coexist. All the spectra show narrower band widths reflecting reduced internal energy (lower vibrational temperature) in comparison with bare H(+)(H(2)O)(6). The Xe-, CH(4)-, and N(2)-mediated spectra show additional band features due to the relatively strong perturbation of the messenger. The observed band patterns in the Ar-, Kr-, Xe-, N(2)-, and CH(4)-mediated spectra are attributed mainly to the "Zundel" type isomer, which is more stable. On the other hand, the Ne- and H(2)-mediated spectra are accounted for by a mixture of the "Eigen" and "Zundel" types, like that of bare H(+)(H(2)O)(6). These results suggest that a messenger sometimes imposes unexpected isomer-selectivity even though it has been thought to be inert. Plausible origins of the isomer-selectivity are also discussed.

Cryogenic ion trap vibrational spectroscopy of hydrogen-bonded clusters relevant to atmospheric chemistry

“…The introduction of IR 2 MS 2 spectroscopy [117] has elegantly solved the UV/VIS chromophore problem, extending the applicability of this technique. Until recently [168,169], it was generally accepted that the protonated water hexamer is the smallest protonated water cluster for which both, an Eigen-type and a Zundel-type isomer (see Figure 2), coexist [170,171]. Here, we take this technique a step further, combining it with the widely tunable radiation from the IR-FEL FELIX to isomer-specifically probe the IR spectrum of mass-selected, cryogenically cooled clusters over nearly the complete IR spectral range [79].…”

“…Johnson and coworkers successfully applied IR 2 MS 2 to the study of mass-selected cluster ions in the spectral region above 2000 cm −1 , where table-top lasers provide sufficiently intense laser pulses [117,[164][165][166][167]. The gas phase vibrational spectroscopy of the isomeric mixture of H + (H 2 O) 6 has been studied previously in the O-H stretching region [170,171]. Protonated water clusters are not only abundantly present in the earth's atmosphere, but they also serve as important model systems for studying solvent-mediated chargedelocalisation and migration processes in aqueous solution.…”

Hydrogen-bonded ring closing and opening of protonated methanol clusters H+(CH3OH)n (n = 4–8) with the inert gas tagging

“…The combination of these two techniques has also recently been employed to obtain a high and constant dissociation yield in dissociation spectroscopy of clusters in a 4 cold ion-trap. 4,[30][31][32][33][34][35][36] Methanol is also a typical protic solvent, and hydrogen-bond (H-bond) network structures of protonated methanol clusters, H + (MeOH) n , have been studied by infrared (IR) spectroscopy and density functional theory (DFT) calculations. 1-5, 8-11, 16-19, 22-29 Unexpected preference of isomer structures, which deviates from the assumption of the simple thermal equilibrium, has been found for the water network with the tagging, and its origin has not yet been fully clarified.…”

“…[32][33][34][35] Additional experiments are performed and the origin of the isomer selectivity is discussed. This anomaly is quite similar to the isomer selectivity found in the tagging of H + (H 2 O) n .…”

Competing Insertion and External Binding Motifs in Hydrated Neurotransmitters: Infrared Spectra of Protonated Phenylethylamine Monohydrate

“…Such ad rastic effect of cooling by Rg tagging on the relative energy ordering and isomer population hasb een noticed previously for hydrated ChemPhysChem 2016, 17,232 -243 www.chemphyschem.org and solvated aromatic cluster ions, [53,60] and protonatedw ater and methanolc lusters. [61][62][63] It further demonstrates the importance of cooling the clusters for the evaluation of the most stable isomers on the potentiale nergys urface, which may either be achieved by Rg tagging of the ions (to enable singlephoton IR photodissociation of cold clusters) or by cooling the bare ions in ac ryogenic ion trap (whicht hen requires IR multiple-photondissociation for the strongly bound cluster ions).…”