title: Gas phase vibrational spectroscopy of the protonated water pentamer: the role of isomers and nuclear quantum effects.
abstract: We use cryogenic ion trap vibrational spectroscopy to study the structure of the protonated water pentamer, H+(H2O)5, and its fully deuterated isotopologue, D+(D2O)5, over nearly the complete infrared spectral range (220-4000 cm-1) in combination with harmonic and anharmonic electronic structure calculations as well as RRKM modelling. Isomer-selective IR-IR double-resonance measurements on the H+(H2O)5isotopologue establish that the spectrum is due to a single constitutional isomer, thus discounting the recent analysis of the band pattern in the context of two isomers based on AIMD simulations 〈W. Kulig and N. Agmon, Phys. Chem. Chem. Phys., 2014, 16, 4933-4941〉. The evolution of the persistent bands in the D+(D2O)5cluster allows the assignment of the fundamentals in the spectra of both isotopologues, and the simpler pattern displayed by the heavier isotopologue is consistent with the calculated spectrum for the branched, Eigen-based structure originally proposed 〈J.-C. Jiang, et al., J. Am. Chem. Soc., 2000, 122, 1398-1410〉. This pattern persists in the vibrational spectra of H+(H2O)5in the temperature range from 13 K up to 250 K. The present study also underscores the importance of considering nuclear quantum effects in predicting the kinetic stability of these isomers at low temperatures.
authors.affiliation: Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstrasse 2, D-04103 Leipzig, Germany. knut.asmis@uni-leipzig.de and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany.
authors.forename: Matias R
authors.initials: MR
authors.lastname: Fagiani
authors.affiliation: Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstrasse 2, D-04103 Leipzig, Germany. knut.asmis@uni-leipzig.de.
authors.forename: Harald
authors.initials: H
authors.lastname: Knorke
authors.affiliation: Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstrasse 2, D-04103 Leipzig, Germany. knut.asmis@uni-leipzig.de.
authors.forename: Tim K
authors.initials: TK
authors.lastname: Esser
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany.
authors.forename: Nadja
authors.initials: N
authors.lastname: Heine
authors.affiliation: Department of Chemistry, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: Conrad T
authors.initials: CT
authors.lastname: Wolke
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany.
authors.forename: Sandy
authors.initials: S
authors.lastname: Gewinner
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany.
authors.forename: Wieland
authors.identifier: http://orcid.org/0000-0003-0564-203X
authors.initials: W
authors.lastname: Schöllkopf
authors.affiliation: Université d'Evry Val d'Essonne, UMR 8587 LAMBE, Boulevard F. Mitterrand, 91025 Evry Cedex, France and CNRS, Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement, UMR 8587, F-91025 Evry Cedex, France.
authors.forename: Marie-Pierre
authors.initials: MP
authors.lastname: Gaigeot
authors.affiliation: Université d'Evry Val d'Essonne, UMR 8587 LAMBE, Boulevard F. Mitterrand, 91025 Evry Cedex, France and CNRS, Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement, UMR 8587, F-91025 Evry Cedex, France.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Spezia
authors.affiliation: Department of Chemistry, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Johnson
authors.affiliation: Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstrasse 2, D-04103 Leipzig, Germany. knut.asmis@uni-leipzig.de.
authors.forename: Knut R
authors.initials: KR
authors.lastname: Asmis