Gas phase vibrational spectroscopy of the protonated water pentamer: the role of isomers and nuclear quantum effects.

Matias R Fagiani,Marie-Pierre Gaigeot,Sandy Gewinner,Conrad T Wolke,Riccardo Spezia,Nadja Heine,Knut R Asmis,Mark A Johnson,Harald Knorke,Wieland Schöllkopf,Tim K Esser

doi:10.1039/c6cp05217g

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)5 isotopologue 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)5 cluster 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)5 in 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.

Highlights

How ions are hydrated has intrigued chemists since the birth of the field of Physical Chemistry.[1]
The widely accepted structural assignment of small protonated water clusters, in particular that of the protonated water tetramer and pentamer, has recently been drawn into question based on the implications of ab initio molecular dynamics (AIMD) simulations.[3,4]
The scaling factor is close to the theoretical value of 1/1.37 that is expected for isolated O–H vs. O–D oscillators

Summary

Introduction

How ions are hydrated has intrigued chemists since the birth of the field of Physical Chemistry.[1] Proton hydration, in particular, is of fundamental importance in chemical, biological and atmospheric processes, but the molecular level interpretation of the anomalously high mobility of protons in aqueous solutions remains hotly debated.[2] Size-selected protonated water clusters, H+(H2O)n, isolated in the gas phase, play an important role in clarifying the spectroscopic markers that encode the collective mechanics involved in long range proton translocation. The advantage of isolated clusters is that they allow direct spectroscopic characterization of key local hydration motifs that are metastable in solution, but can occur naturally in the size- and temperature-dependent structures of the gas-phase clusters. The widely accepted structural assignment of small protonated water clusters, in particular that of the protonated water tetramer and pentamer, has recently been drawn into question based on the implications of ab initio molecular dynamics (AIMD) simulations.[3,4]

Methods

Results

Conclusion